Simply Wood Design of All-Wood Furniture Joints

Master Thesis

Author: Moritz Braun Supervisor: Stergios Adamopoulos, Christer Collin (IKEA of Sweden) Examiner: Krushna Mahapatra Term: VT20 Subject: Engineering with specialization in Innovation

Level: Master Course code: 5TS04E

Abstract

The need for sustainability, increasingly requires developing products according to a cradle to cradle approach. For furniture, wood, being potentially renewable, is a suitable material in this regard. However, most wooden furniture today utilizes steels and synthetic polymers in the joints, which can partially be recycled, but are far from being circular materials. All-wood joints have been used in traditional furniture and construction, but they are not adapted to modern manufacturing techniques and do not fulfill the need for easy assembly. The aim of this thesis is to explore existing solutions for all-wood joints, as well as relevant manufacturing techniques to create an approach for the development of new joints by practitioners. The research questions are: What are the principles used in existing technical solutions and how can they be used to develop new all-wood furniture joints? What are the most important manufacturing techniques for wood today and how can they be considered in the early-stage development? The general approach in this thesis is to abstract the researched existing technical solutions and manufacturing techniques, ideate on this abstract level, and then detail the concepts on a more concrete level. As results, fourteen different principles and six different patterns of transformation were extract- ed from existing solutions and documented in an accessible form. Similarly, seven manufacturing techniques were collected and documented. These were then used in an ideation workshop with practitioners from IKEA, which resulted in six abstract concepts. One of the concepts was further developed into a pre-design and tested with a simulations according to strength and stability requirements from applicable standards. The testing of the pre-design proved its practicality and a team at IKEA is continuing the development of the concept and planning to manufacture a prototype. This is a good indicator for the usefulness of the approach. Even though it worked well, further exploration of the "toolbox" is recommended, as well as the use of different ideation methods. The full environmental benefits of furniture with all-wood joints are not clear, because only resource depletion was considered and the potential effect is small compared to other industries. Despite this, the thesis shows the potential in circular furniture and encourages IKEA and other furniture companies to delve into the topic of circular furniture more deeply.

Keywords: Circularity, Mass-Produced Furniture, Product Development, Wood Joints Acknowledgments

I would like to thank my academic supervisor, Stergios Adamopoulos, and my supervisor at IKEA, Christer Collin, for the mentoring and guidance during the project. Furthermore, I would like to thank the inspirational team at IKEA who participated in the ideation and pre-design, as well as the people who made the collaboration possible, and the family members who supported me during the project. Table of Contents

Abstract ��ii

Acknowledgments ��iii

1 Introduction ��1

1.1 Circularity and Environmental Considerations �� 1

1.2 Conventional Joints in Furniture �� 2

1.3 Sustainability Challenges for Wooden Furniture �� 3

1.4 Purpose, Research Question and Delimitations �� 5

2 Theory ��6

3 Methods ��7

3.1 Research Method for Existing Solutions �� 7

3.2 Research of Manufacturing Capabilities �� 8

3.3 Ideation Method �� 9

3.4 Method for Pre-Design and Testing �� 10

4 Results �� 11

4.1 Principles ��11

4.2 Patterns of Transformation �� 22

4.3 Manufacturing Capabilities �� 25

4.4 Ideation Results �� 29

4.5 Pre-Design of Concept 6 �� 32

4.6 Testing of Pre-Design and Improvements �� 36

5 Conclusion ��39

References ��40 1 Introduction

1.1 Circularity and Environmental Considerations

With the emergence of the environmental movement in North America and Europe in the second half of the 20th century, concerns over the sustainability of societies way of consumption started to arise. In 1972 Meadows et al. conducted a particularly influential study named “The Limits to Growth” where they simulated the interactions between the earth and human systems, consider- ing the variables: population, food production, industrialization, pollution, and consumption of non-renewable natural resources. They concluded:

“If the present growth trends in world population, industrialization, pollution, food production, and resource depletion continue unchanged, the limits to growth on this planet will be reached sometime within the next 100 years. The most probable result will be a rather sudden and uncontrolled decline in both population and industrial capacity.” (Meadows, et al., 1972, p. 23)

The real development of the variables since 1972 has closely matched the simulated trends in the standard simulation run, and a 30-year update of the study came to a similar conclusion as the original study. (Meadows, et al., 2004) To prevent an eventual socioeconomic collapse by exceeding planetary limits, the researchers suggested, among other things, efficient, closed- loop materials systems and technical design that reduces emissions and waste to a minimum. (Meadows, et al. 2004) In their manifesto “Cradle to Cradle: Remaking the way we make things”, Braungart & Mc- Donough (2002) expanded on how the approach of designing and manufacturing products needs to change to consider environmental limits. According to Braungart & McDonough, the main problem here is the cradle to grave pattern, in which products at the end of their life-cycle end-up in landfills, incinerators or aredown-cycled for low-value uses. Instead, they propose a cradle to cradle pattern, i.e. closed loop material systems. In a cradle to cradle approach the materials used in a product are seen as nutrients which never become waste but serve as the building blocks for new products or natural life, mimicking the processes in nature. Biological nutrients, according to the framework, are materials originating from the biosphere, which can safely biodegrade. Technical nutrients, on the other hand, are man- made materials, often from limited resources. Technical nutrients should ideally be used in a way which keeps them out of the biosphere and makes it possible to reuse the materials at the end of life of the product without a loss in material properties. When the possibility exists, that a product or part of a product could end up in the biosphere, it should be made of biological nutrients, which come from renewable sources and can safely biodegrade. (Braungart & McDonough, 2002) According to the cradle to cradle approach, technical nutrients and biological nutrients should therefore not both be used in the same product, unless they can be easily separated. Otherwise, the materials can neither be fully reused for new products or safely become part of the biosphere again. A mix of technical- and biological nutrients such as blended textiles made of polyethylene-

1 Introduction Conventional Joints in Furniture and cotton fibers, allows neither effective recycling nor natural breakdown such as composting. (Braungart & McDonough, 2002) The concept of closed-loop material systems is also part of similar frameworks to "Cradle to Cradle", most notably the framework “Circularity” or “Circular Economy”. (Geissendoerfer, et al., 2017) The frameworks address the Goal 12 in the Agenda 2030 Sustainable Development Goals: Ensure sustainable consumption and production patterns, specifically Target 12.5: By 2030, substantially reduce waste generation through prevention, reduction, recycling and reuse, as well as Goal 13: Take urgent action to combat climate change and its impacts. (UN General Assembly, 2015)

1.2 Conventional Joints in Furniture

A wide variety of different joints are available today for the use in furniture. All-wood joints have been used in traditional carpentry for millennia and can still be found in old building and furniture around the world. Because these often require high-grade wood, high precision and skilled manual labor, traditional all-wood joints were gradually replaced with more economical methods of joining furniture parts, which became available throughout the years. An example of this from the building sector is the gradual replacement of timber frame construction with balloon frame construction. This transition became possible through the availability of industrially manufactured low-cost nails and increasingly automated sawmills. (Cronon, 2018) The nailed connections in balloon frame construction require considerably less skill and precision than the manually cut, tight-fitting joints in traditional timber frame construction. (Cronon, 2018) A similar development has taken place for joints in wooden furniture. Today, the use of all-wood joints in industrialized countries is almost entirely limited to high-cost custom furniture, while joints in industrially produced furniture utilize adhesives or fasteners made of steel or various polymers. These require less skilled manual labor and are generally easier and cheaper to manufacture and assemble than traditional solutions. The most common joints in industrially produced furniture today make use of adhesives, dowels, screws, nuts and bolts, and cam connectors, see Figure 1. (Nutsch, 2005) Much of the furniture sold today is self-assembled by the customers. IKEA, well known for its self-assembly furniture, has a global market share of 8% and is far from being the only manu- facturer and retailer of self-assembly furniture. (Statista 2020, Statista 2021) This self-assembly furniture saves cost in manufacturing and logistics and makes transport easier for the customers because of smaller packaging. As a result, the joints in this furniture must be easy to assemble, possible even for inexperienced customers with only basic tools. According to Inter IKEA Sys- tems (2017), the demand for easier assembly of self-assembly furniture is growing, as customers are increasingly less willing to perform complex assemblies.

A B C D E F

Figure 1 Common connectors used in wooden furniture: cam lock screw (A), cam lock nut (B), barrel nut (C), wood screw (D), plastic shelf pin (E), wooden dowel (F) (Image source: Furnitureparts.com, 2020)

2 Introduction Sustainability Challenges for Wooden Furniture

The mentioned joint types generally fulfill the requirement for easy-assembly, with the exception of adhesives which are therefore usually only used in pre-assembled joints. The basic working principle of the screws, nuts and bolts, and cam connectors is a utilization of the wedge principle to leverage assembly forces to pull the joint surfaces together and stay in place. Tension forces in the assembled joint are then transferred by the connector, while compression forces are transferred by the joint surfaces. Shear forces are transferred by friction between the joint surfaces, the screw or bold, or a dowel. Pins are usually used in joints where the transferred forces are oriented mainly in one direction only, such as in shelf boards. (Nutsch, 2005)

1.3 Sustainability Challenges for Wooden Furniture

Wooden furniture has obvious potential in terms of circularity. If sustainably sourced, the material wood has a negative carbon footprint, as CO2 from the air is sequestered during tree growth. Also, resource depletion through use of wood is marginal, as wood is a renewable material if sustainable forestry practices are used. However, most wooden furniture today not only consist of biological nutrients like wood, but also of technical nutrients. Usually steel, and sometimes other metals like aluminum, is used in connectors, hinges, handles, etc., and various petro-based polymers are used in connectors, coatings, and adhesives. While circular products can also consist of technical nutrients, it has to be possible to separate these and recycle them for high-value uses. There are several issues with this today. For steel parts, and metal parts in general, the separation from the furniture after use does not pose a great difficulty. In Europe wooden furniture is typically ground up into small particles for thermal recycling or recycling in particleboard. (Kharazipour & Kües, 2007) Metal parts can then be separated by utilizing the density differences and steel parts and in particular by utilizing the magnetic properties. (Kharazipour & Kües, 2007) Metals allow infinite recycling in theory, but in practice there are several issues. (Reck & Graedel, 2012) While iron and steel have one of the highest recycling rates of all metals, it is estimated that 70-90% is recycled at end-of-life, while 10-30% are lost. (Graedel et al., 2011) Because of the increased use of steel over time, the recycled steel scrap only accounts for around 40% of the material in steel production. (Graedel et al., 2011) Degrading of material properties also takes place as result of the recycling process and insufficient sorting capabilities. One mechanism for this is that alloying metals such as chrome and nickel, which are indispensable ingredients of high quality alloy steel such as austenitic stain- less steel, get recycled into steel mixes where they do not pose any benefits to material properties. A study by Nakamura et al. (2017) found that only 30-70% (depending on level of scrap sorting) of initial functionality of chrome and nickel can be retained over a period of 100 years. Another problem are so called tramp metals, such as copper and tin in steel, which severely reduce material strength if concentrations are too high. (Reck & Graedel, 2012) Copper, in particular, currently cannot be removed commercially once in the melt and is pervasive in end-of-life steel scrap. (Daehn, Cabrera Serrenho, & Allwood, 2017) Through recycling, the concentration of copper in steel is rising and is projected to reach the tolerable limits in the global steel system in the next decades with current practices. As a result, the fraction of end-of-life scrap which can be used in steel production would be limited. (Daehn, Cabrera Serrenho, & Allwood, 2017) So while steel has one the highest recycling rates among metals, it is currently far from being a circular material. For technical polymers used in wooden furniture the recycling rates are much lower. Partially this is due to a greater difficulty of separation from furniture at end-of-life than for metals, mainly due to the overlap in density spectrum of polymers and wood. (Kharazipour & Kües, 2007) If the furniture is not incinerated or landfilled, but, for example, recycled as particleboard this difficulty to separate can lead to high amounts of polymer particles in the particleboard which can be detrimental to the mechanical properties of the particleboard. (Kharazipour & Kües, 2007) For

3 Introduction Sustainability Challenges for Wooden Furniture the polymer parts which are separated from wooden furniture the standard procedure in Europe is incineration. (Kharazipour & Kües, 2007) If the separated polymer parts instead get into recycling streams, the recycling rates are still relatively low. In a study by Faraca & Astrup (2019) plastic waste at three Danish recycling centers was sampled and characterized. The recycling potential for hard plastics in general was found to be 52%, which can be expected to be lower for separated polymers from wooden furniture because of a larger presence of mixed materials, impurities, and non-thermoplastic polymers. The polymers which are recycled are most likely used in low-quality applications because of the lower chemical purity and decreased mechanical properties compared to virgin materials. (Faraca & Astrup, 2019) From a circularity perspective, petro-based polymers are therefore suboptimal for the use in wooden furniture. An alternative to petro-based polymers are bio-based plastics, adhesives, and coatings. Bio-based plastics are widely used today, especially in packaging, and their use is increasing. (Ramesh Kumar, Shaiju & O’Connor, 2020) In 2018 bio-plastics accounted for around 2 million tonnes (European Bioplastics, 2018) out of 359 million tonnes of plastic produced globally (Plastics Europe, 2019). Generally, similar functionality to the most commonly used conventional plastics can be achieved with bio-plastics. (Ramesh Kumar et al., 2020). Extensive research on renewable, less toxic, and recyclable replacements of the mineral-based adhesives and coatings is ongoing (Pizzi, 2006; Hemmliä, Adamopoulos, et al., 2017) and implementations in the industry can be expected in the coming years. IKEA, for example, is planning a stepwise introduction of renewable adhesives as part of its goal to use 100% renewable or recycled material in the production of furniture by 2030. (Inter IKEA Systems, 2020, p. 22, p.30) However, while use of the non-renewable resource petroleum, as well as green-house-gas emissions are significantly lower for bio-plastics (Weiss et al. 2012), they bring other problems. The negative aspects are mainly results of the use of intensive agriculture for the production of the base materials for bio-plastics. A meta study of 44 life cycle analyses of petro-based and bio- based plastics concluded:

The reviewed literature suggests that one metric ton (t) of biobased materials saves, relative to conventional materials, 55 ± 34 gigajoules of primary energy and 3 ± 1 t carbon dioxide equivalents of greenhouse gases. However, biobased materials may increase eutrophication by 5 ± 7 kilograms (kg) phosphate equivalents/t and strato- spheric ozone depletion by 1.9 ± 1.8 kg nitrous oxide equivalents/t. [...] Additional land use impacts, such as the potential loss of biodiversity, soil carbon depletion, soil erosion, deforestation, as well as greenhouse gas emissions from indirect land use change are not quantified in this review. (Weiss et al. 2012)

While no conclusive evaluations are available for bio-based adhesives and coatings, similar results can be expected because of the similar base materials. So from a circularity perspective it is desirable to limit not only the use of petro-based materials, but also of bio-based plastics, adhesives, and coatings, unless sustainable sourcing can be ensured.

4 Introduction Purpose, Research Question and Delimitations

1.4 Purpose, Research Question and Delimitations

Purpose The purpose of this thesis is to examine the possibilities of completely circular furniture. Wooden furniture has clear potential in this regard, as wood can be a renewable resource. However, most wooden furniture also consists of parts made out of metal and synthetic polymers, which are less favorable than wood in terms of resource depletion. In particular, this thesis examines the possibilities of solid wood joints which do not require the use of metal- or polymer connectors and allow self-assembly by customers. The aim is to explore and categorize existing solutions for these joints as well as to create a guide for practitioners to develop new technical solutions for circular furniture. For such joints to be used in furniture accessible to many people, the manifacturing has to be automated, in a relatively low-cost way. Because of this, the capabilities and limitations of modern, automated manufacturing techniques for wood are also examined and abstracted, so they can be taken into account during early-stage development of new joints.

Research Question What are the principles used in existing technical solutions and how can they be used to develop new all-wood furniture joints?

What are the most important manufacturing techniques for wood today and how can they be considered in the early-stage development?

Delimitations The focus of this thesis is on enabling practitioners to develop new joint types, not on detailed development of joints itself. The more detailed development of a specific joint type in this thesis therefore only serve the purpose of showing how the process can be performed. The result is not a finished product and would require further development to be implemented in practice. While wood-based materials such as particleboards with bio-based adhesives can have similar potential in terms of circularity, this thesis focuses on solid wood and plywood.

5 2 Theory

The methodology is based on the development approach described by Ponn & Lindemann (2011) in “Concept Development and Design of Technical Products” (Konzeptentwicklung und Gestal- tung Technischer Produkte). This development approach focuses on utilizing different product models and their level of abstraction. Ponn & Lindemann (2011, p.34 ff.) divide product models into three different levels of abstraction:

• The Function Level (Funktionsebene) • The Effect Level (Wirkebene) • The Component Level (Bauebene)

Models on the Function Level describe what a product or component does, i.e. why it is needed in the first place. The Effect Level includes models which describe how a product or component works and what physical (or chemical or biological) effects it is utilizing. The Component Level is the most concrete and includes models describing detailed geometries, materials, and how a product or component can be manufactured and assembled. Approaching the development of a product or component on a more abstract level generally allows a broader range of solutions to be considered and allows finding more innovative solutions. (Ponn & Lindemann, 2011, p.34 ff.) For this project, the functional aspects are relatively clear: The furniture joints must be possible to be assembled easily and must reliably transmit forces and restrict movement between the different parts. Since the project is aimed at the development of new types of joints, it is mostly concerned with the Effect Level. This means that found existing solutions need to be abstracted, so that the underlying principles can be used to ideate new solutions on the Effect Level. An example of this abstraction can be found in Figure 2. After ideation on the Effect Level, the resulting abstract concepts need to be detailed to obtain more concrete solutions on the Component Level.

Figure 2 Underlying principles of the IKEA LISABO table joint (Image source: Inter IKEA Systems., 2017).

6 3 Methods

3.1 Research Method for Existing Solutions

Existing solutions for all-wood furniture joints were collected through a literature review of books on traditional joinery and international patent literature. The found solutions were grouped and abstracted to the Effect Level (see previous chapter). The resulting collection of the principles underlying the researched joints can be found in Chapter 4.1 with a description and a reference to the researched joints they are abstracted from. The research on traditional all-wood joints was done in relevant literature, mainly “The complete Japanese joinery” (Sato & Nakahara, 1995), “Wood and wood joints: building traditions of Europe, Japan and China” (Zwerger, 2012), “The complete illustrated guide to Joinery” (Ro- gowski, 2002). Included in the research were all-wood furniture joints, as well as joints which would typically be assembled with adhesives, but where the use is theoretically not necessary. Also included where timber framing joints, which have similar requirements to furniture joints, but their larger size allows for higher geometrical complexity. The research for recent inventions was mainly done through espacenet.com (Search Platform of the European Patent Office), depatisnet.dpma.de (Search Platform of the German Patent Office), and patents.google.com (Google Patents). An example of search parameters used can be found in Figure 3. A few examples of connections which were already familiar to the author were also included. The research was focused on wooden furniture joints with a wide variety of different principles.

no-fastener OR no-metall connector OR glueless OR connection wood furniture AND AND AND OR fastener-free OR assembly OR wooden OR screwless OR construction OR metalfree OR metal-free

Figure 3 Example of search parameters used.

7 Methods Research of Manufacturing Capabilities

division

Figure 4 Example of a “Pattern of Transformation” (Zwerger, 2012, p.96; World Patent No. 2013/104422 A1, 2012; Inter IKEA Systems., 2017).

Chapter 4.1 also includes a list of several patterns of transforming existing solutions into new solutions, as observed in patents. In Figure 4, for example, the joint on the right can be seen as the joint on the left divided into smaller sections. This is similar to the approach used by Genrich Altshuller (1999) to develop a general methodology for inventive problem solving (TRIZ). Altshuller (1999) reviewed around 40’000 patents to derive 40 patterns broadly applicable patterns to inventive problem-solving. Similarly, in this project, the observed patterns of transformation for wood joints were collected to apply them in the development of new wood joints.

3.2 Research of Manufacturing Capabilities

A vast number of different industrial wood manufacturing solutions is available today, seeFigure 5 for a typical example. A detailed consideration of all parameters of all different machine types would be impractical in early-stage development. However, for a cost efficient design it is important that the chosen geometry is adapted to the manufacturing technique which will be used. The chosen approach is therefore to abstract the existing manufacturing technique. In this case not with the aim of developing new techniques, but to allow sufficient consideration of capabilities and limitations of existing manufacturing techniques in early-stage development of wood joints. The focus is, therefore, on the part of manufacturing relevant for shaping the geometry. For solid wood, machining is by far the most relevant type of manufacturing for shaping the geometry, while bending is also commonly used for plywood. While many different automated machining solutions exist, the types of cutting tools used in these are few. In all standard geometry shaping machines offered by the major manufacturers1, the only cutting tools used are drill,

1 See, e.g. Homag.com, IMASchelling.com, Weinig.com, SCMGroup.com, Giben.com, Striebig.com, futura-woodmac.com

Figure 5 Example of a modern wood manufacturing solution (image source: Homag AG., 2020).

8 Methods Ideation Method profiler, mill, lathe, bandsaw, circular saw, planer, and chainsaw. The basics of these cutting tools are described in Chapter 4.3. The list excludes planer, because it is used for improving surface conditions and -tolerances, rather than for shaping geometries. The list also excludes chainsaw, because in automated machines it is only used for large timber frames and has little practical use for furniture.2 The research for existing industrial wood manufacturing solutions was done through a screening of the solutions offered by the major machine manufacturers.1

2 See, e.g. SCM PMT 5-Axis Machining Centre [Accessed: 01-05-2020]

3.3 Ideation Method

As example of how the collected principles and patterns, as well as the abstractions of the manufacturing techniques can be used by practitioners to develop new all-wood furniture joints, an ideation workshop was held. This was done as a 2 hour workshop at the product development center of IKEA with two product developers, two prototyping engineers, and the author. As a starting point of the ideation, the IKEA “Björksnäs” nightstand was used. This product was chosen for that purpose because it utilizes relatively thick solid wood parts and therefore has potential for utilizing all-wood joints. Still, the current model only uses common metal and plastic fasteners. After building up the nightstand at the beginning of the brainstorming, the previously collected principles, effects, and patterns, printed on separate cards, were used to generate new ideas. For this, one card at a time was randomly selected. Then the team tried to find solutions for how that specific principle, effect, or pattern could be applied to a connection in the nightstand or to a previously generated idea, see Figure 6. The abstractions of the manufacturing techniques were provided as openly displayed cards. Out of the results of the workshop, the most promising, innovative ideas were afterwards selected. These can be found in Chapter 4.4

Figure 6 Brainstorming workshop.

9 Methods Method for Pre-Design and Testing

3.4 Method for Pre-Design and Testing

As the last step, a pre-design of one of the previously developed concepts was created and tested. This was done to show how abstract concepts on the Effect Level, ideated through the use of the collected principles and pattern, can be detailed into a more concrete form at the Component Level. While developing a finished solution is beyond the scope of this project, the concept was developed far enough to be able to draw conclusions about its practicality. The focus during the creation of the pre-design was to have a clear logical sequence and show how the different principles can be used to their full effect. Another focus was to show how the design is guided by the constraints from the manufacturing techniques. The testing was done according to the applicable international standards. Manufacturing and testing a physical prototype would have been a possibility for this step with the benefit of being able to test not only strength and stability, but also manufacturability and ease of assembly. How- ever, despite the pre-design being created with cost-efficiency in large-scale production in mind, the manufacturing of a prototype is still relatively complicated. This is mainly due to the need for molds, CNC-milling, and a sandwich construction. Therefore a simulation approach was chosen instead, to test against the most critical strength requirement. For creating the 3D-models and FEM-simulations Autodesk Fusion 360 V.2.0.9439 was used.

10 4 Results

4.1 Principles

The following section describes the different principles found in existing wood joint solutions with references to the solutions from which they are derived. The descriptions do not include detailed failure mechanisms because these vary based on the detailed design.

Form-Fit (Topological Interlock) The most common principle found in traditional all-wood joints is a form-fit (or topological interlock). Two surfaces of the joined parts in contact restrict movement perpendicular to the surfaces resulting in compression forces on the surfaces during loading. Equally, rotational movement is restricted, unless the geometry is axis symmetrical. Joints with form-fit are permanently damaged if the compression stresses exceed the elastic limit of the material on a surface. It can be generally said that the performance of a joint with form fit increases with higher precision. However, a tight form fit can also be achieved through plastic- and elastic compression of uneven parts during forceful assembly. With the principle form-fit alone any number of translatory- and rotational degrees of freedom can be restricted except for all six at once because at least one translatory- or rotational degree of freedom is necessary for assembly. Most joints utilize the principle form-fit to restrict at least some degrees of freedom. All-Wood joints in traditional carpentry which only utilize this principle therefore also utilize gravity to restrict the last degree of freedom, limiting the number of possible applications. An example of the use of only a form-fit in a roof construction can be found in Figure 7, the use in a log cabin can be found in Figure 8. Many solutions for utilizing only a form fit (and gravity) in furniture joints can be found in patent literature, see Figure 9 to Figure 12. However, the use is usually limited to applications in bed frames, cabinets, and shelves.

11 Results Principles

Figure 7 Picture and schematic drawing of a “Watari-ago” joint in hori- Figure 8 Form fit on old wooden house (Rotari, zontal roof members, consisting of two notched beams (Ogawa, 2008). Sasaki, & Yamasaki, 2015).

Figure 9 Connection detail of bedframe (German Figure 10 Connection in a bedframe (German Patent No. 10 2006 012 897 A1., 2008) Patent No. 10 2008 018 692,2008).

Figure 11 Connection with Angled Dowels (German Figure 12 Frame connection (Austrian Patent No. Minor Patent No. G93 02 568.8, 1993). 516858 B1, 2015).

12 Results Principles

Friction Friction between two surfaces causes a force or moment parrallel to the surfaces in the opposite direction of loading. For friction, a pressure between the surfaces is necessary, only a form fit is not sufficient. In traditional carpentry this is usually achieved by forcefully joining parts with an oversized fit, e.g. by hammering a dowel into a slightly smaller hole, elastically compressing the surfaces with the oversized fit. Higher friction is generally beneficial, as it creates a stronger joint. However, the same friction force needs to be overcome during assembly, unless the wedge principle is used, and higher friction makes permanent damage during disassembly more likely. The friction force can be increased by increasing the friction coefficient between the two mating surfaces (μ), by increasing the surface pressure, and by increasing the surface area: The surface pressure can be increased by a more oversized fit, but is limited by the elastic limit in compression of the surface material, and can cause cracking in the female part of the joint if too high. The effective surface area can be increased with higher precision by increasing the share of the surfaces in direct contact. The friction coefficient is dependent on the surface materials used. For solid wood, the friction coefficient is dependent on the combination of grain orientations, decreases with higher hardness, and increases with higher roughness (Xu, Li, Wang, & Lou, 2014). The principle friction is seldomly used alone, because of the lack of strength if assembly forces are to be kept in reasonable limits. Exceptions are dowels or keys as part of a bigger joint. More commonly, the principle friction is used in combination with the principle wedge, see Figure 13.

Figure 13 Wedged tenon (Roth, 1996, p.49).

Keys and Pegs Keys and pegs are thin pieces of wood which are used to restrict the degree- or degrees of freedom necessary to assemble the other parts of the joint with a form fit. The key or peg itself is usually hammered in and is fixated by pressure and friction. Keys are most typically used in traditional timber-frames, see Figure 14 and Figure 15, and are often wedge-shaped to improve the form fit of the joint and create surface pressure. Pegs are most typically used to secure a mortise-and-tenon joint in timber frames, see Figure 16, and less commonly to secure mortise-and-tenon joints in furniture. Especially for small joints, keys and pegs can be challenging to assemble.

Figure 14 “Sao-hikki-doko” - Joint with Figure 15 “Ari kata sanmai hozo komisen Figure 16 Mortise and tenon joint se- two keys in a timber frame uchi” - Through single dovetail cured with pegs (Frameworks (Zwerger, 2012, p. 166). with pin (Sato & Nakahara, Timber, 2020). 1995, p. 204).

13 Results Principles

Wedge A problem when using the principle friction is that the maximum amount of friction later keeping the joint together has to be overcome during assembly. This problem can be reduced by using the principle wedge. A wedge leverages the assembly forces resulting in much larger forces pulling the joint together, creating a tighter form fit and higher surface pressure. This effect is stronger for a smaller wedge slope. A risk when using a wedge is that the forces become too strong and cause the wood to crack. Utilizing the principle wedge does not have to result in a visible wedge like in Figure 17 or Figure 18. The movable piece in Figure 19, for example, has a wedge-shaped cutout in the middle. In Figure 20, the ribbed profile of the female part of the joint runs at a slight angle to the assembly direction to pull the joint tight during assembly, utilizing the wedge principle. Similarly, the round cutout in Figure 21 creates a wedge effect by being in a slightly different shape than the assembly path of the counterpart. An excenter like in Figure 22 also works after the same principle and can be seen as a round wedge.

Figure 17 Through mortise and tenon Figure 18 Bedframe detail with dovetail Figure 19 Wedge lock traditionally been joint with a wedge key and wedge (German Patent No. used in cabinets (Nutsch, (Roth, 1996, p.49). 101 03 798 C2, 2001). 2005).

Figure 20 Grooved joint (World Patent Figure 21 Round wedge in a shelf design Figure 22 Wooden clamp (Ernst No. 2013/104422 A1, 2012). (German Minor Patent No. 20 Dünnemann GmbH & Co.KG, 2019 101 810.0, 2019). 2020).

14 Results Principles

Dovetail A dovetail is a particular form of a form fit. A dovetail has an undercut with a typical V-shape, usually in the main direction of loading. The probable reason for the widespread use of dovetails in wood joints is that it is well adapted to the material properties of wood. The shape distributes the critical shear forces parallel to the grain evenly over a large cross-sectional area, making this failure mode less likely. A negative aspect of the V-shape is that it creates an opening moment on the female part of the joint, which can lead to cracks in the corners. Assembly of dovetail joints is usually done with glue and requires small tolerance for the sliding action to work well and to create a tight form fit. A typical use of dovetails is in drawer fronts, see Figure 23, or in keys, also known as “Butterfly Joint” or “Dutchman Joint”, see Figure 24. A variation of the dovetail is the “Gooseneck” often see in Japanese carpentry, see Figure 25. The V-shape is reversed in this joint, which prevents the bending moment in the female part of the joint, but requires more material. The grooved joint patented by IKEA, see Figure 26, also utilizes the principle dovetail, although less visible. In essence, this joint is a dovetail divided into smaller sections. Two more examples of the use of dovetails can be seen in Figure 27 and Figure 28.

Figure 23 Half-blind dovetail in Figure 24 “Ari Kata” - Butterfly spline and Figure 25 “Mechigai Hozutsuki Kama Tsugi” drawer (Rogowski, “Kine Kata” - Bow-tie spline (Sato - Half-blind mortise and tenon 2002, p. 152). & Nakahara, 1995, p. 193). gooseneck joint (Sato & Nakahara, 1995, p. 177).

Figure 26 Grooved joint (World Patent No. Figure 27 Bedframe detail with dove- Figure 28 Dovetail in frame corner 2013/104422 A1, 2012). tail (German Minor Patent (Roth, 1996, p.49). No. 297 01 033 U1, 1997).

15 Results Principles

Keyhole The keyhole is another particular version of a form fit. In a joint utilizing this principle, the male part is inserted into larger hole and then slid into a tightly fitting grove. Therefore, at least two separate movements are necessary for assembly. Because of the similar loading and grain orientation, a dovetail shape in joints with a keyhole makes sense from a mechanical perspective. This can be seen, for example, in the traditional joint in Figure 29. Two examples of dowels with the principle keyhole can be seen in Figure 30 and Figure 31. A variation with a separate connector can be seen in Figure 32.

Figure 29 Suspended ceiling board Figure 31 Wedge dowel (World Patent No. with “Inago-zashi” (Zwerg- 2013/104422 A1, 2012). er, 2012, p.96).

Figure 30 Sliding dowel and keyhole (Ger- Figure 32 Connector sliding into grooves man Minor Patent No. G 93 17 (German Patent No. 42 09 017 079.3, 1993). A1, 1992).

16 Results Principles

Swelling In joints with this principle the material property of wood to swell up substantially under high moisture content is utilized. In the traditional Japanese technique known as “Kigoroshi” a joint with a form-fit is crafted with an oversized fit. (Moriyama, Sawada, Fujii & Okumura, 2015) The mating surfaces are then carefully tapped with a hammer to plastically compress the wood without damaging the fibres. After joining the pieces, the surfaces swell back to original size over time in a moist environment, creating surface pressure and thus friction between mating surfaces. A variation is found in German Patent No. 42 09 017 A1 (1992), see Figure 33. Here additional means of mechanically compressing the mating surfaces are described, as well as drying the smaller piece and therefore shrinking it before joining. Possible scenarios of failure for joints with this principle are low precision and low swelling resulting in an insufficient form fit and surface pressure. Others are too high swelling resulting in cracking of the female part of the joint or damage of the wood fibres during mechanical compression resulting in a loss of strength. This principle is more suitable for softer wood species where a larger plastic compression is possible.

Figure 33 Connections with a compressed oversized fit (German Patent No. 42 09 017 A1, 1992).

17 Results Principles

Small Teeth Small teeth are an addition to the principles form-fit, and friction. They are little ribs on joint surfaces which get compressed during the assembly process while also compressing the mating surface. An oversized fit of the outer dimensions of a joint with small teeth can ensure a tight form fit and sufficient surface pressure. At the same time, the compressed surface area is reduced through the small teeth, reducing the risk of cracking of the female part and limiting necessary assembly forces. Similar results can be achieved with flat surfaces, but this requires a higher precision for the same reliability. Examples of the utilization of small teeth for reliable, low-cost joints are ribbed dowels, seeFigure 34, and “Hoffmann Dovetail Keys”, seeFigure 35.

Figure 34 Ribbed dowel (Porta Mouldings Pty. Figure 35 Connector in dovetail shape (Hoffmann Ltd., 2020). GmbH Maschinenbau, 2020).

Thread A thread is a combination of the principles Form Fit, Pressure and Friction, and Wedge. Wood threads are prone to stripping, because of the low shear strength of wood parallel to the grain, compared to e.g. steel. For joints in furniture, therefore, they have to be much larger than the thread of steel screws used in a similar joint. Traditionally, wood threads were mainly manufactured with special tools on a lathe, see e.g. Figure 36. A modern alternative is, e.g. the manufacturing with a CNC-mill with a special cutting tool (World Patent No. 2015/185501 A1, 2005). Figure 37shows a wood thread with a conical shape, which has the advantage of the pieces fitting together well even with low tolerances, e.g. due to uneven swelling.

Figure 37 Conical wood thread (World Patent No. 2015/185501 A1, 2005).

Figure 36 Wood thread on traditional spinning wheel.

18 Results Principles

Click Connection A click connection is a principle commonly used for polymer parts. It can, however, also be used for wood joints and has the potential for effortless assembly. Click connections always consist of a spring which is deflected during assembly, usually by a wedge until it retracts in the final position, locking the part in place through an edge. The strain of wood parallel to the grain at the elastic limit is much lower than for most polymers, under 0.5%, compared to e.g. ca. 2.3% for polypropylene (Ashby, 2005, p. 522-525). This means that a wood spring can be elastically deformed substantially less than a polymer spring with the same dimensions. A difficulty for realizing a click connection with wood is, therefore, to have a large enough locking edge without weakening other parts of the joint or creating aesthetic problems through a long spring length. Possible solutions for this are for example using it where a strong joint is not necessary, see Figure 38 and Figure 39, or incorporating a long spring in the aesthetic design (Figure 40).

Figure 40 Spring joint box (Johnson, 2013). Figure 38 Click-connection for Figure 39 Wooden click-connection floorboards (World Patent holding the door shut in a No. 2003/083234 A1, cabinet. 2003).

Folding Structure A folding structure is potentially the most comfortable solution from a user perspective, as it does not require assembly as such by itself. A folding structure always requires some form of a hinge and a mechanism locking it into its final position. While assembly is easy for the user, the need for a hinge can result in a need for pre-assembly, increasing production costs. While not made from only renewable materials, World Patent No. 2018/004416 A1 (2017) describes a low-cost solution where the hinge consists of a thin polymer film, see Figure 41.

Figure 41 Folding mechanism (World Patent No. 2018/004416 A1, 2017).

19 Results Principles

Part in Tension In constructions with this principle, a part is held under constant tension or compression to keep it in place, utilizing a form-fit, and sometimes friction. A counterpart is, of course, necessary to keep a force equilibrium. Figure 42 shows a chair where the adjustable seat and footrest is held in place by compression from parallel metal rods. Figure 43 shows a harness on the same chair which showcases the possibility of using bend laminated veneer as a spring. The harness can be compressed and slid into a fitting groove in the chair.

Figure 42 Chair construction with two boards for Figure 43 Harness on baby chair. sitting held in compression (Norwegian Patent No. 132782C, 1972).

Lever A lever can be used to lock-in a part in the direction of assembly or to amplify assembly forces. A lever allows easy assembly and disassembly. Because of the need for a hinge or pivot, pre-assembly is necessary, leading to an increase in manufacturing costs or assembly complexity. Figure 44 shows a metal locking-lever in a connection intended for frequent assembly and disassembly. Figure 45 shows the use of a lever to amplify assembly forces in a clamp.

Figure 44 Metal furniture locking-lever . Figure 45 Wooden clamp (Ernst Dünnemann GmbH & Co.KG, 2020).

20 Results Principles

Grooved Profile A profile with several interlocking, parallel grooves in the male and female part is used by IKEA in different joints, see Figure 46 and Figure 47. The grooved profile is practically impossible to manufacture with traditional techniques, but economically efficient with modern CNC-routers and special bits. It provides a secure form fit with well-distributed contact surfaces while requiring less precision than for example a traditional dovetail. In the joints in Figure 46 andFigure 47, it also reduces the space necessary compared to a traditional dovetail and thereby reduces the necessary part dimensions.

Figure 46 Grooved joint (World Patent No. Figure 47 Wedge dowel (World Patent No. 2013/104422 A1, 2012). 2013/104422 A1, 2012).

21 Results Patterns of Transformation 4.2 Patterns of Transformation

The following section describes the found patterns of transformation between existing wood joint solutions with references to the solutions from which they are derived.

Division In the pattern division, a standard solution is divided into smaller parts to enhance functionality. By dividing a dowel into four different sections with different diameters, it can be pre-fixated in a matching hole drilled with a special bit and the distance required for hammering it in is reduced, making the task easier, see Figure 48. (U.S. Patent No. 29/183,818, 2004) Similarly, by dividing a dovetail joint into several smaller sections, less space and less precision are required to achieve a tight and secure fit, see Figure 49. (U.S. Patent No. 9,534,623, 2017) The pattern division also includes using two or more components for a function previously performed by one component.

Figure 48 Divided dowel (U.S. Patent No. Figure 49 Ribbed joint (World Patent 29/183,818, 2004; Miller Dowel No. 2013/104422 A1, 2012). Co., 2020).

Change of Shape A simple change in shape alone can sometimes result in a substantial benefit for a joint. German Minor Patent No. 295 03 928.0 (1995) and German Patent No. 10 2006 004 302.2 (2006) (see Figure 50and Figure 51) are examples where a change of shape of the traditional dovetail key (see Figure 52) makes it possible to manufacture the needed groves with a drill press instead of requiring a chisel. World Patent No. 2015/185501 A1 (2005) (see Figure 53) describes a wood thread where the typical shape of a thread has been changed to a conical shape. The conical shape allows assembly even when precision is low, e.g. due to shrinkage or swelling in the wood.

Figure 50 Dovetail key (Echols, 2006). Figure 51 Round connector piece similar to dovetail key (German Patent No. 10 2006 004 302.2, 2006).

Figure 52 Special dowel (German Minor Patent Figure 53 Conical wood thread (World Patent No. 2015/185501 No. 295 03 928.0, 1995). A1, 2005).

22 Results Patterns of Transformation

2D/3D This pattern is the change from a two-dimensional profile to a three-dimensional geometry or vice versa. Figure 54 and Figure 55 show similar shapes, one two-dimensionally, the other three-dimensionally. The similarities would be most visible in a cross-section of the geometry in Figure 55. Possible benefits of utilizing this pattern could be a reduction in manufacturing complexity, more strength or support in more directions, and easier assembly. The geometry in Figure 55, for example, requires higher manufacturing complexity but allows assembly of the pieces meeting in the corner and offers support in more directions.

Figure 54 Two-dimensionally interlocking joint Figure 55 Three-dimensionally interlocking joint (German (Austrian Patent No. 14105 U1, 2015). Patent No. 10 2008 018 692, 2008).

Change to Lateral- or Rotational Movement Especially when a form fit restricting five degrees of freedom is used, the remaining one can be changed from a rotational to a lateral, or vice versa. The movement required for assembly is then a different one. A combination of a lateral and rotational movement is also possible. Considerations for choosing and assembly movement should be ease of assembly, ease of manufacturing, as well as choosing a direction or axis with little loading during use to reduce the risk of unintended disassembly. Two different versions of otherwise the same joint can, e.g. be found in World Patent No. 2013/104422 A1 (2012). One requires a rotational movement (Figure 56) for assembly and the other a lateral movement (Figure 57). Similarly, the wedge principle can be realized through a trianguar shape or an excenter shape.

Figure 56 Wedge dowel with rotational-locking (World Figure 57 Wedge dowel with lateral-locking (World Patent No. 2013/104422 A1, 2012). Patent No. 2013/104422 A1, 2012).

23 Results Patterns of Transformation

Adaptation to Properties of Wood Another pattern seen in recent inventions is to adapt standard fastening solution, usually made out of different materials such as steel or polymers, to the material properties of wood. A good example of this are the “LIGNOLOC®” wood nails manufactured by the Beck Fastener Group, see Figure 58. Only manufacturing wood into the geometry of common nails would make them too weak to be used like steel nails. By compressing the relatively dense wood beech and infusing it with resin, however, the “LIGNOLOC®” wood nails can be shot into wood with a special pneumatic nail gun without pre-drilling, similarly to steel nails. (Beck Fastener Group, 2020)

Figure 58 Compessed, resin-infused wood nails (Beck Fastener Group, 2020).

Inverting In this pattern, features are turned inside-out, upside down, or features are moved from the female to the male part of a joint or vice versa. For example, Figure 59 shows a part with a triangular wedge, while Figure 59 shows a part with a triangular wedge shape cut out in the centre. The first part pushes the other parts of the joints away, while the other pulls them together.

Figure 59 Through mortise and tenon Figure 60 Wedge lock which has tradi- joint with a wedge key (Roth, tionally been used in cabinets 1996, p.49). (Nutsch, 2005).

24 Results Manufacturing Capabilities

4.3 Manufacturing Capabilities

In the following section the available, relevant manufacturing techniques are described. The descriptions are abstract and focused on the possibilities and limitations of the manufacturable geometries. The intention is to allow their consideration in early-stage development.

Drill (Figure 61) A drill is a rotary cutting tool used for making holes with a circular cross-section. Additionally, to the rotational cutting movement, it can only be moved in the direction of the rotational axis while cutting. The holes made by drills are most often in a cylindrical shape, but other geometries are also possible. The shape must be, however, axis-symmetrical and cannot have undercuts in the axis direction. Practically any position and orientation of the hole is possible with modern manufacturing solutions3. Still, an orientation perpendicular to the surface generally results in the lowest manufacturing complexity.

3 See, e.g. Homag CNC-Processing Centre, Available at [Accessed: 01- 05-2020] Figure 61 Abstracted illustration of a drill cutting tool.

Mill (Figure 62) A mill is a cutting tool similar to a drill, with the main difference, that it can also be moved perpendicular to the rotational axis during cutting. In the direction of movement, no undercuts in the cutting edge are possible. With modern CNC-processing centers, practically any movement and rotation of the tool relative to the workpiece is possible, allowing the machining of free-form surfaces.4 To stay within reasonable economic limits for wood joints, however, certain restrictions on the geometry are should be considered. Machine costs can, for example, be reduced if the geometry can be manufactured with a three-axis CNC-machine instead of a five-axis CNC-machine. Machining time can be kept low if the geometry can be machined with only one passing Figure 62 Abstracted illustration of a mill cutting tool. of the milling head, see, e.g. Machining Technique for Conical Thread in World Patent No. 2015/185501 A1 (2005).

4 See, e.g. Homag CNC-Processing Centre, Available at https://www.homag. com/en/product-detail/cnc-processing-center-centateq-p-110 [Accessed: 01-05-2020]; Weining CNC-Processing Centre, Available at [Accessed: 01-05-2020]

25 Results Manufacturing Capabilities

Profiler (Figure 63) A profiler is a tool similar to a mill, which only moves in one straight direction relative to the workpiece, perpendicular to its rotational axis. Profilers are usually used to manufacture long moldings or profiles at a low cost. A profiler can also be in the shape of a planer with profiled blades. Similarly to the drill and mill, undercuts in the cutting edge in the direction of movement are not possible.5

5 See, e.g. Weining Profiling Machine, Available at https://www.weinig.com/< en/solid-wood/planing-machines-and-moulders/unimat-series/unimat-300. html> [Accessed: 01-05-2020]

Figure 63 Abstracted illustration of a profiler cutting tool.

Circular Saw (Figure 64) A circular saw is a rotary cutting tool with a small thickness compared to the diameter and is mostly to cut through a workpiece. Because of the shape of the saw blade, it can only cut straight, and the possible shapes of the cutting edge are limited. Apart from through-cuts, only straight slits can, therefore, be machined with a circular saw. These can be positioned and oriented freely when, e.g. utilizing a modern CNC-Processing Centre.6

6 See, e.g. Homag CNC-Processing Centre, Available at [Accessed: 01- 05-2020])

Figure 64 Abstracted illustration of a circular saw cutting tool.

26 Results Manufacturing Capabilities

Bandsaw (Figure 65) The continuous band of a band saw allows only through-cutting of workpieces. Bandsaws are usually used for straight cuts, but some machines will enable the blade to move in smooth curves perpendicular to the cutting movement.7

7 See, e.g. MZ Project CNC Band Sawing Centre, Available at [Accessed: 01-05-2020]

Figure 65 Abstracted illustration of a bandsaw cutting tool.

Lathe (Figure 66) In a lathe, the cutting movement is performed by rotating the workpiece while the cutting tool is held in place. The cutting edge can have different shapes and can be moved freely relative to the workpiece. Usually, axis-symmetrical geometries are machined with a lathe, but spiraling geometries are also possible, as well as non-symmetrical geometries by linking the tool movement to the rotations.8

8 See, e.g. Schnitzer CNC Lathe, Available at [Accessed: 01-05-2020]

Figure 66 Abstracted illustration of a lathe cutting tool.

27 Results Manufacturing Capabilities

Laminated Bending (Figure 67) For laminated bending, veneer slices are press glued in a mold. For mass production, usually, a two-part, heated metal mold is used, and the veneer slices with glue applied are stacked crosswise at an angle of 90°. After hardening and cooling of the part, the outlines are usually CNC-milled to achieve the final geometry. (Navi & Sandberg, 2011, p.320 ff) A mold consisting of more parts than two is possible but increases manufacturing complexity. Using veneer slices allows for thick bends of much smaller radius than for solid wood bending. The minimum radius possible according to Navi & Sandberg (2011) is about 30 times the veneer thickness and typically veneer with a thickness between 1 and 5 mm is used. A study of minimum bending radii of different veneers gives similar values. (Lun- guleasa, Cosereanu, Budau, & Matei, 2013) The recommended values for furniture are a bending radius between 37 and 60 mm for veneer thicknesses between 1 and 1.5 mm, and a radius higher than 60 mm for thicknesses between 1.5 and 2.5 mm. Figure 67 Abstracted illustration of laminated bending. The bending geometry can be two-dimensional or three-dimensional, whereas the latter results in increased manufacturing complexity. (Navi & Sandberg, 2011, p.320 ff) The possibility to use bio-based and biodegradable adhesives for this manufacturing method is here assumed to be possible soon to allow the use of this method for circular furniture.

28 Results Ideation Results 4.4 Ideation Results

In the following, the results of the ideation are presented. The graphics intentionally contain no details. They should not be seen as manufacturing drawings, but just as representations of the working principle and of how the different principles and patterns are utilized.

Concept 1 Divided Click

form fit

grooved profile

click

division

Figure 68 Concept 1 - “Divided Click”.

Concept 1 (Figure 68) is a joint with a form fit and a click connection. The new idea behind it is to decrease the necessary spring deflection of the click connection by dividing the front part. With this, more edges are keeping the components in place, so each edge can be smaller than if only one edge were used. At the tip of the male part, the edges could be slightly bigger, because there the component deflects slightly more than further back.

Concept 2 Wedged Tenon

form fit wedge

grooved profile

small teeth click

division

Figure 69 Concept 2 - “Wedged Tenon”.

Concept 2 (Figure 69) is a modification of the traditional wedged tenon joint. The classic wedged tenon joint is held together through friction (and sometimes glue) at the outer surface. This joint, instead, keeps the joint together with a form fit achieved through a grooved profile at that surface. The click principle is used for pre-assembly, and the form fit is completed by a wedge driven in from the top. The wedge should be relatively thick and with teeth to make assembly easier, compensate tolerances, and avoid having to trim the top.

29 Results Ideation Results

Concept 3 Bend Laminated Click click adapting to wood

laminated bend veneer keyhole

Figure 70 Concept 3 - “Bend Laminated Click”.

Concept 3 (Figure 70) aims to solve a common problem when using the keyhole principle. When using only a keyhole, one degree of freedom is left open because it is needed for assembly. Al- though it is usually not the main direction of loading, this could lead to unintended disassembly. In this concept, the movement is restricted through a click principle realized with a small piece of laminated veneer U-shape. Similar solutions already exist out of metal or polymers. In some joints, disassembly could be made possible through a hole, allowing compression of the bend veneer piece.

Concept 4 Click Drawer Front keyhole

click

spring

Figure 71 Concept 4 - “Click Drawer Front”.

Concept 4 (Figure 71) is similar to Concept 3, but the click principle is realized differently. It is intended for drawer fronts. For the click connection here, a chamfer on the drawer front acts as a wedge, and a groove locks the drawer bottom in place after assembly. The spring could, e.g. be realized through a long strip of laminated bend veneer and is located at the back of the drawer base. The spring would also have the benefit of keeping the drawer base from moving during use despite low tolerances.

30 30 Results Ideation Results

Concept 5 Finger Joint Hinge

adapting to wood

folding structure

wood nail

Figure 72 Concept 5 - “Finger Joint Hinge”.

Concept 5 (Figure 72) is a folding mechanism similar to a piano hinge. The joint has the shape of a traditional finger joint with one edge rounded off. The pieces are connected through a rod. The rod could be glued with bio-adhesives or welded to one of the parts. The joint would, for example, be suitable for easy-assembly drawers in combination with Concept 4.

Concept 6 Table Legs Locking in Tension

grooved profile

click

bend laminated veneer in tension

part in tension

Figure 73 Concept 6 - “Table Legs Locking in Tension”.

Concept 6 (Figure 73) is a concept for joints in a table which are held together by the tension of the legs made out of bend laminated veneer. For easier assembly, the concept utilizes a click mechanism with the legs acting as the springs. The concept also uses a grooved profile to increase the strength and stiffness of the joint, especially in shear. This concept could also be inverted, with the legs in compression, locking joints on the inside of the tabletop.

31 Results Pre-Design of Concept 6

4.5 Pre-Design of Concept 6

The following contains a more detailed version of one of the previously presented abstract concepts. The purpose is to show how the concepts can be brought from the Effect Level to the Component Level and how the manufacturing techniques can be considered for the selection of the joint geometry. It has to be noted, that the presented results do not represent the final development step and further prototyping, testing, and refining is necessary.

click

Figure 74 Assembly of Concept 6.

Material Selection (Figure 75 and Figure 76) The working principle of this concept is mainly a combination of the principles “part in tension” and “click”. In order for these two principles to work, the legs need to act as springs. The easiest solution in terms of manufacturability to create the spring effect is to use a bend laminated veneer profile for the legs. A critical mechanical requirement for the table joint is sufficient strength and stiffness against bending moments in the joint due loads parallel to the table surface. To reduce the beding defor- mations in the tables short direction, the legs were designed in a v-shape. The grooved profile, which was already part of the concept at the Effect Level, increases the strength and stiffness against loads in the tables long direction. Additionally, the strength and stiffness can be increased by a relatively large height and width of the joint. A relatively large height is also beneficial for the click-mechanism. This height could be realized through a support frame under the table top, but since this could require additionally assembly, a thick sandwich structure was chosen instead. This sandwich structure consists of a cardboard honeycomb center, veneer top and bottom, and solid wood edges which are glued together in a manufacturing plant.

top sandwich structure (solid wood edges, veneer top and bottom, honeycomb center)

legs bend laminated veneer

Figure 75 Material selection for Concept 6.

32 Results Pre-Design of Concept 6

legs act as springs • allows for a click-mechanism • permanent holding force

Figure 76 Joint geometry for Concept 6.

Consideration of Manufacturing Constraints For the manufacturing of the legs, an important consideration in terms of cost effectiveness is chosing a geometry which requires few mold parts for the laminated bending process. The v-shape of the legs is beneficial for this, because it enables the use of a two-part mold, seeFigure 78. The cost effectiveness can also be increased by molding a wider piece first and cutting it into several leg pieces afterwards. To enable this, a constant width was chosen for the legs. Another manufacturing consideration for the legs was the use of a minimum radius of 50 mm to allow the manufacturing with veneer thicknesses between 1 and 1,5 mm, see Chapter 4.3. For the manufacturing of the relatively complex grooved joint geometry on the table and legs, CNC-milling was chosen as manufacturing method. To reduce manufacturing time and cost, the geometry was chosen so that the grooved part can be machined with one passing of a customized milling head, see Figure 77. A second milling step with a different milling head is necessary to cut a groove which provides a support edge for the table top, see also Figure 77.

Figure 77 Milling of joint geometry. Figure 78 Manufacturing of legs.

33 Results Pre-Design of Concept 6

Pre-Dimensioning The initial dimensioning of the joint geometry is based on the clamping force of the spring mechanism after assembly and the required assembly force. The aim is to maximize the clamping force which is beneficial for the strength and stiffness of the joint, and to limit the required assembly force to allow easy assembly of the joint by all customers. As limit of the assembly force 100 N were chosen. L1 For the pre-dimensioning, the calculations are performed for one half of a leg part, making use of the symmetry. To calculate the relationship between deflection at the joint and holding and assembly force, the part was simplyfied as two connected cantilever beams, see Figure 79. L2 The clamping force of the spring mechanism is then influenced by the bending stiffness of the legs and the displacement at Figure 79 Beam model of leg. the top of the legs. The relationship between the displacement and the force is determined as following.

The side part of the leg is modeled as cantilever beam u1 loaded by a perpendicual force at the end, see Figure 80. The relationship between the force and the displacement at the top, according to Grote & Feldhusen (2007, p. C18) is 4 3 FL1 . u1 3EI4 (1)

The bottom part of the leg is also modeled as cantilever beam, but loaded by a constant moment, see Figure 81. The relationship between the moment and the deformation angle at the tip, according to Grote & Feldhusen (2007, p. C18) is Figure 80 Deflected beam model of side part of leg.

ML4 2 . EI4 (2)

u2 Since the moment in this case is equal to the force F from eq. (1) times the length of the left side of the leg L1, eq. (2) can be formulated as 44 FL12L . MF 4L1 EI4 (3) θ For a small angle θ, the deflection at the top of the leg due to deformation of the bottom part is therefore

44 1 FL12L .  uL214L1 EI4 (4) Figure 81 Deflected beam model of bottom part of leg.

34 Results Pre-Design of Concept 6

The total deflection u at the top for a given force F is thus

4 3 442 2 FL1 FL1 L21L C L1 S. uu 12u F D L2 T 33EI4 EI4 EI4 E U (5)

The aim is to calculate the clamping force for a given imposed deflection of the top of the legs due to assembly of the joint. We therefore solve eq. (5) for F, to get the relationship between displacement at the top and clamping force

EI4 Fu . 2 C L1 S L1 D L2 T E 3 U (6)

The displacement of the top of the leg is dependent on the chosen joint geometry. The initial choice of the path of the click mechanism can be seen in the cross-sectional view in Figure 82. To allow for easy assembly, the undeformed leg should fit on the top part, as shown in Figure 82, before force is applied. Maximum deformation is reached just before the leg "clicks-in", see Figure 83. The final deformation can be seen in Figure 84. For the initially chosen geometry, the

maximum deflection of the top of the leg umax is 8 mm and the final deflection ufinal is 5mm.

umax

ufinal

Figure 82 Cross-sectional view of joint Figure 83 Cross-sectional view of joint Figure 84 Cross-sectional view of assem- during assembly, undeformed during assembly, maximum bled joint with final deformation legs. deformation of legs. of legs.

35 Results Testing of Pre-Design and Improvements

With the chosen basic leg dimensions as shown in Figure 85, L = 711 mm, L = 230 mm, the moment 1 2 70 of inertia around the bending axis, I = 1.8∙105 mm4. 0 The modulus of elastisticity was chosen as E = 12 GPa as average of the values given iny grain direction for different wood species by Forest Products Laboratory (2010, p.54). With the pre- 0 60 35 viously given deflection values, this results in a 70 maximum clamping force per side, according to

Equation 6, of Fmax = 87 N, and a final clamping force Ffinal = 55 N. The total maximum assembly force resulting from the maximum clamping force has a frictional component and a component from the wedge 10° mechanism and can be calculated as

42 4sin, FFassembly,max max (7) Figure 85 Basic leg dimensions chosen. where μ is the friction coefficient for wood on wood. With a typical wood on wood friction coefficient of μ = 0.4 (Xu, Li, Wang & Lou, 2014), a wedge angle of α = 10°, and the previously calculated maximum clamping force of Fmax = 87 N, the resulting total assembly force per leg part is Fassembly,max = 100 N. This is equal to the previously chosen limit for the assembly force.

4.6 Testing of Pre-Design and Improvements

The following describes the testing of the previously created pre-design of Concept 6 according to requirements set in applicable international standards. This is done to get an indication of the concepts viability and possible improvements.

Requirements for the stability and strength of tables for domestic use can be found in EN 12521:2015 and EN 1730:2012. (European Committee for Standardization, 2015; European Committee for Standardization, 2012) These were used as the basis of the testing of the pre-design.

Stability To test for stability, EN 1730:2012 requires a test with the setup shown in Figure 86, with a vertical force of up to 400 N, depending on the length of the table. (International Organization for Standardization, 1988; European Committee for Standardization, 2015) Figure 87 shows clearly that the pre-design with an assumed mass of 20 kg does not fulfill this requirement. As possible improvement the angle of the legs could be changed, so that the contact point with the ground lies further outwards.

Figure 86 Setup of vertical stability test according to EN 1730:2012 (European Committee for Standard- ization, 2012, Figure 9, p.27)

36 Results Testing of Pre-Design and Improvements

contact point

Figure 88 Setup of test with horizontal static load according Figure 87 Side view of table with contact point to EN 1730:2012 (European Committee for Stan- with ground, vertical stability test load dardization, 2012, Figure 2a, p.9) and gravitation force.

Strength For testing the strength, EN 1730:2012 requires a test with a vertical static load and a test with a horizontal static load. More critical in this case is the test with a horizontal static load for which the required test setup is shown in Figure 88. For the test the legs are fixed on one side against horizontal sliding, the top is loaded with a mass of 50 kg, and a horizontal force of 400 N is sub-sequentially applied centric to each side. (European Committee for Standardization, 2015; European Committee for Standardization, 2012) This test was simulated with a horizontal force pushing on the long side of the table to determine if the joint would open. To verify the simulation model, a model of a leg part, loaded with the analytically determined final clamping force was created, see Figure 89. In the analytical calculations a displacement of 5 mm resulted in a clamping force of 55 N per side, see Chapter 4.5. In the simulation model, the loading with a force of 55 N per side resulted in a displacement of 4.925 mm, see Figure 89. The simulation approach can therefore be considered reliable. The simulation setup of the test with a horizontal static load according to EN 1730:2012 can be found in Figure 90. The leg parts were modeled as initially undeformed, with the clamping force applied as external load. The loaded 50 kg mass and an assumed table mass of 20 kg were modeled as vertical force of 700 N in the center of the table top. Contact constraints between the joint surfaces were set to allow separation and sliding, but no penetration. An elastic modulus of 12 GPa was used. The resulting displacements can also be found in Figure 90. The maximum displacement of the table under the horizontal static load test according to the simulation is 4.882 mm. In theory, since the top of the legs is displaced 5 mm during assembly, this means that the joint would not open and therefore fulfill the strength requirement. In practice, however, a safety factor of ca. 1 is very little for an approximate model where e.g. tolerances, moisture related shrinkage and swelling, and loss of clamping force due to creep are not considered.

Figure 89 Displacement contour and setup of a FEM model of the pre-designed leg part loaded with the calculated clamping force. The visible deflection of the geometry is exaggerated.

37 Results Testing of Pre-Design and Improvements

Figure 90 Displacement contour and setup of a FEM model of the table under conditions of the horizontal static load test according to EN 12521:2015 and EN 1730:2012 (European Committee for Standardization, 2015; European Committee for Standardization, 2012). Clamping force of legs is modelled as external force and a table weight of 20 kg is assumed.

To improve the strength of the joint, i.e. to decrease its tendency to open, the clamping force could be increased. This could be achieved by increasing the deflection of the legs during assembly through using a different sliding path for the click mechanism. However, an increase in the clamping force generally also leads to an increase in the assembly force. This can be limited by flattening out the sliding path towards the point of the maximum deflection, such as shown in Figure 91, to decrease the wedge component of the assembly force, see Equation 7. Since the limit of the assembly force was chosen arbitrarily as 100 N, it should be tested in practice and the design adjusted according to the outcome. Another possible improvement to prevent the joint from opening up due to a horizontal load on the table could be to add a form fit in this direction, such as shown in Figure 92. Also possible would be to use the joint in a smaller table for which the requirements are lower. Tables with a height lower than 600 mm, have to be tested with a horizontal static force of only 200 N. (European Com- mittee for Standardization, 2015)

Figure 91 Cross-sectional view Figure 92 Cross-sectional view of joint geometry with of joint geometry with changed sliding path additional form fit

38 5 Conclusion

With the purpose of exploring the possibilities of circular furniture, an approach for the development of all-wood furniture joints was created. This approach consist of a collection of fourteen principles and six patterns of transformation, which were abstracted from existing solutions found in patent literature and literature about traditional joinery. The approach also consists of an overview of the most relevant manufacturing techniques, simplified in a way to allow the consideration of their constraints in early-stage development. To demonstrate how this can be used to create new joint types, an ideation workshop was held at the IKEA product development center. Out of the resulting six concepts, one was refined further into a pre-design and tested with simulations. While the research and abstraction of existing solutions and manufacturing methods resulted in no insights on its own, it proved to be a valuable basis for the development of new joints. The ideation workshop clearly showed that the collection of principles and patterns serves as an inspiration. The level of abstraction is suitable to generate a broad range of different ideas without getting lost in details and is accessible to practitioners with non-engineering background. The overview of the relevant manufacturing technique has its use not as an inspiration, but by making the development process more accessible to less technical practitioners like product designers. The main benefits to practitioners lie in offering a structured approach instead of trial-and-error, inspiration beyond already tried solutions, and the possibility of making this type of development a trans-disciplinary process. The exact use of the principles, patterns, and overview of manufacturing techniques is not set, it should rather be seen as a "toolbox". It should also not be seen as complete, and should grow, as new principles, patterns and manufacturing techniques are discovered. The ideation workshop also only represents one possibility of working with it. The ideation workshop, however, proved to be a useful method and resulted in six different novel concepts. The detailing of one of the concepts into a pre-design showed its practicality, despite further improvements and testing being necessary. The plan of the IKEA team to manufacture a prototype and continue the development of the concept is an indication of the usefulness of the approach for developing innovative all- wood joints. While the development approach was successful and it was shown, that novel all-wood joint adapted to modern manufacturing and easy assembly can be developed in a structured process, conclusions about the environmental benefits are not easy to draw. For one, the amount of non-renewable resources which could potentially be saved is very small compared to amounts used in e.g. the construction industry. Also, the number of furniture which can be replaced with all- wood joint is most likely quite limited and a need for coatings and adhesives exists nevertheless. Additionally, the only aspect of environmental sustainability discussed so far is resource use, while e.g. energy use and carbon emissions during manufacturing and transport have not been discussed. Further limitations result from the literature research, in which it was not possible to cover the whole patent literature and review every traditional joint solution. The list of principles and pattern can therefore be expected not to include every possible principle and pattern. Additionally, new principles and patterns which are not used in existing solutions, as well as new manufacturing techniques could possibly be found. Despite this, the project is still a step in the right direction towards circular furniture. The project showed that the development of new all-wood joints might also result in other benefits, such as easier assembly higher perceived value of the products, which might be reflected in more care for them. To conclude, this thesis encourages IKEA and other furniture manufacturers to delve more deeply into the topic of circular furniture with more sustainable products to be seen.

39 References

Patents Anderson, B. (2017). World Patent No. 2018/004416 A1. Geneva, World Intellectual Property Organization Anderson, B., Eriksson, A., & Sjöstedt, G. (2015). World Patent No. 2015/185501 A1. Geneva, World Intellectual Property Organization Anderson, B., Eriksson, A., & Sjöstedt, G. (2012). World Patent No. 2013/104422 A1. Geneva, World Intellectual Property Organization Berthold, K. (1992). German Patent No. 42 09 017 A1. Munich, Deutsches Patent und Markenamt Fa. C. Höhn (1993). German Minor Patent No. G 93 17 079.3. Munich, Deutsches Patent und Markenamt Grube, P. (2001). German Patent No. 101 03 798 C2. Munich, Deutsches Patent und Markenamt Huhn, H. (2008). German Patent No. 10 2008 018 692. Munich, Deutsches Patent und Markenamt Jocham, P. (2015). Austrian Patent No. 14105 U1. Vienna, Österreichisches Patentamt Krug, W. (1995). German Minor Patent No. 295 03 928.0. Munich, Deutsches Patent und Markenamt Mikamoco Germany (2019). German Minor Patent No. 20 2019 101 810.0. Munich, Deutsches Patent und Markenamt Miller, M. R. (2004). U.S. Patent Application No. 29/183,818. Mueller, C. (2006). German Patent No. 10 2006 012 897 A1. Munich, Deutsches Patent und Markenamt Olle & Manz GmbH (1997). German Minor Patent No. 297 01 033 U1. Munich, Deutsches Patent und Markenamt Opsvik, P. (1972). Norwegian Patent No. 132782C. Oslo, Norwegian Industrial Property Office Pervan, D. (2003). World Patent No. 2003/083234 A1. Geneva, World Intellectual Property Organization Pervan, D., Hakansson, N. (2012). World Patent No. 2012/154113 A1. Geneva, World Intellectual Property Organization Schmidt, W. (2006). German Patent No. 10 2006 004 302.2. Munich, Deutsches Patent und Markenamt Strzygowski, R. (1993). German Minor Patent No. G93 02 568.8. Munich, Deutsches Patent und Marken- amt Wolfschwenger, D. (2015). Austrian Patent No. 516858 B1. Vienna, Österreichisches Patentamt

Books Altshuller, G. S. (1999). The innovation algorithm: TRIZ, systematic innovation and technical creativity. Technical innovation center, Inc. Ashby, M. (2005). Material selection in mechanical design. Third Edition. Butterworth-Heinemann. Oxford

40 Forest Products Laboratory (2010). Wood handbook: wood as an engineering material. General Technical Report FPL-GTR-190. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 508 p. Grote, K. H., & Feldhusen, J. (2007). Taschenbuch für den Maschinenbau, 22. Auflage. Berlin: Springer. Kharazipour, A., & Kües, U. (2007). Wood production, wood technology, and biotechnological impacts. Chapter 20. Recycling of Wood Composites and Solid Wood Products, pp. 509-533, Universitätsverlag Göttingen. Meadows, D., Randers, J., & Meadows, D. (2004). Limits to growth: The 30-year update. Chelsea Green Publishing. Navi, P., & Sandberg, D. (2011). Thermo-hydro-mechanical wood processing. CRC Press. Ponn, J., & Lindemann, U. (2011). Konzeptentwicklung und Gestaltung technischer Produkte: system- atisch von Anforderungen zu Konzepten und Gestaltlösungen. Springer-Verlag. Rogowski, G. (2002). The complete illustrated guide to Joinery. Taunton Press Inc. ISBN: 1-56158-401-0 Roth, K. (1996). Konstruieren mit Konstruktionskatalogen: Band 3: Verbindungen und Verschlüsse, Lösungsfindung. Second Edition. Springer-Verlag. Sato, H., & Nakahara, Y. (1995). The complete Japanese Joinery. Hartley & Marks. Zwerger, K. (2012). Wood and wood joints: building traditions of Europe, Japan and China. Walter de Gruyter.

Papers Daehn, K. E., Cabrera Serrenho, A., & Allwood, J. M. (2017). How will copper contamination constrain future global steel recycling?. Environmental Science & Technology, 51(11), 6599-6606. Faraca, G., & Astrup, T. (2019). Plastic waste from recycling centres: Characterisation and evaluation of plastic recyclability. Waste Management. 95. 388-398. Geissdoerfer, M., Savaget, P., Bocken, N., Hultink, E. (2017). The Circular Economy – A new sustainability paradigm?. Journal of Cleaner Production. 143: 757–768. DOI:10.1016/j.jclepro.2016.12.048 Graedel, T. E., Allwood, J., Birat, J. P., Reck, B. K., Sibley, S. F., Sonnemann, G., ... & Hagelüken, C. (2011). UNEP (2011) Recycling Rates of Metals-A Status Report. A Report of the Working Group on the Global Metal Flows to the International Resource Panel, United Nations Environment Programme. Hemmilä, V., Adamopoulos, S., Karlsson, O., & Kumar, A. (2017). Development of sustainable bio-adhesives for engineered wood panels–A Review. Rsc Advances, 7(61), 38604-38630. Lunguleasa, Cosereanu, Budau, & Matei. (2013). Contributions to the curvature radius and bending capacity of veneers. Wood Research, 59(5), 843-850. Moriyama, Y., Sawada, Y., Fujii, Y., & Okumura, S. (2015). The efficacy of traditional skills on wooden bridge to prevent rain water from entering joint of deck boards. J-Stage. Wood Preservation 2015 Volume 41, Issue 5 p. 213-219. Available at: Nakamura, S., Kondo, Y., Nakajima, K., Ohno, H., & Pauliuk, S. (2017). Quantifying recycling and losses of Cr and Ni in steel throughout multiple life cycles using MaTrace-alloy. Environmental Science & Tech- nology, 51(17), 9469-9476. Ogawa, K., Sasaki, Y., & Yamasaki, M. (2015). Theoretical modeling and experimental study of Japanese “Watari-ago” joints. Journal of wood science, 61(5), 481-491. Pizzi, A. (2006). Recent developments in eco-efficient bio-based adhesives for wood bonding: opportuni- ties and issues. Journal of adhesion science and technology, 20(8), 829-846. Ramesh Kumar, S., Shaiju, P., & O’Connor, K. E. (2020). Bio-based and biodegradable polymers-State- of-the-art, challenges and emerging trends. Current Opinion in Green and Sustainable Chemistry, 21, 75-81.

41 Reck, B. K., & Graedel, T. E. (2012). Challenges in metal recycling. Science. 337(6095), 690-695. Weiss, M., Haufe, J., Carus, M., Brandão, M., Bringezu, S., Hermann, B., & Patel, M. K. (2012). A review of the environmental impacts of biobased materials. Journal of Industrial Ecology, 16, S169-S181.) Xu, M., Li, L., Wang, M., & Luo, B. (2014). Effects of surface roughness and wood grain on the friction coefficient of wooden materials for wood–wood frictional pair. Tribology Transactions, 57(5), 871-878.

Standards European Committee for Standardization. (2015) Furniture - Strength, durability and safety - Require- ments for domestic tables. (EN 12521:2015). European Committee for Standardization. (2012) Furniture - Tables - Test methods for the determination of stability, strength and durability. (EN 1730:2012).

Online Beck Fastener Group. (2020). The Collated Nail Made Of Wood - Lignoloc. Available at: [Accessed 2 April 2020]. Cronon, W. (2018). Built Environments of Domesticity and Commerce. Available at: < https://www. williamcronon.net/courses/469/handouts/469-built-environments-of-domesticity-and-commerce.html> [Accessed 26 April 2020]. Inter IKEA Systems. (2017). Story of the Wedge Dowel. Available at: < https://ikea.today/story-of-the- wedge-dowel/> [Accessed 24 March 2020] Johnson, R. (2013). Spring Joint Box. popularwoodworking.com. Available at: [Accessed 2 April 2020].

Other European Bioplastics. (2018). Bioplastics Market Data. Available at: https://www.european-bioplastics. org/wp-content/uploads/2016/02/Report_Bioplastics-Market-Data_2018.pdf Inter IKEA Systems. (2020). IKEA Sustainability Report FY19. Available at: Plastics Europe. (2019). Plastics – the Facts 2019. Available at: https://www.plasticseurope.org/application/ files/9715/7129/9584/FINAL_web_version_Plastics_the_facts2019_14102019.pdf Statista. (2020). Value of the furniture market worldwide from 2020 to 2027. Available at: [Accessed 19 January 2021] Statista. (2021). Annual revenue of the IKEA Group worldwide from 2001 to 2020 Available at: [Accessed 19 January 2021] UN General Assembly. (2015). Transforming our world : the 2030 Agenda for Sustainable Development. A/RES/70/1

Image Sources Echols, G. (2006). How to Cut a Butterfly Key. [online] Available at: https://www.finewoodworking. com/2006/08/10/a-lesson-in-butterfly-keys> [Accessed 06 May 2020] Ernst Dünnemann GmbH & Co.KG (2020). KLEMMSIA Clamps. [online]. Available at: [Accessed 2 April 2020]. Frameworks Timber (2020). Timber Frame Homes. [online] Frameworks. Available at: [Accessed 10 April 2020]. Furnitureparts.com. (2020). [online] Available at: [Accessed 29 Sep- tember 2020].

42 Hoffmann GmbH Maschinenbau. (2020). Hoffmann-Schwalbe. [online] Available at: maple-12-length/> [Accessed 2 April 2020].

Homag AG. (2020). [online] Available at: . [Accessed: 26. May 2020] Miller Dowel Co. (2020). Miller Dowels, Wood Joinery. [online] Available at: [Accessed 2 April 2020]. Nutsch, W. (2005). Verbindungsbeschläge Für Den Möbelbau: Aufschraubbare Verbinder. [online] BM online. Available at: [Accessed 10 April 2020]. Porta Mouldings Pty. Ltd. (2020). Round Multi-Groove Dowel. [online] Available at: oak/> [Accessed 2 April 2020]. Rotari, D., (2008). Dovetail Joint. [online] En.wikipedia.org. Available at: [Accessed 10 April 2020].