Int J Adv Manuf Technol (1996) 11:18-26 © 1996 Springer-Verlag London Limited

The Intt, m,~nal Journal of

fldvanced manuPacturinu Technolou[I

Development of a Computer Numerical Control (CNC) Modular Fixture - Machine Design of a Standard Multifinger Module

K. C. Chan and C. S. Lin School of Mechanical and Manufacturing Engineering, University of New South Wales, PO Box 1, Kensington, Australia

This paper presents the frst phase of our ongoing development of a computer numerical control (CNC) modular fixture involving the machine design of a standard multifinger module. The aim is to develop a modular fixture which consists of only one type of standard multifinger CNC module which can provide locating, supporting and clamping functions. The module consists of four fingers with eight degrees of freedom to conform to any arbitrary workpiece surfaces. The eight motions of the four fingers are controlled by one motor through the use of two transmission and clutch systems. Different fixture layouts or reconfigurations can be made by different combinations of a multiplicity of such modules for major product family changes; while adjustments" to finger positions of individual modules can be made by direct computer numerical control for minor product changes within a product family. The machine design and operation of the module are presented in the paper. A prototype was built and tested and satisfactory performance was achieved. Finally, two examples are provided to illustrate the flexibility and use of the standard multifinger module.

Keywords: Computer numerical control fixture; Fixture; Flex- ible automation; Flexible fixture; Modular fixtures; Workhold- ing technology

1. Introduction

Although the traditional dedicated fixturing approach is most economical for mass production, it would cause long lead time and high fixturing cost in small-to-medium sized batch production. Modular fixtures, which consist of a set of standard fixture components, have been a solution to small batch production since the 1960s. Although modular fixtures can provide flexibility in accommodating parts of different sizes and shapes, they are still not suitable for flexible manufacturing systems (FMS) owing to the fact that they must be reconfigured

Correspondence and offprint requests to: K. C. Chan, School of Mechanical and Manufacturing Engineering, University of New South Wales, PO Box t, Kensington, NSW 2033, Australia.

and assembled manually, and cannot be set-up automatically by any software means.

It is generally agreed that flexible fixtures are necessary to realise the true benefits of FMS. Since the early 1980s, a number of novel flexible fixtures have been developed [1-11]. Extensive surveys and detailed discussions on these novel fixtures are available in the literature and will not be repeated here [12, 13]. However, these fixtures are very different in regard to their designs, functions and applications. Although these systems may be very distinctive, they all serve one major purpose, i.e. to provide flexibility to accommodate parts of different sizes and shapes. These fixtures are usually designed for a specific type of product or manufacturing process.

The modular flexible approach has been adopted by many researchers in the past in the development of flexible fixtures [3-11]. These modular fixturing systems usually consist of a number of standard components: a base plate (magnetic/ hole/T-slot), vertical and/or horizontal locators (fixed size/ adjustable size), V-blocks (fixed width/variable width) and clamps (toggle/mechanical/hydraulic/pneumatic). These components can be arranged in different combinations to assemble different fixtures according to the sizes and shapes of parts. All of them require an external device (usually a robot) for layout reconfiguration. In other words, the reconfiguration is done indirectly by programming the external device/robot, and there is no means available to program the fixture operation directly. The other drawback of this approach is that the fixture accuracy would be limited by the external device/robot accuracy, especially for magnetic or T-slot baseplates.

This paper proposes a new approach that combines modu- larity and computer numerical control in the design of flexible fixtures. The major difference of the modular CNC approach to the other modular approaches is that only one single type of standard CNC multifinger module is designed to provide all the locating, supporting and clamping functions. In other words, rather than have a large inventory of locators, V- blocks, clamps, etc., a multiplicity of standard multifunction modules is provided. In addition, computer numerical control of these modules allows direct programming and control of fixture operation without the need of an external device/robot

Development of a CNC Modular Fixture 19

t

Mo%or / W side ~ E side

/

/ ~ ~ E ~ Fingerl Fingem3 iI__q,~ [ ]

I Finger'2 Fin~er4

o

Fig. 1. Multifinger module.

in the system. Therefore, the accuracy of the CNC modular fixture is independent of the accuracy of the external device/ robot. For other modular fixturing approaches, the fixture itself must be reconfigured physically by the external device/ robot or manually whenever there is any change made to the product. The CNC modular fixture can be modified simply by programming the locations of the finger positions to accommodate minor product changes, and thus, the CNC modular approach is more suitable for FMS. Of course, if there is a radical change in the product that is beyond the capacity of the finger movement limits, the fixture has to be redesigned and reconfigured manually. However, since most FMSs are implemented for specific product families, it is anticipated that most changes in products can be accommo- dated by finger adjustments within a particular fixture layout.

In this paper, the first phase of our ongoing development of a CNC modular fixture, which is the machine design of a standard multifinger module, will be presented. The module consists of four fingers with eight degrees of freedom to conform to the surface of a workpiece. The eight motions of the four fingers are controlled by one motor through two transmission and clutch systems. Different reconfigurations are achieved by different combinations of a multiplicity of such modules, and thus high flexibility can be achieved. Numerical control of these modules allows direct programming and control of fixture operations without the need for an external device/robot in the system. The design, operation, and prototype development of this standard module is presented.

2. Multifinger Module

The design of the multifinger module is fundamentally based on the design requirements of a CNC modular fixture. Therefore, the general design requirements of a CNC modular fixture will first be presented. Then the design of the multifinger module will follow.

2.1 Design Requirements of CNC Modular Fixture

Different fixture layouts can be achieved by different combinations of these standard modules. These modules are multifunctional and can be used either as locators or clamps.

2. Surface Conformable. The standard module must be able to conform to different shapes of surfaces at a number of contact points. In other words, the module has adjustable contact points.

3. Programmable. The positions of the contact points of the module must be programmable by a computer. If there is any change in the part, and the change is within the adjustable limits of the contact points, the new set-up can be found automatically (numerical control) without altering the locations of the modules.

4. Reconfigurable. If there is any significant change in the geometry or size of the part, and the change is beyond the limits of the contact points, the fixture must be reconfigured either manually or by a robot. Such fixture reconfiguration should only happen occasionally.

2.2. Structure

The standard multifinger module consists of a motor and a mechanical structure (Fig. 1). The mechanical structure is shown in Fig. 2, and can be divided into two sides. One is the W side that is proximate to the motor, the other is the E side. The structure of the E side and the W side is basically the same (the difference will be discussed in Section 2.4). In the first design, there are two fingers on each side, that is, four fingers in one module. These four fingers are driven by a single motor to move individually in the X and Y directions. In other words, one motor is used to control eight different motions. During fixture reconfigurations, these finger move- ments are adjusted sequentially by the control computer. As indicated in Fig. 2, the mechanical system of each side consists of two parts. One is the finger movement mechanism. The other is the power transmission system which transfers power to the selected finger shaft to drive the finger to move in either the X or Y direction individually.

The design requirements of the CNC modular fixture were set as follows:

1. Modular. The fixture must be composed of standard modules. Only one type of module will be designed.

2.3 Finger Movements

The movements of each finger are controlled by a finger movement mechanism. The detailed structure of the mechan-

20 K. C. Chan and C. S. Lin

Fig. 2. Mechanical structure of multifinger module.

ism is shown in Fig. 3. This mechanism consists of a threaded shaft to control the finger motion along the Y direction, and a square shaft in conjunction with a worm gear set to control the finger motion along the X direction.

2.3. 1 Finger Movements in the X Direction

The finger has an external thread, and fits into the shaft of the worm gear that has an internal thread. The shaft of the worm gear is a press fit into the worm gear which is driven by the worm that is installed on a square shaft. The sequence of operations is that first the square shaft is driven, then the

worm will turn along the Y axis and the worm gear and its shaft will turn along the X axis, and consequently the finger will move forward or backward in the X direction.

The purpose of the worm gear is to resist the reactive force that occurs during a clamping action. This is due to the fact that the worm can transfer power to the worm gear, but the worm gear cannot drive the worm in reverse. Therefore, the reactive force of the clamping does not affect the worm gear set mechanism and an accurate position can be maintained. Furthermore, a thrust bearing is used on the shaft of the worm gear to resist the reactive force of the clamping.

/

~Sc~ppo~-z ra lQck/- - -Wor 'm g e a r

~ / wo~'m 9ec~r

--inge~ ~lg

/ ~ e a r l n g " - - ~ ~nger 4__ ]3~'onze beo, r 'ng

Fig. 3. Finger movement mechanism.

Development of a CNC Modular Fixture 21

I d l e r g e a r

g e a r

F-Q ge

Fa ge

lOLer gear

M~in gear

sleeve

Face gear6

F a c e g e e r 7

Face gear8

Fig. 4. Powertrans~rsystem.

2.3.2 Finger Movements in the Y Direction

The finger, worm and worm gear are installed on a support block that is driven by a threaded shaft. When the threaded shaft is turned, the whole support block including the finger moves in the Y direction.

As Fig. 3 shows, the finger is carried by a square shaft and a threaded shaft. These shafts are driven individually by a motor through the power transmission. The design of the power transmission will be presented in the following section.

2.4 Power Transmission

The purpose of power transmission is to transfer the power of the motor to the individual shaft to control finger movements. Each multifinger module has two power trans- missions that are installed on the E side and W side of the module. The power to the W side is directly provided by the motor; while the power to the E side is transferred from the W side through a round shaft. There are nine shafts in a module. The round shaft for power transfer is common to both sides. Each side has four shafts (two square, two threaded) as drivers for fingers.

The power transmission is explained in terms of three systems. They are the power transfer system, the shaft change system, and the clutch system.

2.4.1 Power Transfer System

The power transfer system is a mechanism that transfers power to the selected shaft. For example on the W side, the system consists of a main gear, an idler gear and several face

gears, as in Fig. 4. The shaft of the motor is extended by a shaft sleeve so that all parts can be assembled. The main gear is fixed on the shaft sleeve of the motor. The face gear 3 is fixed on the idler gear, and face gears 4-8 are installed on the shafts of the four fingers and the round shaft of the power transfer.

When face gear 3 is engaged with one of the face gears 4-8 (by the clutch system described later), the power of the motor will be transferred to the identified shaft through the shaft sleeve, main gear, idler gear, face gear 3 and the face gear of the selected shaft (face gear 5 in the example in Fig. 4).

2.4.2 Shaft Change System

The purpose of the shaft change system is to select the shaft to be driven. This means that when one of the five shafts is chosen, the gear arm will be turned to the position of the identified shaft, as in Fig. 5. The system consists of a pair of face gears 1 and 2, and an idler gear arm. One end of the idler gear arm is fitted to face gear 2, the other end of the idler gear arm is fitted to face gear 3, which is fixed on the idler gear. The subassembly of the idler gear arm is shown in Fig. 6.

The motor drives face gear 1 that is installed on the shaft sleeve of the motor. When face gears 1 and 2 are engaged (by the clutch system described later), the idler gear arm will be rotated by the motor to the position of the identified shaft. Then the ball end pin will prevent the idler gear arm from moving. The ball end pin is also shown in Fig. 6.

22 K. C Chan and C. S, Lin

c~pm ;itlon

T~II ~,A 0,1"

7C~C

]ec

rO.C

Fig. 5. Shaft change system,

Idter IdLer

~ 6' O. K"

Posit ion Betlend pln

Zdenti¢ied fi~ce gear

Face gear3

~rm

Bakkend pln

~ce 9eem2

Fete 9e~rl

Ctukch ~ m

Fig. 6. A subassembly of the idler gear arm.

2. 4.3 Clutch System

The clutch system consists of a clutch arm and two solenoid switches A and B (Fig.7). Solenoid switch A pulls the clutch arm which pushes the idler gear arm to force face gears 1 and 2 to engage. Solenoid switch B pushes the clutch arm which pushes the idler gear arm to force face gears 3 and 5 (or 4, 6, 7, 8) to engage.

2.4.4 Difference Between the E Side and W Side of Power Transmission

The mechanism of movement for each finger is structurally identical. However, the mechanism of power transmission for two sides is a little different, as shown in Fig. 8. The main difference is the power source. The W side is driven directly by the motor, while the E side is driven by a gear which is driven by a transfer shaft from the W side.

2.5 Operation

The structure of the multifinger module and the functions of individual parts have been described above. During the fixture reconfiguration process, each finger is adjusted to the required positions sequentially. To explain the sequence of operations, we use an example in which finger 1 is required to move in the X direction. The procedure is as follows:

1. Select the square shaft which is responsible for transfer of power to move the finger.

Development of a CNC Modular Fixture 23

Solenoid switch A activates, which forces face gears 1 and 2 to engage (Fig. 5).

The motor rotates 30 ° clockwise, which turns the idler gear arm 30 °, so that face gear 3 reaches the position of face gear 4, which is the square shaft of finger 1. Then the position pin will fix the idler gear arm in the right position. The idler gear arm is therefore not moved when the motor is rotating.

Solenoid switch A cuts off, which forces face gears 1 and 2 to release. This released position of the motor will be stored in the memory of the controller as the "previous position of face gear 1".

2. Transfer the power to square shaft.

The motor rotates to the "previous position of face gear 4" of the square shaft which is provided by the controller. Face gears 3 and 4 therefore can be engaged (Fig. 4).

Solenoid switch B activates, and this forces face gears 3 and 4 to engage (Fig. 7). The power can be transferred from the motor through the main gear, idler gear and face gears 3 and 4 to the square shaft.

The motor rotates until the required finger position is achieved.

Solenoid switch B cuts off, which forces face gears 3 and 4 to release. This released position will be stored in the controller as the "previous position of face gear 4".

3. Return of the idler gear arm to the original position.

The motor rotates to the "previous position of face gear 1" in order to allow face gears 1 and 2 to engage. This position has been saved in step 1 above.

Engage o r r e l e a s e Face 9 e a r s 3 & 5

m

Pw DOi

2[U 3.rF

oid . . . . . . . . . .

. . . . . . of 0 swi±ch B

Fig. 7. Clutch system.

24 K. C. Chan and C. S. Lin

W r ' ~ . . . . . . . A z

Fig, 8. Difference between the E side and W side of power transmission.

Solenoid switch A activates, which forces face gears 1 and 2 to engage.

The motor rotates 30 ° counterclockwise, which turns the idler gear arm to the original position. In the mean time, the position pin will fix the idler gear arm in the right position.

Solenoid switch A cuts off, and the whole procedure is finished.

The procedure for the movement of finger 1 in the Y direction and the movements of finger 2 in both the X and Y directions is the same. The only difference is the position of the finger shaft that affects rotation angle of the motor. For fingers 3 and 4, the procedure is basically the same, except that the power is transmitted from the W side through a round shaft, as explained in Section 2.4.

Fig. 9. Prototype of the multifinger module.

z

aYIE E

Hoto r

Fig. 10. CNC machine vice.

2.6. Prototype

A prototype of the multifinger module was built and tested (Fig. 9). The overall performance of the module was satisfactory. The finger-movement mechanism and the trans- mission worked as expected. However, the main problem

Development of a CNC Modular Fixture 25

found was the rigidity of the structure. If the damping force is in the X direction, the module is able to resist the damping force. If the clamping force component in the Y direction is too large and the finger is extended too far, the finger will bend. In general, the muttifinger module is stiff in the X direction but complient in the Y direction. It is proposed to use larger fingers and stronger support blocks for the next prototype.

3. Examples of CNC Fixtures

This section will provide two examples of CNC fixtures using the mulfifinger module. The first is a CNC machine vice and the second is a CNC modular fixture.

3.1 CNC Machine Vice

The design of the CNC machine vice is shown in Fig. 10. The vice consists of two multifinger modules, a baseplate, and a set of quick release cylinder systems, all controlled by a computer. The fingers are first driven to the identified positions conforming to the surface of the workpiece, then the cylinder is actuated to clamp the workpiece securely.

3.2 CNC Modular Fixture for Assembly

The design of the CNC modular fixture is shown in Fig. 11. The system consists of three multifinger modules mounted on

HuLti Finger module ~s Loca%ors

Hut t i -Pingep module as clamp

Workp iece

Cyl inder

Fig. 11. CNC modular fixture for assembly.

26 K. C. Chan and C. S. Lin

a magnetic baseplate, and a set of quick release cylinder systems. This system is intended to be used for families of parts. When there is any change in the part, the fixture can be reconfigured automatically by changing the positions of the fingers. This approach can reduce set-up time significantly. However, if there is any radical change in the product, the fixture must be reconfigured either manually or by a robot.

4. Conclusions

This paper presents the design of a multifinger module for modular CNC fixtures. The design concept is distinct from other approaches presented in the literature. This research was aimed at developing a standard multifinger module so that different fixture layouts can be achieved simply by putting these modules together in different combinations. The multifinger approach was chosen because of its surface conformability and flexibility in accommodating a large variety of parts. The module is computer numerically controlled. Therefore, it is well suited to FMS, and automatic fixture reconfiguration can be achieved without any human inter- vention during the hardware set-up.

The design and operation of the module were presented. A prototype was built and tested and satisfactory performance was achieved. Finally, two examples were provided to illustrate the flexibility and usage of the module.

Acknowledgement

The work was supported by a Faculty Research Grant. Assistance provided by G. Smith and F. Hall in manufacturing the prototype is gratefully acknowledged.

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