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Robotized Assembly of Modular Fixtures
Robotized Assembly of Modular Fixtures F. Giusti ( l ) , M.Santochi (2), G. Dini, Institute of Mechanical Technology, University of Pisa/ltaly Received on January 14,1991
SUIQC4RY. A prototype plant for the robotized assembly of modular fixtures for NC machining centers is described. After presenting a brief summary of the present tendencies relative to the design, preparation and management of the fixtures, the plant and its components are described in their general structure, posing particular focus on the geometric and functional features of the modular elements, designed in order to facilitate the automatic operations of manipulation and assembly. The plant is controlled by a software structured for design of fixture configuration, planning of the assembly sequence and off-line programming of the robot. Results obtained in assembly tests are also discussed, and emphasis is placed on the more critical aspects linked to the automation of this important operation. In conclusion, the possible applications of the automatic assembly of fixtures for FMS plants are proposed.
KEY WORDS: Assembly, Planning, Fixtures.
INTRODUCTION The international trend in the field of mechanical manufacturing systems seems to be towards FMS plants: the diffusion and success of these plants is tied, in addition to the well known economic and reliability problems, to the flexibility that can be obtained and to the gradual elimination of human presence where it is still necessary. Two typical areas of FMS, that do not have the levels of automation and flexibility obtained elsewhere, must be taken into consideration: the tool-room and the fixtures preparation area. The automation of the tool-room has already been (6) demonstrating the studied by the authors feasibility of an automatic assembly and disassembly system, managed by an appropriate software, of tools in their toolholders for NC machining centers. The other area is represented by the preparation of the fixtures. This area involves the study of flexibility of such fixtures and their automated management. The problem is more complex with respect to that of the tools, because of the huge variety of the workpieces (shapes and dimensions), the considerable influence of the fixture design on the quality of the product to be machined and the high financial investments required. Nevertheless, the problem is not adequately considered and is left to be studied by a few specialists. The automation of the phases that characterize this (ll), has process, "loading" and "clamping" essentially focused on the former, due to the use of robots, and on the latter only for rotational parts. However, considerable problems remain in the area of clamping of more complex workpieces, generally known as "prismatic parts". In this case, various solutions have been proposed: magnetic devices, systems with plastic resins or metals with a low melting point that can be adapted to any type of shape, and NC systems that automatically change the position of the supporting and clamping elements (11). The main system used today, and by now standardized, for the management of workpieces in FMS, is the pallet system, that allows to adequately solve the problem of loading and transporting in the plant. The fixture is mounted manually on each pallet: although, in the case a small batches, it must then be frequently assembled and disassembled with high investments of personnel and mechanical components. Another system of fixtures, proposed f o r a long time but recently reconsidered because of its advantages in variable productions, is based on modular elements. These elements, designed in various shapes and dimensions to obtain the functions of reference, support and clamping, are assembled manually on a baseplate connected to the pallet. The baseplate can be equipped with a pattern of holes or else with a series of T slots. According to several experiences in the industrial environment (5), the use of modular elements has allowed for considerable savings on the cost of fixtures. In addition, the use of such systems is today facilitated by the storing of the available modular elements in graphic libraries, that can be used in CAD systems and that give the user the possibility of an interactive selection of the necessary components and their position on the baseplate, a visualization of the entire fixture and an assembly list for the operator. Interest in problems pertaining to the assembly of modular fixtures is also demonstrated by studies on the ergonomic organization of an area predisposed for (5) that utilizes automatic such operations magazines, robots for the handling of plates and fixtures located in order to ensure a fast and efficient manual assembly. As far as the automation of assembly of fixtures is concerned, some experiences (3,7) were carried out on
the positioning of workpieces without, however, discussing the practical details of the operation. From the point of view of the software adopted for the design of the fixtures, an intense amount of research has been conducted in the field of expert systems (2,3,8,9,12). These systems have covered some types of workpieces without, however, taking into consideration the modular fixtures. With these premises in mind, the authors are convinced that an appropriate solution to the problems related to fixtures in FMS, characterized by varied productions, is represented by the robotized assembly of modular elements. This must be in total integration with CAPP techniques to automatically plan the machining and assembly phase. Therefore, two aspects characterize this technique: 1) the design of the fixture on the basis of the raw part CAD model, the machining plan and the graphic library of the modular elements available; 2 ) the automatic assembly and disassembly of the fixture and the workpiece, with automatic generation of the robot part-program. This work reports essentially on the results obtained during the experimental tests relative to point Z), with a prototype plant at the Institute of Mechanical Technology of the University of Pisa. DESCRIPTION OF THE PROTOTYPE PLANT The Fig.1 schematically illustrates the structure of the prototype plant realized for the automatic assembly of the fixtures. This operation is carried out directly on a pallet of a NC machining center: the pallet was predisposed to receive the clamping and reference modular components. This is fully integrated with a software system for the design of the fixture and for the planning of the operations that the robot has to perform. The machining center has a horizontal spindle, with a 5-axes CNC, and is equipped with an automatic exchange device of the pallets by a translation _.
..
. .
WORKPIECES
ELEMENTS RACKS
ASSEMBLY ROBOT
, -
U
ASSEMBLY PLANT
---
CAPP SYSTEM YOOULC.
-
.. . _
S O F T W R E SYSTEM
Fig.1
MODULE FOR PLANNINO OF FIXTURES ASSEMBLY
MODULE FOR FIXTURES DES10N
-
- Schematlc representation of the prototype assembly plant.
Annals of the CIRP, Vol. 40/1/7991
17
shuttle that alternatively allows the presence of one of these outside the work area and, therefore, inside the robot operating range. A baseplate designed to receive and lock the single modular elements is mounted on a pallet and its features facilitate the robotized assembly operations. This plate can dispose its surface in two positions: a horizontal position, assumed during the assembly phase, in order to make possible coupling operations; a vertical position, assumed after having completed the fixture, mounted and clamped the workpiece, allowing the machining with the horizontal spindle. If the workpiece needs all the five axes for the machinin the baseplace can be direcrly mounted on the 5qn axis, taking advantage of the rotation of the axis for the baseplate orientation. The assembly robot was equipped with a sensorized gripper, a system for fingers automatic exchange and a screwing pneumatic unit. A conveyor for raw parts, one for machined parts, and some racks utilized to house all the modular elements are present in the assembly area. The fundamental part of this study was the design of the nodular elements, where the classical criteria of "Design for Assembly'' was applied. The elements necessary f o r reference, for supporting and clamping of some typical parts of the motorcycle industry, have been desLgned and built at the current srage of research. The essential geometrical and functional features of the modular elements designed and tested are (Fig.2): 1 - each element was designed in order to make available one or more surfaces of simple shape (flat or cylindrical) and easily accessible for a correct grasping by the gripper: 2 - the coupling interface of each element with the baseplate was designed to obtain an easy assembly in the vertical direction. In fact, after having taken into account the different coupling systems proposed by different manufacturing firms (T slots, alternate reference and threaded holes), the final configuration of the baseplate was obtained by a pattern of holes, each one characterized by a bored and a threaded surface. This solution has several advantages: - combines in a single hole both the reference and the locking of t h e modular element, allowing to obtain a greater rimer of possible positions; - with respect to T slots systems, it does not need a previous insertion of T nuts in the slots (difficult operation with automatic devices):
1 - Elements with a centering pin f o r a coupling the baseplate. The Fig.3 into the holes of iliustrates three examples of these elements. The Fig.3.a represents a cylindrical element that can be used either as a support or as a reference element. The element. in the Fig.3.b is similar to the previous one, but presents an additional reference surface and a rotating clamp, and was creaced to reference and clamp workpieces with a small thickness or workpieces that had been previously equipped with suitable
n
7-7.3
-
~ l e s of m c c h : s r
elzme?.ts d e s i
rototlzad a s s e
clamping surfaces (a detail is shown in the Fig.4). The Fig.3.c illustrates a reference and clamping element equipped with an expandable collet, suited for workpieces with finished holes. 2 Elements, having a prismatic shape, designed in order to be placed in any point of the baseplate, independently from the position of the holes. In the commercial systems this problem is mainly solved with adaptation plates positioned upon the baseplate, on which the user makes the holes where the assembly of the elements is required. Obviously, this solution cannot be proposed in an automatic assembly plant. Therefore, some elements were designed in order to be assembled onto the baseplate, interposing the guide illustrated in Fig.3.d: the guide is positioned and oriented on a hole of the plate, while the el.ement is mounted adjusting its position along the guide and fixing it with the fastening screw. The Fig.3.e and the Fig.3.f respectively illustrate a prismatic element with an adjustable stop and an element for lateral clamping.
-
YODULAR ELEMENT
t MUEPLATE
F ~ q . 2-
Geamecrical
nr.c
f'inctional f e a t u r e s of t7.e
m o d c l a r elemrzts.
- reduces the number of components necessary for the reference and locking of each modular element ( o n l y a fastening screw is necessary): 3 - the locking of the elements on the baseplate is obtained with screws inserted and screwed only in the vertical direction of assembly; 4 - the elements used to apply a clamping force on the workpiece (mechanical, not hydraulic, for an easier assembly) have been designed in order to handle the clamping screw along a vertical axis: 5 - on the lower surface of each element, in direct contact with the baseplate, an elastic element has been placed. Such a solution was necessary in order to increase the friction between the two surfaces, without damagmg the plate and avoiding a possible slip due to the torque applied during the screwing phase. Since the grid of holes requires the assembly of the elements in pre-determined points of the baseplate, that does not guarantee any configuration of the fixture, it was necessary to design and built two types of elements:
18
PLANNING SOFTWARE The programming of all the operations necessary to accomplish the assembly of modular fixtures requires a long procedure: the operations of the robot are numerous (picking-up and placing of the objects, finger exchanging, screwing, etc.). Furthermore, the planning of the trajectories requires long and repetitive self-learning sessions directly in the assembly area. In this case, particularly evident is the necessity to provide the plant with a software system for the off-line programing of the robot. The plant was linked to a software program, presently in the development stage, having the following aims (Fig.1): 1) to design the configuration of the fixture: 2 ) to plan the assembly and to off-line program the robot. The design of the fixture is conducted at a CAD terminal as a function of the geometrical characteristics of the workpiece and the machining operations required. This software is, therefore, closely linked to the automatic planning of the manufacturing, and is in fact structured to be used as a module of a general CAPP system already existing (10)* Through the subdivision of the CAD model in its facets, the system is able to identify the areas of the workpiece that satisfy the geometrical and
elements of tie fixture.
Tlg.4
-
CAf. n o d e l 0 5 t i e x c r ' c ~ i e c eand C A 3 nodel o f the ?::
technological features required f o r their use as reference and clamping surfaces. As a function of these surfaces, the system selects the modular elements available in a graphic library, evaluating and visualizing their position on the baseplate. The software f o r the programming of the assembly phases was developed on the basis of previous experiences carried out by the authors in this field (6). The software provides the following functions: a) Generation of the assembly sequences: of all the possible sequences identification according to which the various elements on the baseplate can be assembled to obtain the same fixture. The starting element of this evaluation is the CAD model of the fixture (defined by the previous module). The system calculates the precedence relationships between the modular elements, by means of a disassembly simulation procedure, and deduces the possible assembly sequences. b) Planning the robotized actions: the system plans and simulates both the operations and trajectories that the robot must accomplish to manipulate and assembly each component. c) Selection of the best assembly sequence characterized by the minimum execution time. d) Automatic generation of the part-program to be sent to the robot NC unit.
center. The robot proceeds to assemble the single modular elements, picking them up from the racks and positioning them on the baseplate (Fig.5). At the end the robot deposits the workpiece on the reference elements (Fig.6), and locks it with the clamping arms by means of the torque controlled screwing unit. In the assembly of the fixture illustrated in the Fig.4, 5 and 6 , 16 modular elements have been manipulated and positioned: this required 3 finger exchange operations, with a total assembly time of 4'16". The assembly of the workpiece required a time of 5 0 " . The Fig.7 shows another example of an automatically assembled fixture during a machining operation. The tests conducted put into focus some critical and specific aspects of this application: 1) the precision of positioning of the modular elements on the baseplate; 2 ) the depositing path of the workpiece on the fixture; 3 ) the precision of positioning of the workpiece on the fixture.
EXPERIMENTAL TESTS The Fig.4 illustrates an example, taken from the experimental tests, of a CAD model of a workpiece: it is a die-cast element, manufactured in an Italian motorcycle firm, that substantially requires milling and drilling operations executed in a NC machining
Fig.7
- Exanple of fixture, assembled
b y the robot,
during the machining 1! The first aspect involved the precision of positioning the modular elements on the baseplate. Obviously, this problem does not involve the elements equipped with a centering pin whose position is univocally determined by the holes on the baseplate, but is present exclusively for those modular elements that constitute a lateral reference for the workpiece and whose position is not connected to the presence of holes (Fig.3.e). The possible sources of errors in positioning can be identified in the following: - the repeatability of the robot (generally not less than t0.05 mm), that involves both the pick-up phase of the element from the rack and the deposition phase on the plate; the precision of the gripper, due to the accuracy of the grasping surfaces, and to the presence of clearances in the opening and closing mechanism (error estimated at about tO.05 mm);
-
Fig.5
-
Assembly of the modular elements on the baseplate.
19
- the positioning accuracy of the pallet in the assembly area that, having been positioned with dowels, can be neglected: - the rotation of the modular element during the assembly of the fastening screw, caused by the twisting deformation of the antislip elastic element, estimated at about 0.025iO.05 degree. The resulting error, in correspondence to the lateral reference can then reach values comprised between 0.1 to 0.25 mm, values that for certain types of machining operations are unacceptable. The solution to this problem can be a probe inspection that measures this error (translation and rotation of the workpiece with respect to its theoretical position). This operation can be carried out directly on the NC machine tool, measuring along unparallel directions, the displacement of three workpiece surfaces, automatically selected during the CAPP stage. This allows to correct the position of the axes of the machine (operation that can be easily done in modern CNC). 2 ) Another critical phase pertains to the deposition path of the workpiece on the fixture: no obstacles should exist that force the robot to place the workpiece through complicated trajectories, difficult to be planned with the "off-line" methods: the fixture must be designed in such a way the positioning of the workpiece occurs by simple translation along the vertical assembly axis. For this purpose, the modular elements that apply the clamping force from the top, and that would constitute an obstacle during the phase of vertical positioning of the workpiece, have been constructed with the clamping arm orientable around a vertical axis (Fig.6). Once the workpiece has been released, the robot orients and clamps the arms on the selected surfaces.
3) An equally delicate phase is the clamping of the workpiece on the fixture. During this operation it is in fact necessary that all the surfaces of the workpiece make contact with the reference elements, and this is not possible by simply depositing the workpiece on the fixture with the robot gripper. To solve this problem, two different solutions have been adopted: the first consists in the design of the fixture in such a way the clamping elements force the workpiece against the reference surfaces: the second, to be adopted in case the previous one cannot be applied, is the use of elastic elements that push the workpiece to the stop surfaces during the clamping phase. However, the application of these solutions strongly depends on the workpiece morphology; a more general method consists in the use of a robot assisted by a 3-axes manipulator: when the workpiece is to be assembled, the manipulator can hold it in position on the reference elements and the robot can simultaneously put into action the clamping devices. APPLICATIONS AND DEVELOPMENTS OF THE SYSTEM The robotized assembly of fixtures through the use of modular elements can find an application in FMS plants, where production is quite varied and it is often necessary to substitute the fixture. There are two possibilities: MBEYBLED TIXTURES WUFFER
Fig.8
- Proposal o f
a "set-up room": a fully automated plant for assembly and handling of fixtures.
1) The assembly conducted in an environment defined as a "set-up room", where a highly automated plant carries out the following operations (Fig.8): assembly of the fixtures through use of modular elements on plates mounted directly on the pallets, assembly of raw parts on the fixtures, sending to the FMS plant after completing control and coding, disassembly of the machined parts returning from the FMS plant, washing of pallets, and eventual disassembly of fixtures. This system should work on three shifts in order to cover every need of variation in production. The plant could be
20
completely automated, or else the human presence could be limited to only one operator with the supervision tasks of correct positioning of modular elements and workpieces on the pallets, this being :he most delicate phase of the robotized assembly. However, this operation could be also accomplished by a fully automated control station, where a measuring machine conducts the probe inspection, mentioned above, and stores the corrections on magnetic chips, which also contain the codes of the workpieces loaded onto the pallet. 2 ) Robotized assembly of the fixture directly on each machine. In this configuration the robot near the machine tool could conduct the assembly and disassembly of the fixtures and workpieces. Every machine of the FMS plant could be specialized f o r certain types of workpieces and therefore have a magazine of different modular components. In this configuration it would not be possible to imagine the presence of an operator for the control of the correct positioning of the workpiece, and this operation should be completely managed by sensors: this would entail a greater expenditure of time, taken away from the machining phase. Such a solution would have the advantage of reducing the number of circulating pallets in the plant. The selection between the two solutions should therefore be subject to a careful analysis of both technical and economical factors, even though the authors tend to favour the former.
CONCLUSIONS This research represents a first approach to the problem of the automatic assembly of modular fixtures for NC machining centers, and constitutes a first step towards the creation of more flexible and complex structures that can be inserted in the production process of a mechanical industry. The integration of the plant with a software system able to design and plan the assembly of the fixture, has allowed to optimize the assembly phase and to create the part-program in a few seconds. At the present stage of development, this system has been tested on several types of workpieces. To obtain this result, modular elements were designed to be used in the robotized assembly. The tests carried out gave positive results, and allowed to identify and solve some critical aspects associated to the automatic set-up of fixtures. REFERENCES (1) - Asada H., By A., 1985, Kinematic analysis of workpart fixturing for flexible assembly with automatically reconfigurable fixtures, IEEE Jou. Robot. Automation RA-1, 86-93. (2) - Boerma J.R., Kals H.J.J., 1988, FIXES, a system for automatic selection of set-ups and design of fixtures, Annals of the CIRP, Vo1.37/1, 443-446. Englert P.J., Wright P.K., 1986, Applications (3) of artificial intelligence and the design of fixtures for automated manufacturing, Proc. IEEE Int. Conf. on Robotics and Automation, San Francisco CA, April 7/10, 345-351. ( 4 ) - Eversheim W., Buchollz G . , Xnauf A., 1985, Rechnerunterstutze, konstruktion von baukastenvorrichtungen, Industrie Anzeiger, N.10 1.2, 13-15. (5) - Gallien D., Hammer H., 1983, Efficient and cost effective use of modular fixture kits at the machine site, TZ fur Metallbearbeitung, 77 N.5. (6) - Giusti F., Santochi M., Dini G., 1990. An integrated and flexible system for automatic tool assembly and disassembly, Annals of the CIRP, VO1.39/1, 29-32. (7) - Haynes L . S . , Graham H.M., 1988, A formal approach to specifying assembly operations, Int. Jou. Mach. Tools Manufact. Vol.28 N.3, 281-298. ( 8 ) , - Nee A.Y.C., Bhattachaqya N., Po0 A.N., 1987, Applying A1 in jigs and fixtures design, Robotics h Computer Integrated Hanufact., Vo1.3 N.2, 195-200. (9) - Pham D.T., de Sam Lazaro A., 1990, Autofix: an expert CAD system for jigs and fixtures, Int. Jou. Mach. Tools Manufact., V01.30, No.3, 403-411. (10) - Santochi M., Dini G., 1989, An example of CAD/CAM integration: fully automatic programming of a CNC machining center, La Meccanica Italiana, N.224, 54-62 (in Italian). (11) - T u f f e n t s m e r K., 1981, Automatic loading of machining systems and automatic clamping of workpieces, Annals of the CIRP, Vo1.30/2, 553-558. (12) - Van Brussel H., 1990, Planning and scheduling of assembly systems, Annals of the CIRP, vo1.39/2, 637-644.
-
ACXNOWLEDGMENTS This research is supported by the National Research Council "Progetto Finalizzato Robotica" (contract n.89.00517.67) and by the Ministry of the University and Scientific and Technological Research.
SUIQC4RY. A prototype plant for the robotized assembly of modular fixtures for NC machining centers is described. After presenting a brief summary of the present tendencies relative to the design, preparation and management of the fixtures, the plant and its components are described in their general structure, posing particular focus on the geometric and functional features of the modular elements, designed in order to facilitate the automatic operations of manipulation and assembly. The plant is controlled by a software structured for design of fixture configuration, planning of the assembly sequence and off-line programming of the robot. Results obtained in assembly tests are also discussed, and emphasis is placed on the more critical aspects linked to the automation of this important operation. In conclusion, the possible applications of the automatic assembly of fixtures for FMS plants are proposed.
KEY WORDS: Assembly, Planning, Fixtures.
INTRODUCTION The international trend in the field of mechanical manufacturing systems seems to be towards FMS plants: the diffusion and success of these plants is tied, in addition to the well known economic and reliability problems, to the flexibility that can be obtained and to the gradual elimination of human presence where it is still necessary. Two typical areas of FMS, that do not have the levels of automation and flexibility obtained elsewhere, must be taken into consideration: the tool-room and the fixtures preparation area. The automation of the tool-room has already been (6) demonstrating the studied by the authors feasibility of an automatic assembly and disassembly system, managed by an appropriate software, of tools in their toolholders for NC machining centers. The other area is represented by the preparation of the fixtures. This area involves the study of flexibility of such fixtures and their automated management. The problem is more complex with respect to that of the tools, because of the huge variety of the workpieces (shapes and dimensions), the considerable influence of the fixture design on the quality of the product to be machined and the high financial investments required. Nevertheless, the problem is not adequately considered and is left to be studied by a few specialists. The automation of the phases that characterize this (ll), has process, "loading" and "clamping" essentially focused on the former, due to the use of robots, and on the latter only for rotational parts. However, considerable problems remain in the area of clamping of more complex workpieces, generally known as "prismatic parts". In this case, various solutions have been proposed: magnetic devices, systems with plastic resins or metals with a low melting point that can be adapted to any type of shape, and NC systems that automatically change the position of the supporting and clamping elements (11). The main system used today, and by now standardized, for the management of workpieces in FMS, is the pallet system, that allows to adequately solve the problem of loading and transporting in the plant. The fixture is mounted manually on each pallet: although, in the case a small batches, it must then be frequently assembled and disassembled with high investments of personnel and mechanical components. Another system of fixtures, proposed f o r a long time but recently reconsidered because of its advantages in variable productions, is based on modular elements. These elements, designed in various shapes and dimensions to obtain the functions of reference, support and clamping, are assembled manually on a baseplate connected to the pallet. The baseplate can be equipped with a pattern of holes or else with a series of T slots. According to several experiences in the industrial environment (5), the use of modular elements has allowed for considerable savings on the cost of fixtures. In addition, the use of such systems is today facilitated by the storing of the available modular elements in graphic libraries, that can be used in CAD systems and that give the user the possibility of an interactive selection of the necessary components and their position on the baseplate, a visualization of the entire fixture and an assembly list for the operator. Interest in problems pertaining to the assembly of modular fixtures is also demonstrated by studies on the ergonomic organization of an area predisposed for (5) that utilizes automatic such operations magazines, robots for the handling of plates and fixtures located in order to ensure a fast and efficient manual assembly. As far as the automation of assembly of fixtures is concerned, some experiences (3,7) were carried out on
the positioning of workpieces without, however, discussing the practical details of the operation. From the point of view of the software adopted for the design of the fixtures, an intense amount of research has been conducted in the field of expert systems (2,3,8,9,12). These systems have covered some types of workpieces without, however, taking into consideration the modular fixtures. With these premises in mind, the authors are convinced that an appropriate solution to the problems related to fixtures in FMS, characterized by varied productions, is represented by the robotized assembly of modular elements. This must be in total integration with CAPP techniques to automatically plan the machining and assembly phase. Therefore, two aspects characterize this technique: 1) the design of the fixture on the basis of the raw part CAD model, the machining plan and the graphic library of the modular elements available; 2 ) the automatic assembly and disassembly of the fixture and the workpiece, with automatic generation of the robot part-program. This work reports essentially on the results obtained during the experimental tests relative to point Z), with a prototype plant at the Institute of Mechanical Technology of the University of Pisa. DESCRIPTION OF THE PROTOTYPE PLANT The Fig.1 schematically illustrates the structure of the prototype plant realized for the automatic assembly of the fixtures. This operation is carried out directly on a pallet of a NC machining center: the pallet was predisposed to receive the clamping and reference modular components. This is fully integrated with a software system for the design of the fixture and for the planning of the operations that the robot has to perform. The machining center has a horizontal spindle, with a 5-axes CNC, and is equipped with an automatic exchange device of the pallets by a translation _.
..
. .
WORKPIECES
ELEMENTS RACKS
ASSEMBLY ROBOT
, -
U
ASSEMBLY PLANT
---
CAPP SYSTEM YOOULC.
-
.. . _
S O F T W R E SYSTEM
Fig.1
MODULE FOR PLANNINO OF FIXTURES ASSEMBLY
MODULE FOR FIXTURES DES10N
-
- Schematlc representation of the prototype assembly plant.
Annals of the CIRP, Vol. 40/1/7991
17
shuttle that alternatively allows the presence of one of these outside the work area and, therefore, inside the robot operating range. A baseplate designed to receive and lock the single modular elements is mounted on a pallet and its features facilitate the robotized assembly operations. This plate can dispose its surface in two positions: a horizontal position, assumed during the assembly phase, in order to make possible coupling operations; a vertical position, assumed after having completed the fixture, mounted and clamped the workpiece, allowing the machining with the horizontal spindle. If the workpiece needs all the five axes for the machinin the baseplace can be direcrly mounted on the 5qn axis, taking advantage of the rotation of the axis for the baseplate orientation. The assembly robot was equipped with a sensorized gripper, a system for fingers automatic exchange and a screwing pneumatic unit. A conveyor for raw parts, one for machined parts, and some racks utilized to house all the modular elements are present in the assembly area. The fundamental part of this study was the design of the nodular elements, where the classical criteria of "Design for Assembly'' was applied. The elements necessary f o r reference, for supporting and clamping of some typical parts of the motorcycle industry, have been desLgned and built at the current srage of research. The essential geometrical and functional features of the modular elements designed and tested are (Fig.2): 1 - each element was designed in order to make available one or more surfaces of simple shape (flat or cylindrical) and easily accessible for a correct grasping by the gripper: 2 - the coupling interface of each element with the baseplate was designed to obtain an easy assembly in the vertical direction. In fact, after having taken into account the different coupling systems proposed by different manufacturing firms (T slots, alternate reference and threaded holes), the final configuration of the baseplate was obtained by a pattern of holes, each one characterized by a bored and a threaded surface. This solution has several advantages: - combines in a single hole both the reference and the locking of t h e modular element, allowing to obtain a greater rimer of possible positions; - with respect to T slots systems, it does not need a previous insertion of T nuts in the slots (difficult operation with automatic devices):
1 - Elements with a centering pin f o r a coupling the baseplate. The Fig.3 into the holes of iliustrates three examples of these elements. The Fig.3.a represents a cylindrical element that can be used either as a support or as a reference element. The element. in the Fig.3.b is similar to the previous one, but presents an additional reference surface and a rotating clamp, and was creaced to reference and clamp workpieces with a small thickness or workpieces that had been previously equipped with suitable
n
7-7.3
-
~ l e s of m c c h : s r
elzme?.ts d e s i
rototlzad a s s e
clamping surfaces (a detail is shown in the Fig.4). The Fig.3.c illustrates a reference and clamping element equipped with an expandable collet, suited for workpieces with finished holes. 2 Elements, having a prismatic shape, designed in order to be placed in any point of the baseplate, independently from the position of the holes. In the commercial systems this problem is mainly solved with adaptation plates positioned upon the baseplate, on which the user makes the holes where the assembly of the elements is required. Obviously, this solution cannot be proposed in an automatic assembly plant. Therefore, some elements were designed in order to be assembled onto the baseplate, interposing the guide illustrated in Fig.3.d: the guide is positioned and oriented on a hole of the plate, while the el.ement is mounted adjusting its position along the guide and fixing it with the fastening screw. The Fig.3.e and the Fig.3.f respectively illustrate a prismatic element with an adjustable stop and an element for lateral clamping.
-
YODULAR ELEMENT
t MUEPLATE
F ~ q . 2-
Geamecrical
nr.c
f'inctional f e a t u r e s of t7.e
m o d c l a r elemrzts.
- reduces the number of components necessary for the reference and locking of each modular element ( o n l y a fastening screw is necessary): 3 - the locking of the elements on the baseplate is obtained with screws inserted and screwed only in the vertical direction of assembly; 4 - the elements used to apply a clamping force on the workpiece (mechanical, not hydraulic, for an easier assembly) have been designed in order to handle the clamping screw along a vertical axis: 5 - on the lower surface of each element, in direct contact with the baseplate, an elastic element has been placed. Such a solution was necessary in order to increase the friction between the two surfaces, without damagmg the plate and avoiding a possible slip due to the torque applied during the screwing phase. Since the grid of holes requires the assembly of the elements in pre-determined points of the baseplate, that does not guarantee any configuration of the fixture, it was necessary to design and built two types of elements:
18
PLANNING SOFTWARE The programming of all the operations necessary to accomplish the assembly of modular fixtures requires a long procedure: the operations of the robot are numerous (picking-up and placing of the objects, finger exchanging, screwing, etc.). Furthermore, the planning of the trajectories requires long and repetitive self-learning sessions directly in the assembly area. In this case, particularly evident is the necessity to provide the plant with a software system for the off-line programing of the robot. The plant was linked to a software program, presently in the development stage, having the following aims (Fig.1): 1) to design the configuration of the fixture: 2 ) to plan the assembly and to off-line program the robot. The design of the fixture is conducted at a CAD terminal as a function of the geometrical characteristics of the workpiece and the machining operations required. This software is, therefore, closely linked to the automatic planning of the manufacturing, and is in fact structured to be used as a module of a general CAPP system already existing (10)* Through the subdivision of the CAD model in its facets, the system is able to identify the areas of the workpiece that satisfy the geometrical and
elements of tie fixture.
Tlg.4
-
CAf. n o d e l 0 5 t i e x c r ' c ~ i e c eand C A 3 nodel o f the ?::
technological features required f o r their use as reference and clamping surfaces. As a function of these surfaces, the system selects the modular elements available in a graphic library, evaluating and visualizing their position on the baseplate. The software f o r the programming of the assembly phases was developed on the basis of previous experiences carried out by the authors in this field (6). The software provides the following functions: a) Generation of the assembly sequences: of all the possible sequences identification according to which the various elements on the baseplate can be assembled to obtain the same fixture. The starting element of this evaluation is the CAD model of the fixture (defined by the previous module). The system calculates the precedence relationships between the modular elements, by means of a disassembly simulation procedure, and deduces the possible assembly sequences. b) Planning the robotized actions: the system plans and simulates both the operations and trajectories that the robot must accomplish to manipulate and assembly each component. c) Selection of the best assembly sequence characterized by the minimum execution time. d) Automatic generation of the part-program to be sent to the robot NC unit.
center. The robot proceeds to assemble the single modular elements, picking them up from the racks and positioning them on the baseplate (Fig.5). At the end the robot deposits the workpiece on the reference elements (Fig.6), and locks it with the clamping arms by means of the torque controlled screwing unit. In the assembly of the fixture illustrated in the Fig.4, 5 and 6 , 16 modular elements have been manipulated and positioned: this required 3 finger exchange operations, with a total assembly time of 4'16". The assembly of the workpiece required a time of 5 0 " . The Fig.7 shows another example of an automatically assembled fixture during a machining operation. The tests conducted put into focus some critical and specific aspects of this application: 1) the precision of positioning of the modular elements on the baseplate; 2 ) the depositing path of the workpiece on the fixture; 3 ) the precision of positioning of the workpiece on the fixture.
EXPERIMENTAL TESTS The Fig.4 illustrates an example, taken from the experimental tests, of a CAD model of a workpiece: it is a die-cast element, manufactured in an Italian motorcycle firm, that substantially requires milling and drilling operations executed in a NC machining
Fig.7
- Exanple of fixture, assembled
b y the robot,
during the machining 1! The first aspect involved the precision of positioning the modular elements on the baseplate. Obviously, this problem does not involve the elements equipped with a centering pin whose position is univocally determined by the holes on the baseplate, but is present exclusively for those modular elements that constitute a lateral reference for the workpiece and whose position is not connected to the presence of holes (Fig.3.e). The possible sources of errors in positioning can be identified in the following: - the repeatability of the robot (generally not less than t0.05 mm), that involves both the pick-up phase of the element from the rack and the deposition phase on the plate; the precision of the gripper, due to the accuracy of the grasping surfaces, and to the presence of clearances in the opening and closing mechanism (error estimated at about tO.05 mm);
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Fig.5
-
Assembly of the modular elements on the baseplate.
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- the positioning accuracy of the pallet in the assembly area that, having been positioned with dowels, can be neglected: - the rotation of the modular element during the assembly of the fastening screw, caused by the twisting deformation of the antislip elastic element, estimated at about 0.025iO.05 degree. The resulting error, in correspondence to the lateral reference can then reach values comprised between 0.1 to 0.25 mm, values that for certain types of machining operations are unacceptable. The solution to this problem can be a probe inspection that measures this error (translation and rotation of the workpiece with respect to its theoretical position). This operation can be carried out directly on the NC machine tool, measuring along unparallel directions, the displacement of three workpiece surfaces, automatically selected during the CAPP stage. This allows to correct the position of the axes of the machine (operation that can be easily done in modern CNC). 2 ) Another critical phase pertains to the deposition path of the workpiece on the fixture: no obstacles should exist that force the robot to place the workpiece through complicated trajectories, difficult to be planned with the "off-line" methods: the fixture must be designed in such a way the positioning of the workpiece occurs by simple translation along the vertical assembly axis. For this purpose, the modular elements that apply the clamping force from the top, and that would constitute an obstacle during the phase of vertical positioning of the workpiece, have been constructed with the clamping arm orientable around a vertical axis (Fig.6). Once the workpiece has been released, the robot orients and clamps the arms on the selected surfaces.
3) An equally delicate phase is the clamping of the workpiece on the fixture. During this operation it is in fact necessary that all the surfaces of the workpiece make contact with the reference elements, and this is not possible by simply depositing the workpiece on the fixture with the robot gripper. To solve this problem, two different solutions have been adopted: the first consists in the design of the fixture in such a way the clamping elements force the workpiece against the reference surfaces: the second, to be adopted in case the previous one cannot be applied, is the use of elastic elements that push the workpiece to the stop surfaces during the clamping phase. However, the application of these solutions strongly depends on the workpiece morphology; a more general method consists in the use of a robot assisted by a 3-axes manipulator: when the workpiece is to be assembled, the manipulator can hold it in position on the reference elements and the robot can simultaneously put into action the clamping devices. APPLICATIONS AND DEVELOPMENTS OF THE SYSTEM The robotized assembly of fixtures through the use of modular elements can find an application in FMS plants, where production is quite varied and it is often necessary to substitute the fixture. There are two possibilities: MBEYBLED TIXTURES WUFFER
Fig.8
- Proposal o f
a "set-up room": a fully automated plant for assembly and handling of fixtures.
1) The assembly conducted in an environment defined as a "set-up room", where a highly automated plant carries out the following operations (Fig.8): assembly of the fixtures through use of modular elements on plates mounted directly on the pallets, assembly of raw parts on the fixtures, sending to the FMS plant after completing control and coding, disassembly of the machined parts returning from the FMS plant, washing of pallets, and eventual disassembly of fixtures. This system should work on three shifts in order to cover every need of variation in production. The plant could be
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completely automated, or else the human presence could be limited to only one operator with the supervision tasks of correct positioning of modular elements and workpieces on the pallets, this being :he most delicate phase of the robotized assembly. However, this operation could be also accomplished by a fully automated control station, where a measuring machine conducts the probe inspection, mentioned above, and stores the corrections on magnetic chips, which also contain the codes of the workpieces loaded onto the pallet. 2 ) Robotized assembly of the fixture directly on each machine. In this configuration the robot near the machine tool could conduct the assembly and disassembly of the fixtures and workpieces. Every machine of the FMS plant could be specialized f o r certain types of workpieces and therefore have a magazine of different modular components. In this configuration it would not be possible to imagine the presence of an operator for the control of the correct positioning of the workpiece, and this operation should be completely managed by sensors: this would entail a greater expenditure of time, taken away from the machining phase. Such a solution would have the advantage of reducing the number of circulating pallets in the plant. The selection between the two solutions should therefore be subject to a careful analysis of both technical and economical factors, even though the authors tend to favour the former.
CONCLUSIONS This research represents a first approach to the problem of the automatic assembly of modular fixtures for NC machining centers, and constitutes a first step towards the creation of more flexible and complex structures that can be inserted in the production process of a mechanical industry. The integration of the plant with a software system able to design and plan the assembly of the fixture, has allowed to optimize the assembly phase and to create the part-program in a few seconds. At the present stage of development, this system has been tested on several types of workpieces. To obtain this result, modular elements were designed to be used in the robotized assembly. The tests carried out gave positive results, and allowed to identify and solve some critical aspects associated to the automatic set-up of fixtures. REFERENCES (1) - Asada H., By A., 1985, Kinematic analysis of workpart fixturing for flexible assembly with automatically reconfigurable fixtures, IEEE Jou. Robot. Automation RA-1, 86-93. (2) - Boerma J.R., Kals H.J.J., 1988, FIXES, a system for automatic selection of set-ups and design of fixtures, Annals of the CIRP, Vo1.37/1, 443-446. Englert P.J., Wright P.K., 1986, Applications (3) of artificial intelligence and the design of fixtures for automated manufacturing, Proc. IEEE Int. Conf. on Robotics and Automation, San Francisco CA, April 7/10, 345-351. ( 4 ) - Eversheim W., Buchollz G . , Xnauf A., 1985, Rechnerunterstutze, konstruktion von baukastenvorrichtungen, Industrie Anzeiger, N.10 1.2, 13-15. (5) - Gallien D., Hammer H., 1983, Efficient and cost effective use of modular fixture kits at the machine site, TZ fur Metallbearbeitung, 77 N.5. (6) - Giusti F., Santochi M., Dini G., 1990. An integrated and flexible system for automatic tool assembly and disassembly, Annals of the CIRP, VO1.39/1, 29-32. (7) - Haynes L . S . , Graham H.M., 1988, A formal approach to specifying assembly operations, Int. Jou. Mach. Tools Manufact. Vol.28 N.3, 281-298. ( 8 ) , - Nee A.Y.C., Bhattachaqya N., Po0 A.N., 1987, Applying A1 in jigs and fixtures design, Robotics h Computer Integrated Hanufact., Vo1.3 N.2, 195-200. (9) - Pham D.T., de Sam Lazaro A., 1990, Autofix: an expert CAD system for jigs and fixtures, Int. Jou. Mach. Tools Manufact., V01.30, No.3, 403-411. (10) - Santochi M., Dini G., 1989, An example of CAD/CAM integration: fully automatic programming of a CNC machining center, La Meccanica Italiana, N.224, 54-62 (in Italian). (11) - T u f f e n t s m e r K., 1981, Automatic loading of machining systems and automatic clamping of workpieces, Annals of the CIRP, Vo1.30/2, 553-558. (12) - Van Brussel H., 1990, Planning and scheduling of assembly systems, Annals of the CIRP, vo1.39/2, 637-644.
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ACXNOWLEDGMENTS This research is supported by the National Research Council "Progetto Finalizzato Robotica" (contract n.89.00517.67) and by the Ministry of the University and Scientific and Technological Research.