IMPROVEMENTS IN OR RELATING TO GRINDING

Abstract: A method and apparatus for grinding a cam (12) with a cam lobe is provided. Rotation of the cam (12) is synchronised with the infeed/outfeed movement of a grinding wheel (10). Control means measures the relative rotational and lateral positions of the cam (12) and grinding wheel (10). The movement of the grinding wheel (10) relative to the cam (12) is controlled to roughly grind a desired cam profile. Finish grinding may be by way of grinding wheels or abrasive belt. Surface hardening may be applied to the ground cam before the finish grinding step.

Description: Title Improvements in or relating to Grinding Field of the invention This invention relates to a method of grinding, and more especially to the rough grinding of a workpiece having one or more lobes, such as a camshaft cam, and also to a grinding machine for performing such method. Background to the invention Rough grinding of fixed workpieces, such as cams on a camshaft, is conventionally carried out by a progressive infeed/outfeed movement of the grinding wheel of increasing depth, so that parts of the workpiece requiring maximum grinding are only worked upon at the end of the progression of feed movements. In addition to being very time consuming, the conventional method has the disadvantage that a retraction mark is left on the workpiece at the point where the wheel and workpiece finally separate, especially since the workpiece profile at the point of retraction will be a point of maximum depth of grinding. It is an object of this invention to provide an improved method of grinding which can overcome the above-described disadvantages of the conventional technique. The invention According to one aspect of the invention, there is provided a method of rough grinding a cam within a maximum of two rotations of a workpiece relative to a grinding wheel, wherein rotation of the cam and operation of the cam/grinding wheel infeed/outfeed are synchronised to cause the grinding wheel first to contact the workpiece ahead of the intended position of the peak of the cam lobe therearound, to engage the workpiece and as infeed continues, to remove material so that the radius of the peak is attained at the intended position of the peak as it becomes aligned with the grinding wheel with rotation of the workpiece, and thereafter, by continued rotation of the workpiece and infeed/outfeed operation, to remove material from the workpiece so as to attain the required contour of the remainder of the cam, and finally to break contact with ground face of the cam at or after the peak of the lobe has become aligned with the grinding wheel for a second time but before the said third such alignment. The above-described method produces a rough ground profile ready for finish grinding. Finish grinding may be carried out with the same or a different grinding wheel, as by means of a belt grinder. Surface hardening may be interposed between rough grinding and finish grinding. According to another aspect of the invention, there is provided a grinding machine for carrying out the above defined- method, equipped with control means for synchronising workpiece rotation and infeed/outfeed movement of the grinding wheel and cam as aforesaid. The cam, for example, may be a cam on a camshaft, which typically requires to be ground with a plurality of lobed cams. According to a further feature of the invention, therefore, camshaft indexing along its axis from one cam workpiece to the next is synchronised relative to the infeed/outfeed movements of the grinding wheel and to the camshaft so that the next cam workpiece is engaged by the grinding wheel. According to a further aspect of the invention there is provided a camshaft grinding machine for carrying out the above defined method, comprising a bed, means for supporting and rotating a camshaft to be ground relative to a grinding wheel, infeed/outfeed drive means for moving the cam and grinding wheel relative to each other, first position sensing means for determining the distance between the longitudinal axis of the shaft and the nearest point thereto on the surface of the grinding wheel, second position sensing means for determining the angular position of the rotating camshaft, control means responsive to signals from the two position sensing means to control the drive means to cause the wheel and camshaft to engage just prior to when the region of maximum radius of the cam lobe to be ground becomes aligned with a plane containing the axis of the wheel and the axis of the shaft, means containing radius data relative to rotational shaft position and means for utilising the radius position data to control the infeed/outfeed drive means to remove more or less material from the camshaft to attain the desired radius profile around the camshaft in one but not more than two rotations of the camshaft. The method in accordance with the invention has the advantage that rough grinding can be performed much more quickly, specifically in little more than one turn of the cam workpiece. Moreover, because the grinding wheel separates from the cam at the nose position, the presence of a retraction mark can be substantially avoided. Thus, if desired, the grinding wheel can dwell for a short period at the depth of the nose, separa ion being produced by the continued rotation of the rough ground cam. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 4 are diagrammatic views showing a sequence of stages in a rough grinding process; Figure 5 is a diagrammatic view of a rotating cam; and Figure 6 is a flow chart describing the synchronisation of rotating cam and grinding wheel. Description of embodiment Referring to the drawings, Figure 1 shows a grinding wheel 10 and a cam workpiece 12. A grinding machine on which the grinding wheel 10 is located comprises a bed, means for supporting and rotating a camshaft to be ground relative to a grinding wheel and infeed/outfeed drive means for moving the cam and grinding wheel relative to each other. First position sensing means is provided for determining the distance between the longitudinal axis of the shaft and the nearest point thereto on the surface of the grinding wheel 10 and second position sensing means for determining the angular position of the rotating camshaft 12. A control means responsive to signals from the two position sensing means controls the drive means to cause the wheel 10 and camshaft 12 to engage just prior to when the region of maximum radius of the cam lobe to be ground becomes aligned with a plane containing the axis of the wheel and the axis of the shaft. Means containing radius data relative to rotational shaft position and means for utilising the radius position data are used to control the infeed/outfeed drive means to remove more or less -material from the camshaft to attain, the desired radius profile around the camshaft in one but not more than two rotations of the camshaft. The grinding wheel 10 is rotating clockwise about its axis, and the cam workpiece 12 is rotating anti-clockwise about its axis. In addition, a controlled infeed/outfeed movement of the grinding wheel is taking place by relative displacement of the grinding wheel axis and the axis of the workpiece. The workpiece 12 is pre-formed ready for rough grinding to produce a cam having a lobe 14. The material to be ground away from the workpiece is indicated by the shaded border 16, the interior dashed line indicating the size and shape required for the rough ground cam. In accordance with the invention, the infeed/outfeed movement of the grinding wheel 10 is synchronised with the rotary movement of the workpiece 12 so that the grinding wheel first engages the workpiece just ahead of the nose of the required cam lobe 14. Figure 1 shows the condition just before initial engagement takes place. The two movements are synchronised throughout, and the infeed/outfeed controlled, so that the grinding wheel 10 bites relatively deeply into the workpiece 12. Thus, as shown in Figure 2, the depth of "cut" reaches the required radius at the nose of the required lobe 14 exactly when the nose is aligned with the axis of the infeed/outfeed movement. As the synchronised movements continue, the grinding wheel removes all workpiece material down to the required shape and profile of the cam 16. Figure 3 shows the condition when the workpiece has rotated through about 180 degrees from the condition shown in Figure 2. One side of- the workpiece has been wholly ground away, and material 16A to be ground away remains only on the other side of the workpiece. This- material 16A is ground away as rotation of the cam workpiece 12 continues in synchronised relationship to the controlled infeed/outfeed movement, until the starting position of the Workpiece is reached. Grinding then continues to remove the sloping step initially cut into the workpiece ahead of the nose of the lobe. When the nose of the now rough ground cam again comes into alignment with the axis of the infeed/outfeed movement, the grinding wheel 10 is allowed for a short period to dwell without movement relative to the cam, so that disengagement of the wheel and cam is produced by the continued rotation of the cam. The rough ground cam is thereby fully formed in just over one complete rotation of the workpiece. Typically, the cam may be one cam of a series along a camshaft. Thus, following the condition shown in Figure 4, relative longitudinal indexing of the shaft and grinding wheel may be carried out, in synchronised relationship to the infeed/outfeed movement and shaft rotation, so that commencement of grinding of the next cam workpiece occurs when the next cam reaches the condition shown in Figure 1, i.e. in less than one rotation of the shaft. In this way, all the cams on a camshaft may be rough ground in considerably less time that has been the case hitherto. Conventional finish grinding may be carried out subsequently, possibly by the same grinding wheel, and possibly after a surface hardening treatment carried out on the rough ground cams. Although workpiece 12 is described as rotating anticlockwise, it is to" be understood that it may alternatively rotate in a clockwise sense. Figure 5 shows the position of zero degree reference on a cam 18 which defines the position from which relative angles for the control of the synchronisation between the cam and the grinding wheel are determined. The cam 18 has a maximum radius with 'respect to the centre of cam rotation as shown by line 20. This radius is chosen to be a zero degree reference line. The cam 18 is defined at zero degrees with respect to the grinding wheel 22 when the cam zero reference is pointing horizontally towards the grinding wheel centre 22. The zero degree reference line 20 is further referenced to an angular measuring system coincident with the axis of cam rotation. Rotation of the reference line 20 relative to its initial position produces an angular rotation which is measured by the angular measuring system. The relative angle is read by a computer controlling the grinding machine and a signal fed from the computer to the grinding machine so as to synchronise movement of the cam 18 and the grinding wheel 22. Figure 6 describes the steps required to synchronise the grinding of the cam 18 with rotation of the grinding wheel 22. The cam 18 is indexed to zero, plus or minus an angular offset. The offset is stored in the controlling computer by a machine operator. The size of the offset depends on the direction of rotation of the cam 18 during grinding, i.e. whether the rotation is clockwise or anti-clockwise. After the initial cam position is stored in the computer, the grinding wheel 22 is moved forward until its surface is positioned so as to contact the rotating cam 18 when the maximum throw of the cam, as defined along line 20 in Figure 5, aligns with the line joining the centre of rotation of the cam 18 and the grinding wheel- 22. Cam rotation in the desired direction of grinding is started and when the cam 18 is at zero degrees with respect to the grinding wheel 22, the latter 22 is moved linearly with respect to the horizontal axis along which the zero degree reference to the cam was initially defined. The grinding wheel movement is controlled by (cam angle/grinding wheel position) coordinates as specified by the controlling computer in accordance with the cam profile to be ground. After 360 degrees of rotation when the maximum radius of the cam is back at the zero degrees reference position, the linear motion of the grinding wheel 22 is stopped. The cam 18 is rotated through an angle controlled by the machine control computer, as specified by a machine operator, and stopped. The grinding wheel 22 no longer contacts the cam 18 at any position and the wheel 22 is retracted away from the cam 18. The roughly ground cam is then removed.

Claims: Claims 1. A method of rough grinding a cam within a maximum of two rotations of a workpiece relative to a grinding wheel, wherein rotation of the cam and operation of the cam/grinding wheel infeed/outfeed are synchronised to cause the grinding wheel first to contact the workpiece ahead of the intended position of the peak of the cam lobe therearound, to engage the workpiece and as infeed continues, to remove material so that the radius of the peak is attained at the intended position of the peak as it becomes aligned with the grinding wheel with rotation of the workpiece, and thereafter, by continued rotation of the workpiece and infeed/outfeed operation, to remove material from the workpiece so as to attain the required contour of the remainder of the cam, and finally to break contact with the ground face of the cam at or after the peak of the lobe has become aligned with the grinding wheel for a second time but before the said third such alignment. 2. A method of rough grinding according to claim 1, further comprising the step of finish grinding using a grinding wheel. 3. A method of rough grinding according to claim 1, further comprising the step of finish grinding using an abrasive belt. 4. A method of rough grinding a cam having a cam lobe according to claims 1, or 2 or 3 comprising the further step of surface hardening the cam before the finish grinding step. 5. A grinding machine for carrying out the method of claim 1, wherein control means for synchronising workpiece rotation and infeed/outfeed movement of the grinding wheel and cam is provided so as to grind the cam workpiece to a required contour. 6. A grinding machine according to claim 5, wherein camshaft indexing along the axis from one cam workpiece to the next is synchronised relative to the infeed/outfeed movements of the grinding wheel and to the camshaft so that the next cam workpiece is engaged by the grinding wheel. 7. A cam shaft when ground in accordance with the method of any of claims 1 to 4. 8. A camshaft grinding machine comprising a bed, means for supporting and rotating a camshaft to be ground relative to a grinding wheel, infeed/outfeed drive means for moving the cam and grinding wheel relative to each other, first position sensing means for determining the distance between the longitudinal axis of the shaft and the nearest point thereto on the surface of the grinding wheel, second position sensing means for determining the angular position of the rotating camshaft, control means responsive to signals from the two position sensing means to control the drive means to cause the wheel and camshaft to engage just prior to when the region of maximum radius of the cam lobe to be ground becomes aligned with a plane containing the axis of the wheel and the axis of the shaft, means containing radius data relative to rotational shaft position and means for utilising the radius position data to control the infeed/outfeed drive means to remove more or less material from the camshaft to attain the desired radius profile around the camshaft in one but not more than two rotations of the camshaft. 9. A method for rough grinding a cam substantially as herein described with reference to the accompanying drawings. 10. Apparatus for rough grinding a cam substantially as herein described with reference to the accompanying drawings.

Inventors: BARTLETT CHRISTOPHER DAVID, MIDGLEY CLIVE JOHN SIMMONS, BARTLETT, CHRISTOPHER, DAVID, MIDGLEY, CLIVE, JOHN, SIMMONS
Applicants: BARTLETT CHRISTOPHER DAVID, MIDGLEY CLIVE JOHN SIMMONS, WESTERN ATLAS UK LTD, BARTLETT, CHRISTOPHER, DAVID, MIDGLEY, CLIVE, JOHN, SIMMONS, WESTERN ATLAS UK LIMITED
Application year: 1995
Main classification: B24B19
IPC codes: B24B19