1. Process Planning and Process Plan

Computer Aided Process Planning In Machinery Industry

Ivan Kuric 1. PROCESS PLANNING AND PROCESS PLAN

Definitions of process planning Process planning can be defined as engineering activities assigned to manufacture or assembly the products. Process Planning is the activity in a production enterprise that determines which processes, materials, and instructions will be used to fabricate a product. Process planning dictates a manufacturing facility, processes and parameters which are to be used to convert materials from an initial form to a predetermined final form. Process planning is an act of preparing detailed process instructions to produce a part. Process planning is a result of engineering planning activities of process planners which prepare a list of processes needed to convert a raw material shape as a starting point into a predetermined final shape. The aim the of process planning is to convert design specification into manufacturing instructions and to make products within the function and quality specification at the lowest costs. Process planning is a recipe for making products. Process planning in manufacturing may be generally defined as the development of set of instructions describing all the operations required to convert a design into a product.

1.1 Process plan and process planning Engineering parts are manufactured by some of engineering technologies – machining, forming, casting or welding. Production is carried out according to prescribed process instructions and operations. The prescription is stated in a process plan. The process plan is documentation determining all machining operations, order of the operations, all machine equipment, conditions and requirements to producing a part. Process plan consists of preparing a detailed plan with instructions specifying how a product is to be manufactured or assembled, taking into account the characteristics of both the product and the available equipment and processes.

Process plan Documentation

Process planning Engineering activities Figure 1.A: Process plan and process planning

RAW STOCK PART

Machinery operations type of machine type of cutting tool type of fixture cutting condition

time, cost

DESISION

Figure 1.B: Process plan as a recipe for transformation of raw stock to the final part

1.2 Purpose of process planning Process planning deals with the selection of the processes and the determination of conditions of the processes. The selected operations and conditions have to be realised in order to transform raw material into a given shape. All the specifications and conditions of operations are included in the process plan. The process plan is a document such as engineering drawing. Both the engineering drawing and the process plan present the basic document for the manufacturing of products. Process planning meaningly influences time to market and productions cost. Therefore the planning activities have a great meaning for competitive advantage. Process planning

Time to market

Price Production cost

Competitive advantage

Figure 1.C: Influence of process planning on the competitive advantage Process planning is the planning activity of an engineer or a planner. One can see planning activities in various application areas, for example:  food industry,  chemistry industry,  building industry  engineering industry. In the engineering industry following activities for various technologies are planned:  machining,  forming,  casting,  welding,  heat treating,  assembling. The product is manufacturable in many ways. Manufacturing methods depend on several parameters of part and production:  geometrical properties (shape and dimensions) of part,  material properties of part,  total amount of parts,  batch,  available manufacturing equipment,  production facilities,  constraints of manufacturing environment,  time to manufacture,  cost to manufacture. A human has to consider the mentioned parameter and to propose an optimal process plan. The optimal process plan has to fulfil the selected production criteria (cost, time, quality). The manufacturing environment and the production facilities are subjected to relatively small changes over time. This means that processes, machine tools, cutting tools, measuring machines, fixtures, jigs and other equipment are not subjected to great changes over time. However, the planner must take into account the constraints of the manufacturing environment. During process planning a planner operates with:  information located in engineering drawings and bills of material,  manufacturing knowledge and  information of machine equipment. Process planning is an engineering activity with a great amount of decisions. It calls for good knowledge and experience on the part of the planner participating in the process plan making. Engineering drawing , Bill of material

Manufacturing knowledge Information and - manufacturing posibilities knowledge about machine of machines and tools Planner equipment - technological constraints - geometrical constraints - type of machine and - manufacturing methods cutting tools - heat and chemical treatment - parameters of machines - fixturing - fixture, jigs - methodology of process plan - measurement equipment creation - information about - theoretical knowledge, manufactured and cutting experience material

Manufacturing documentation

Process plan Assembly process plan List of tools, NC code, route sheet

Figure 1.D: Information and knowledge needful for process plan creation The process plan is realised step by step in individual planning tasks. According to the process planning there are multilevel planning activities. Into output of a previous task influences the following planning tasks. Individual planning steps are shown in the following picture. Analysis of engineering documentation - engineering drawing - BOM - bill of material

Analysis of production - manufacturing programme - profil - production schedule - manufactured parts - existing machine equipment

Design of raw stock (type, dimension, form, etc)

Design of order of process operations

Selection of machine equipments - machine tools - cutting tools - jigs, fixtures - measurement tools and equipment Optimal process plan

Technological and cutting conditions Evaluation

Evaluation of technical and economic properties Variants of process plans

Figure 1.E: Individual tasks of the process planning

1.3 Manufacturing process planning In a scientific literature the term - process planning is also known as  manufacturing planning,  material processing,  process engineering,  machine routing.

Process planning in manufacturing may include the following activities:  selection of raw-stock,  determination of machining methods,  selection of machine tools,  selection of cutting tools,  selection or design of fixtures and jigs,  determination of set-up,  determination of machining sequences,  calculations or determination of cutting conditions,  calculation and planning of tool paths,  processing the process plan. The level of development of process documentation depends on production type (piece, batch or mass production). If the product is to be manufactured on Numerical Controlled (NC) machines, process planning includes also the generation of NC programs. The degree of detail of a process plan varies from industry to industry. The job-shop type of manufacturing environment usually requires the most detailed process plans since the design of tools, jigs and fixtures and manufacturing sequence etc., are dictated directly by the process plan.

1.4 Process plan as a set of manufacturing features The process plan involves process operations. The process operation is possible to consider as a manufacturing feature involving the machine, cutting tools and fixture. Subsequently the process plan is a set of the manufacturing features. It is important to determine the condition of utilising the machine equipment and environment. The cutting condition (cutting speed, feed and depth of cut) represents significant condition for machine equipment.

1.5 Macro and micro process planning There is methodological possibility to segment process planning activity in two following section:  macro process planning,  micro process planning. Output of macro process planning activities is a scheme of machine equipment and process operation sequencing. Micro process planning includes determination of cutting condition. micro planning g n

i micro n planning n a l p

n a o l r p

c s a s e c m o r p

micro planning

machine cutting equipment condition

Figure 1.F: Micro and macro process planning Macro Process Planning includes:  review of common machining processes,  accessibility of manufacturing features,  process and machine selection,  operations sequencing methods,  workpiece set-up planning. Micro Process Planning are activities as following:  advanced process models for prediction of machining performance  process optimization,  optimum tool path planning algorithms,  analysis of Part-process-workholding design trade-off. Oftentimes process planning is accepted as a manufacturing recipe with specified manufacturing equipment and conditions. However, the process plan is necessary to understand as a complex task to transform raw-stock not only to final product but to the customer. It comes to this, that in the complex process plan the following problems are involved:  machining,  transportation,  assembly,  metrology,  unit and system test,  packaging and  export. Recently, and also often nowadays, experts have mainly performed these tasks. This work involves both creativity and skill. Many of activities are routine based. When creativity and skill are not sufficient to solve a process planning problem, the process planner will usually communicate the problem to the designer in order to try to solve the problem. Therefore, process planning is a team job which includs experts from the individual application area (tool designer, fixture designer, process planner, cutting condition planner, etc.). Communication among the process planning team has great balance, a good process plan often depends on joint co-operation and team-work. During the process planning the following human abilities play an important role:  theoretical knowledge,  skill,  experience. Goals of process planning are:  meet quality requirements,  maximize production efficiency,  minimize production cost,  protect working conditions,  improve technology. The process planning job, except the basic task of plan generation, should check design feasibility and also generate cost estimates and bid product information. 1.6 Input and output of process planning It is possible to consider process planning as a closed subsystem with input and output. Following basic input parameters for the process plan generation are:  manufacturing resources,  engineering specifications,  design drawings,  bills of material (BOM). Output from the process planning is a set of the following:  material and blank specifications,  enterprise and machine routings,  machine settings,  operating instructions for each manufacturing task,  measurement equipment and measuring activities,  tool and fixture descriptions,  required process capabilities and  time and cost estimates.

1.7 Type of process plan The type of the process plan depends on the production type. The same part may have an other process plan in a different enterprise. A different process plan belongs to the same part in piece, batch and mass production. The level of process plan elaboration depends above all on the type of the production (piece, batch and mass). There are several varieties of process plans:  textual process plan,  pictographic process plan,  NC code,  simulation code. Process plan

Textual Pictographics NC code code for process plan process plan simulation

Figure 1.G: Types of external representation of process plan

1.8 Computer support at process planning activities

1.8.1 Why computer support for process planning? Competitive surroundings and environment force manufacturing enterprises to produce in smaller series, with shorter lead time and with decreasing costs. Flexibility is one of the keys to solve the current state of production. Process plan includes standard process operations. The process planning task is to retrieve and to synchronise these operations into an effective and optimal process plan. Synchronising of the process operation is a high mental activity for the process planner. As process planning is a complex task, it is needful to take into consideration the affecting factors. Parallel reflection and reasoning is a characteristic feature of the human. Possible type and range of activities in process planning:  intuitive activities (from 1 to 5 %),  formal logical activities (from 25 to 50 %),  routine activities (from 45 to 74 %). The routine activities are solved with known algorithms. A more complicated matter is the computer solving of problems with unknown algorithms (e.g. selection of acceptable machines and tools, selection of operation sequence, etc.). Definite algorithm for some specification of the process plan solving is not known in advance. The process planner often utilizes heuristic solving methods, knowledge, experience and intuition. These activities have been practiced for many years and knowledge of research in the area is often of great importance in the decision process. The manual process planning is often a tedious and time demanding engineering process and it is one of the labour activities in the preparatory stage of manufacturing. There are many routine, heuristic, deciding and intuitive activities used by a planner. There is an effort for these activities to be supported by computer. Computer support can markedly help to solve some planning activities. Computer aided process planning (CAPP) system is software for the automated design of route sheet. 1. 2. variant variant

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Manual process planning

Figure 1.H: Manual process planning

Computer Aided Process Planning

Figure 1.I: Computer support in process planning A number of reasons can be identified for the advance of CAPP systems. There are amount of process planning departments in small batches where a skilled workforce is scarce. Many companies have different process planners make different process plans for the same parts, resulting in inconsistencies and extra paper work. CAPP systems can help in overcoming these inconsistencies. Computer Aided Process Planning aids in creation of process plans for manufacturing and increases the flexibility of manufacturing. Process planning is a task which requires a significant amount of both time and experience. Computer support or computerised process planning systems can help reduce a process planning time and increase plan consistency and efficiency. model of part decision model making and of process logic operation model of machine equipment

Figure 1.J: Computer aided process planning

1.9 Benefits of Computer-aided Process Planning The use of CAPP systems has the following potential advantages:  reduced demand on the skilled planner,  reduced process planning time,  reduced process planning and manufacturing cost,  created more consistent plans,  produced accurate plans,  increased productivity,  increased high flexibility,  attained high efficiency,  attained adequate high product quality,  possibility of integration with the other automated functions and systems.

1.10 Methods of automated manufacturing process planning The advantage of automated manufacturing process planning is undisputed. There are two approaches for creation and processing of process plan based on computer support and advanced planning methods. The first approach is based on Group technology utilising (variant method), the second approach is the exact mathematical principle based on modelling of part, manufacturing knowledge and process plan (generative method). In the history of computer aided process planning two different ways of obtaining the process plan can be observed:  variant process planning,  generative process planning.

Process planning

Manual Automated (Computer aided)

Variant Hybrid Generative approach approach approach

Figure 1.K: Kind of process planning COMPUTER AIDED PROCESS PLANNING

Group technology Exact approach approach

Generative Variant CAPP CAPP

Similarity of parts Mathematic and process plans modelling

Sysklass Part (CAPP software) (CAPP software)

Textual process All form of plan process plan

Figure 1.L: Two basic approaches for CAPP

1.10.1 Variant Process Planning Variant process planning implements a coding and classification scheme by which a process plan for a previously planned part is retrieved. The retrieved plan is based on the similarity to the new part. The process plan is then manually modified as required for the new part design. There is high probability that similar parts have similar process plans. This is a basic assumption of utilising the variant process planning method. Variant methods assume that the user is able to determine the appropriate classification codes needed to retrieve appropriate plans, and that plans exist and include features which are closely analogous to those of the new part. Because variant systems are based on the process plans on historic data, it is also assumed that the factory configuration is stable, with only minimal workstation or process capability changes. The variant approach to CAPP was the first approach used in computer process planning. Variant CAPP is based on the concept that similar parts have similar process plans. The computer is used as a tool to assist in identifying similar process plans, as well as in retrieving and editing the plans to suit the requirements for specific parts. Variant CAPP is related to part classification and Group Technology coding. In these approaches, parts are classified and coded based upon several characteristics or attributes. A Group Technology code can be used for the retrieval of process plans for similar parts.

1.10.2 Generative Process Planning The process plan is created according to a very complicated methodology. The description of part and knowledge bases are necessary for the process plan creation. In a generative process planning system, an adequate (usually feature-based) part model is utilised to build a process plan from scratch. From the fully described geometry, parameters are derived to determine applicable processes and resources. Generative process planning allows a high level of flexibility for building the process plan according to varying resource conditions. Generative CAPP came into development in the late seventies. It aims at the automatic generation of process plans, starting from scratch for every new workpiece description. Often, the workpiece description is a CAD solid model, as this is an unambiguous product model. A manufacturing database, decision making logics and algorithms are the main ingredients of a generative CAPP system. In the early eighties, knowledge based CAPP made its introduction using AI techniques. The generative CAPP systems are often called expert CAPP systems because these systems utilise the expert process planning methodology.

1.11 System approaches in CAPP Developments in computer based planning are attempts to free the human from the planning process and to eliminate decisions required during design and planning. CAPP has various levels of human intervention according to used approaches. The two principles are principally different. In the first principle - Group Technology based approach or so called variant approach - a planner retrieves the plan for similar components using coding and classifications of parts. The planner edits the retrieved plan to create a variant to suit the specific requirements of the component being planned. This technique is based on the principle that geometrical and technological similar parts have similar process plans. The computer aid is used to assist in identifying similar plans, retrieving them and editing the plans according to the geometrical difference. In variant CAPP system the process plan is assigned for the whole part according to the global part information. Parametric information between the technological operations and the part feature does not exist. In the second approach – generative method - the process plan for a new part is automatically synthesized. The generative CAPP system, also called exact system, creates the process plan from information available in manufacturing databases according to a CAPP methodology. The CAPP system operates without or with little human intervention. Each of the part features can be manufacturable by several technological operations. For individual feature from manufacturing knowledge base, the technological operations (technological transformer) ensure the required part properties are generated. From a set of convenient technological transformers an optimal aggregate of technological transformers is extracted. There is parametric information between the technological operation and the part feature. The manufacturing knowledge is one of the basic information bases for automated process planning. The manufacturing knowledge in the variant CAPP systems is placed in standard plans for each family group. Knowledge is complexly expressed in manufacturing, fixturing and heat treatment instructions. The knowledge is represented in textual or coded form and is not systematicly divided into knowledge bases. The knowledge in the generative CAPP systems is placed in the individual bases. They should consist of information on manufacturing methods, manufacturing equipment, fixturing, heat treatment, product feature structure, etc. The individual bases are mutuall in relation. The knowledge is directly expressed and is represented in various representation schemes (production rules, frames, decision trees, decision tables, semantic nets). The two basic approaches require the different describing of the part properties. The generative CAPP system needs the unambiguous description of the geometrical, topological and technological part properties. For variant CAPP approach it is convenient to have ambiguous part information, for example some of well-know GT codes (Opitz, CODE, Miclass, Dclass code).