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Development of a fixture-free machining center for machining block-like components
Journal of Materials Processing Technology, 39 (1993) 405-413 Elsevier
405
Development of a fixture-free machining center for machining block-like components B.P. B a n d y o p a d h y a y , T. Hoshi, M.A. L a t i e f and T. H a n a d a
Toyohashi University of Technology, Department of Production Systems Engineering, Tempaku-cho, Toyohashi 440, Japan (Received July 13, 1992; accepted February 12, 1993)
Industrial S u m m a r y The cost of designing and manufacturing fixtures constitutes a major factor in the FMS-type unattended operation of machining centers. The efficiency of operation of an FMS can be enhanced greatly by the developing of a fixture-free machining center. The concept of such a "fixture-free" machining center for the machining of block-like components has been developed and is presented in this paper. This machining center, supported by an automated CAM capability, feeds bar-form material automatically and positions it for unattended machining of all of the six faces of block-like components.
1. Introduction In the c o m p e t i t i v e world of m a n u f a c t u r i n g , m a n u f a c t u r i n g e n g i n e e r s are u n d e r p r e s s u r e to modify t h e i r p r o d u c t s quite f r e q u e n t l y , p r o d u c t designs being c h a n g e d c o n s t a n t l y to m e e t the needs of the consumer. Some examples of these p r o d u c t s are cars, special-purpose m a c h i n e tools, cameras, video recorders, etc. The m a n u f a c t u r i n g division of a c o m p a n y has to m e e t these c h a l l e n g e s of c o n s t a n t l y - c h a n g i n g p r o d u c t lines with new designs, Flexible M a n u f a c t u r i n g Systems (FMS) being a viable s o l u t i o n to this problem. T r a d i t i o n a l l y , F M S fills the gap b e t w e e n high-volume t r a n s f e r lines and h i g h l y flexible n u m e r i c a l l y - c o n t r o l l e d (NC) machines. F M S are e c o n o m i c a l in the mid v o l u m e r a n g e [1], b e c a u s e of the time and cost involved for the "set-up" to a c c o m m o d a t e a newly designed c o m p o n e n t . The set-up in this case includes the p r e p a r a t i o n of the w o r k p i e c e h o l d i n g fixtures, the p r e p a r a t i o n of c u t t i n g tools and the g e n e r a t i o n of a n NC p a r t program.
Correspondence to: Dr. B.P. Bandyopadhyay, Department of Mechanical Engineering, University of North Dakota, Box 8214, University Station, Grand Forks, ND 58202, USA. 0924-0136/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
406
B.P. BapTdyopadhyay et al./A fixture-free machitzing center
The successful operation of FMS for low volume production and high product variability will depend greatly on the management of the set-up. The latter has been estimated to comprise about 20% of the cost of new manufacturing systems, whilst for FMS the proportions may be even higher, since the set-up includes high-cost fixtures [2-4]. Despite the high set-up costs, unfortunately not much attention has been paid by researchers to these issues. However, following the financial disasters of many FMS [5], machine tool builders and manufacturing industries have realized recently that tooling can have a significant impact on the effective performance of an FMS. It has been suggested that the tooling investment must be controlled carefully before the company can realize the full potential of any automated manufacturing system [6]. Therefore, research is needed in the area of an effective method of fixture development for the success of a FMS producing a limited quantity of parts and operating with high product variability. In mass production, the cost of specially designed fixtures can be justified easily. However, in small batch production, the cost of manufacturing fixtures and the lead time are quite significant on a part basis. Manufacturers have attempted to solve this problem by the use of modular and adaptable fixtures for holding parts of various shapes and sizes [7,8]. Fixture design has been accomplished traditionally by human effort, but with the advent of CAD/CAM, a significant amount of research has been conducted by investigators into the automatic design of fixtures [9-15]. However, these studies have not revealed any cost-effective method for the manufacturing of the various fixtures that are essential for low volume FMS. Investigations have shown that prismatic parts (non rotational) constitute a very important group amongst the various kinds of workpieces machined [12]. The present authors have developed the concept of a "fixture-free" machining center for the machining of block-like components. The principle objective of this research is to create a fully automatic machine that facilitates the automated low-volume production of block-like components without the need of preparing any workpiece holding fixtures external to the machine, and which is supported by automated CAM capability. The present paper describes the fundamental principle of the Block Machining Center that automatically feeds and positions the material (in bar form) for the fixture-free unattended machining of all of the six faces of a block-like component in a single process.
2. Method of study The most important requirement for the effective operation of an FMS for low-volume production with a high product-mix is to improve the productivity, which latter should be accomplished by the machining of the major part of the components by unattended operation. It is a popular belief that flexibility and productivity are inversely related, but it is the authors' intention to prove that this is not necessarily true. Flexibility has a great advantage
B.P. Bandyopadhyay et al./A fixture-free machining center
407
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~
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2 0 Z
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O
R
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10 100 1000 TOTALNUMBERPRODUCED LOWVOLUME ,~
~
S^ ial-.'--
se
1 10 100 1000 MONTHLY PRODUCTION103pc ~-HIGHVOLUME
Fig. 1. Low volume FMS. when many varieties of products are being machined by the system and will reduce the machine idle time significantly, thereby increasing the overall productivity. Existing FMS are capable of unattended machining for a relatively higher volume of production where each component is produced to repeated orders of several hundred or more. However, many industries such as special-purpose machine tools, automatic welding positioners, etc., do not produce to such great volume. For these types of companies with low-volume production and a high product-mix, FMS is not economical today, especially for those industries with machining centers producing prismatic (non-rotational) parts, because of the difficulties in the workpiece-holding technology. It is critical, therefore, to develop a technology where the workpiece-holding technology can be designed and fabricated, without relying on external sources, but instead relying on the job shops themselves, with short lead time and low cost. The durability of the fixture may be limited to machining up to, say, one hundred pieces. Another approach would be to develop a new machining technology where the use of fixtures will be eliminated if the geometry of the components so permits. In this case, the machine tool will hold the workpiece directly. This new technology would lead to a new concept of low volume prismatic FMS, where human tasks would be focused on the "set-up", including the preparation of cutting tools and the generation of the NC part program, which latter is essential for the subsequent unattended operation. Figure 1 shows that by enhancing the workpiece-holding technology, the manufacturing cost per piece for low volume FMS can be reduced greatly [12].
408
B.P. Bandyopadhyay et al./A fixture-free machining center
3. Workpiece-holding technology In existing prismatic FMS, workpiece-holding fixtures are designed and manufactured that can withstand repeated use. Fixtures are designed currently such that reference surfaces that contact the workpiece repeatedly are hardened and ground. They are made in tool shops, resulting in a longer lead time prior to actual production. The situation is complicated by one or more fixtures being required for each design of the component to be produced. An exception to the foregoing are large-size prismatic workpieces that are machined in many shops today by five-face cutting machining centers, unattended during the night shift. These large workpieces are, in most cases, set on the work table manually by using reference blocks and other fixturing elements. The complete machining process, which may take several hours, is generally completed in one operation by special-purpose machining centers. Thus unattended operation is easy to achieve simply by setting up a few workpieces on the Automatic Work Table Changer (AWC) system of the machine tool. However, in addition to large-size components, the production requirement may include a large number of prismatic components of mediumto-small sizes. New workpiece designs arrive for production at a higher rate in low-volume and high product-mix job shops, hence it is not economical to prepare workpiece-holding fixtures for quasi-permanent use. In present job shops, the workpieces are set and removed manually using either universal-type fixtures such as machine vices or reference blocks and other fixturing elements. Therefore, operation of machining centers needs human attendance except for the last workpiece of the day, which may be left for unattended machining. In the case of medium-to-small workpieces, however, the machining of the last piece is finished in few minutes and then the machine stands idle until the operator reports to work the following day.
4. Fixture-free machining center A special workpiece-holding device has been designed and fabricated within the authors' laboratory. This device, placed on the table of a vertical machining center, constitutes the concept of the fixture-free machining center as is illustrated in Fig. 2. The principle of operation is as follows. A workpiece in bar form is supplied from the material feeder device and held by the material clamping mechanism. To improve the rigidity the other end - which is to be machined - is further supported by the automated workpiece holding vise. The material clamping mechanism allows the rotation of the workpiece about its longitudinal axis, which permits the machining of the four circumferential faces. The outside axial face is machined using the angular tool attachment of the machining center. Following the final parting of the workpiece from the bar stock material, the sixth face can be machined
B.P. Bandyopadhyay et al./A fixture-free machining center
409
Material Feed Device
Material Clamping
I M a c h i n i n g Head
Automated workpiece Holding Vise
Fig. 2. Block machining center.
after rotating the automated workpiece holding vise 180 degrees around the vertical axis. 4.1. Workpiece-holding device
Figure 3 describes the assembly design of the workpiece-holding device. The material is fed through this mechanism and clamped by the spanring (7) on both sides. Positioning of all four sides is accomplished by clamping the square head (4) with two-piece vises (11). The square head can be rotated freely since it is supported by two tapered roller bearings (8). The two-piece vises, which are held on the frame (10), can move forwards and backwards by means of a linear slide bearing (12). The rigidity of the machined part is increased further, by supporting the other end - to be machined - by a pair of free-jaw vises (14). This mechanism allows the rotation of the workpiece about its longitudinal axis, this rotation being essential for the machining of four circumferential faces. Following the final parting-off of the workpiece from the bar, the remaining axial face is machined after rotating the workpiece holding vise 180 degrees about the vertical axis by the pivot-pin (15). This device, although presently operated manually, demonstrates the fundamental principle of a fixture-free machining center. The machining center, supported by an automated CAM capability, automates the material feed and positions it for fixture-free unattended machining of all six faces of block-like parts in a single process.
410
B.P. Bandyopadhyay et al./A fixture-free machining center
~,l [' L.:.J
L....J
t """l ~ /l ~.,
Fig. 3. Workpiece-holding device: 1 - base plate; 2 - housing; 3 cover plate; 4 - head, square; 5 nut; 6 head, cylinder, 7 spanring; 8 bearing, taper; 9 spacer,ring; 10- holder, vise; 11 two-piecevises; 12 slide bearing; 13 base, vise; 14 free-jawvises; 15 pin, pivot.
5. CAM capability Figure 4 shows the flow diagram for data used in the CAM system currently being developed at the authors' laboratory [16]. The input data, labeled "product data", are processed by the CAM system, resulting in the output of two sets of data. One set consists of the NC data: the "product data" are used first to generate an APT part program through the use of internal CAM system software, the APT part program, as input data, being in turn processed by a ready-made post processor, G-Post, which produces NC commands suitable for the machining center. The second set of data to be generated contains the "operator instructions": these data are to be presented to the operator with the information needed for preparation prior to starting the machining process. As an example, Operator instructions which are generated for use with the CNC milling machine are shown in the lower-right corner of the figure. During the "tool preparation" stage a particular type of tool, to be used for the machining process, along with pertinent tool data, is displayed. "Origin finding" displays the origin position as determined by the CAM system when generating the APT data, thereby directing the operator to set the tool at the right position. "Instructions by operation" displays other preparatory information, such as the tool identification number, the spindle speed, the coolant and the NC part program number. Upon completion of the machining process, the APT part program, the NC data, and the operator instruction data are deleted from the computer and the NC memory storage. The reasons for not storing these data are as follows:
B.P. Bandyopadhyay et al./A fixture-free machining center
411
COMPUTER _._J
U
TOOL PRESETTER
on DISPLAY
i~Tr~r-u~n-i / 1.Tool preperation 2.Origin finding I 3.Instruction
by ol~r~on TERMINAL CNC MILLING MACHINE
Fig. 4. CAM system capability.
Fig. 5. Examples of block-like machined parts.
TOOl number Spindle soeed Coolant
___#NC . . . . . . .
412
B.P. Bandyopadhyay et al./A fixture-free machining center
Fig. 6. An example of the process sequences for a block-like part: (from left to right) facing 1,3; facing 2,4; pocketing, drilling, boring, parting off; and axial facing 5,6 (rotate view 18oo).
(1) Because the CAM system automatically generates the NC and the operator instruction data each time it processes the product data, there is no need to store these data. (2) The advantages of storing tool-specific NC and operator instruction data are lessened due to the possibility of changes in the actual tool dimensions and cutting parameters, which necessitates the rewriting of the NC data. (3) By not storing these data, computer and NC memory storage space are conserved.
6. Example of machined parts Figure 5 shows examples of various prismatic components manufactured, whilst Fig. 6 shows the machining operation of one of them. These examples demonstrate that a very complex part can be machined by the fixture-free machining center.
7. Conclusions Even though it is presently operated manually, this workpiece-holding device mounted on the table of the milling machine demonstrates the principle of a fixture-free machining center. Integrated with the CAD/CAM System with fully automated CAM capability, the system proves the future possibility of highly automated production of low-volume high-product variability of blocklike prismatic parts. This type of machining center will find wide application for manufacturing fixtures and rapid prototyping.
Acknowledgement The authors wish to express their sincere thanks to the industrial members of the research proj ect "Fixture Center CAD/CAM" for their financial support. Special thanks are due to the Ministry of Education, Government of Japan,
B.P. Bandyopadhyay et al./A fixture-free machining center
413
from Dr. B.P. B a n d y o p a d h y a y for i n v i t i n g him for a period of one y e a r from the U n i v e r s i t y of N o r t h D a k o t a , USA, for this r e s e a r c h project.
References [1] M.P. Groover, Automation, Production Systems and Computer Aided Manufacturing, Prentice Hall, Inc., NJ, 1980. [2] K. Amaoko-Gyampah, J.R. Meredith and A. Raturi, A comparison of tool management strategies and part selection rules for a FMS, Int. J. Prod. Res., 30 (4) (1992) 733-748. [3] P. Tomek, Tooling strategies related to FMS management, FMS Magazine, (1986) 102-107. [4] N. Nagarur, Some performance measures of FMS, Int. J. Prod. Res., 30 (1992) 709-809. [5] A.S. Kiran and R.J. Krason, Automating tooling in a FMS, Ind. Eng., 20 (1988) 52-57. [6] S.A. Melnyk, Tool management and control: an introduction, Proc. Tool Management and Control Conf., Grand Rapids, MI, October, 1988, pp. 34-47. [7] M.V. Gandhi and B.S. Thomson, The integration of CAD and CAM in adaptive fixturing for FMS, Proc. Computers in Engineering, ASME, 1985, pp. 301-305. [8] P.C. Miller, Modular assembly speeds fixture building, Tooling Prod., (1983) 6749. [9] Y.C. Chou and M.M. Barash, Computerized fixture design from solid models of Workpieces, Integrated and Intelligent Manufacturing, ASME, 21 (1986) 133-141. [10] Y.C. Chou, V. Chandru and M.M. Barash, A mathematical approach to automatic design of fixtures: analysis and synthesis, Intelligent and Integrated Manufacturing Analysis and Synthesis, ASME, 25 (1987) 11-27. [11] A. Hayashi and K. Yamazaki, Development of fixture design and fabrication system by means of CAD/CAM, Proc. 16th NAMRC, May, 1988, pp. 424-436. [12] T. Hoshi, Low volume FMS, Proc. Manufacturing International 1990, Vol. III, Atlanta, Georgia, 1990, pp. 21-29. [13] W. Eversheim and H. Esch, Automated generation of process plans for prismatic parts, Ann. CIRP, 32(1) (1983) 361-364. [14] X. Dong and W.R. De Vries, Featured-based reasoning in fixture design, Ann. CIRP, 40(1) (1991) 111-114. [15] A.Y.C. Nee and A. Senthil Kumar, A framework for an object/rule-based automated fixture design system, Ann. CIRP, 40(1) (1991) 147-151. [16] T. Hanada and T. Hoshi, Block-like Component CAD/CAM system for fully automated CAM processing, Ann. CIRP, 41(1) (1992).
405
Development of a fixture-free machining center for machining block-like components B.P. B a n d y o p a d h y a y , T. Hoshi, M.A. L a t i e f and T. H a n a d a
Toyohashi University of Technology, Department of Production Systems Engineering, Tempaku-cho, Toyohashi 440, Japan (Received July 13, 1992; accepted February 12, 1993)
Industrial S u m m a r y The cost of designing and manufacturing fixtures constitutes a major factor in the FMS-type unattended operation of machining centers. The efficiency of operation of an FMS can be enhanced greatly by the developing of a fixture-free machining center. The concept of such a "fixture-free" machining center for the machining of block-like components has been developed and is presented in this paper. This machining center, supported by an automated CAM capability, feeds bar-form material automatically and positions it for unattended machining of all of the six faces of block-like components.
1. Introduction In the c o m p e t i t i v e world of m a n u f a c t u r i n g , m a n u f a c t u r i n g e n g i n e e r s are u n d e r p r e s s u r e to modify t h e i r p r o d u c t s quite f r e q u e n t l y , p r o d u c t designs being c h a n g e d c o n s t a n t l y to m e e t the needs of the consumer. Some examples of these p r o d u c t s are cars, special-purpose m a c h i n e tools, cameras, video recorders, etc. The m a n u f a c t u r i n g division of a c o m p a n y has to m e e t these c h a l l e n g e s of c o n s t a n t l y - c h a n g i n g p r o d u c t lines with new designs, Flexible M a n u f a c t u r i n g Systems (FMS) being a viable s o l u t i o n to this problem. T r a d i t i o n a l l y , F M S fills the gap b e t w e e n high-volume t r a n s f e r lines and h i g h l y flexible n u m e r i c a l l y - c o n t r o l l e d (NC) machines. F M S are e c o n o m i c a l in the mid v o l u m e r a n g e [1], b e c a u s e of the time and cost involved for the "set-up" to a c c o m m o d a t e a newly designed c o m p o n e n t . The set-up in this case includes the p r e p a r a t i o n of the w o r k p i e c e h o l d i n g fixtures, the p r e p a r a t i o n of c u t t i n g tools and the g e n e r a t i o n of a n NC p a r t program.
Correspondence to: Dr. B.P. Bandyopadhyay, Department of Mechanical Engineering, University of North Dakota, Box 8214, University Station, Grand Forks, ND 58202, USA. 0924-0136/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
406
B.P. BapTdyopadhyay et al./A fixture-free machitzing center
The successful operation of FMS for low volume production and high product variability will depend greatly on the management of the set-up. The latter has been estimated to comprise about 20% of the cost of new manufacturing systems, whilst for FMS the proportions may be even higher, since the set-up includes high-cost fixtures [2-4]. Despite the high set-up costs, unfortunately not much attention has been paid by researchers to these issues. However, following the financial disasters of many FMS [5], machine tool builders and manufacturing industries have realized recently that tooling can have a significant impact on the effective performance of an FMS. It has been suggested that the tooling investment must be controlled carefully before the company can realize the full potential of any automated manufacturing system [6]. Therefore, research is needed in the area of an effective method of fixture development for the success of a FMS producing a limited quantity of parts and operating with high product variability. In mass production, the cost of specially designed fixtures can be justified easily. However, in small batch production, the cost of manufacturing fixtures and the lead time are quite significant on a part basis. Manufacturers have attempted to solve this problem by the use of modular and adaptable fixtures for holding parts of various shapes and sizes [7,8]. Fixture design has been accomplished traditionally by human effort, but with the advent of CAD/CAM, a significant amount of research has been conducted by investigators into the automatic design of fixtures [9-15]. However, these studies have not revealed any cost-effective method for the manufacturing of the various fixtures that are essential for low volume FMS. Investigations have shown that prismatic parts (non rotational) constitute a very important group amongst the various kinds of workpieces machined [12]. The present authors have developed the concept of a "fixture-free" machining center for the machining of block-like components. The principle objective of this research is to create a fully automatic machine that facilitates the automated low-volume production of block-like components without the need of preparing any workpiece holding fixtures external to the machine, and which is supported by automated CAM capability. The present paper describes the fundamental principle of the Block Machining Center that automatically feeds and positions the material (in bar form) for the fixture-free unattended machining of all of the six faces of a block-like component in a single process.
2. Method of study The most important requirement for the effective operation of an FMS for low-volume production with a high product-mix is to improve the productivity, which latter should be accomplished by the machining of the major part of the components by unattended operation. It is a popular belief that flexibility and productivity are inversely related, but it is the authors' intention to prove that this is not necessarily true. Flexibility has a great advantage
B.P. Bandyopadhyay et al./A fixture-free machining center
407
HIGHRATE
,, LU O LU E0 z
~
ManualMachineTools "~ NC Ma;/hFnMe:Ools
"C~= B~ A ~ - ' ~
2 0 Z
W
O
R
K
P
10 100 1000 TOTALNUMBERPRODUCED LOWVOLUME ,~
~
S^ ial-.'--
se
1 10 100 1000 MONTHLY PRODUCTION103pc ~-HIGHVOLUME
Fig. 1. Low volume FMS. when many varieties of products are being machined by the system and will reduce the machine idle time significantly, thereby increasing the overall productivity. Existing FMS are capable of unattended machining for a relatively higher volume of production where each component is produced to repeated orders of several hundred or more. However, many industries such as special-purpose machine tools, automatic welding positioners, etc., do not produce to such great volume. For these types of companies with low-volume production and a high product-mix, FMS is not economical today, especially for those industries with machining centers producing prismatic (non-rotational) parts, because of the difficulties in the workpiece-holding technology. It is critical, therefore, to develop a technology where the workpiece-holding technology can be designed and fabricated, without relying on external sources, but instead relying on the job shops themselves, with short lead time and low cost. The durability of the fixture may be limited to machining up to, say, one hundred pieces. Another approach would be to develop a new machining technology where the use of fixtures will be eliminated if the geometry of the components so permits. In this case, the machine tool will hold the workpiece directly. This new technology would lead to a new concept of low volume prismatic FMS, where human tasks would be focused on the "set-up", including the preparation of cutting tools and the generation of the NC part program, which latter is essential for the subsequent unattended operation. Figure 1 shows that by enhancing the workpiece-holding technology, the manufacturing cost per piece for low volume FMS can be reduced greatly [12].
408
B.P. Bandyopadhyay et al./A fixture-free machining center
3. Workpiece-holding technology In existing prismatic FMS, workpiece-holding fixtures are designed and manufactured that can withstand repeated use. Fixtures are designed currently such that reference surfaces that contact the workpiece repeatedly are hardened and ground. They are made in tool shops, resulting in a longer lead time prior to actual production. The situation is complicated by one or more fixtures being required for each design of the component to be produced. An exception to the foregoing are large-size prismatic workpieces that are machined in many shops today by five-face cutting machining centers, unattended during the night shift. These large workpieces are, in most cases, set on the work table manually by using reference blocks and other fixturing elements. The complete machining process, which may take several hours, is generally completed in one operation by special-purpose machining centers. Thus unattended operation is easy to achieve simply by setting up a few workpieces on the Automatic Work Table Changer (AWC) system of the machine tool. However, in addition to large-size components, the production requirement may include a large number of prismatic components of mediumto-small sizes. New workpiece designs arrive for production at a higher rate in low-volume and high product-mix job shops, hence it is not economical to prepare workpiece-holding fixtures for quasi-permanent use. In present job shops, the workpieces are set and removed manually using either universal-type fixtures such as machine vices or reference blocks and other fixturing elements. Therefore, operation of machining centers needs human attendance except for the last workpiece of the day, which may be left for unattended machining. In the case of medium-to-small workpieces, however, the machining of the last piece is finished in few minutes and then the machine stands idle until the operator reports to work the following day.
4. Fixture-free machining center A special workpiece-holding device has been designed and fabricated within the authors' laboratory. This device, placed on the table of a vertical machining center, constitutes the concept of the fixture-free machining center as is illustrated in Fig. 2. The principle of operation is as follows. A workpiece in bar form is supplied from the material feeder device and held by the material clamping mechanism. To improve the rigidity the other end - which is to be machined - is further supported by the automated workpiece holding vise. The material clamping mechanism allows the rotation of the workpiece about its longitudinal axis, which permits the machining of the four circumferential faces. The outside axial face is machined using the angular tool attachment of the machining center. Following the final parting of the workpiece from the bar stock material, the sixth face can be machined
B.P. Bandyopadhyay et al./A fixture-free machining center
409
Material Feed Device
Material Clamping
I M a c h i n i n g Head
Automated workpiece Holding Vise
Fig. 2. Block machining center.
after rotating the automated workpiece holding vise 180 degrees around the vertical axis. 4.1. Workpiece-holding device
Figure 3 describes the assembly design of the workpiece-holding device. The material is fed through this mechanism and clamped by the spanring (7) on both sides. Positioning of all four sides is accomplished by clamping the square head (4) with two-piece vises (11). The square head can be rotated freely since it is supported by two tapered roller bearings (8). The two-piece vises, which are held on the frame (10), can move forwards and backwards by means of a linear slide bearing (12). The rigidity of the machined part is increased further, by supporting the other end - to be machined - by a pair of free-jaw vises (14). This mechanism allows the rotation of the workpiece about its longitudinal axis, this rotation being essential for the machining of four circumferential faces. Following the final parting-off of the workpiece from the bar, the remaining axial face is machined after rotating the workpiece holding vise 180 degrees about the vertical axis by the pivot-pin (15). This device, although presently operated manually, demonstrates the fundamental principle of a fixture-free machining center. The machining center, supported by an automated CAM capability, automates the material feed and positions it for fixture-free unattended machining of all six faces of block-like parts in a single process.
410
B.P. Bandyopadhyay et al./A fixture-free machining center
~,l [' L.:.J
L....J
t """l ~ /l ~.,
Fig. 3. Workpiece-holding device: 1 - base plate; 2 - housing; 3 cover plate; 4 - head, square; 5 nut; 6 head, cylinder, 7 spanring; 8 bearing, taper; 9 spacer,ring; 10- holder, vise; 11 two-piecevises; 12 slide bearing; 13 base, vise; 14 free-jawvises; 15 pin, pivot.
5. CAM capability Figure 4 shows the flow diagram for data used in the CAM system currently being developed at the authors' laboratory [16]. The input data, labeled "product data", are processed by the CAM system, resulting in the output of two sets of data. One set consists of the NC data: the "product data" are used first to generate an APT part program through the use of internal CAM system software, the APT part program, as input data, being in turn processed by a ready-made post processor, G-Post, which produces NC commands suitable for the machining center. The second set of data to be generated contains the "operator instructions": these data are to be presented to the operator with the information needed for preparation prior to starting the machining process. As an example, Operator instructions which are generated for use with the CNC milling machine are shown in the lower-right corner of the figure. During the "tool preparation" stage a particular type of tool, to be used for the machining process, along with pertinent tool data, is displayed. "Origin finding" displays the origin position as determined by the CAM system when generating the APT data, thereby directing the operator to set the tool at the right position. "Instructions by operation" displays other preparatory information, such as the tool identification number, the spindle speed, the coolant and the NC part program number. Upon completion of the machining process, the APT part program, the NC data, and the operator instruction data are deleted from the computer and the NC memory storage. The reasons for not storing these data are as follows:
B.P. Bandyopadhyay et al./A fixture-free machining center
411
COMPUTER _._J
U
TOOL PRESETTER
on DISPLAY
i~Tr~r-u~n-i / 1.Tool preperation 2.Origin finding I 3.Instruction
by ol~r~on TERMINAL CNC MILLING MACHINE
Fig. 4. CAM system capability.
Fig. 5. Examples of block-like machined parts.
TOOl number Spindle soeed Coolant
___#NC . . . . . . .
412
B.P. Bandyopadhyay et al./A fixture-free machining center
Fig. 6. An example of the process sequences for a block-like part: (from left to right) facing 1,3; facing 2,4; pocketing, drilling, boring, parting off; and axial facing 5,6 (rotate view 18oo).
(1) Because the CAM system automatically generates the NC and the operator instruction data each time it processes the product data, there is no need to store these data. (2) The advantages of storing tool-specific NC and operator instruction data are lessened due to the possibility of changes in the actual tool dimensions and cutting parameters, which necessitates the rewriting of the NC data. (3) By not storing these data, computer and NC memory storage space are conserved.
6. Example of machined parts Figure 5 shows examples of various prismatic components manufactured, whilst Fig. 6 shows the machining operation of one of them. These examples demonstrate that a very complex part can be machined by the fixture-free machining center.
7. Conclusions Even though it is presently operated manually, this workpiece-holding device mounted on the table of the milling machine demonstrates the principle of a fixture-free machining center. Integrated with the CAD/CAM System with fully automated CAM capability, the system proves the future possibility of highly automated production of low-volume high-product variability of blocklike prismatic parts. This type of machining center will find wide application for manufacturing fixtures and rapid prototyping.
Acknowledgement The authors wish to express their sincere thanks to the industrial members of the research proj ect "Fixture Center CAD/CAM" for their financial support. Special thanks are due to the Ministry of Education, Government of Japan,
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from Dr. B.P. B a n d y o p a d h y a y for i n v i t i n g him for a period of one y e a r from the U n i v e r s i t y of N o r t h D a k o t a , USA, for this r e s e a r c h project.
References [1] M.P. Groover, Automation, Production Systems and Computer Aided Manufacturing, Prentice Hall, Inc., NJ, 1980. [2] K. Amaoko-Gyampah, J.R. Meredith and A. Raturi, A comparison of tool management strategies and part selection rules for a FMS, Int. J. Prod. Res., 30 (4) (1992) 733-748. [3] P. Tomek, Tooling strategies related to FMS management, FMS Magazine, (1986) 102-107. [4] N. Nagarur, Some performance measures of FMS, Int. J. Prod. Res., 30 (1992) 709-809. [5] A.S. Kiran and R.J. Krason, Automating tooling in a FMS, Ind. Eng., 20 (1988) 52-57. [6] S.A. Melnyk, Tool management and control: an introduction, Proc. Tool Management and Control Conf., Grand Rapids, MI, October, 1988, pp. 34-47. [7] M.V. Gandhi and B.S. Thomson, The integration of CAD and CAM in adaptive fixturing for FMS, Proc. Computers in Engineering, ASME, 1985, pp. 301-305. [8] P.C. Miller, Modular assembly speeds fixture building, Tooling Prod., (1983) 6749. [9] Y.C. Chou and M.M. Barash, Computerized fixture design from solid models of Workpieces, Integrated and Intelligent Manufacturing, ASME, 21 (1986) 133-141. [10] Y.C. Chou, V. Chandru and M.M. Barash, A mathematical approach to automatic design of fixtures: analysis and synthesis, Intelligent and Integrated Manufacturing Analysis and Synthesis, ASME, 25 (1987) 11-27. [11] A. Hayashi and K. Yamazaki, Development of fixture design and fabrication system by means of CAD/CAM, Proc. 16th NAMRC, May, 1988, pp. 424-436. [12] T. Hoshi, Low volume FMS, Proc. Manufacturing International 1990, Vol. III, Atlanta, Georgia, 1990, pp. 21-29. [13] W. Eversheim and H. Esch, Automated generation of process plans for prismatic parts, Ann. CIRP, 32(1) (1983) 361-364. [14] X. Dong and W.R. De Vries, Featured-based reasoning in fixture design, Ann. CIRP, 40(1) (1991) 111-114. [15] A.Y.C. Nee and A. Senthil Kumar, A framework for an object/rule-based automated fixture design system, Ann. CIRP, 40(1) (1991) 147-151. [16] T. Hanada and T. Hoshi, Block-like Component CAD/CAM system for fully automated CAM processing, Ann. CIRP, 41(1) (1992).