GB2193127A

GB2193127A - Dressing of grinding wheels

Info

Publication number: GB2193127A
Application number: GB08716779A
Authority: GB
Inventors: John Malcolm Barnard
Original assignee: Unicorn Industries Ltd
Current assignee: Unicorn Industries Ltd
Priority date: 1986-08-01
Filing date: 1987-07-16
Publication date: 1988-02-03
Also published as: EP0255267A3; GB8716779D0; EP0255267A2

Abstract

In a method of dressing a grinding wheel 14 to form thereon a peripheral surface of a required cross-sectional profile, the grinding wheel is rotated at a predetermined speed, and rotated about a parallel axis is a dressing wheel (10) having a hard peripheral forming surface with a complementary cross-sectional profile. The peripheral surface of the grinding wheel 14 is brought into pressing engagement with the peripheral surface of the dressing wheel 10, and while maintaining the pressing engagement the speed of rotation of the dressing wheel is controlled in a manner to provide a difference in speed between the engaging peripheral surfaces, The speed of rotation may be controlled by an electric servo-motor 12 driving the dressing wheel 10, or by controlled braking of the dressing wheel, For a given composition of grinding wheel and dressing wheel the surface characteristics of the finished wheel may thus be determined by pre-setting the dressing wheel speed. <IMAGE>

Description

1 GB2193127A 1 SPECIFICATION traverse, as well as the force between the

wheels and their relative rates of rotation.

Dressing of grinding wheels However, where the grinding wheel is required to have a non-cylindrical profile relative tra- The invention relates to the dressing of grind- 70 versing movement is not possible and the ing wheels, by which is meant the shaping of shaping of the grinding wheel profile is ef the outer peripheral surface of a grinding fected by a generally crushing or rotary dress wheel to ensure that the wheel is perfectly ing action of the dressing wheel. While such circular and to provide its peripheral surface processes achieve the necessary trueing and with an accurate cross-sectional profile having 75 shaping of the grinding wheel it is difficult to the desired surface characteristics. The procontrol in a predictable manner the condition cess of making the grinding wheel perfectly ing of the surface of the grinding wheel.

circular is normally referred to as---trueing- The present invention sets out to provide a and the process of providing the required sur- new method and apparatus for dressing a face characteristics of the periphery of the 80 grinding wheel, providing a further parameter grinding wheel is normally referred to as which may be varied to vary the conditioning conditioning-. of the surface of the grinding wheel.

The invention is particularly applicable to the According to one aspect of the invention, dressing of so-called -superabrasive- grinding therefore, there is provided a method of wheels, as will be described hereinafter, but it 85 dressing a grinding wheel to form thereon a may also be applied with advantage to the peripheral surface of a required cross-sectional dressing of conventional grinding wheels. profile, the method comprising rotating the Profiled grinding wheels are used for the ac- grinding wheel at a predetermined speed curate shaping of many different types of roabout its central axis, rotating about a parallel tary machine parts, such as engine camshafts. 90 axis a dressing wheel having a hard peripheral A grinding wheel which has been dressed to forming surface the crosssectional profile of perform a certain operation will become worn which is the complementary inverse of said in the course of its use and may require re- required cross-sectional profile, effecting rela dressing many times during its working life to tive movement between the dressing wheel remove worn abrasive material and metal par- 95 and grinding wheel in a direction perpendicular ticles from the peripheral surface of the wheel to their axes of rotation to bring the peripheral as well as to restore the required exact cross- surface of the dressing wheel into pressing sectional profile to the peripheral surface. engagement with the peripheral surface of the Grinding wheels are normally manufactured grinding wheel, maintaining said pressing en- from grains or particles of abrasive material 100 gagement, and controlling the speed of rota- held together and moulded into disc form by a tion of the dressing wheel in a manner to bonding agent. Typical abrasive materials are provide a difference in speed between said aluminium oxide and silicon carbide. Various engaging peripheral surfaces of the dressing bonding agents are used such as synthetic re- wheel and grinding wheel. Preferably the sins, metals or vitrified materials. 105 dressing wheel is controlled in such manner The peripheral suace of the dressing wheel that its peripheral speed is less than the per must obviously be formed from material which ipheral speed of the grinding wheel.

can cut or abrade, without undue wear itself, It is found, using this method, that the sur the hard surface of the grinding wheel, Typi- face conditioning of the grinding wheel may cally, therefore, dressing wheels consist of 110 be varied by adjusting the speed of rotation of diamond particles embedded in a hard material the dressing wheel and thereby varying the such as nickel, tungsten carbide or other hard speed difference between the engaging sur matrix. faces of the wheels. Accordingly, for a given It has become increasingly common, in composition of grinding wheel and dressing some applications, to use superabrasive grind- 115 wheel, the surface characteristics of the fin ing wheels in which the abrasive material is ished wheel may be determined in a predicta diamond or cubic boron nitride, for example in ble fashion by setting an appropriate predeter a vitrified bonding agent, and difficulties have mined speed of rotation for the dressing arisen in conditioning such wheels using con- wheel. The effect of the surface speed differ ventiopal methods. In the case where the pro- 120 ence will be to cause rotational -scuffing- of file of the grinding wheel is flat, i.e. where the the surface of the dressing wheel relatively to peripheral surface of the wheel is perfectly the surface of the grinding wheel, in addition cylindrical, acceptable conditioning of the sur- to the normal action. It is this scuffing, the face has been achieved by traversing the interextent of which depends, for given wheels, on engaging surfaces of the two wheels relatively 125 the predetermined value of the speed of rota to one another in the axial direction while they tion of the dressing wheel, which determines are rotated in engagement with one another. the surface characteristics of the finished The surfa ce characteristics of the grinding wheel.

wheel, that is to say its conditioning, may As previously mentioned, the invention is then be varied by varying the relative rate of 130 particularly applicable where the grinding 2 GB2193127A 2 wheel is a superabrasive wheel comprising, for from said speed sensing means in a manner example, cubic boron nitride or diamond in a to tend to return the speed of rotation to vitrified bonding agent, and at least the per- wards a predetermined value when it departs ipheral surface of the dressing wheel may from said value.

comprise diamond particles embedded in a 70 In the case where the speed of rotation of bonding material, such as nickel or tungsten the dressing wheel is controlled by brake carbide. means, the means for rotating the dressing The speed of rotation of the dressing wheel wheel about its central axis may comprise a may be controlled by driving the wheel by rotatably mounted shaft having means for means of a speed-controllable electric servo- 75 mounting the dressing wheel coaxially thereon, motor. The speed of rotation of the servo- the brake means being operable on said shaft motor may then be continuously controlled so or a part rotatable therewith. For example, the as to maintain the speed of rotation of the brake means may comprise a disc brake dressing wheel at a substantially constant pre- mechanism, the disc of which is mounted on determined value. Alternatively, the speed of 80 the shaft or part rotatable therewith.

rotation of the servo-motor, and hence of the The brake mechanism may be pneumatically dressing wheel, may be varied in a cyclic actuated, and in this case said control means fashion. may include an air control valve which con- In an alternative arrangement according to trols the supply of pressurised air to the brake the invention, the speed of rotation of the 85 mechanism.

dressing wheel may be controlled by applying The control means may further include a dif thereto a braking load when the speed of ro- ferential control device for comparing signals tation of the dressing wheel is greater than a from the speed sensor with a pre-set datum predetermined value, the magnitude of the indicative of said required predetermined braking load being variable and being such as 90 speed, and actuating said air control valve in a to tend to reduce the speed of rotation of the manner to reduce the difference between said dressing wheel towards said predetermined signals and the datum.

value. Preferably the air control valve is solenoid The pressing engagement between the peri- operated and is actuated by electrical signals pheries of the grinding wheel and dressing 95 from said differential control device.

wheel may be maintained, in known manner, The means for rotating the dressing wheel by feeding one wheel towards the other at a about its central axis may include an electric predetermined rate. In accordance with a fur- motor drivingly connected to the shaft on ther feature of the present invention, however, which the dressing wheel is mounted. In this this feed rate may be controlled in accordance 100 case the differential control device is prefera with the speed of rotation of the dressing bly also connected to the electric motor and wheel, in such manner that the feed occurs at is arranged to switch off the motor when the a first rate when the speed of rotation of the speed of rotation of the dressing wheel rises dressing wheel is below said predetermined above the aforesaid predetermined value.

value, and at a second, lower rate when the 105 The following is a morp detailed description speed of rotation ii above said value. Thus, of embodiments of the invention, by way of the speed of rotatin of the dressing wheel example, reference being made to the accom will be at or below the predetermined value panying drawings in which:

before the wheels are brought into engage- Figure 1 is a diagrammatic representation of ment and the Wheels may thus be fed to- 110 one form of apparatus in accordance with the wards each other comparatively rapidly. How- invention, ever, as soon as the wheels come into en- Figure 2 is a similar repres entation of an gagement the grinding wheel will accelerate alternative form of apparatus, and the dressing wheel above the. predetermined Figure 3 is a sectional view through appara- rate of rotation with the result that the feed 115 tus in accordance with Figure 2 for mounting, rate will be switched to a lower value appro- driving and braking the dressing wheel, priate for the wheels being in operative enReferring to Figure 1, a dressing wheel 10 gagement. is mounted on the end of a rotary shaft 11 The invention also provides apparatus for which is driven by an electric servo-motor 12 dressing a grinding wheel comprising means 120 through a suitable coupling.

for rotating the grinding wheel at a predeter- The grinding wheel 14 to be dressed is mined speed about its central axis, means for mounted on a rotatable shaft 15 driven by an rotating a dressing wheel about a parallel axis, electric motor 16. The dressing wheel 10 and means for effecting relative movement be- grinding wheel 14 are rotatable about parallel tween the dressing wheel and-grinding wheel 125 axes and the assembly comprising the grinding in a direction perpendicular to their axes of wheel and its motor is so mounted that the rotation, means for sensing the speed of rota- grinding wheel may be fed towards the dress tion of the dressing wheel, and control means ing wheel in a direction perpendicular to the for controlling the speed of rotation of the axes of rotation of the two wheels. The feed dressing wheel in accordance with signals 130 ing mechanism, which is electrically driven, 3 GB2193127A 3 may be of any well known kind and will not ranged to control the rate of feed of the be described in further dbtail. grinding wheel towards the dressing wheel so Adjacent the periphery of the dressing that the feed rate drops, as soon as the wheel 10 is a speed sensor 17 which is elec- dressing wheel speeds up over 750 rpm, from trically connected to a differential control dean initial rapid indexing rate to a lower operat vice 18 to which it sends signals indicative of ing rate.

the speed of rotation of the dressing wheel. A Alternatively, or additionally, the rate of feed revolution counter 19 is connected to the con- of the grinding wheel may be controlled in trol device to indicate the speed of rotation. accordance with the positions of the axes of The differential control device is connected to 75 rotation of the grinding wheel and dressing the electric servo-motor 12 driving the dress- wheel, the rate of feed being reduced when ing wheel 10 and is arranged to maintain the the axes of the wheels reach a position where speed of rotation of the dressing wheel at a the peripheries of the wheels will be in con predetermined value. This value is adjustable tact. The relative positions of the axes of ro and may be pre-set in the differential control 80 tation of the wheels may be sensed, for ex device. Alternatively, the control device may arrimple, by using an optical grating, linear vol be such that it can be pre-programmed to tage differential transformer (LVDT), capacitive vary the speed of rotation of the dressing transducers, inductosyn transducers, or the wheel in cyclic fashion. like. The signals from such devices are then The arrangement is such that when the 85 used to control the rate of feed of the grind- speed of rotation of the dressing wheel 10 ing wheel, so that the feed rate is reduced departs from the value pre-set in the differen- when a predetermined relative position of the tial control device, as determined by the de- wheels is reached.

vice comparing signals from the speed sensor During continued operation the grinding 17 with the pre-set value, the servo-motor 12 90 wheel 14 constantly tries to rotate the dress is automatically controlled in a manner to re- ing wheel 10 at a speed greater than the pre store the speed of the dressing wheel to the determined 750 rpm, and the servo-motor 12 pre-set value. Servo-motors and control sys- is constantly controlled to maintain the pre-set tems to operate in this fashion are readily speed.

available commercially and the control system 95 As the grinding wheel 14 is fed towards the will not therefore be described in detail. dressing wheel 10, the dressing wheel acts In operation, the electric motors 12 and 16 on the periphery of the grinding wheel with a are initially switched on. The dressing wheel crushing/dressing action so as to "true" the 10, while it is not subjected to any external wheel, Le. render it perfectly circular, and to load, then rotates at the predetermined speed 100 shape the profile of the peripheral surface of set in the differential control device which the wheel in accordance with the profile of may, for example, be 750 rpm. The grinding the peripheral surface of the dressing wheel.

wheel 14 may, for example, be rotating at a At the same time, however, the lower per speed of a 1000 rpm. ipheral speed of the dressing wheel provides a Once the dressing wheel and grinding wheel 105 scuffing action between the peripheral surface are rotating the grinding wheel 14 is fed to- of the dressing wheel and the peripheral sur wards the dressing wheel until the peripheries face of the grinding wheel urged against it.

of the two wheels pome into contact. When The surface characteristics of the grinding contact occurs the grinding wheel increases wheel produced by this combined crushing the speed of rotation of the dressing wheel 110 and scuffing action vary according to the ex above 750 rpm. (The relative diameters of the tent of the scuffing action, and this in turn wheels may be such that if the dressing wheel depends on the pre-set rotational speed of the were freely driven by the grinding wheel it dressing wheel. Accordingly, for a given type would reach a speed of something over 3000 of dressing wheel and grinding wheel, and a rpm). However, as Soon as the speed of rota- 115 given rotational speed of the grinding wheel, tion of the -dressing: wheel exceeds the predet- the surface characteristics of the finished ermined 750 rpm the differential control de- grinding wheel can be varied in a predictable vice 18 responds to reduce the speed of the fashion by adjusting the pre- set rotational servo-motor and hence of the dressing wheel. speed of the dressing wheel or by varying the The speed sensor 17 may be of any suit- 120 speed in a cyclic fashion.

able form. For example, it may operate by The invention thus allows predictable condi- optical sensing of a shaft encoder or by opti- tioning of-the grinding surface of a grinding cal sensing of the wheel periphery using a wheel where the profile of the surface, and/or stroboscope. Alternatively the sensor may use the material of the wheel, makes such predic- one or more transducers such as.accelero- 125 table conditioning difficult or impossible using meters, piezo-electric strain gauges or capaci- conventional methods.

tive transducers arranged so that the signals In the alternative arrangement of Figure 2 therefrom vary in accordance with the speed the speed of rotation of the dressing wheel of rotation of the dressing wheel. 10 is controlled by a brake mechanism 13 The differential control device is also ar- 130connected between the electric motor 12 and 4 GB2193127A 4 the shaft 11 of the dressing wheel, and not feed of the grinding wheel may also be con by direct control of the electric motor 12. trolled in accordance with the positions of the Corresponding components of the two ar- axes of rotation of the wheels.

rangements are indicated by the same refer- During continued operation the grinding ence numerals. 70 wheel 14 constantly tries to rotate the dress In the Figure 2 arrangement the differential ing wheel 10 at a speed greater than the pre control device 18 is again connected to the determined 750 rpm, and the brakes are con electric motor 12 driving the dressing wheel stantly applied to the dressing wheel to re and is arranged to switch off the motor duce its rotational speed. The control of the when the speed of rotation of the dressing 75 brakes is such that the braking force increases wheel reaches or exceeds a predetermined in dependence on the excess speed of the value. As before, this value is adjustable and dressing wheel.

may be pre-set in the differential control de- The braking of the dressing wheel, and the vice. hunting effect of the dressing wheel being In the modified arrangement of Figure 2, 80 constantly braked towards the pre-set rota- however, the differential control device 18 tional speed, provides the aforementioned scu also controls a solenoid actuated air control ffing action between the peripheral surface of valve 20 which controls the supply of air from. the dressing wheel and the peripheral surface an air supply source 21 through a regulator of the grinding wheel urged against it. Accord- 22 to a manifold 23 which distributes the air 85 ingly, for a given type of dressing wheel and supply to a number of pneumatically operated grinding wheel, the surface characteristics of brakes in the coupling and brake apparatus 13 the finished grinding wheel can again be associated with the drive to the dressing varied in a predictable fashion by adjusting the wheel 10. The pneumatically operated brakes, pre-set rotational speed of the dressing wheel.

of which there may be any suitable number, 90 Figure 3 shows in greater detail a typical are indicated diagrammatically at 24. construction for the driving and braking appa The arrangement is such that when the ratus for the dressing wheel in the embodi speed of rotation of the dressing wheel 10 is ment of Figure 2.

greater than the value pre-set in the differen- Referring to Figure 3, the dressing wheel 10 tial control device, as determined by the de- 95 is mounted on one end of the shaft 11 the vice comparing signals from the speed sensor opposite ends of which are rotatably mounted 17 with the pre-set value, the air control valve in bearings 25 and 26 in a casing 27.

is operated so as to actuate the brakes 24 The end of the shaft 11 opposite to the and thus reduce the speed of the dressing dressing wheel 10 is connected by a coupling wheel 10. 100 28 to the output shaft of the aforementioned In operation, the electric motors 12 and 16 electric motor 12. Adjacent the coupling 28 are initially switched on. The dressing wheel there is mounted on the shaft 11 the disc 10, while it is not subjected to any external plate assembly of a pneumatically actuated load, then rotates at the predetermined speed disc brake mechanism. One of the pneumati- set in the differential control device which 105 cally actuated brake pad mechanisms of the may, as in the previous embodiment, be 750 disc brake is indicated at 30 and a suitable rpm. The grinding wheel 14 may, as before, number of such brake pad assemblies are dis be rotating at a speed of a 1000 rpm. posed around the periphery of the disc. The Once the dressing wheel and grind.ing wheel brake mechanism may be of a kind which is are rotating the grinding wheel 14 is fed to- 110 commercially available where the number of wards the dressing wheel until the peripheries brake pad assemblies may be selected accord of the two wheels come into contact. When ing to the braking power required. As previ contact occurs the grinding wheel increases ously mentioned, the brake mechanism is pre the speed of rotation of the dressing wheel ferably of a kind where the braking torque is above 750 rpm. However, as soon as the 115 proportional to the operating air pressure.

speed of rotation of the dressing wheel ex

Claims

ceeds the predetermined 750 rpm the differ- CLAIMS ential control device

18 responds and swit- 1. A method of dressing a grinding wheel ches off the electric motor 12 and operates to form thereon a peripheral surface of a re the valve 20 to actuate the pneumatic brakes 120 quired cross-sectional profile, the method and apply a braking load to the dressing comprising rotating the grinding wheel at a wheel. predetermined speed about its central axis, ro.

As in the previous embodiment, the differ- tating about a parallel axis a dressing wheel ential control device is also arranged to con- having a hard peripheral forming surface the trol the rate of feed of the grinding wheel 125 cross-sectional profile of which is the comple towards the dressing wheel so that the feed mentary inverse of said required cross-sec rate drops as soonas the dressing Wheel tional profile, effecting relative movement be speeds up over 750 rpm, from an initial rapid tween the dressing wheel and grinding wheel indexing rate to a lower operaling rate. Also, in a direction perpendicular to their axes of as in the previous embodiment, the rate of 130 rotation to bring the peripheral surface of the GB2193127A 5 dressing wheel into pressing engagement with occurs at a first rate as the wheels approach the peripheral surface of the grinding wheel, a predetermined relative position, and at a maintaining said pressing engagement, and second, lower rate after the wheels have controlling the speed of rotation of the dress- reached said position.

ing wheel in a manner to provide a difference 70 11. Apparatus for dressing a grinding wheel in speed between ' said engaging peripheral comprising means for rotating the grinding surfaces of the dressing wheel and grinding wheel at a predetermined speed about its cen wheel. tral axis, means for rotating a dressing wheel
2. A method according to Claim 1, wherein about a parallel axis, means for effecting rela- the dressing wheel is controlled in such man- 75 tive movement between the dressing wheel ner that its peripheral speed is less than the and grinding wheel in a direction perpendicular peripheral speed of the grinding wheel. to their axes of rotation, means for sensing
3. A method according to Claim 1 or Claim the speed of rotation of the dressing wheel, 2, wherein at least the peripheral surface of and control means for controlling the speed of the dressing wheel comprises diamond par- 80 rotation of the dressing wheel in accordance ticles embedded in a bonding material. with signals from said speed sending means
4. A method according to Claim 3, wherein in a manner to tend to return the speed of the bonding material is selected from nickel rotation towards a predetermined value when and tungsten carbide. it departs from said value.
5. A method according ' to any of Claims 1 85 12. Apparatus according to Claim 11, to 4, wherein the speed of rotation of the wherein the means for rotating the dressing dressing wheel is controlled by driving the wheel about its central axis comprise a rotata wheel by means of a speedcontrollable electric bly mounted shaft having means for mounting servo-motor. the dressing wheel coaxially thereon, brake
6. A method according to Claim 5, wherein 90 means being operable on a part rotatable with the speed of rotation of the servo-motor is said shaft.

continuously controlled so as to maintain the 13. Apparatus according to Claim 12, speed of rotation of the dressing wheel at a wherein the brake means comprise a disc substantially constant predetermined value. brake mechanism, the disc of which is
7. A method according to Claim 6, wherein 95 mounted on the shaft.

the speed of rotation of the servo-motor, and 14. Apparatus according to Claim 13, hence of the dressing wheel, is varied in a wherein the brake mechanism is pneumatically cyclic fashion. actuated, said control means including an air
8. A method according to any of Claims 1 control valve which controls the supply of to 4, wherein the speed of rotation of the 100 pressurised air to the brake mechanism.

dressing wheel is controlled by applying 15. Apparatus according to Claim 14, thereto a braking load when the speed of ro- wherein the control means further include a tation of the dressing wheel is greate_r than a differential control device for comparing sig predetermined value, the magnitude of the nals from the speed sensor with a pre-set braking load being variable and being such as 105 datum indicative of said required predeter to tend to reduce the speed of rotation of the mined speed, and actuating said air control dressing Wheel towards said predetermined valve in a manner to reduce the difference value. between said signals and the datum.
9. A method according to any of Claims 1 16. Apparatus according to Claim 14, to 8, wherein the pressing engagement be- 110 wherein the air control valve is solenoid oper tween the peripheries of the grinding wheel ated and is actuated by electrical signals from and dressing wheel is maintained by feeding said differential control device.

one wheel towards the other at a predeter- 17. Apparatus according to Claim 15, mined rate, the feed rate being controlled in wherein the means for rotating the dressing accordance with the speed of rotation of the 115 wheel about its central axis include an electric dressing wheel, in such manner that the feed motor drivingly connected to the shaft on occurs at a first rate when the speed of rota- which the dressing wheel is mounted, and the tion of the dressing wheel is below said pre- differential control device is also connected to determined value, and at a second, lower rate the electric motor and is arranged to switch when the speed of rotation is above said 120 off the motor when the speed of rotation of value. the dressing wheel rises above the aforesaid
10. A method according to any of Claims 1 predetermined value.

to 9, wherein the pressing engagement be- 18. A method of dressing a grinding wheel tween the peripheries of the grinding wheel substantially as hereinbefore described with and dressing wheel is maintained by feeding 125 reference to Figure 1 or Figures 2 and 3 of one wheel towards the other at a predeter- the accompanying drawings.

mined rate, the feed rate being controlled in 19. Apparatus for dressing a grinding wheel accordance with the relative positions of the substantially as hereinbefore described with axes of rotation of the grinding wheel and reference to Figure 1 or Figures 2 and 3 of dressing wheel, in such manner that the feed 130 the accompanying drawings.

6 GB2193127A 6 Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.