GB2183515A - Centreless grinding - Google Patents

Abstract

Centreless grinding tends to produce lobed profiles. This is controllable by adjusting machine parameters during grinding. Thus a workpiece is supported by a blade 8 and contacted by a grinding wheel 1 and a regulating wheel 2. Sensors 3, 4 monitor the diameters of the wheels 1, 2 and provide inputs to a computer 5 which controls the height and/or angle of the blade 8. Thus the computer may control two adjusting motors 7, 10 which rotate threaded shafts 6, 11 which engage complementary bores in displacement means coupled to a blade carrier to effect, respectively, vertical displacement and pivoting about a horizontal axis. For vertical displacement the shaft 6 acts to displace a wedge having a cam surface which engages the blade carrier and is profiled to provide a desired displacement ratio. <IMAGE>

Description

SPECIFICATION Centreless grinding This invention relates to a device and methodforuse in centreless grinding, particularlyforcontrolling the contact geometry.

In centreless grinding, a componentW is supported on a fixed blade X and rotated by frictional contact with a regulating wheel Yas shown in Figure 1.

The contact geometry may be defined by two vari ables -'ee',the blade angle relative to the tangent line 'G' of component and grinding wheel Z, and 'E', the angle between the tangent lines 'G' and 'R' ofthe grinding and regulating wheels respectively.

This contact geometry has been determined in practice be setting the component height 'h' relative to the line connecting the centres Cz, Cy ofthe grinding and regulating wheels. The bestsetting height for a particular size of component is determined em pirically by a trial and error process.

It is observed that components produced by the centreless grinding process are not, in general,truly circular but lobed in profile. An exaggerated profile envelope is shown in Figure 2. The frequency and amplitude of lobing is variable but is known to be influenced by the blade angle and height setting.

On a long production run the contact geometry may change owing to wearofthe grinding and regulating wheels, with the result that components produced at the beginning of a run may be acceptable while those produced later, having a different load profile, may be unacceptable.

The underlying causesforthis behaviour are not fully appreciated by the industry in spite ofthefact that research has shown that the lobe generation mechanism in centreless grinding can be modelled mathematically.

We have established theoretically the V process is susceptible to instabilityovera much wider range of contact geometries than was previously realised, andthatthefinal profile depends very critically on the initial component profile and on parameters which may change significantly during a production run. Consequently we base our proposed control strategies on a new method of interpreting the mathematical model as follows.

It is convenientto express Figure 1 in a form in which machine parameters can be more easily recognised (Figure3). The absolute shoe or blade angle is defined as (3,the grinding and regulating contact ang les as Eg, Er respectively and the radii of grinding wheel, regulating wheel and component as rg, rr and rc respectively. The effect of a change in rg or rr is shown in Figure 4to result in changes in e9 or er which alterthe contact geometry.

The effect of altering the above parameters on the overall stability of the process is shown in Figure 5 for a blade angle 150 r 80. Stability boundaries have been calculated forthe various lobe frequencies shown. An operating line can be plotted for given values of rg, rr and rc. This line AB is an example of a lineforwhich rg = 183, rr = 128 and rc = 25 units. A possible operating point X, for h = 30 units, lies on this line. If rg and rrare reduced to 256 and 165 units, the operating line rotates to position CD and the new operating point is at 'Y'.The stability diagram indi catesthata change in lobe frequency from 25to 17 lobeswould be expected. PointZ indicates a stable region. Such regions are defined as exteriorto ellipti cal boundaries.

It would be useful to provide a means by which the most stable grinding configuration can be pred etermined given r9, rr, rc and ss.

It would also be useful to provide a means of con trolling angles 'a' and 'e' in Figure 1 to compensate for changes in rg and rw. For most practical purposes this can best be achieved by controlling height 'h'.

Itwould also be useful to be ableto interfere with the feedback mechanism in orderto inhibit the growth of unstable lobe frequencies at any blade angle or height setting. Our research indicates that this may be achieved by oscillation of the blade aboutthe component axis.

It would also be useful to be able to determineac curatelythefeed/rev and numberofgrinding revolutions required for a particularcomponentto attain maximum roundness, given the rotational speed and finish grinding allowance, since we have foundthattheformer parameters also depend on 'a' and 'e'.

Preferred embodiments ofthe invention enable one or more ofthese advantageous resultsto beobtained.

According to the present invention there is provided a centreless grinding machine comprising a grinding wheel, a regulating wheel and a blade for supporting a component between the grinding wheel and the regulating wheel; the heightofthe blade relative to a line between the centres ofthe grinding wheel and the regulating wheel and/orthe angle of inclination ofthe blade being adjustable during operation ofthe machine.

Preferably, means are provided for adjusting the heightand/ortheangleofthe bladeautomaticallyin response to sensed parameters of the machine. The sensed parameters may be the diameters of the grinding and regulating wheels. The machine may therefore have sensorsforsensing the diameters of the grinding and regulating wheels, drive meansfor adjusting the height and/or angle ofthe blade, and control means for controlling the drive means in response to signals from the sensors.

Another aspect of the present invention provides a method of controlling a centreless grinding machine comprising a grinding wheel, a regulating wheel and a blade for supporting a component between the grinding wheel and the regulating wheel,the method comprising adjusting the height and/orthe angle ofthe blade during operation ofthe machine, in a periodicfashion in orderto interferewiththeun- stable feedback mechanism and prevent growth of unstable lobe frequencies.

Preferably, this method comprises the steps of: (a) providing data representing the stability or instability of the grinding process as a function of parameters ofthe process, one of these parameters being the height and/or angle of the blade; (b) sensing at least one ofthe other parameters; and (c) adjusting the height and/or angle ofthe blade in response to the other parameter or parameters, therebyto avoidorameliorateworsening ofthest ability of the grinding process as one or more ofthe other parameters changes.

In a third aspectthe invention provides a support/ adjustment assemblyforthe supporting centreless grinding machine, particularly for use with a machine and/or in a method according to the earlier aspects.

Fora better understanding ofthe presentinvention, and to show more clearly how it may be carried into effect, some embodiments will now be described with reference to Figures 6 to 8 of the ac- companying drawings, of which drawings: Figures 1 to Shave already been described; Figure 6 is a schematic diagram showing the elements of a contact geometry control system emb odying the invention; Figures 7A and B are front and side elevations of meansforvertical blade height adjustment either pre-process, in-process or post-process in an embodimentofthe invention; and Figures BA andBarefrontand side elevations of meansforbladeangleadjustmentand/orheight adjustment either pre-process, in-process or postprocess in a second embodiment of the invention.

Figure 6 shows a control system which embodies the invention and comprises a grinding machine (not shown) containing a grinding wheel 1 and regulating wheel 2, and a blade support system 9 comprising an angle sliding and/or rotating blade 8. Two tra nsducers 3 and 4, which may be attached to the dressing mechanismorotherpartofthemachine, monitor the wheel diameters and provide electrical signals to a microcomputer or microprocessor 5.

This provides control outputs to motors 7 and 10 which are arranged with suitable drive systems 6 and 11 and gear boxes 22 and 23 to effectthedesired change in the blade height 'h' or blade angle 'ss' respectively.

Element 5 is provided with software orfirmware which can determine, by appropriate algorithms, what action should betaken on receiptofthe initial data, inputfrom a keyboard, or changes in signals from transducers 3 and 4.

In the embodiment shown in Figure 7, the base element9 is slotted to receive a wedge-like sliding member 21 which is rotatably attached via antibacklash coupling 17 to the screwed drive 6, which in turn is rotatably attached to a gearbox 22, which in turn is rotatably attached to the motor 7. An upper slot in element9 locates a sliding block 12 which is slotted to receive the blade 8 and attached to locking blocks 19 with fasteners 20. The geometry of the upper end of blade 8 is determined bythemachine configuration butthe lower end is dimensioned to slide vertically in element 12. A slotted plate 13 guidesthe blade in element 12 while allowing movement in the vertical direction. A pin 14fixed to element8 projects into element 13 and engages the upper end of spring 16.Afurther pin 15 fixed to element 13 engages the lower end of spring 16. Provision ismadeforattaching blocks similarto element 12 or of different function in other positions along the slot in element 9, whilst maintaining an accurate relationship with the first block.

Vertical displacement of blade element 8 is achieved by mechanical displacement of the wedge (element 21) which is in sliding contact with the specially shaped lower end of the blade. This dis placement is effected by rotation ofthe motor7 and the associated gearbox 22 and drive system 6. The wedge element 21 is specially profiled to provide the correct displacement ratio with element8 and is gui ded and sealed with a cover plate 18.

Afurther embodiment is described in accordance with Figure 8. Assembly 24 comprises elements sim ilarto those described in the first embodiment, Figure 7, but is provided with pivots at the upper end and an extension at the lower end. Each pivot is loc ated in a bearing 26 contained within a block 27 which is constrained to slide vertically in the base unit 25. Fine adjustment for the pivot axis is provided by adjusting screw 29 which is rotatably attached to block 27 and secured to it bya pin 30. The vertical height of the pivot axis of assembly may be locked by means of nuts 28 whilst allowing free angulardis placement ofthe assembly about the pivot axis.

The lower extension of assembly 24 is equipped with a block 31 which is guided vertically in the extension by a slot and cover plate 32. A screwed drive shaft 11 is rotatably attached to block 31 via trunnion 33 and passes through a furtherslot in the extension.

The drive shaft is in turn rotatably attached to gearbox 23, which in turn is rotatably attached to motor 10 which is fixed to the base element 25.

It will be appreciated that initial height'h'setting can now be achieved by vertical adjustment ofthe pivot axis, whilst accommodation forcomponents of different radii is incorporated in the design of blade element 8 and the nominal radial positionofitssup- port face relative to the pivot axis. The embodiment is thus capable of several different modes of operation as follows.

1. With drive shaft 6 stationary, drive shaft 11 is rotated via motor 10 and gearbox 23 in responseto signals from the control unit5 (Figure 6), and assembly 24 rotates aboutthe pivot axis, which is also the component axis, to a new blade angle.

2. As 1butwith reciprocation ofthe blade angle movement between limits.

3. With drive shaft 11 stationary, drive shaft 6 is rotated to effect changes in blade heig ht as described in thefirst embodiment, Figiure7,thusdisplac- ing the component axis from the pivot axis.

4. With component and pivot non-coaxial, rotation ofthe drive shaft 11, as in mode 1, in one direction will cause a decrease in component diameter; and in the other direction an interruption to the grinding action. Mode 4 is therefore a convenient means bywhich component sizing and processtimecan be controlled.

It should be noted that the embodiments described above are onlytwo of many possible and the precise arrangement and specification ofthe elements is dependent on the application of the invention. In addition it will also be appreciated that the successful application of the device will require some interaction between its control system and that of the machine to which it is applied.

Claims (14)

1. A centreless grinding machine comprising a grinding wheel, a regulating wheel and a blade for supporting a component between the grinding wheel and the regulating wheel; the height of the blade relative to a line between the centres of the grinding wheel and the regulating wheel and/orthe angle of inclination of the blade being adjustable during operation of the machine.

2. Acentreless grinding machine according to claim 1 including means for adjusting the height and/ orthe angle of the blade automatically in response to sensed parameters ofthe machine.

3. A centreless grinding machine according to claim 2 in which said sensed parameters comprise the diameters ofthe grinding and regulating wheels.

4. A centreless grinding machine according to claim 3 including sensors for sensing the diameters ofthe grinding and regulating wheels, drive means for adjusting the height and/or angle ofthe blade, and control means for controlling the drive means in responsetosignalsfromthesensors.

5. A centreless grinding machine according to any preceding claim having means for adjusting the height ofthe supporting blade comprising horizontallydisplaceablewedgemeanswhich provideacam surface coupled to the blade so that horizontal displacement of the wedge means urges displacement of the blade in the height direction.

6. A centreless grinding machine according to claim 5 having wedge displacement means comprising a horizontally extending rotaryshaftthreadedly engaged with the wedge means, and a motor coupledtotheshaftto provide controllable rotation.

7. A centreless grinding machine according to claim 5 or 6 having locking means for locking the blade at a desired height.

8. Acentrelessgrinding machine according to any preceding claim having means for adjusting the angle ofthe supporting blade comprising a blade carrier pivotally mounted with a horizontally extending axis; and displacement means coupled to the carrierforeffecting controlled pivoting thereof.

9. A centreless grinding machine substantially as any herein described with reference to and as il lustrated in Figures 5 to 8 of the accom panying drawings.

10. Amethodofcontrolling acentrelessgrinding machine comprising a grinding wheel, a regulating wheelandabladeforsupporting acomponentbetween the grinding wheel and the regulating wheel, the method comprising adjusting the height and/or the angle of the blade during operation of the machine, in a periodic fashion in orderto interfere with the unstable feedback mechanism and prevent growth of unstable lobe frequencies.

11. A method according to claim 10 which comprises the steps of: (a) providing data representing the stability or instability of the grinding process as a function of parameters ofthe process, one ofthese parameters being the heightand/orangle ofthe blade; (b) sensing at least one ofthe other parameters; and (c) adjusting the heightand/orangle ofthe blade in response to the other parameter or parameters, therebytoavoidorameliorateworsening ofthestability ofthe grinding process as one or more ofthe other parameters changes.

12. A method of controlling a centreless grinding machine substantially as any herein described with reference to and as illustrated in Figures 5 to 8 of the accompanying drawings.

13. Asupport/adjustmentassemblyforthesup- porting blade of a centreless grinding machine comprising a grinding wheel, a regulating wheel and a blade for supporting a component between the grinding wheel and the regulating wheel such that the heightofthe blade relativetoa line between the cen tresofthegrindingwheel andthe regulating wheel and/orthe angle of inclination of the blade is adjustable during operation of the machine.

14. Asupport/adjustment assembly according to claim 13 for use in a machine according to any of claims 1 to 9 and/or in a method according to any of claims 10to 12.