LICENTIATE THESIS 1992:22 L DIVISION OF MATERIALS PROCESSING ISSN 0280 - 8242
ABRASIVE WATER JET CUTTING
An experimental and theoretical investigation
LARS OHLSSON
TEKNISKA HÖGSKOLAN I LULEA LULEÅ UNIVERSITY OF TECHNOLOGY 1992 ··11- D 4 LICENTIATE THESIS 1992:22L �IBLIOTEKET
ABRASIVE WATER JET CUTTING An experimental and theoreticalinvestigation
Lars Ohlsson
Division of Materials Processing Lulel!.University of Technology 1992
J
ISSN 0280 - 8242 i
PREFACE
Since 1990 I have had the pleasure and opportunity of carrying out research work on Abrasive Water Jet (AWJ) Cutting. AWJ cutting is a rather new industrial process, the first commercial system was introduced to the market place as late as 1983, although Water Jet cutting has been available since 1968. In recent years the AWJ technique has been developed considerably and is now used with great economical and technical success world- wide.
The present work is a result of four projects carried out at Luleå University of Technology, Division of Materials Processing.
Special thanks are due to my supervisor Professor Claes Magnusson for his guidance and support during this work, and to Dr. John Powell from whom I have received invaluable guidance, discussions and suggestions. I will also use the opportunity to thank our reference group on AWJ cutting for their valuable suggestions and ideas.
I also wish to thank all members in the Sheet Metal Group and specially the head of the group, Dr. Ulla Öhman, for her interest in and support of my work.
The present work has been financially supported by The Research Council of Norrbotten, Sweden.
Luleå, November 1992
Lars Ohlsson 11
ABSTRACT
Although a great deal of experimental and theoretical work has been published on the subject of Abrasive Water Jet Cutting (AWJ), there is very little information available concerning practical engineering problems as well as a detailed description of the cutting process itself.
This thesis consists of four parts;
Part Iis a comparison between AWJ cutting and laser cutting. The objective of part I is to introduce the abrasive water jet technique and compare the method with a complementary established technique. The section presents a discussion of the advantages and disadvantages of AWJ profiling as compared with laser cutting.
Part II and HI gives a detailed examination of two important engineering problems, the generation of start-up holes and profiling of corners. A theoretical discussion is conducted and solutions to the problems are presented together with experimental results. This work involved detailed analysis of the AWJ - material interactions concerned and this generated results which were important both scientifically and commercially. For example it has been demonstrated that the piercing event can be accelerated by a factor 10 if the correct movement parameters are employed. A similar analysis of corner profiling generated technical guide-lines on the production of accurate, sharp corners in thick section materials.
The last part of the thesis, Part 1111, presents a detailed investigation of the forces exerted on the workpiece during cutting and the relation between cut front geometry and the forces acting on the material. An extensive experimental program has been conducted where the cutting force and the surface of the cut edge were measured. The result of the work is a more detail understanding of the cutting process and the relation between cutting forces and the quality of the surface of the cut edge. iü
CONTENTS
Page
ABSTRACT i
PREFACE ii
CONTENTS 1
PAPER I Comparison between Abrasive Water Jet Cutting and Laser Cutting 1
L. Ohlsson, J. Powell, A. Ivarson, C. Magnusson
Published in Journal of Laser Applications, 3(1991)3, pp. 46 - 50.
PAPER II Optimisation of the Piercing or Drilling Mechanism of Abrasive Water Jets 13
L. Ohlsson, J. Powell, C. Magnusson
Published in Proceedings of the 11th International Conference on Jet Cutting Technology, St. Andrews, Scotland, Sept 8 - 10, 1992, pp. 359 - 370.
PAPER III The Profiling of Sharp Corners by Abrasive Water Jet Cutting 26
J. Powell, L. Ohlsson, C. Magnusson
Submitted for publication (1992) in International Journal of Water Jet Technology.
PAPER IIII Cut Front Geometry and the Forces Exerted on the Workpiece During Abrasive Water Jet Cutting 36
L. Ohlsson, J. Powell, C. Magnusson
Submitted for publication (1992) in International Journal of Water Jet Technology. 1
PAPER 1
COMPARISON BETWEEN ABRASIVE WATER JET CUTTING AND LASER CUTTING 2
COMPARISON BETWEEN ABRASIVE WATER JET CUTTING AND LASER CUTTING
L. Ohlsson+, J. Powellll+ * A. Ivarson+, C. Magnusson+
Luleå University of Technology Division of Materials Processing S-951 87 Luleå, Sweden
*Laser Expertise Ltd. Acorn Park Industrial Estate Nottingham NG7 2TR, UK
ABSTRACT
This paper is intended to demonstrate the advantages and disadvantages of laser profiling techniques as compared with the Abrasive Water Jet (AWJ). The growth of AWJ as a cutting tool has provided engineers with a new profiling technique which often offers great technical and commercial advantages over more traditional methods. However, AWJ cutting is not the best solution to all profiling problems. There are a number of techniques which compete with or complement the process and the optimum profiling method can be difficult to identify, W. The following paper serves as a general guide-line comparing two competitive cutting methods (CO2 laser cutting and Nd:YAG laser cutting) with AWJ cutting. The subject of cutting covers a great many more processes than can be reviewed in one article but the techniques to be discussed were chosen because they all involve profiling using an axially symmetric energy beam of some sort.
ABRASIVE WATER JET CUTTING, AWJ
In 1983 Flow Systems Inc sold the first commercial AWJ cutting system in the world which makes AWJ cutting a rather new profiling technique,[31. The basic mechanism of AWJ cutting is quite simple. A hydraulic pump is used to pressurise water in an intensifier to pressures of the order of 3800 bar. This pressurised water is then ejected from a small diameter nozzle (0.12-0.33 mm). The jet velocity, up to 800 m/s, depends only on the water pressure. The high pressure water jet thus produced then passes through a chamber where it 3 is mixed with abrasive powder, for example silica or garnet, before passing through the focusing nozzle to interact with the workpiece. The intensifier achieves very high water pressure by using a large diameter, hydraulic oil driven piston to drive a smaller diameter piston acting on a chamber full of water. The pressure on the water exceeds the pressure of the oil by the ratio of the surface areas of two cylinders. This ratio can be of the order of 20:1 and thus a 200 bar hydraulic oil system can produce a pressure of 4000 bar in the water. These intensifiers have a predetermined stroke length and are often designed to be reciprocating to maintain as continuous a supply of pressurised water as possible. Even though this reciprocating action produces pressurised water when the piston is travelling in either direction, the flow is not continuous as the piston must stop between changes of direction. To reduce the consequences of this intermittent flow on the cutting action the system includes an accumulator which acts as a reservoir of high pressure water which then supplies the cutting nozzle. The abrasive can be mixed into the water by injecting a blown powder or by filling the mixing chamber with a water and abrasive slurry through which the high pressure jet can pass.
The actual cutting mechanism is one of abrasion of the workpiece by the high velocity particles in the water jet. The cutting process consists of two modes as figure 1 shows. The first is the cutting wear mode, in which material is removed by particle impact at shallow angles, as in a micromachining process. The second is the deformation wear mode, which is characterized by material removal due to excessive plastic deformation by impacts at large angles,[4].
WATER VELOCITY Uo PARTICLE VELOCITY Vo UD Vo
CUTTING WEAR ZONE
PARTICLE TRAJECTORIES i.
DEFORMATION WEAR ZONE
STEP REMOVAL BY DEFORMATION WEAR
Figure 1. Cutting mechanism for AWJ,[4].
The abrasive particles used are made of silica, garnet or aluminium oxide and are generally used in grit sizes of 250 to 500 gm with consumption rates of 250 g/min to 2 kg/min. Fine abrasives with a grit size of 500 gm (Mesh 120) may be used when the best surface finish is required and have been used with excellent results when cutting glass. Larger grit sizes are more effective for steel cutting and even coarser sizes (e g Mesh 36 & 16) have given the best results for cutting rocks and concrete [5].
Table 1 gives an idea of the cutting speeds available by this process. These figures must be used as a guide-line only as the process involves a large number of independent and interrelated variables all of which have an effect on the effectiveness of the process. These parameters include water pressure, orifice diameter, focusing nozzle diameter, grit type, grit 4 size and the feed rate of abrasives. Figure 2 and 3 gives some examples of how the cutting speed is related to some of the processing parameters.
Table 1. Typical cutting speeds for AWJ method, [5,6,71. Material Thickness [mm] Cutting speed [mm/min] Cutting speed [m/min]
Mild Steel 1.6 500 0.50 13.0 100 0.10 50.0 38 0.038 180.0 10 0.010
Stainless Steel 5.0 400 0.40 13.0 150 0.15 25.0 76 0.076
Aluminium 1.6 1300 1.30 6.0 500 0.50 25.0 130 0.13 100.0 25 0.025
Titanium 3.0 500 0.50 6.0 400 0.40 12.0 100 0.10
Glass 13.0 1300 1.3 19.0 600 0.6 25.0 130 0.13
Marble 50.0 400 0.4
Concrete 250.0 25 0.025
18 18 ( d=0.25 mm Mild Steel d=0.25 mm Grey Cast Ironl E>=0,8 mm 86g/min 16 - -D=0.8 mm 16- -L=51 mm frifIl 145g/min - 14- 150mm/min 14- L51 E m=145 g/min v=150 mm/min E 12 - E 12 - ,Gamet Mesh 80, Gamet Mesh 73 10- 10 - '5 -- 8- a. 6 - 6- 4-- 300mm/min 4- 32g/min 2- 450mm/min 2- 0 0 1500 2000 2500 3000 3500 4000 1500 2030 2500 3000 3500 4030 Pressure (bar) Pressure (bar) Figure 2. Effect of pressure, abrasive flow rate and traverse speed on depth of cut in mild steel and gray cast iron,[6].
Inside the waterjet the abrasive grit particles are accelerated to velocities of the order of 800 m/s. At this velocity the impact of the hard particles on the cutting front causes local failure of the workpiece and controlled erosion takes place. Because high particle velocities are needed over the whole of the cutting front, the velocity and momentum of the water jet is only partially reduced during passage through the cut zone. It is therefore important to dissipate the high energy jet leaving the bottom of the cut zone by either catching the water 5 in a hard ceramic tube long enough to dissipate the jet, or for production systems where the jet is moving rather than the workpiece, an energy absorbing replaceable bed is used to support the workpiece.
50 g 50 = 1C(1 mm/min 2 p =2400 bar 20mm/min 3000bar 0 40 — E 50mm/min a 2400bar gx I 103mm/min co 30— a 2000bar 5»•.. 30 — 2COmm/min a 2°- 1030bar a) cgo 10-- • lo 0 cx 0g 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 5 10 15 20 25 30 35 40 0 10 20 30 40 50 60 70 80 Measuring position y (mm) Measuring position y (mm)
I-JET noel d=0.25mm focus L=50mm D=1.2mm f al stand-off s=2mm abrasives m=8g/s y ))) test material AlMgSi0.5 Figure 3. Roughness of the cut as a function of pressure and traverse rate, [71.
The advantages of AWJ cutting over laser cutting are:
I. The maximum thickness of material which can be cut is an order of magnitude or more than is possible by CO2 laser. Steel cutting in thicknesses up to 100 mm is common. Most materials can be cut in sections of several tens of millimetres.
II. All materials can be cut irrespective of such physical properties as melting point and thermal conductivity. Materials which are impractical to laser cut such as marble or concrete are cut with great effectiveness.
III. As the process is non-thermal there is no heat affected zone associated with the cut edge. This feature is particularly attractive when susceptible materials such as titanium alloys are being profiled.
IV. For some metals the cut edge quality can be superior. Dross is not a problem.
The features of the AWJ process which compare badly with laser cutting are:
I. Kerf widths are substantially larger than for laser cutting and usually exceed one millimetre. This naturally restricts the amount of detail which can be cut.
II. The process is much slower for cutting metals in the range of thicknesses where the two compete, i.e. between 1.0 - 15.0 mm.
III. The capital and running costs of the two processes are of the same order of magnitude but the cost per unit length of cut will be greater for the AWJ process when thin section metals are cut as a result of the reduced cutting speed. 6
IV. AWJ cutting is much noisier and generally messier than laser cutting.
In summary, the AWJ process is an excellent method of cutting thick section materials of all types and for cutting heat susceptible materials but speeds are low and fine detail is not possible.
Capital and running costs for AWJ cutting
The running cost for an AWJ system is highly dependent on the type of material to be cut and the required kerf quality. Hard materials require the use of more expensive abrasives as well as the increase of the water pressure which will lead to higher running costs. Figure 4 gives an example of the average capital and running costs for a normal AWJ cutting system with one cutting head and an intensifier which has a water flow rate of 3.1 1/min at a pressure of 3800 bar.
Water 8( Electricity 1.8% Abrasives 16%
Operator 33.4% Waste Removal 0.8% Nozzles 3.6%
Maintenance 6.9%
Capital 37.5%
Total Cost: $ 122.75 / h Figure 4. Average cost per hour for AWJ cutting.
CO2 LASER CUTTING
The basic mechanism of CO2 laser cutting is simple and can be summarised as follows. A high intensity beam of infrared light with a wavelength of 10.6 p.m is generated by a CO2 laser. This beam is focused onto the surface of the workpiece by means of a lens. The focused beam heats the material and establishes a very localised melt throughout the depth of the sheet. The molten material is ejected from the area by a pressurised gas jet acting coaxially with the laser beam. With certain materials this gas jet can accelerate the cutting process by doing chemical as well as physical work. For example, steels are generally cut in a jet of pure oxygen. The oxidation process initiated by the laser heating generates its own heat and this greatly adds to the efficiency of the process. This localised area of material 7 removal is moved across the surface of the sheet thus generating a cut. Movement is achieved by manipulation of the focused laser spot or by mechanically moving the sheet on a CNC X-Y table.The first industrial use of CO2 lasers was cutting of plywood die boards for the packaging industry. Since this early application in 1971 the technology has developed enormously and lasers are now used with great commercial and technical success to cut almost any material. A cursory review of the literature available will reveal the enormous scope of application, from drilling the holes in baby feeder teats to cutting superalloy panels for the aerospace industry.
The advantages of CO2 laser cutting over AWJ cutting are:
I. The kerf width is extremely narrow (0.1 to 1.0 mm) and so very detailed work can be carried out without the restriction of a minimum internal radius imposed by AWJ cutting.
II. The process cuts at high speed compared to AWJ cutting (see table 2). For example a typical 1200 W laser will cut 2 mm thick Mild Steel at 6 m/min. If AWJ cutting is used the speed will be approximately 0.5 milmin.
III. Although the capital cost of a laser cutting machine is substantial the cost per unit length of cut will be much lower compared with AWJ cutting (see figure 6).
IV. The process is extremely quiet and clean compared to AWJ cutting, a factor which improves the working environment and the efficiency of the operating staff.
V. The hardness of the material to be cut does not affect the cutting speed and the quality of the kerf when CO2 laser cutting is used as cutting method.
The features of the CO2 laser cutting which compare badly with AWJ cutting are:
I. The maximum thickness of any material which can be cut is limited, e g with a laser power of 1000 W: Mild Steel —10 mm, Aluminium —3 mm, Acrylic —50 mm.
II. CO2 laser cutting is a thermal process so there will be a heat affected zone even if it is small.
III. It is difficult to cut highly reflective materials using CO2 lasers, e g Al, Cu, Ag etc. 8
Table 2. Typical cutting speeds for CO2 laser cutting, [8,9,10]. Material Thickness Cutting Cutting Laser Cutting Gas Speed Speed Power Gas Pressure
[mml [mm/mini [m/min] [WI [bal Mild Steel 1.5 4800 4.80 800 02 3.0 3.0 2400 2.40 800 02 2.3 5.0 1300 1.30 800 02 1.5 8.0 600 0.60 800 02 1.0
Stainless Steel 1.0 5800 5.80 800 02 6.5 2.0 3100 3.10 800 02 6.5 3.0 1900 1.90 800 02 6.5 5.0 1150 1.15 800 02 8.0
Aluminium 0.70 1500 1.50 500 N2 2.0 1200 1.20 1000 N2 3.20 1000 1.00 1250 N2
Titanium 1.0 7500 7.50 600 02 3.20 1500 1.50 1500 CO2 6.40 1500 1.50 750 02
Copper 0.60 500 0.50 500 02
Acrylic 5.0 4000 4.00 500 N2 10.0 1600 1.60 500 N2 20.0 700 0.70 500 N2
Glass 2.0 800 0.80 350 Air
Capital and running costs for CO2 laser cutting.
Figure 5 gives an example of the average capital and running costs for a CO2 laser cutting system. In this case the figures are based upon a system with a 1200 W laser, a CNC controlled cutting station and 1600 hiyear. 9
Electricity, Water & Gas 8.8% Maintenance 8( Lenses 4.4% Operator 30.1%
Total Cost: $136.00 / h Figure 5. Average cost per hour for CO2 laser cutting.
Nd:YAG LASER CUTTING
Nd:YAG lasers, usually referred to as simply YAG lasers, are capable of cutting and welding metals in much the same way as CO2 lasers, [21. The output beam is generally of a lower power than that generated by CO2 machines and this tends to restrict cutting speeds and maximum material thicknesses. The light generated by a Nd:YAG laser has a wavelength of 1.06 gm which is, by coincidence, one tenth of the wavelength of CO2 lasers. This shorter wavelength light has a number of advantages over CO2 laser output.
Glass is transparent at this wavelength and so high quality glass optics can be employed to focus the beam. Glass lenses are generally of a higher optical quality than zinc selenide or gallium arsenide lenses used for CO2 lasers and are also cheaper. Complex multicomponent optical arrangements can be employed to minimise the focus spot diameter and it is possible to fit an optical microscope to the focusing head to observe the welding or cutting process directly through the same lens which is focusing the laser beam. In addition to these advantages it is possible to guide the beam down optical fibres.
A more intense focused spot can be achieved than is possible for CO2 lasers. The diameter of a focused spot is proportional to the wavelength of the light being focused. As the Nd:YAG laser generates light with a wavelength of 1.06 um rather than 10.6 gm the theoretical minimum focal spot diameter will be approximately one tenth of the CO2 value. In practice this reduction in spot size is not possible from a high power Nd:YAG laser because these lasers produce a poor quality mode which does not focus as effectively as the CO2 laser beam. The two effects of lower wavelength and poor mode tend to cancel each other out. 10
The absorptivity of metals to infrared light increases as the wavelength decreases. The increase in absorptivity for the shorter wavelength Nd:YAG laser implies that these machines would be suitable for processing highly reflective materials. In practice this has been found to be true and Nd:YAG machines have found applications in the jewellery industry cutting gold sheet.
In spite of all these optical advantages Nd:YAG lasers occupy only a small part of the general laser cutting market. The majority of Nd:YAG machines are sold for welding and drilling applications which is a reversal of the CO2 laser market. The reasons why CO2 lasers dominate the field are as follows.
Although cutting speeds for most metals are similar for both Nd:YAG and CO2 machines at any particular laser power, the usual range of powers for Nd:YAG cutting machines is much lower (50-400 W) than it is for CO2 machines (50-2000 W). A major part of the cutting market involves the profiling of polymers. CO2 laser radiation is very effectively absorbed by these materials but they are generally transparent and therefore uncuttable at the Nd:YAG wavelength of 1.06 gm. Although Nd:YAG lasers cannot cut polymeric materials they can be used to cut ceramics with great success by scribing or full penetration cutting. The maximum speeds of scribing are lower than for CO2 machines because the pulse repetition rate is lower for Nd:YAG (i e maximum 500 Hz).
Nd:YAG lasers have found a market in applications where fine detailed work or highly reflective metals need to be cut. Kerf widths of tens of microns can be achieved and dross is not generally a problem as the amount of residual melt left on the cut edge is small. Cutting takes place as a result of repeated pulsing of the beam. Although the laser power is quoted in Watts this refers to the average power over several seconds. Table 3 gives a selection of cutting speeds for various materials using Nd:YAG lasers.
Table 3. Typical cutting speeds for Nd:YAG laser cutting, 11 12 . Material Thickness Average Cutting Cutting Pulse Cutting Laser Power Speed Speed Conditions Gas
[mm] f wl [mm/mini [m/min] [ms] Mild Steel 2.5 350 559 0.559 Oxygen 5.0 350 127 0.127 Oxygen 10.0 350 10 0.010 Oxygen
Stainless Steel 0.5 120 1000 1.0 0.5 Oxygen 2.5 350 180 0.18 Oxygen 5.0 350 38 0.038 Oxygen 10.0 350 2.5 0.0025 Oxygen
Aluminium 1.0 120 500 0.5 0.5 Oxygen 3.0 120 50 0.05 1.5 Oxygen
Copper 1.0 120 500 0.5 0.2 Oxygen 3.0 120 50 0.05 2.0 Oxygen
Titanium 1.0 120 1000 1.0 0.5 Argon 3.0 120 300 0.3 1.5 Argon
11
CONCLUSIONS
If we compare AWJ cutting with CO2 laser cutting and Nd:YAG laser cutting it becomes clear that the strongest points of AWJ cutting are:
I. The range of materials to be cut is almost unlimited. It is also possible to cut very thick sections of material.
II. The material to be cut will not be affected by any heat during the cutting process because the cutting method is a non-thermal profiling technique.
AWJ is not ideally suited to cutting conventional materials in thin sections due to the low cutting speed. Figure 6 shows the cost per unit length of cut, with similar kerf quality, for three different materials and as we see CO2 laser is always cheaper if we do not take into account the negative effects such as heat affected zones and dross problems.
AWJ cutting is an excellent profiling technique for thermally susceptible materials such as titanium alloys and for cutting thick sections of material with a good kerf quality and without heat effected zones. AWJ is to be seen as a complimentary cutting method to laser cutting rather than a competing profiling technique.
50-7 7 =CO2-1000 W MS=Mild Steel 11. =AWJ-35C0 bar SS=Stainless Steel =YAG-400 W K o
30— o) C c OMI (1) I 2 2 20— ‘,M c
I FCnt\ o
a. NOT ECONOMIC 17, 10— 0 0 o z < (.) _m7.3 -11 11Z msl MS5 MS10 SS1 SS5 SS10 All A15 A110 111 Ti5 1110 Material thickness (mm) Figure 6. Comparison between cost per unit length of cut for AWJ [5,6,7], CO2, [8,9,10] and Nd:YAG [2,11,12]. 12
REFERENCES
1. Powell, J., Wykes, C.: A comparison between CO2 laser cutting and competitive techniques, Proc. 6th Int. Conf. Lasers in Manufacturing, May 1989, pp. 135 - 153.
2. Powell, J.: Guidelines and Data for Laser Cutting, The Industrial Annual Handbook, 1990, Ed: Belforte, D., Levitt, M., PennWell Books, Oklahoma, USA, pp. 56 - 67, ISBN 0-87814-359-9.
3. Mason, F.: Special Report 807, Water and Sand Cut it, American Machinist 133 (1989) 10, pp. 84- 95.
4. Hashish, M.: A Model for Abrasive-Waterjet (AWJ) Machining, Journal of Engineering Materials and Technology, 111(1989) 2, p. 159.
5. Hashish, M.: Aspects of Abrasive-Waterjet Performance Optimization, Proc. 8th Int. Sym. on Jet Cutting Technology, Durham, England, 9-11 Sept 86, pp. 297 - 308.
6. Hashish, M.: Pressure Effects in Abrasive-Waterjet (AWJ) Machining, Journal of Engineering Materials and Technology, 111(1989) 3, P. 226.
7. Blickweld, H., Guo, N.S., Haferkamp, H., Louis, H.: Prediction of Abrasive Jet Cutting Efficiency and Quality, Proc. of 10th Int. Sym. on Jet Cutting Technology, Amsterdam, 31 Oct- 2 Nov, pp. 163 - 179.
8. Schuöcker, D.: Laser Cutting, AGA Int. Cutting & Welding Seminar, Essen, 1989, pp. 93 - 100.
9. Powell, J., Ivarson, A., Ohlsson, L., Magnusson, C.: Energy redistribution in laser cutting, Accepted for publication in Welding in the World, 1991.
10. Arecchi, F.T., Schulz-Dubois, E.0.(Ed): Laser Handbook Vol. 2, North-Holland Publishing Company, 1972, p. 1631.
11. BeNorte, D., Levitt, M.(Ed): The Industrial Laser Annual Handbook 1988, Penwell Books Oklahoma USA, 1988, ISBN 0.87814.333.5.
12. Industrial Processing Applications: Raytheon Nd:YAG Lasers, Raytheon Laser Centre, Burlington M.A. 01803, USA. 13
PAPER 2
OPTIMISATION OF THE PIERCING OR DRILLING MECHANISM OF ABRASIVE WATER JETS 14
OPTIMISATION OF THE PIERCING OR DRILLING MECHANISM OF ABRASIVE WATER JETS
*4. * L. Ohlsson*, J. Powell , A. Ivarson , C. Magnusson*
*Luleä University of Technology Division of Materials Processing S-951 87 Luleå, Sweden
+Laser Expertise Ltd. Acorn Park Industrial Estate Nottingham NG7 2TR, UK
ABSTRACT
Although Abrasive Water Jet drilling is an industrial application in its own right its most common use is the production of a start-up hole for a subsequent cutting operation. This paper presents the results of an experimental analysis of the piercing process concentrating on the improvement in penetration time possible if the abrasive water jet is moving rather than stationary. Linear and circular movements of the jet have been investigated and it is shown that penetration times can be reduced by an order of magnitude. From the results the authors have developed a phenomenalogical model which explains the generally superior performance of moving jet piercing.
INTRODUCTION
In most cases where abrasive water jet cutting is to be carried out the jet must first be employed to pierce the workpiece. This piercing is usually achieved using a stationary jet which can take several seconds to penetrated the material [1,2,3,4]. This time contributes to the cost of the eventual cut component and therefore has an effect on the commercial viability of the process. A number of experienced engineers have discovered that movement of the cutting jet (or the workpiece) during this drilling operation reduces the time needed to penetrate the material [5,6]. This paper gives experimental evidence of the remarkable improvements in piercing time possible and presents guide-lines on the optimum movement technique. 15
Two different movement types were compared to stationary jet piercing, these were: linear and circular. Optimising the linear movement piercing is simply a matter of trying a number of speeds but circular movement is rather more complex. In the case of circular movement during piercing there are two major parameters: 1. the diameter of the circle and 2. the velocity of the movement. The diameter of the circle in this case means the CNC pro- grammed diameter (D) of the movement of the jet. The area of influence of the jet will be a combination of this circular movement and the diameter of the jet itself (d), see figure 1. Throughout this experiment the diameter of the jet (d) and all other jet related variables were kept constant. Our investigation of circular movement drilling involved changing values for the diameter of the circular movement (D) and the velocity of movement (v). It was discovered that an optimum range of D and v exists within which it is possible to achieve very short penetration times.
Figure 1. The relationsh p between the diameter of the abrasive water jet (d) and the diameter of the CNC programmed movement (D). The area of influence of the jet during piercing is a combination of these two.
EXPERIMENTAL PROCEDURE
Throughout the experimental program the equipment was set up as follows:
Equipment: Cutting parameters: High pressure pump: FLOW 9X-Single Pump pressure: 3585 bar (52 000 psi) Abrasive system: FLOW PASER II Abrasive flow rate: 500 g/min (8.3 g/s) CNC system: NUM 720F Abrasive type: Olivine Mesh 60 Water/Abrasive nozzle: 1.2 mm diameter Water jet orifice: 0.33 mm diameter
Note: In the following figures the abrasive nozzle of 1.2 mm diameter and its water jet orifice of 0.33 mm diameter have been labelled 1.2/0.33 mm.
All the samples used for the penetration trials were cut from the same bar of Mild Steel (SS 1312) to allow direct comparison of the results. (The formulation of SS 1312 is; Fe 99%, C 0.20%, Si 0.05%, Mn 0.4 - 0.7%, P 0.050%, 5 0.050%). 16
RESULTS AND DISCUSSION
Linear movement piercing The first experimental program involved measuring the time needed to pierce a 25 mm sample of steel over a range of linear velocities from 0.01 minis to 1.25 mm/s. The results of this investigation are presented in figure 2.
110 25 mm Mild Steel 100-1' P=3585 bar 90- m=500 g/min 8o- ,Nozzle=1.2/0.33 mm
) 70- (s 60-
time 50- ing ao- rc 30- Pie 20- 10- 0 0:2 0:4 0.6 0.8 1 1:2 1.4 Linear speed (mm/s) Figure 2. Penetration times as a function of movement speed for 25 mm thick section of steel.
The time taken to penetrate this section of steel by a stationary jet was found to be —90 seconds and it is clear from figure 2 that this value can be greatly reduced under the correct jet movement conditions. The piercing the lowest movement speed (0.01 mm/s) is similar to that for the stationary jet but this value rapidly falls as the movement speed is increased. As the speed is raised to 0.5 mm/s the piercing time is reduced to a minimum level of approximately 9.0 seconds. This represents a piercing mechanism which is ten times more effective than a stationary jet. This rapid piercing rate is stable over a large range of move- ment speeds and does not begin to rise again until the speed is increased to above 1.1 mm/s.
The rapid reduction in piercing time as the movement speed is increased from zero to 0.5 mm/s may be attributable to a change in the material removal process at the bottom of the piercing hole shown in figure 3. 17
Figure 3. A schematic comparison of the material removal mechanism for a stationary jet (a) and a moving jet (b) during piercing. a: High impact pressure but low abrasive flow - low material removal rate. b: High abrasive flow - high material removal rate. Compare figure 3b with the example shown in figure 6.
When a stationary jet is used there is a great deal of pressure exerted on the bottom of the hole but the level of abrasive flow across the bottom of the hole is low. The reasons for the low level of erosive activity can be summarised as follows: a. The incident jet and the exhaust stream act against each other. Inside the blind hole (see figure 3a) the incident jet is in direct contact with the exhaust stream. This conflict will disrupt the integrity of the incident jet and give rise to a zone of complex turbulent flow towards the base of the hole. This type of flow is a less efficient source of erosion than a highly directional jet. b. The erosion zone is almost perpendicular to the incident jet. Effective AWJ erosion depends upon high velocity particles impinging on the workpiece at a glancing angle. Figure 4 demonstrates that if the impingement angle approaches 90° the material removal process is frustrated. A secondary consideration is that a "glancing angle" abrasive water jet auto- matically cleans the debris generated by particle impact out of the erosion zone. This allows the erosion process to continue as subsequent particles have clear access to the workpiece. A perpendicular jet will not wash the area as effectively and a protective debris layer may be formed. One final disadvantage of a perpendicular jet is the possibility that fragments of hard abrasive particles may become embedded in the erosion zone surface. These particles would protect the underlying workpiece and further frustrate the piercing process.
As figure 3b shows, the jet-material interaction is completely different if there is a smooth transition from incident jet to exhaust stream. This transition is made possible by moving the jet during penetration. The erosion zone is now inclined with respect to the incident jet and the particle-material interaction will be of the desired type shown in figure 4a. 18
a) impingement at a glancing angle 19) perpendicular impingement
1) 1) 0 ,e 0 4,
imbedd.ed fra ment
Figure 4. A schematic demonstrating that the most effective material removal rates can be expected when the high velocity abrasive particles impinge upon the workpiece at a glancing angle (a). As the incident angle approaches 900 the erosion process becomes frustrated (b).
The shape of the graph given as figure 2 can now be explained:
Stage 1 (low speeds < 0.4 mm/s): At the lowest speeds the gradual elongation of the piercing hole happens too slowly to change the basic flow pattern from figure 3a type to figure 3b. The exhaust stream leaves the hole coaxially with the incident jet in the same way as in a stationary jet situation. As the movement speeds are increased the material removal mechanism changes to the figure 3b type and the piercing times drop rapidly.
Stage 2 (moderate speeds 0.4 - 1.1 mm/s): An optimum piercing mechanism is achieved as the movement speed is raised above a lower threshold value (in this case —0.4 mm/s). The material removal mechanism is now of the figure 3b type. As the movement speed is increased the piercing time remains almost constant which indicates some sort of equilibrium has been established. As the movement speed is accelerated the elongation of the pierced hole will increase. This means that the volume of material which must be removed during the piercing event will increase but this will be balanced by the improved efficiency of the material removal process as the incident and exhaust jets become more separated (see figure 3b).
Stage 3 ( excessive speeds> 1.1 mm/s): As the speed of movement is increased beyond the equilibrium condition (stage 2) the amount of material needed to be removed from the slot continues to grow even though the material removal process has achieved an optimum efficiency. Under these conditions an acceleration of the movement speed merely gives the penetrating jet more work to do and this will increase the piercing time.
The second part of the experimental program involved using the optimum range (figure 2) movement speeds to penetrate a number of different thicknesses of steel. The result from these trials are given in table 1 together with the penetration times for a stationary jet. 19
TABLE 1 Piercing times for a range of material thicknesses and movement speeds. Material Linear Linear Linear Linear Stationary Optimum thickness movement movement movement movement jet moving jet 0.33mm/s 0.50mm/s 0.67mm/s 0.83mm/s time as [mm] [s] [s] [s] [s] [s] % of stationary 5 2.34 1.89 1.86 1.72 3.08 56% 10 4.31 3.76 3.33 3.14 18.30 17% 15 5.80 4.66 4.26 5.39 41.94 10% 20 8.53 6.34 5.24 5.79 59.12 9% 25 9.67 8.24 7.90 8.21 91.97 9% 30 13.05 9.98 10.40 11.19 125.63 8% 40 18.96 13.53 13.83 14.03 203 7% 50 23.34 19.36 19.55 26.10 324 6% Note: Optimum moving jet times are shown in bold print.
Comparison of these figures clearly shows that the moving jet penetration times are much lower than their stationary jet counterparts. The figures at the right hand side of table 1 express the optimum moving jet result as a percentage of the stationary jet times. The relative improvement of performance of the moving jets as the material thickness is increased can be clarified by presenting the information in the form of average penetration rates as shown in table 2.
TABLE 2 Average penetration rates for stationary and moving abrasive water jets. Material thickness mml 5 10 15 20 25 30 40 50 Optimum moving jet [mm/s] 2.90 3.18 3.52 3.81 3.16 3.00 2.90 2.60 Stationary jet [mm/s1 1.62 0.54 0.36 0.34 0.27 0.23 0.20 0.15
Table 2 demonstrates that the penetration rates for the moving jet increase to a maximum and then gradually decrease as the material section rises. The stationary jet rates show a progressive decline from an initial maximum value at thin sections. This decrease of the penetration rate for the stationary jet is attributable to the increase in input - output jet interference as the penetration hole becomes deeper. The rise and fall of the performance of the moving jet can be explained by reference to figure 3b. When thin sections are being pierced the jet-material interaction time is so small that the elongation of the hole is insufficient to separate the incident jet and the exhaust flow. Interference of the two tends to slow down the penetration process although the performance is still superior to a stationary jet. As the interaction times increase for thicker sections the geometry of the bottom of the cut zone changes to the figure 3b type and optimum material removal result in maximum penetration rates. As the material thickness is increased further the penetration rate decreases as a result of the gradual reduction in the amount of energy left in the jet by the time it reaches the bottom of the hole. (For deep holes a significant portion of the jet energy will be consumed in eroding the leading edge of the slot).
Table 1 shows that there is a general tendency for the optimum movement speed to decrease as the material thickness is increased. This phenomenon has only a minor effect on the observed penetration times and the results of this experiment would not have been 20 substantially changed if a fixed movement speed of either 0.50 or 0.67 mm/s had been used. There is however, an optimum range of speed which tends to decrease as the material thickness is increased. The reasons for this can be extracted from our earlier discussion:
1. For thin sections the pierced slot is not elongated enough to allow separation of the incoming and outgoing jets. Under these conditions a higher movement speed would help to establish a figure 3b removal mechanism.
2. For thick sections the energy of the jet available to the bottom of the slot is diminished by the interaction of the jet with the leading edge of the slot. More energy can be made available to the piercing process if the movement speed is reduced.
Circular movement piercing The first circular movement piercing trials involved moving the jet in small diameter circles at a variety of speeds to penetrate a 25 mm thick steel section. The programmed circle diameter (D, see figurel) was varied between 0.3 and 3.6 mm and the results are summarised in table 3.
TABLE 3 Circular movement niercing times for 25 mm thick steel. Diameter Circular Circular Circular Circular Circular Circular speed speed speed speed speed speed 1 mm/s 2 mm/s 3 mm/s 5 mm/s 7.5 mm/s 10 mm/s [mml [s] [s] [s] [si [s] [s] 0.3 55.68 62.69 0.6 37.20 42.00 39.38 39.01 • 0.9 22.78 27.17 32.36 30.45 . • 1.2 19.88 16.46 20.30 20.43 • 1.8 13.96 11.81 12.06 9.89 9.59 • 2.4 13.48 12.14 12.57 11.09 10.80 9.84 3.0 13.87 14.34 14.82 13.40 13.06 12.59 3.4 15.51 17.18 17.57 15.08 15.22 15.36 Note: During these trials the programmed velocity was checked against the actual velocity because CNC machines generally slow down for small diameter circles. lt was discovered that some of the small high speed circles could not be executed by the equipment available which explains the absence of information in the top right hand section of table 3.
It can be seen that all the piercing times for the moving jet are substantially lower than the 92 seconds required by the stationary jet technique. Penetration times of less than ten seconds (similar to the optimum level for linear movement) are achieved with programmed diameters of 1.8 and 2.4 mm at velocities of 5 and 7.5 mm/s. Although an optimum speed of movement is identifiable for each circle diameter the variation in penetration time from speed to speed is minor when compared to the variation from diameter to diameter. Table 4 demonstrates this feature clearly. 21
TABLE 4 The variation in average Diercing time with circle diameter. Programmed circle diameter (D, see figure 1) 0.3 0.6 0.9 1.2 1.8 2.4 3.0 3.6 1mm] Average piercing time (from table 3) 59 39 28 19 11.4 11.7 13.7 16.0 [s]
This dominance of the circle diameter as a variable controlling the piercing mechanism is demonstrated very clearly in figure 5 which utilises all the information available in table 3.
80 -•, -.-- 1 MM/S ----- 7.5 mm/s A 2 mm/s ---/--- 10 mm/s
-0-- 3 mm/s — 0 5 mm/s 60 • Mild Steel 25 mm P=3585 bar -.7*) m=500 g/min a) Nozzle=1.2/0.33 mm ._E .2 ab 40 . ,c • 0 a) 0 a:
• 20— 4
•, Z>
0 , 0 1 2 3 4 Diameter (D) (mm) Figure 5. The variation of piercing times as a function of programmed circle diameter (D) for a range of movement speeds.
Figure 5 shows that there is an optimum range of circle diameters between 1.8 and 2.4 mm. Below this range the incoming and outgoing jets will interfere with each other in the same way as very low speed linear movement or stationary jets do. As the diameter of the movement is increased to just over the diameter of the incident jet the piercing process would be able to achieve a complex version of figure 3b type flow and rapid penetration could be achieved. (In this case the AWJ nozzle diameter was 1.2 mm which would generate a water jet of a slightly larger diameter —1.4 mm).
If the programmed circle diameter is further increased the penetration time will also rise as a result of the larger volume of material which must be removed from the jet-material interaction zone. This feature is demonstrated towards the right hand side of figure 5. 22
Table 5 summarises the results of a further circular movement penetration trial which used optimum conditions from the table 3 experiments to penetrate different thicknesses of steel. Table 5 compares these results with those for stationary and linear movement jets.
TABLES Minimum niercing times for each method. Method Material thickness Stationary Circular Linear (1.8mm/7.5mm/s) [mm] [s] [s] [s] [v in mm/s] 5 3.08 2.09 1.72 [0.83] 10 18.30 3.52 3.14 10.83 15 41.94 4.71 4.26 [0.67] 20 59.12 6.06 5.24 j0.67 25 91.97 8.95 7.90 [0.67] 30 125.63 13.34 9.98 [0.50] 40 203 25.16 13.53 [0.50] 50 324 65.84 19.36 [0.50]
It is clear that the circular movement can improve penetration rates by an order of magnitude compared with the stationary jet. For the thickest sections the circular movement method is substantially slower than the linear movement technique and this must be attributable to a change in the material removal mechanism. The interference of the exhaust stream with the incident jet will be greater in the case of circular movement because the exit path is continuously changing. This effect will become more severe when the workpiece thickness is increased as the sides of the hole will tend to converge towards the bottom. This reduction in penetration rate as compared to the linear technique is a minor consideration when it is appreciated that both techniques are an order of magnitude more effective than the use of a stationary jet.
Circular movement also has one advantage over linear movement which is the localisation of the piercing event. Comparing the two techniques for the 50 mm thick example given in table 5: a. Dimensions of entry hole for circular movement = —D+d = —3.2 mm diameter circle. b. Dimensions of slot for linear movement = —11.0 x —1.4 mm slot.
This large start-up slot may not be appropriate to many cutting applications and so circular movement may be preferred. Circular movement may, of course, also be used to "drill" circular holes.
One further difference between linear movement and the stationary or circular movement techniques is that the linear movement pierced hole is not always immediately usable as the start point of a cut. Figure 6 demonstrates this point and shows a cross section of a pierced hole taken along the axis of movement. The "figure 3 type b" erosion zone geometry can clearly be observed in figure 6. Although this geometry helps to accelerate the penetration rates it also gives rise to an inclined pierced hole. This inclination means that the bottom of the hole is not beneath the incident jet. Before cutting can commence the exit hole must be almost directly below the jet as it is in the case of the circular movement or stationary jet 23 techniques. Before normal cutting can continue after a linear movement piercing event the jet must be returned almost to its start point. After this return cutting can continue in any direction although it may be preferable to begin cutting in a different direction to the original linear piercing movement. The return to origin move can be carried out at maximum traverse speed and without turning off the abrasive water jet. The amount of time needed is therefore very small and should not exceed one or two seconds. This extra time is trivial when compared with the great savings available in the original piercing event.
Figure 6. A cross section of a linear movement penetration hole taken along the direction of travel. (50 mm thick Mild Steel penetrated in 26.1 seconds at a linear speed of 0.83 mmis. Direction of jet movement is from left to right in this photograph.) 24
SUMMARY
Figure 7 summarises the result of this experimental program and demonstrates the superior effectiveness of moving AWJ's as piercing tools. This information is of commercial as well as technical interest because processing times can be substantially reduced and AWJ cutting can become more cost effective by using these techniques.
325 7 Stationary jet 300- Circular movement 275- Linear movement 0,83 mm/s 250- A Linear movement 0,67 mm/s 225- Linear movement 0,50 mm/s
) 200 >- (s
e Mild Steel 175— P=3585 bar tim 150 . m=500 g/min
ing NozAe=1.2/0.33 mm 125- Pierc 100- -.7 75- 50- 25— o o 10 15 20 25 30 35 40 45 50 Material thickness (mm) Figure 7. Minimum piercing time for the three methods as a function of material thickness.
As a rough example let us take a 25 mm thick steel plate which needs 14 holes of 25 mm diameter cut before profiling the outside edge which has a line length of 2 meters. Assuming a cutting speed of 0.7 mm/s the overall cutting time would be of the region of 75 minutes excluding the piercing times. To this must be added the total piercing time of 15 x 90 seconds (22.5 minutes) for the stationary jet or 15 x 9 seconds (2.25 minutes) for the moving jet. The total profiling time is therefore: - with stationary jet piercing: 97.5 minutes - with moving jet piercing: 77.2 minutes
As the moving jet technique reduces the total time by 21 % it must also reduce the cost of the process by the same amount. 25
CONCLUSIONS
1. Movement of the jet during AWJ piercing of materials reduces the penetration time.
2. Linear and circular movement can both reduce the penetration time by up to an order of magnitude but linear movement times are generally smaller than those for circular movement.
3. There is an optimum range of speed for linear movement penetration (a good rule of thumb would be to use 0.7 minis for steel).
4. There is an optimum range of speeds and programmed diameters for circular movement penetration. The most important choice is that of programmed diameter which should be between 1.5 and 2.0 times the nozzle diameter. Movement speeds for steel should be of the order of 5 - 10 mm/s.
5. The reason for the improvement in penetration rates for moving jets is a change in the fluid dynamics in the bottom of the penetration hole. This point is demonstrated in figure 3. The two basic principles are, 1. the eroding jet must strike the erosion zone at a glancing angle and 2. the exhaust stream must leave the interaction zone without direct interference with the incident jet.
6. As a result of the geometry of a linear movement pierced hole it may be necessary to return the jet to its original position before cutting can commence, figure 6. This movement can be made at maximum traverse speed of the table without turning the AWJ off.
REFERENCES
1. Gates, E.M., Toogood, R.W., Simms, B.W.: A Model for Drilling by High Pressure Water Jet, Proceedings of the 7th International Symposium on Jet Cutting Technology, Ottawa, Canada,26 - 28 June, 1984, pp. 221 - 236.
2. Hunt, D.C., Kim, T.J., Sylvia, J.G.: Parametric Study of Abrasive Waterjet Processes by Piercing Experiment, Proceedings of the 8th International Symposium on Jet Cutting Technology, Durham, England, 9 - 11 September, 1986, pp. 287 - 295.
3. Yanagiuchi, S., Yamagata, H.: Cutting and Drilling of Glass by Abrasive Jet, Proceedings of the 8th International Symposium on Jet Cutting Technology, Durham, England, 9 - 11 September, 1986, pp. 323 - 329.
4. Mural, H., Nishi, S.: Structure of Water Jet and Erosion of Materials, Proceedings of the 5th American Water Jet Conference, Toronto, Canada, 29-31 August, 1989, pp. 89 - 98.
5. Kimblad, Sven, Kimblad Technology AB, Box 183, S-175 23 Järfälla, Sweden. (personal communication)
6. Ström, Sven-Erik, PROJET AB, Telegatan 4, S-372 31 Ronneby, Sweden. (personal communication) 26
PAPER 3
THE PROFILING OF SHARP CORNERS BY ABRASIVE WATER JET CUTTING 27
THE PROFILING OF SHARP CORNERS BY ABRASIVE WATER JET CUTTING
J. Powell*+, L. Ohlsson*, C. Magnusson*
* Luleå University of Technology Division of Materials Processing S-951 87 Luleå, Sweden
+Laser Expertise Ltd. Acorn Park Industrial Estate Nottingham NG7 2TR, UK
ABSTRACT
This paper presents the results of an experimental investigation into the problems which can arise if abrasive water jets are used to profile sharp corners. The changes in jet- material interaction which occur when corners are cut have been identified and analysed. The authors have established the cause of incomplete cut penetration and cut line distortion which are associated with sharp corners cut at the highest speeds. The paper concludes with straight forward guide-lines for the avoidance or minimisation of sharp comer imperfections.
INTRODUCTION
Over the past few years abrasive water jet (AWJ) cutting has established itself as a powerful and versatile profiling technique [1,2,31. The process is capable of cutting a wide range of materials up to considerable thicknesses but the nature of the AWJ-material interaction is such that problems can arise if high levels of dimensional accuracy are required. The primary source of dimensional discrepancies is the lag between the top part of the cut front and the bottom (see figure 1). At commercially profitable cutting rates this lag is often considerably larger than the kerf width and the diameter of the water jet. Under these conditions the turning of sharp corners will disrupt the steady sate material removal mechanism. This disruption of the cut front geometry and its effects on the dimensional accuracy of the finished cut product is the main subject of this paper. An experimental 28 program has been carried out to identify the extent of the problem and to explain the phenomena involved. The paper concludes with suggestions for maximum accuracy profiling.
< Cutting direction AWJ
a >.
Figure 1. The lag (a) between the top part of the cut front and the bottom.
EXPERIMENTAL PROCEDURE
Throughout the experimental program the equipment was set up as follows:
Equipment: Cutting parameters: High pressure pump: FLOW 9X-Single Pump pressure: 3585 bar (52 000 psi) Abrasive system: FLOW PASER II Abrasive flow rate: 500 g/min (8.3 g/s) CNC system: NUM 720F Abrasive type: Olivine Mesh 60 Water/Abrasive nozzle: 1.2 mm diameter Water jet orifice: 0.33 mm diameter
All the samples used for the corner trials were cut from the same bar of Mild Steel (SS 1312) to allow direct comparison of the results. (The formulation of SS 1312 is; Fe 99%, C 0.20%, Si 0.05%, Mn 0.4 - 0.7%, P 0.050%, S 0.050%).
RESULTS AND DISCUSSION
Figure 2 shows photographs of the top and bottom of cut corners profiled from 15 mm thick Mild Steel at two speeds (0.5 mm/s and 1.0 minis). Under each pair of photographs is a line drawing showing the discrepancy between the top and bottom lines. Considering first of all the slower of the two speed (samples A - C), it is clear that all the cut corners suffer from the same type of dimensional discrepancies to differing degrees. Increasing the speed (samples D - F) accentuates the problems observed at lower speeds and even leads to a interruption in the jet penetration on the entry line of the comer. (In the following discussion each corner will be taken as consisting of an entry and exit line i.e. the line taken by the AWJ to approach the comer point and depart from it). The general information provided by figure 2 is quite clear: 29 a/ Material removal from the bottom of the cut zone becomes less efficient and can be interrupted as the corner is approached. b/ On the bottom of the cut the exit line from the corner becomes extended back into the workpiece as the corner is left behind.
900 Top A 60° Top B 30° Top C _ ., .
:
Cutting speed 0.5 mm/s Cutting speed 0.5 mm/s Cutting speed 0.5 mm/s 90° Bottom 60° Bottom 30° Bottom
7 / \
. \
Cutting speed 0.5 mm/s Cutting speed 0.5 mm/s Cutting speed 0.5 mm/s
90° Top D 60° Top E 30° Top F ,
. ,
Cutting speed 1.0 mm/s Cutting speed 1.0 mm/s Cutting speed 1.0 mm/s 90° Bottom 60° Bottom 30° Bottom
, » L #red-ssze .. ,, -- -e. 7 . , .. 4•1',;ei . . .s. Cutting speed 1.0 mm/s Cutting speed 1.0 mm/s Cutting speed 1.0 mm/s
Figure 2. The top and bottom views of cut corners profiled from 15 mm thick Mild Steel. 30
Preliminary observations of these phenomena might lead to the assumption that they are directly related and caused by a complex three dimensional cut front being set up as the top, leading edge of the cut front turns the corner before the lower, trailing edge. Detailed analysis of the situation has however, revelled that the two features (a+b) are independent of each other and are generated by the stop - start nature of corner turning. The two phenomena can therefore be discussed separately:
A. Inefficient! incomplete material removal on the corner entry line.
This problem is a result of the change in geometry of the cut front when an abrasive water jet is suddenly stopped. These changes in geometry were first observed by Hashish [4] and follow the pattern demonstrated schematically in figure 3.
A. Steady state cutting B. Movement stopped
Step generation
C. Drilling D. Piercing
Uncut region
Figure 3. The changes in cut front geometry if the movement of the jet is stopped. N.B. The existence of an uncut region will only be associated with a cut front lag (see A) which is substantially larger than the kerf width.
Figure 3A shows the geometry of the cut front during cutting and the gradual change in inclination from OT to O. As soon as the movement of the jet is stopped the erosion will rapidly reduce OT to zero and a small step will be generated on the cut front as shown in
31 figure 3B. The inclination and position of this step means that it becomes the zone of maximum erosion and the cut front geometry shown in figure 3C is quickly established. This " scooped" geometry acts as the major erosion zone and acts to deflect the AWJ away from the original bottom of the cut front which remains uneroded. The final piercing event therefore leaves behind a small amount of uncut material towards the bottom of the cut (figure 3D).
On an experimental level this series of events was verified in two ways:
1. Direct observation of the termination point of straight line cuts (see figure 4).
2. Drilling through a specially prepared "cut front sample".
Figure 4. The uncut region left at the end of straight line cuts made under the same conditions as those in figure 2 D,E and F.
Figure 4 clearly demonstrates that the incomplete penetration phenomenon is not connected to the corner turning event but is attributable solely to the sudden halting of the water jet movement.
The "cut front sample" was produced by profiling a shape with the same profile as the cut drag lines observed on the edges of the samples produced for figure 2 D,E and F. The drag lines are shown in figure 5a and the sample is shown as 5b.
a X b
Figure 5. a/ The drag lines on the cut edge which reflect the geometry of the cut front. b/ The sample produced to replicate the cut front geometry. 32
The abrasive water jet was positioned at position X in figure 5b and fired to represent the final erosion stages of the cut as shown in figure 3. The result is demonstrated in figure 6 and it can clearly be seen that an uncut, uneroded section is left behind after the water jet has pierced the sample.
Figure 6. The drilled "cut front sample" showing the existence of the uncut zone.
The generation of an uncut zone at the end of a cut line is clearly a function of the geometry of the original cut front as demonstrated in figure 3. If the difference between OT and OB is substantial and the cut front lag is large then the uncut material phenomenon is likely to occur. This type of cut front geometry is associated with the highest possible cutting speeds and thick workpieces (e.g. 10 mm +). If the workpieces are of a thinner section or cutting speeds are well below maximum the cutting front will have a smaller lag and the piercing event will involve the whole of the cut front. In this case the bottom of the end of the cut line will fully penetrate the material but may be of a slightly smaller diameter, a feature which is shown clearly in figure 3A and 3B.
B. Elongation of the cut bottom along the corner exit line.
This elongation is shown schematically in figure 7 and is clearly visible in the "bottom" photographs in figure 2
Cut line elongation Incomplete/inefficient cuffing discussed earlier
Bottom view of cut
Figure 7. Elongation of the cut line at the bottom of the cut along the comer exit line. 33
This elongation of the cut line is a result of the generation of a curved cut front as the jet moves away from the corner. The exhaust jet from this curved front is propelled backwards to erode material as shown in figure 8.
AWJ AWJ
Figure 8. The mechanism of erosion by the exhaust jet which elongates the cut line.
This effect can be clearly demonstrated by cutting a line at right angles to an existing cut line as shown in figure 9.
Figure 9. An extension of cut line on the bottom of a T-shape cut in two operations. Cutting parameters: 1 mm/s, material thickness 15 mm; Mild Steel.
In the case of figure 9 the cross piece of the T was cut first and the jet was then returned to the middle of that line before moving off to cut the upright line. The top of the cut shows the correct T-shape but the bottom demonstrates the type of secondary erosion shown in figure 8.
GUIDE-LINES FOR THE PRODUCTION OF SHARP CORNERS
The previous discussion demonstrated that both types of cut corner imperfection can be attributed to the shape of the cut front as it approaches and leaves the desired turning point. If a good quality male and female corner component is required it will be necessary to change the cut front in the approach - departure zone. A gradual deceleration - acceleration
34
cycle will be necessary in order to achieve a close to perpendicular erosion front in this zone. When, for example, cutting substantial section (10 mm +) metals at maximum speed the desired result could be achieved by reducing the speed by 10% per millimetre for the last five millimetres of the corner approach. The CNC would then bring the jet to a standstill at the corner point and from this zero velocity the acceleration rate could be 20% per millimetre until the original speed is achieved after 5 mm. The results of such an acceleration - deceleration program are demonstrated by the sample shown in figure 10.
Top view Bottom view
Figure 10. The improved corner cutting possible if the correct acceleration -deceleration cycle is used. Deceleration - acceleration from 1.0 to 0.5 =Vs over the 5 mm nearest to the turning point (compare with figure 3).
If a sharp male corner is the only requirement and the female part of the corner remains in the scrap material, good results can be achieved by overshooting the corner as shown in figure 11.
60° Top A 600 Top B
• ._
, •
Cutting speed 0.5 mm/s Cutting speed 1.0 mm/s 600 Bottom 60° Bottom - .. . — .-: -+
,•
.,, ,. ., ,. • ...... - . •.:,,K. . • • : ,.... , . . Cutting speed 0.5 mm/s Cutting speed 1.0 mm/s
Figure 11. Top and bottom view of sharp corners made using overshooting, 15 mm Mild Steel. 35
Figure 11 clearly demonstrates that if the overshooting method is used a clean corner is produced in the "male" component as all the corner turning problems are confined to the waste material.
CONCLUSIONS
1. When cutting sharp corners with abrasive water jets at maximum speed the finished cut suffers from two defects:
a/ Incomplete penetration on the corner approach. b/ Elongation of the bottom of the cut line along the corner exit line.
2. Both these problems are a result of the stop - start nature of the CNC corner turning process.
3. Both these problems are minimised at low cutting velocities.
4. Corner defects can also be eliminated by the correct choice of acceleration - deceleration cycle.
5. If only the male part of the comer is required the problems can be eliminated by overshooting the comer.
ACKNOWLEDGMENTS
The authors would like to thank The Research Council of Norrbotten, Sweden, for the financial support of this project.
REFERENCES
1. Hashish, M.: A Modeling Study of Metal Cutting With Abrasive Waterjets, Engineering Materials and Technology, 106(1984)1, pp. 88 - 100.
2. Hashish, M.: Application of Abrasive-Waterjets to Metal Cutting, Proceedings of the Nontraditional Machining Conference, ASM, Cincinnati, OH, Dec. 1985, pp. 1 - 11.
3. Mason, F.: Special Report 807, Water and Sand Cut it, American Machinist, 133(1989)10, pp. 84 - 95.
4. Hashish, M.: Visualization of the Abrasive-Waterjet Cutting Process, Experimental Mechanics, June 1988, pp. 159 - 169.
5. Hashish, M.: Three-Dimensional Machining with Abrasive-Waterjets, Proceedings of the 11th International Conference on Jet Cutting Technology, St. Andrews, Scotland, Sept 8 - 10, 1992, pp. 605 - 620. 36
PAPER 4
CUT FRONT GEOMETRY AND THE FORCES EXERTED ON THE WORKPIECE DURING ABRASIVE WATER JET CUTTING 37
CUT FRONT GEOMETRY AND THE FORCES EXERTED ON THE WORKPLECE DURING ABRASIVE WATER JET CUTTING
L. Ohlsson*, J. Powell*+, C. Magnusson*
*Luleå University of Technology Division of Materials Processing S-951 87 Luleå, Sweden
+Laser Expertise Ltd. Acorn Park Industrial Estate Nottingham NG7 2TR, UK
ABSTRACT
This paper presents the results of an investigation into the forces acting on the cut front during abrasive water jet cutting. Identification of the average forces and their fluctuations has enabled the authors to postulate a possible material removal mechanism related to the cut front geometry. The basis for the mechanism is multiple step formation on the cut front. Each step acts as a zone of high erosion rate. The authors also estimate that a vibrating or pulsing AWJ system may cut more efficiently as step initiation would be enhanced.
1 INTRODUCTION
Abrasive Water Jet cutting is a method of profiling material by localised erosion under the action of a high pressure water jet containing particles of abrasive (see figurel). 38
Abrasive Hopper High Pressure Water
...
nre:We
Water Orifice Mixing Chamber Abrasive Feed • Abrasive Water Jet
Abrasive Nozzle Cutting Zone Workpiece
Figure 1. A schematic of Abrasive Water Jet cutting.
This paper investigates the forces exerted on the workpiece during this erosion process. It will be shown that the process exerts pressures in the vertical place and in the direction of cutting which are dependant upon the overall geometry of the cut front. This geometry is determined by the type of material being cut and the cutting speed. The sensors used during this experiment have also identified rapid periodic fluctuations in the average pressures exerted. These fluctuations can be attributable to the establishment and removal of localised areas of high erosion (steps) on the cut front.
2 EXPERIMENTAL PROCEDURE
The experimental set up is described in figure 2. The metal bar to be cut was supported by pressure transducers to give readings of the force exerted in the Z direction (vertical) and the Y direction (the direction of cutting).
Figure 2. The experimental set up for erosion force measurement.
The transducers sampled the impingement pressure 1000 times per second and this data was stored on the computer for reference and analysis. 39
2.1 Equipment used
The equipment used through out the experiment was as follows:
Equipment: Cutting parameters: High pressure pump: FLOW 9X-Single Pump pressure: 3585 bar (358.5 MPa) Abrasive system: FLOW PASER II Abrasive flow rate: 500 g/min (8.3 g/s) CNC system: NUM 720F Abrasive type: Olivine Mesh 60 Water/Abrasive nozzle: 1.2 mm diameter Water jet orifice: 0.33 mm diameter Stand-off distance: 2 mm
Force measurement equipment: Data collection and analysis: Kistler 9257A Dynamometer Macintosh Classic with Kistler 5001 Charge Amplifier MacLab System
2.2 Material cut
Two types of material were cut: a. Mild Steel (SS 1312) Formulation: Fe 99%, C 0.20%, Si 0.05%, Mn 0.4 - 0.7%, P 0.050%, S 0.050%. b. Aluminium (SS 4212) Formulation: Al 97.4%, Mg 0.9%, Mn 0.7%, Si1.0%.
The sample thickness was kept constant at 10 mm and two cutting speeds were employed for each material. The speed used were: a. The maximum for good quality cutting with the equipment used and, b. half of the maximum. (i.e. 3.33 and 1.67 mm/s for Mild Steel and 11.17 or 5.58 mm/s for Aluminium.)
3 RESULTS AND DISCUSSION
3.1 Forces exerted during the piercing event
The force exerted during the stationary jet piercing event was found to be unaffected by material type and was equal to 40 N in the vertical direction only. It is not surprising that this value remained constant as it merely reflects the stagnation pressure of the incident jet. Naturally the time of penetration changed with material type (18 seconds for Mild Steel, 6 seconds for Aluminium) as more work needs to be done to pierce Mild Steel than the softer Aluminium alloy.
NB. Piercing times can be reduced by 90% by moving the workpiece or the jet during the penetration event [1]. 40
3.2 The average force acting upon the workpiece during cutting
The average forces acting on the workpieces during the cutting process are given in table 1.
Table 1. The average values of the forces acting on the workt.ieces during cutt n Material Thickness [mm] Cutting speed [mm/si Average Force IN] Fy Fz Mild Steel 10 1.67 8.94 7.26 10 3.33 15.34 14.62 Aluminium 10 5.58 9.92 6.84 10 11.17 15.80 17.15
From the information given in table 1 two things are clear:
1. The forces on the workpiece in both Z and Y directions (see figure 2) increase with increasing cutting speed.
2. The forces acting on the material for the "fast" and "slow" cuts are very similar even though the materials have changed.
Both of these phenomena are attributable to the fact that the forces acting on a workpiece are a function of the general geometry of the cut front. Figure 3 presents photographs of the cut edges of the four samples in question. The striations on these cut edges have a slope which is indicative of the original inclination of the cut front during cutting. It is clear from these photographs that there are strong similarities between the shape of the cut fronts for either material when cutting at maximum or 50% maximum speeds. The point is demonstrated more clearly by figure 4.
Figure 3. Photographs of the cut edges produced during the experimental program. (Each photograph is accompanied by a sketch of the general inclination of the cut front estimated from the striation pattern on the cut edge.)
41
Cutting Direction AWJ AWJ AWJ AWJ
A B Cutting Speed: 0 50%Max Max Max+ Average Force: -9.5N (Fy) -15.6N (Fy) -7.1N (Fz) -15.9N (Fz) Figure 4. The influence of cut front geometry on the average force exerted on the workpiece during cutting (see table 1).
Figure 4 shows schematically how the cut front geometry changes as the cutting speed increases from zero to more than the maximum possible. As the inclination of the cut front increases with the cutting speed it interacts more effectively with the incident water jet which therefore exerts more of a pressure on the workpiece. At the maximum cutting speed for full penetration the combined force of the jet in the Y and Z direction approaches the stagnation pressure of the jet.
It will be noted from table 1 that the forces in the Y and Z directions are approximately equal. This may at first appear surprising as the direction of the original jet is in the Z (vertical) direction only. This split of the force into two directions is a function of the material removal mechanism from the cut zone. Hashish [2] first postulated the creation of steps on the cut surface which then underwent rapid erosion as a result of their geometry. This principle is shown schematically in figure 5.
Cutting Direction >
A J A J A J A J
1 2 3 4 Step Step growth Rapid Step initiation and removal erosion initiation Figure 5. The generation and erosion of steps on the cut front.
The results given in table 1 support the idea of step formation and removal as they imply that the average inclination of the cut front is 45°. (A face inclined at 45° to an incident jet would split the force of that jet equally in the vertical and horizontal directions.) However the sketches of the cut fronts given as part of figure 3 show that a macroscopic view of the 42 cut front would give an average inclination of approximately 18° degrees for the maximum speed cuts and 10° degrees for the cuts carried out at half the that speed.
The macroscopic inclination of the cut front is therefore far less than the 45° needed to give the almost equal Z and Y values of force given in tablel. It is however, possible to achieve an average cut front inclination of 45° if the cut front is covered in several steps all undergoing the cycle described in figure 5. This principle is described in figure 6.
Figure 6. A cut front made up of several steps each undergoing the cycle described in figure 5.
This type of cut front is in dynamic equilibrium and combines the steady state aspects of the overall cut front geometry (see figure 3) with a multiple step formation - erosion cycle. As the following section shows this multiple cycle gives rise to rapid fluctuations in the incident force measurements obtained from the pressure transducers.
3.3 Cyclic fluctuations in the forces exerted on the cut front
Figure 7 shows typical detailed results obtained from the pressure sensors over a cutting period of half a second.
43
Mild Stee1,10 mm, 1.67 minis Fy Fz 40 — Average Force 35 — 30 — 25 — [N] 20 — 15 — 10 5 — fi' t f()It V fi 1•'1\ I 1St .-',19 4 h 4 \\*4t V 1 0 1 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Mild Steel, 10 mm, 3.33 mnVs Fy Fz 40 — - Average Force 35 30 25 I [N] 20 l 15 ta\ - -ii ' •„ \ VA l:Sil\t , iAger 4 it ,$s I y os‘ 10 et / 5 0 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Aluminium, 10 mm, 5.58 minis Fy Fz 40 - Average Force 35 30 25 [N] 20 15 p„ 10 totK, ire _ i if t IF IV le lip ,,C. e- \, 4 vv# 5 , Y 11 \ er 0 0 0.1 0.2 0.3 0.4 0.5 Cutting lime [s]
Aluminium, 10 mm, 11.17 mmis Fy Fz 40 — - - - - Average Force 35 — 30 — 25 — [N] 20 — _ 15 qIii$V t,NieÄ 4' \ 10 1 5 o 7 o 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Figure 7. Typical 0.5 second samples of readings from the pressure sensors. 44
The information presented in figure 7 reveals rapid fluctuations in the force acting on the cut front in both the vertical and horizontal directions. Although the fluctuation of the horizontal and vertical components have similar frequencies in each case they are not exactly in phase. This implies a change in geometry of the step or steps on the cut front as they progress downwards. This fits in well with the idea of a growth and rapid erosion cycle as this would involve a change in the angle of inclination of the step with respect to the incident jet. The vertical and horizontal pressure components reach maximum values at different angles of incidence, a principle which is demonstrated in figure 8.
Although there is a strong cyclic element in the fluctuations the graphs do not easily lend themselves to frequency analysis. It is clear however that the pressure signal is occasionally dominated by a single step event but that a multiple event is more common. The frequency range of all the graphs given as figure 7 lies between 40 to 80 Hz and it is likely that the general frequency for any cutting system will be generated by a combination of two factors: 1. A natural step generation frequency which results purely from the fluid dynamics of the jet and the physical properties of the material and, 2. cycles imposed upon the jet - material interaction as a result of mechanical vibrations set up in the cutting equipment.
Although this second sort of vibration may be seen to be a weakness in the system it is possible that these vibrations positively assist the cutting process by providing step initiation stimuli. Development of this principle may lead to the assessment of cutting machines which a deliberate vibration system in the nozzle or the X - Y movement.
Figure 9 shows the results of a profilometry survey of the four cut surfaces generated during the experiment. The profiles in each case refer to a typical half seconds worth of cutting so that the cut surfaces can be directly compared with the pressure sensor results given in figure 7. More general views of the edges showing the profile over 25 mm of cut edges are presented in figure 10.
Angle of incidence: 0 15 30 45 60 75 90 MAX Vertical component of incident force (Fz)
MAX
Horizontal component of incident force (Fy)
Angle of incidence: 0 15 30 45 60 75 90 AWJ AWJ AWJ AWJ AWJ
110° 190°
Figure 8. The effect of changing angles of incidence on Fy + Fz (see table 1). This simplified model assumes a moving jet which transfers all its energy to the work- piece. In practice a large portion of the jet energy passes straight through the cut zone at low angles of incidence and does not exert any force on the cut front. 45
Mild Steel, 10 mm, 1.67 mm/s
60 40 20 -----
-40 -60 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Mild Steel, 10 mm, 3.33 mm/s
60 40
20 ...... --- 2 -20° -40 -60 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Aluminium, 10 mm, 5.58 mm/s
60 40 20
-40 -60 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Aluminium, 10 mm, 11.17 mm/s
60 40 20 7
2 CI -20 -40 -60 0 0.1 0.2 0.3 0.4 0.5 Cutting Time [s]
Figure 9. Results of a profilometry survey. The profiles in each case refer to a half second of cutting. 46
Mild Steel, 10 mm, 1.67 minis
60 40 20 J 0 . -20 -40 -60 0 5 10 15 20 25 Measuring Distance [mm]
Mild Steel, 10 mm, 3.33 mm/s
60 40 t A 20 i \111%. I 0 -20 i'l\ts.NP -40 -60 0 5 10 15 20 25 Measuring Distance [mm]
Aluminium, 10 mm, 5.58 mm/s
60 40 20 ä
I 0 \i'll ii! -20 -40 -60 0 5 10 15 20 25 Measuring Distance [mm]
Aluminium, 10 mm, 11.17 mm/s
60 40 20
I ° -20 iel tfetiAt\t\i/ VSiiiiiii\j"\Vit -40 -60 0 5 10 15 20 25 Measuring Distance [mm]
Figure 10. The surface profile over 25 mm of cut edge. 47
The information presented in figures 9 and 10 reveals the following points: a. The cut edges have profiles which have at least two superimposed periodic components combined with a random element. b. The high frequency components of the aluminium profiles (see figure 9) have a frequency of the same order of magnitude as the related pressure cycles noted in figure 7. The high frequency component in the case of Mild Steel has been largely smoothed out probably as a result of the longer jet - material interaction at the lower cutting speeds. c. The lower frequency periodic components have frequencies of between 5 and 20 Hz and are probably the result of mechanical vibrations set up in the cutting machine. d. Although the aluminium samples are slightly rougher (see figure 10) the general appearance of the "slow" and "fast" pairs of cut profiles is similar even though the materials have changed. This implies that the roughness of the eventual cut edge is greatly influenced by the macroscopic geometry of the cut front (see figure 3).
3.4 Summary
The information produced by this experimental program implies that the mechanism of material removal from the cut zone is highly dependant on the macro and micro geometry of the cut front. Macroscopically the cut front is only slightly inclined from the vertical although this slope increases with the cutting speed. On a more microscopic level the fluctuations in applied pressure and the ripples on the cut surface reveal a dynamic equilibrium on the cut front. This equilibrium involves the generation and erosion of steps on the cut front. These steps all undergo an initiation - growth and erosion cycle the rate of which is determined by the characteristics of the material (hardness, strength etc.) and these of the AWJ (abrasive type, pressure etc.).
It may be possible in the future work to influence the rate of generation of the steps (and therefore the cutting speed) by oscillating the incident pressure of the AWJ by mechanical means.
4 CONCLUSIONS
1. The force impingent on a material during abrasive water jet cutting acts in the direction of the cutting as well as the vertical direction.
2. The magnitude of the pressure acting in both directions is determined primarily by the geometry of the cut front - not by the material properties of the workpiece.
3. The cut front has a geometry made up of two components: a. A general slope inclined at a shallow angle to the abrasive water jet and: b. A smaller scale series of steps on this slope which act as areas of high erosion rate. 48
4. The rapid erosion of these steps is a major contributor to the effectiveness of abrasive water jet cutting but is also the reason that the cut edge has a periodic roughness or waviness.
5. The steps are initiated by two methods: a. By localised fluid dynamic effects. b. By fluctuations in the jet - material interaction as a result of mechanical vibration of the cutting machine or nozzle etc.
6. If is possible that a specially designed deliberately vibrating system could aid step initiation and thereby accelerate the cutting process.
5 ACKNOWLEDGMENTS
The authors would like to thank The Research Council of Norrbotten, Sweden, for the financial support of this project.
6 REFERENCES
1. Ohlsson, L., Powell, J., Ivarson, A., Magnusson, C.: Optimisation of the Piercing or Drilling Mechanism of Abrasive Water Jets, Proceedings of the llth International Conference on Jet Cutting Technology, St. Andrews, Scotland, Sept 8 - 10, 1992, pp. 359 - 370.
2. Hashish, M.: Visualization of the Abrasive-Waterjet Cutting Process, Experimental Mechanics, June 1988, pp. 159 - 169.
3. Li, H.Y., Geskin, E.S., Chen, W.L.: Investigations of forces exerted by an abrasive water jet on a workpiece, Proceedings of the 5th American Water Jet Conference, Toronto, Canada, Aug 29 - 31, 1989, pp. 69- 77.
4. Blickweld, H., Guo, N.S., Haferkamp, H., Louis, H.,: Prediction of Abrasive Jet Cutting Performance and Quality, Proceedings of the 10th International Symposium on Jet Cutting Technology, Amsterdam, Netherlands, Oct 31 - Nov 2, 1990, pp. 163 - 179.
5. Chao, J., Geskin, E.S., Chung, Y.: Investigations of the dynamics of surface topography formation during abrasive waterjet machining, Proceedings of the 11th International Conference on Jet Cutting Technology, St. Andrews, Scotland, Sept 8 - 10, 1992, pp. 593 - 603.