Background to the invention Workrests are a common feature on grinding machines set up to grind otherwise unsupported elongate workpieces. Workrests are a regular feature in grinding machines set to grind crankshafts of internal combustion engines particularly when grinding journal regions of such crankshafts. Typically such workpieces are mounted between centres.

Historically single wheel grinding machines were employed and the workpiece was either indexed relative to the wheel or the wheel indexed relative to the workpiece to allow the wheel to register with each of the plurality of journal regions along the length of the crankshaft which required to be ground. In such situations, a single workrest tended to be provided to be moved with the wheelhead (where the wheel was indexed relative to the workpiece), or mounted in a fixed position opposite a fixed wheel in the event that the workpiece was indexed relative to the wheel. The workrest was set up so as to resist bending forces exerted on the workpiece during grinding and the arrangement worked reasonably well since the workrest was always located opposite the grinding wheel irrespective of where the journal region was located along the length of the workpiece.

In an attempt to reduce grinding cycle times, it has become popular to use multi-wheel grinders and in particular an arrangement in which the same number of wheels are located at axially spaced positions along a common shaft as there are journal regions along the crankshaft. Thus if the crankshaft includes N journal regions, N grinding wheels are provided, one for each journal region. The N wheels are located at axially spaced positions along the common drive shaft and the wheels are dressed so as to produce N accurately ground journal regions in a single grinding operation.

Since the journal regions are usually located approximately symmetrically relative to the mid-position of such a shaft, and since both ends of the shaft are supported, it has been conventional in multi-wheel grinders to retain the single workrest arrangement which hitherto has been located opposite the one grinding wheel, and to arrange for the workrest to engage a central journal region of the workpiece.

It has always been assumed that the centrally located workrest has eliminated any errors at the centre of the workpiece, and for the purpose of determining whether the workrest is performing correctly or requires resetting up, it has been conventional to gauge journal regions near the two opposite ends of the workpiece after a test workpiece has been ground, and to adjust the workrest accordingly.

By regularly gauging the same outboard regions of subsequently ground workpieces (such as every untel workpiece), any workrest wear has been identified and the workrest has been repositioned radially of the workpiece, as appropriate. Usually the workrest was hydraulically operated, and repositioning merely involved adjusting the hydraulic pressure in the workrest.

Whilst in theory this procedure should produce accurate results, in practice unexpected or uneven wear and/or thermal changes can result in errors arising, especially over long crankshafts where the errors can be considerable.

The process of adjusting the position of the workrest to take account of wear and other factors is typically referred to as workrest compensation.

Object of the invention It is an object of the present invention to provide an improved arrangement for controlling workrest compensation in multiwheel grinding of journal regions along the length of a workpiece using a multiwheel grinding machine.

Summary of the invention According to the present invention there is provided a method of operating a multiwheel grinding machine having a single workrest engageable with an elongate workpiece, comprising the steps of supporting the workpiece at opposite ends by a headstock and a tailstock, positioning the workrest adjacent a region to be ground at or near the mid- position of the workpiece, driving the workpiece in rotation about its longitudinal axis, gauging the region during or after grinding, and adjusting the position of the workrest in a direction perpendicular to said longitudinal axis of the workpiece so as to reduce any gauged errors noted in the ground region.

The invention further provides a multiwheel grinding machine comprising a single workrest adjustable in position relative to a first journal region of an elongate workpiece to be ground, drive means for adjusting the position of the workrest at least in a direction perpendicular to the elongate workpiece, headstock and tailstock means for supporting the workpiece therebenveen, the workpiece having a plurality of other journal regions including said first journal region along its length, which are to be ground simultaneously by a corresponding plurality of grinding wheels mounted for rotation about a common axis upon a wheelhead, gauge means for gauging the diameters of the first journal region during or after grinding, and computing means to which the gauged diameter signals are supplied, the computing means being programmed to compare the gauged diameter with reference dimensions stored in a memory within the computing means, or by means of an algorithm process for the gauged diameter signals, and the computing means being further programmed to generate control signals for the workrest drive means for adjusting the position thereof in a direction perpendicular to the workpiece so as to compensate for any grinding errors in the first region, depending on the comparison or algorithm processing of the gauged diameter signals.

The grinding machine may advantageously further comprise a wheelhead position gauge means adapted to gauge displacements of the wheelhead from a first datum position, and a workrest surveillance gauge means adapted to gauge displacements of the workrest from a second datum position. Displacements signals are supplied to the computing means, which is programmed to generate control signals for the workrest drive and the wheelhead drive for adjusting the positions thereof in directions perpendicular to the axis of rotation of the workpiece so as to permit the use of one or more feed rates during a grinding cycle and to compensate for wear of the workrest over one or more grinding cycles.

In the simplest arrangement, the workpiece is rotated slowly during a gauging process and a plurality of diameters are measured by the gauge at different angular positions of the workpiece during its rotation. A simple mean ground diameter may be computed from the measurements so made and this compared with a target diameter for the region.

If oversize, positioning control signals are derived from a computer programme supplied with the mean gauged diameter, and the position of the workrest adjusted so as to exert rather more force on the workpiece during the grinding process so as to ensure that slightly more material is removed from the region during the grinding process than previously.

If the opposite is the case, the position of the workrest is adjusted in an opposite sense (ie away from the workpiece) so that the central region of the workpiece can deflect by a slightly greater amount during grinding thereby causing slightly less material to be removed during the grinding process.

According to a further aspect of the invention, a dynamic drive is provided for the workrest which is capable of being adjusted incrementally during grinding, and instead of producing a mean ground diameter for a ground component, the computer stores each of a plurality of gauged diameters for known angular positions of the workpiece relative to a datum, and the gauged diameters are compared with a set of ideal diameters for the region being ground. Any departure from the ideal dimension, is logged together with the angular position at which the error has been detected, and a dynamic control signal is built up by means of a computer programme supplied with the angular positional data and the measured errors so as to move the workrest dynamically during each rotation of the workpiece so as to advance and retract the position of the workrest in strict synchronism with the rotation of the workpiece so as to compensate for any out of roundness or other errors detected by the gauging.

Since the workpiece is held firmly at each end by the headstock and tailstock, the application of a workrest to the mid-position halves the free length of workpiece available to distort under the influence of machining force, and by accurately dressing the grinding wheels and thereby ensuring consistent grinding of the journal regions along the length of the workpiece, the maximum error due to deflection under machining forces is limited to the quarter points of the shaft length. Since any out-of-roundness errors at the mid- position, caused for whatever reason, will be removed by the active dynamic control of the workrest provided by the invention, any out-of-roundness errors at any other journal position along the length of the workpiece will be due to the lack of stiffness of the workpiece at the different journal regions, and since the free length of the workpiece has been halved by the positioning of the workrest at the mid-point, the errors should be reduced to a fraction of what they would be if the workrest were not present and were not active in removing any out-of-roundness at the mid-position of the workpiece.

In addition to ollt-of-roundness, the journal regions not engaged by the workrest may become tapered, where the surface of a journal region is not parallel to the axis of rotation of the workpiece, due to varying deflections of the workpiece at different points along the length thereof under the influence of machining forces. As stated above, such deflection is a maximum at the quarter points of the workpiece and a minimum at the points engaged by the workrest, headstock and tailstock. The invention therefore also lies in taper compensation, whereby the feed rate of the grinding wheels is reduced to zero as the workpiece nears its finished size to reduce the deflection of the workpiece and thereby to remove tapers. The axis of the crankshaft and the axis of the common shaft for the multiple grinding wheel may divert from their desired parallel relationship due mainly to thermal conditions, and this would manifest itself in the crankshaft journals being ground slightly tapered as between the two ends thereof.

By providing left-hand and right-hand sizing gauges, the invention enables this situation to be monitored and to be corrected for, in that the common shaft for the grinding wheels is pivoted by an appropriate angle to restore the parallelism.

Brief description of the drawings A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows by way of a flow chart the steps carried out by a workrest and wheelhead, under the control of a computer, and the control functions carried out by a computer, during a grinding cycle from the initial advance of the wheelhead and workrest at a medium feed rate, (step 1C and step 12 respectively) to the conclusion of a period of slow feed (step 36); -Figure 2 follows on from Figure 1 and shows the steps carried out by the workrest and the control functions carried out by the computer during a procedure to check and, if necessary, correct the position of the workrest (steps 38,48,50,52 and 54); Figure 3 follows on from Figure 2 and shows the steps carried out by the workrest and wheelhead, and the control functions carried out by the computer from detection by the computer that the journal regions have been ground to their final size to retraction of the wheelhead and workrest to their initial positions (steps 40,42,44,46,56 and 58) ; and Figure 4 is a plan view of a crankshaft to be ground showing the workrest and gauges. In the preferred arrangement there is provided a wheelhead, which comprises multiple grinding wheels, a common shaft on which the grinding wheels are mounted and a drive means for said shaft. Referring particularly to Figure 4, a workrest 60 comprises a workrest surveillance gauge 62 operable to measure the displacement of the workrest towards the workpiece (crankshaft 63) by measuring the diameter of the central journal region of the workpiece that is engaged by the workrest 60. Similar sized gauges 64 and 66 are positioned respectively at the left-hand and right-hand journal regions.

The surveillance gauge 62 and workpiece sizing gauge are operable to communicate their measurement values to a computer. The computer comprises a memory in which predetermined reference values of the wheelhead and workrest displacements and journal region diameter are stored. The computer is operable to compare the measurements of wheelhead and workrest displacements and journal region diameter with said reference values.

The sequence of steps comprising a grinding cycle in the preferred arrangement is shown in Figures 1 to 3. Initially the wheelhead and the workrest are advanced simultaneously under CNC control, in the X-axis direction at a medium feed rate to engage the workpiece, as shown in steps 10 and 12 respectively of Figure 1.

-The workrest advances at the medium feed rate until the computer detects that the workrest has moved, relative to an initial datum, to a position corresponding to the medium feed endpoint (step 14), when the workrest is haited and a period of feed dwell is initiated (step 16).

The workrest feed dwell continues for a predetermined time (step 18) whereupon the workrest starts to advance at a slow feed rate (step 20). The workrest advances at the slow feed rate until the computer detects that the workrest has moved, relative to the initial datum, to a position corresponding to the slow feed endpoint (step 22), when the workrest is halted (step 24). The wheelhead advances at the medium feed rate while the surveillance workpiece sizing gauge communicates measurements of the diameter of the journal region engaged by the workrest to the computer, until it detects that the journal region has been ground to a reference diameter (step 26), when the wheelhead is halted and a period of feed dwell is initiated (step 28).

The wheelhead feed dwell continues for a predetermined time (step 30) after which the wheelhead starts to advance at a slow feed rate to engage the workpiece (step 32). Thus grinding at the slow feed rate causes less distortion of the workpiece than grinding at the medium feed rate.

If any discrepancy is detected between the readings from the left-hand gauge 64 and the right-hand gauge 66, a so-called taper compensation now takes place, in that the angle of the grinding wheel shaft is adjusted accordingly.

Grinding at the slow feed rate continues until the computer detects that the journal region diameter is at a size corresponding to a reference value (step 34), when taper compensation is halted (step 36) and a check of the workrest position is initiated (step 38), as shown in Figure 2.

'The computer contains reference values corresponding to the upper and lower limits of the range of workrest displacement measurements expected from the workrest surveillance gauge when the journal region diameter reaches the size at which the computer initiates the check of the workrest position. Provided that the workrest displacement measurement lies between said limits, grinding at the slow feed rate continues until the computer detects that the journal regions have been ground to a diameter corresponding to a reference value (step 40), when the wheelhead is halted and a period of feed dwell is initiated, as shown at step 42 of in Figure 3. The wheelhead feed dwell permits the grinding wheels to"spark out"from the journal regions. The"spark out"feed dwell continues until the computer detects that the workpiece has been ground to its final size (step 44) whereupon the wheelhead starts to retract at an initial breakaway rate (step 46). If, however, the workrest displacement measurement is not within the upper and lower limits, the computer compares the measurement with a reference value that corresponds to the finished journal region diameter (step 48). If the reference value and the workrest displacement measurement are equal, then the wheelhead starts to retract at the initial breakaway rate (step 46).

If the workrest displacement measurement is not within the upper and lower limits, the computer determines whether the workrest displacement measurement is greater than the upper limit (step 50), indicating that the grinding wheels have removed less material than desired, and the workrest is advanced by a predetermined amount (step 52). If, on the other hand, the computer determines that the workrest displacement measurement is less than the lower limit, indicating that the grinding wheels have removed more material than desired, the workrest is retracted by a predetermined amount, as shown in step 54 of Figure 2.

The wheelhead retracts at the initial breakaway rate until the computer detects that the wheelhead has retracted to a breakaway rate endpoint (step 56). At the breakaway rate endpoint, both wheelhead and workrest start to retract at a rapid rate (step 58), to return to their respective datum positions and complete the grinding cycle.