Field of invention

This invention concerns grinding methods and grinding machines particularly for grinding the main cylindrical bearing support surfaces and their adjacent side-walls and the off- axis cylindrical crankpins and their adjacent side- walls, of a crankshaft.

Background

A plunge grinding method is described in US patent 4,603,514 (Toyoda). This method involves a sequence of plunge grinds in which during at least some of the plunge grinds the relative movement between grinding wheel and workpiece is such that if the workpiece is considered to be stationary the wheel will be seen to move along a trajectory that is at an acute angle to the axis of the cylindrical surface it is to grind, and during grinding the wheel removes metal from both the cylindrical surface and an adjoining side-wall.

Summary of the invention

According to the present invention in such a method the angle of wheel advance is selected so that the side-wall grind will be completed ahead of the cylindrical grind and after the side-wall grind has been completed the wheel is stood-off from the side-wall, so that only the external cylindrical circumferential surface (the face) of the wheel is in grinding engagement at least at the start of the final part of the plunge grind leading to the final diameter of the cylindrical surface being ground.

Thus the present invention provides a method of operating a grinding machine including a grinding wheel to grind a cylindrical surface on a workpiece, the cylindrical surface being bounded at one or both ends with a radial flange or side wall which also has to be ground to size, the method comprising the steps of:-

(1) selecting the angle of wheel advance so that the side-wall grind will be completed ahead of the cylindrical grind,

(2) after the side-wall grind has been completed standing off the wheel from the side- wall, so that only the external cylindrical circumferential surface (the face) of the wheel will make grinding engagement with the workpiece at least at the start of the final part of the grind, and

plunge grinding the cylindrical surface to achieve the final diameter required of the cylindrical surface.

The stand-off may be achieved by axially displacing the wheel relative to the workpiece or the workpiece relative to the grinding wheel by a small distance.

During the final part of the plunge grind the relative axial movement may be stopped and the movement reversed for a small distance to create the stand-off so that thereafter the trajectory of the wheel is normal to the cylindrical surface to be ground. Alternatively in some circumstances after creating the stand off the wheel may be advanced along an angled path similar to that employed during the previous part of the grind, the stand-off preventing contact with, and metal removal from, the side-wall during the final angled approach of the wheel to complete the grind.

Preferably during the final part of the plunge the workspeed is reduced from the speed at which it is rotated during the previous part of the plunge to assist in achieving a desired grinding quality of the cylindrical region. Preferably during the final part of the plunge the coolant flow rate is reduced from that which is employed during the previous part of the plunge, so as to achieve a desired grind quality of the cylindrical region.

The method is applicable to grinding the cylindrical crank-pins or the main bearing support surfaces of a crankshaft, but can be employed when grinding any cylindrical surface bounded at one or both ends with a radial flange which also has to be ground to size.

In a method embodying the invention a diameter and two side-walls are ground using a succession of two angled plunge grinds albeit with the side face of the grinding wheel stood off from the side-wall of the workpiece during a final part of each plunge grind. During each angled plunge grind, feed rate, dwells, workspeed, coolant pressure and flow rate are controlled in relation to end-points of the grind, so that a side-wall at one end of the cylindrical surface and the adjacent part of the latter are ground to size in the manner hereinbefore described.

After grinding the side-wall and diameter by the first plunge grind at one end, the wheel is retracted and if necessary indexed laterally before a second angled plunge grind is performed this time moving the wheel along a trajectory towards the side-wall at the other end of the cylindrical surface, thereby to grind the side-wall and diameter at the other end of the cylindrical surface.

Preferably the lateral indexing is not more than 2/3 of the wheel width so as to ensure overlap on the diameter between plunges.

If the wheel width is insufficient for the whole length of the cylindrical surface to be ground to diameter by the two angled plunge grinds, one or more perpendicular plunge grinds may be performed between the first and second angled plunge grinds or after the second angled plunge grind.

Preferably the process employs a dressed profiled wheel. Information from the dressing of the wheel and the position of the wheel faces allows the side-walls and diameters to be ground accurately.

Since the side faces are each employed in turn the wheel can have an equal depth of CBN grit on each of the two side faces, and by selecting an appropriate depth of CBN grit around the cylindrical face of the wheel, the wheel should wear uniformly in use so that all of the CBN grit around the wheel should be utilised before the wheel has to be replaced.

If the quantity of metal to be removed on the diameter is approximately 50% of that to be removed from the side-walls, then using a 35.00mm wide CBN wheel, the thickness of the CBN layers can be as follows :-

CBN layer on (cylindrical) face of the wheel = 5.0mm. - CBN layer on left side face of the wheel = 5.0mm. - CBN layer on right side face of the wheel = 5.0mm.

This gives a total dressable width on the wheel of 10.0mm.

The invention also lies in a grinding machine and programmable computer based control system therefor, programmed to grind a cylindrical region bounded by at least one radial side wall on a workpiece by way of a series of plunge grinds in which a grinding wheel of the machine grinds the or each side-wall to size before the adjoining cylindrical surface is ground to the correct diameter, and in which the wheel is caused to stand-off from the ground to size side-wall at least at the start of a final plunge grind of the diameter.

A two-wheel grinding machine may be employed each wheel being controlled to perform an angled grind with lateral stand-off thereafter, prior to the final part of each grind provided the two wheels are independently controllable along the X and Z axes of the machine. A method embodying the invention and part of apparatus for performing the method is shown in the accompanying Figs 1-14.

A typical computer controlled grinding machine is shown in Fig 15, a diagrammatic illustration of the external parts of the grinding machine is shown in Fig 16 and the computer program steps involved in operating the machine to perform the side-wall grind in accordance with the diagonal grinding proposed by the invention, is shown in Fig 17.

The method will first be described with reference to Figs 1-14.

In Fig 1 the wheelhead is moved axially relative to the crankshaft 4 in the direction marked "A" so as to be positioned adjacent the region to be ground for first plunge grind. This is referred to as the lateral start position of the wheel 2.

In Fig 2 the wheel 2 is shown after a rapid plunge move in the direction marked "B" to an end point in which the wheel is at an equal distance from the side-wall 6 and cylindrical region 8 to be ground.

In Fig 3 the wheel is shown during an angled plunge to the right in the direction marked "C" using programmable feeds in the directions of the RH side-wall 6 and the cylindrical region 8, to programmable end points. Ideal angle of feed is 45° with the side- wall end point 6' being reached before that of the central region diameter 81 by a distance "x" of approximately 0.010mm. Other functions controlled during the plunge by means of staggered control end points are dwells, multi-stage feedrates, coolant pressure control and workspeeds, to gain a desired grind quality. The part rotation speed is typically 80RPM (journal) or 40RPM (pin).

In Fig 4 the wheel is shown in the stand-off position (after movement in the direction marked "D") required before the remainder of the right hand plunge grind is performed. The stand-off positions the wheel a distance of "y" 0.050mm clear of the side-wall. The workspeed and coolant flow rate are reduced and the wheel continues to advance perpendicularly to the axis of the cylindrical region. Typically the workpiece is rotated at 20RPM. By arresting the axial displacement of the workpiece (employed during the previous part of the grind to achieve the 45° effective angle of feed) the stand-off is preserved during the final part of the plunge grind.

In Fig 5 the wheel is shown at the end point of the final part of the plunge after movement in the direction marked "E", during which the sequence continues to use the staggered control end points, controlling dwells, multistage feedrates, coolant pressure control and workspeeds, to ensure the desired grind quality for the cylindrical surface.

In Fig 6 the wheel is shown retracted in direction "F" clear of the workpiece to a programmable safe position from the wheelhead to allow lateral indexing of the wheelhead towards the LH end of the region being ground.

In Fig 7 the wheel is shown after lateral indexing in direction "G" to an initial position for the second angled plunge grind.

In Fig 8 the wheel is shown after a rapid plunge movement in direction "H" to the end point from which the LH angled plunge grind is to begin. This corresponds to the start position of Fig 2 for the RH angled plunge.

In Fig 9 the wheel is shown during the angled LH plunge grind during which programmable feeds in the directions of the LH side-wall and central region diameter are used, to move the wheel in direction "I" to programmable end points. Again the ideal angle of feed is 45°, with the LH side-wall end point being reached before that of the central region diameter by approximately 0.010mm. As with the RH angled plunge other functions controlled during the plunge by means of staggered control end points are dwells, multi-stage feedrates, coolant pressure control and workspeeds, to gain the desired grind quality. Typically the part rotation speed is 80RPM (journal) or 40RPM (pin). Fig 10 shows the wheel in its second stand-off position - in which the wheel is once again stood off in direction "J" by 0.050mm (distance "z") this time from the LH side-wall, and the workspeed and coolant flow rate are reduced. The speed of rotation may be reduced to 20RPM, typically. If the axial movement of the workpiece is arrested during the final part of the grind, the grinding trajectory will be perpendicular to the axis of the cylindrical region, instead of at 45°.

In Fig 11 the wheel is shown at the end point of the final part of the plunge in direction "K", during which the sequence continues to use the staggered control end points, controlling dwells, multi-stage feedrates, coolant pressure control and workspeeds, to ensure the desired grind quality for the cylindrical surface.

In Fig 12 the wheel is shown retracted clear of the workpiece in direction "L" to a programmable safe position ready for subsequent lateral indexing of the wheelhead relative to that region of the workpiece if a further plunge grind is required, (typically without simultaneous axial movement of the workpiece) should the width of the wheel be insufficient for the whole of the axial length of the cylindrical region to have been ground to final diameter.

In Fig 13 the wheel 2 is shown further retracted so that the wheelhead can index to the next part of the crankshaft 4 which is to be ground, and the multiple plunge grind sequence is repeated from Fig 1 above.

Fig 14 shows a crankshaft workpiece 4 mounted between headstock 10 and tailstock 12 with the wheelhead 14 ready to advance to the first grinding position, but parked in a position clear of the workpiece to allow the latter to be demounted or mounted.

Fig 15 shows a grinding machine 68. The machine shown includes two grinding wheels 70,72 driven by motors 74,76 and mounted on wheelheads 78,80 for separate and simultaneous movement towards and away from a workpiece 82 along linear tracks 84,86 under the control of wheelfeed drive motors 88,90. The workpiece is mounted between centres in a tailstock 92 and a headstock 94 which also houses a motor (not shown) for rotating the workpiece 82 via a chuck 96. The workpiece shown is a crankshaft of an internal combustion engine and includes offset crankpins such 98 which are to be ground to size, each of which constitutes a cylindrical workpiece for grinding.

Although two grinding wheels are shown on the machine of Fig 15 it is to be understood that one of the wheels, wheelheads and drive motors can be omitted so that the machine contains only one grinding wheel (e.g. 70) as shown in Figs 1-14.

A computer 100 running a suitable programme controls the operation of the machine and inter alia moves the wheelhead 78 (or both wheelheads 78,80) towards and away from the workpiece 82 as the workpiece rotates, so as to maintain contact between the wheel and the crankpin being ground, as the latter rotates circularly around the axis of the workpiece centres.

A gauge, not shown, may be carried by the wheelhead assembly for in-process gauging the diameter of the crankpin as it is ground.

At 102 is mounted a hydraulically or pneumatically operated worksteady having a base 104 and movable cantilever arm 106 adapted at the right hand end as shown to ' engage a cylindrical journal bearing region of the crankshaft workpiece 82. Controlling signals for advancing and retracting 106 are derived from the computer 100.

Wheel diameter sensing gauges may be included, signals from which are supplied back to the computer 100.

In accordance with the present invention the wheelhead 78 is movable along an axis parallel to the workpiece axis (the Z axis) by a further drive.

In Fig 16 the essential parts of the machine are shown namely a wfleelhead 200 having a wheel drive motor 202, a Z-axis feed drive motor 204 and an X-axis feed drive motor 206. The X and Z axes are denoted by labelled arrows. A grinding wheel 208 is mounted to one side of the wheelhead 200 and movement of the wheelhead is controlled by signals from a computer based control system 210.

A workpiece 212 is shown mounted between headstock 214 and tailstock 216. The former includes a C-axis drive motor (not shown) for rotating the workpiece about its lengthwise axis. The workpiece includes radial flanges at 218, 220 between a cylindrical region 222 and the purpose of the grind is to finish grind the opposed side-walls denoted by 224, 226 and the diameter of the cylindrical region 222.

In accordance with the invention the wheelhead is moved relative to the workpiece so that one side-wall of the grinding wheel is brought into grinding contact with one of the two side-walls (for example 224) of the workpiece, the feed movement of the wheelhead being controlled along both X and Z axes, so that the wheel and wheelhead describe a trajectory which makes an acute angle with the workpiece axis - typically 45°, until the side-wall grind is complete, after which the wheel is stood off from the side- wall 224 and the wheelhead is advanced so as to finish the grind the diameter of region 222, whilst maintaining the stand off between the side-wall of wheel and the side-wall of workpiece 224.

The other workpiece side-wall 226 is then ground using the other side-wall of the grinding wheel, and the remainder of the cylindrical region 222 is ground, whilst the wheel is stood off from the second workpiece side-wall 226.

Fig 17 shows the steps required to be performed by the computer 210 in response to feedback from gauges and or X and Z axis position signals.

Grinding is initiated by step 228 which causes wheel feed 206 to move the wheelhead 200 parallel to the X-axis. The X-feed 206 is controlled so that a specific diameter of the region 222 will be ground. The X-feed is monitored and when the wheel has reached the side-wall grind start position (well before the wheel has reached the region 222) step 230 produces a YES signal to initiate a side- wall grind.

Here it will be assumed that the first side-wall to be ground is 226 of Fig 16.

Step 282 of the programme causes Z-axis movement towards the chosen side-wall (226 in the example under consideration) which is simultaneous with the X-axis movement initiated by step 228.

Step 232 provides two outputs one to a Z-axis drive controlling step 234 and one to a monitoring logic step 236 which determines whether the X-axis feed has achieved the desired diameter of the region 222.

The Z-axis movement is monitored by step 238 which produces a YES signal when the combined X and Z axes movement has resulted in the side-wall 226 having been ground to size (measured in the Z direction).

A YES signal from step 238 triggers step 240 to instigate a reverse Z-axis motion to back off the side-wall of the wheel 208 from contact with the side-wall 216, now ground to size. An output signal from 240 indicates back off is completed.

A logic stage 242 provides a YES signal if the output signal from 240 indicates back off is complete and the X-axis movement has achieved the desired position in region 222.

A similar logic stage 244 provides a YES signal if the side-wall cycle including back-off is complete when the monitoring step 235 confirms that the region 222 has been ground to size.

When both 242 and 244 provide a YES signal, step 246 reverses the X-feed drive 206 to retract the wheel. Grinding of the other side-wall 224 is achieved by reversing the Z-axis commands to the Z-axis drive 204 whilst performing similar X-axis movements in response to signals from the computer 210.