GRINDING METHOD AND APPARATUS

Abstract: In a method of and apparatus for grinding a cylindrical workpiece, the speed of the grinding wheel (14) and/or of the workpiece (18) is changed during the final dwell period in which sizing occurs, and the following additional steps are carried out: the rotational speed of the wheel and/or of the workpiece is measured, the target ratio of wheel speed to workpiece speed is calculated, and the speed of the wheel and/or workpiece is adjusted by a servo-controller (22) such that the desired ratio is altered to 0.25 if the mantissa of the target ratio is 0.5 or less, and if the mantissa is above 0.5, it is altered to 0.75.

Description: Title Grinding Method and Apparatus Field of the invention This invention relates to a method of grinding a workpiece having a cylindrical surface and to apparatus for carrying out said method. Background to the invention It is well known that, when a cylindrical workpiece is being ground, the grinding process produces vibrations which generate lobes on the workpiece surface. This phenomenon is commonly known as chattering, and the resulting distortions on the workpiece are known as chatter-marks. To some extent chatter-marks can be reduced by absolute truing of the grinding wheel, but this is time consuming and expensive, especially when the grinding surface is a super-abrasive such as diamond or CBN. It is an object of the present invention to provide a method of and apparatus for reducing the tendency for generation of chatter-marks during grinding. In our UK Patent Application, Publication No GB 2268895A, there is described and claimed a method of grinding a cylindrical workpiece surface in which the speed of rotation of the grinding wheel and/or the speed of rotation of the workpiece is changed one or more times during the final stage of grinding, the period of such rotational speed variation including at least part of the final dwell period in which sizing occurs, thereby to reduce chatter-marks on the workpiece. Summary of the invention It has now been discovered that said method can be further improved by performing the steps of: measuring the speed of rotation of the wheel and/or of the workpiece during said final stage of grinding at each such desired change of speed, calculating the target ratio of the speed of the wheel to the speed of the workpiece and adjusting the speeds of the wheel and/or workpiece in such a manner that if the mantissa or remainder of the target ratio is 0.5 or less the mantissa of the desired ratio is altered to 0.25, and if the mantissa is above 0.5, it is altered to 0.75. In this way it is ensured that the ratio of the or each changed speeds is not a whole integer or in the ratios of the second or third harmonics, and thereby any chatter marks in the workpiece are further reduced. Preferably it is the workpiece speed which is adjusted, while that of the grinding wheel is maintained constant. The invention also extends to apparatus for grinding a cylindrical workpiece surface in accordance with the foregoing method. The method and apparatus of the invention may be applied to internal as well as external grinding. Moreover, it is to be noted that the speed variation effected is not dependent on vibration sensing of the grinding wheel or the workpiece. The rotational speed change is preferably practised by rotational speed change which includes a change in the rotational speed of the workpiece. If the speed variation includes change in the rotational speed of the grinding wheel, any change made will usually be decelerative. Preferably, however, the rotational speed of the grinding wheel is kept constant. The rotational speed change may be initiated either after a predetermined number of rotations of the workpiece where the number of turns per grinding cycle is controlled and/or at a predetermined feed distance before the feed position for final sizing, as indicated in more detail below. Thus, the possibilities for workspeed variation are limitless if its rotational speed is servo-controlled relative to time, grinding wheel feed position, component size and/or number of component turns. The speed change effected may therefore be linear, reduce or increase at a rate relative to the above variables, or be of other controlled forms such as sinusoidal. However, the speed change is normally initiated during the slow approach to size feed and always extends into the final dwell period in which sizing occurs. Description of embodiment The practical implementation of the method and apparatus of the invention is exemplified with reference to the accompanying drawings, in which:- Figure 1 is a graph showing the feed of the grindingwheel, in terms of its distance from the workpiece, against time; and Figure 2 is a much simplified diagram of a grinding machine. Figure 1 is a typical feed diagram for the grinding wheel in a grinding process for grinding a cylindrical workpiece. The grinding wheel feed includes a fine feed period 10 immediately prior to a stationary final dwell period 12 (size dwell) in which sizing occurs. In accordance with the invention, at least one change in the rotational speed of the workpiece is initiated in the fine feed period, and this speed change is extended into at least part of the final dwell period 12. Means are provided for measuring the speed of the grinding wheel immediately prior to a change of speed of the workpiece, and the value of wheel speed is fed into a microprocessor associated with a servo controller for altering the speed of the workpiece. The target speed of the workpiece is also fed into the processor, which then calculates the ratio of the speed of the wheel to that of the workpiece. If the fractional part of the ratio (ie the mantissa) is 0.5 or less, the workpiece speed which was demanded by the operator of the grinding machine is adjusted slightly so that the mantissa becomes 0.25; alternatively if the calculated mantissa is above 0.5 it is adjusted to 0.75. In this manner any tendancy for chatter marks to form on the workpiece is significantly improved, in that the harmonics of the speed ratios of wheel/workpiece up the third harmonics are eliminated. In the simplified grinding machine shown in Figure 2, wherein the grinding wheel is referenced 14, the wheel feed indicated at 16, the workpiece at 18 and the workpiece support at 20, a means 22 is incorporated for changing the rotational speed of the workpiece 18, as aforesaid. The means 22 is preferably constituted by a servo-controller, which automatically initiates a change in the workspeed at the aforesaid time and for the aforesaid period, for example dependently on the feed position, the number of turns of the workpiece in the grinding cycle, and/or component size. The change effected is preferably not sudden, but is a relatively slow change which may be linear, sinusoidal or of any other desired form.

Claims: CLAIMS ; 1. A method of grinding a cylindrical workpiece surface in which the speed of rotation of the grinding wheel and/or the speed of rotation of the workpiece is changed one or more times during the final stage of grinding, the period of such rotational speed variation including at least part of the final dwell period in which sizing occurs, thereby to reduce chatter- marks on the workpiece, characterised by performing the steps of: measuring the speed of rotation of the wheel and/or of the workpiece during said final stage of grinding at each such desired change of speed, calculating the target ratio of the speed of the wheel to the speed of the workpiece, and adjusting the speeds of the wheel and/or workpiece in such a manner that if the mantissa or remainder of the target ratio is 0.5 or less the mantissa of the desired ratio is altered to 0.25, and if the mantissa is above 0.5, it is altered to 0.75. 2. A method according to claim 1 in which the rotational speed variation is practised by a rotational speed change which includes a change in the rotational speed of the workpiece. 3. A method according to claim 1 or claim 2 in which the speed variation includes a change in the rotational speed of the grinding wheel, such speed change being decelerative. 4. A method according to claim 1 in which the workpiece speed is adjusted, while that of the grinding wheel is maintained constant. 5. A method according to any one preceding claim in which the rotational speed variation is initiated either after a predetermined number of rotations of the workpiece, where the number of turns per grinding cycle is controlled, and/or at a predetermined feed distance before the feed position for final sizing . 6. A method according ro any one preceding claim in which the rotational speed of the workpiece is servo-controlled relative to the variables of time, grinding wheel feed position, workpiece size and/or number of workpiece rotations. 7. A method according to any one preceding claim in which the speed change effected is linear, or reduces or increases at a rate relative to said variables, or is of another controlled form such as sinusoidal. 8. A method according to any one preceding claim in which the speed change is initiated during the slow approach to size feed, extending into the final dwell period in which sizing occurs. 9. Apparatus for grinding a cylindrical workpiece surface in accordance with the method of any one preceding claim. 10. A method and apparatus according to any one preceding claim in which internal and/or external grinding is performed on the cylindrical workpiece. 11. A method of grinding a cylindrical workpiece surface substantially as herein described with reference to, and as illustrated in, the accompanying drawings. 12. Apparatus for grinding a cylindrical workpiece surface substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

Inventors: PICKLES JOHN DAVID, PICKLES, JOHN, DAVID
Applicants: PICKLES JOHN DAVID, WESTERN ATLAS UK LTD, PICKLES, JOHN, DAVID, WESTERN ATLAS U.K. LIMITED
Application year: 1995
Main classification: B24B1
IPC codes: B24B1, B24B41, B24B49