GB2608940A

GB2608940A - A grinding machine including a workrest and methods of operation thereof

Info

Publication number: GB2608940A
Application number: GB2214947.0A
Authority: GB
Inventors: Almack Douglas; Rowling Anthony; Thornton Steven
Original assignee: Fives Landis Ltd
Current assignee: Fives Landis Ltd
Priority date: 2018-07-23
Filing date: 2019-03-27
Publication date: 2023-01-18
Anticipated expiration: 2039-03-27
Also published as: GB2608940B; GB2576962A; GB201904234D0; GB202214947D0; GB201811991D0; GB2576962B

Abstract

A grinding machine (22 fig.4) includes a machine base (24 fig.4) and a workrest 56 for supporting a journal bearing 50 of a workpiece (44 fig.5) which may be a crankshaft. The workrest has workpiece engaging components 80, 82 which may be rollers. The machine has a controller (38 fig.4) which receives a runout signal (40 fig.4) related to the displacement of the bearing due to runout and outputs a drive signal in response to the runout signal. The drive signal raises the workrest and bearing until the midpoint (12 fig.3) of the vertical displacement (d' fig.3), which is the product of the bearing runout and sag due to gravity, to the same height as a longitudinal rotational machine reference axis (6 fig.3). A slide 104 may allow horizontal movement or oscillation of the workpiece along a path (70 fig.6). An actuator 92 may drive a rigid support structure 58, comprising the rollers 80, 82 around a pivot 90.

Description

Title: A Grinding Machine including a Workrest and Methods of Operation thereof

Field of the invention

The present invention relates to a grinding machine including a workrest for supporting a central region of a workpiece More particularly, it is directed at such a grinding machine and methods of operation thereof which seek to increase the precision of the grinding and reduce the time taken for grinding processes. It may be suitable for grinding a crankshaft for example.

Background of the invention

Grinding machines for machining crankshafts support the crankshaft at each end. The central region of the crankshaft will tend to sag due to the effects of gravity, particularly in larger crankshafts over 1500mm in length which may exhibit poor intrinsic stiffness. Forces exerted on the crankshaft during grinding will add to the gravitational deflection and cause the axis about which the central region of the crankshaft rotates to be pushed further from the intended location. These effects may be reduced by applying workrests which act on journal bearings of the crankshaft to provide support and to counteract grinding forces.

However, poor roundness in an unfinished journal bearing can cause the crankshaft to rotate unpredictably in a workrest and result in poor surface integrity, roundness, chatter or runout when grinding other surfaces of the crankshaft.

Summary of the invention

The present invention provides a grinding machine for grinding a workpiece, the workpiece comprising two opposite ends and a first journal bearing between the ends, the first journal bearing having a central longitudinal axis, wherein the grinding machine comprises: a machine base; two end supports carried by the machine base for supporting respective ends of a workpiece and allowing rotation of the workpiece about a longitudinal machine reference axis; a workrest carried by the machine base for supporting the first journal bearing, the workrest comprising workpiece engaging components for engaging a workpiece, and a drive for changing the height of the workpiece engaging components above the machine base; and a controller configured to: receive a runout signal related to displacement of a first journal bearing of a o workpiece due to runout during rotation thereof; and output a drive signal to the drive in response to the runout signal, wherein the drive signal causes the drive to raise the workpiece engaging components until the midpoint of the vertical displacement of the central longitudinal axis of the first journal bearing due to runout is at substantially the same height as the longitudinal machine reference axis.

The runout signal may be related to a vertical or horizontal radial displacement(s) of the first journal bearing of the workpiece due to runout during rotation thereof. The controller may receive vertical and horizontal runout signals which are related to the vertical and horizontal displacements, respectively, of the first journal bearing of the workpiece due to runout during rotation thereof The runout signal may be related to the radial displacement of the first journal bearing of the workpiece due to runout which occurs during a whole rotation thereof.

The displacement of the first journal bearing may be the distance travelled by the journal bearing in a given straight line direction (the direction lying in a plane perpendicular to the central longitudinal axis of the first journal bearing) during a whole rotation thereof.

The drive signal may cause the drive to raise the workpiece engaging components until the midpoint of the displacement in the vertical direction of the central longitudinal axis of the first journal bearing due to runout during rotation thereof is

J

substantially the same height as the longitudinal machine reference axis. The height of the longitudinal machine reference axis may be determined during calibration of the driven axes of the grinding machine.

The location of the central longitudinal axis of the first journal bearing may be determined with reference to a parameter related to the radius of the first journal bearing. For example, the controller may receive a dimensional signal which is related to the radius of the first journal bearing. The dimensional signal may be indicative of the diameter of the first journal bearing. The dimensional signal may be to outputted by a dimensional sensor operable to measure a dimension of the first journal bearing.

Accordingly, the machine may be able to reduce the effects of sag of the workpiece due to gravity as well as the effects of radial runout. A measurement of the vertical is displacement of the first journal bearing due to runout enables the extent of the shift in the location of the central longitudinal axis of the first journal bearing due to gravity to be determined. The machine then raises the first journal bearing so as to counteract the shift in its position due to gravity. This serves to minimise the effect of radial runout of a rough finished journal bearing. By counteracting the effects of runout, this allows the amount of stock material left by earlier stages of manufacture of the shaft to be reduced, leading to a shorter grinding time.

The runout signal may be related to, or indicative of displacement of the first journal bearing due to runout during rotation thereof (or more particularly indicative of a value proportional thereto). The displacement may be the range of the displacement measured during a complete rotation of the first journal bearing.

Preferably, the workrest comprises an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components; 3o and a slide or guideway which enables the upper body portion to move relative to the lower body portion along a horizontal reference axis perpendicular to the longitudinal machine reference axis. The slide may be a slideway which guides and facilitates motion of the upper body portion relative to the lower body portion along that axis.

With the raised workrest in contact with the first journal bearing, bearing runout will tend to cause oscillation of the bearing in a horizontal direction.

Runout of part of a workpiece (such as a journal bearing) may be measured in the horizontal direction during rotation of the workpiece whilst the workpiece part is supported by the workrest, as the workrest permits horizontal motion of the workpiece part.

The slide enables the workpiece engaging components of the workrest to move freely o in the horizontal direction relative to the machine base. This may have the effect that any runout of the unground, first journal bearing is not overly constrained and so minimises any additional runout motion of the shaft which would otherwise be imparted to other journal bearings as they are machined whilst the first journal bearing is supported by the workrest. With the first journal bearing supported in this manner, is at least a semi-finish operation may be performed on another journal bearing, preferably the adjacent journal bearings, on the workpiece. The semi-finished bearing(s) may then be constrained by workrest(s) allowing the remaining journal bearings to be ground with minimum runout along the length of the crankshaft.

In a preferred embodiment, the slide comprises a rolling element slide.

The workrest may include a biasing assembly configured to exert a biasing force on the upper body portion along the horizontal reference axis. Whilst the slide enables the upper body portion to move freely relative to the lower body portion along the horizontal reference axis, the biasing assembly is provided to urge the upper body portion away from the grinding region with sufficient force to move it to a withdrawn position without materially affecting the forces exerted on the workpiece by the workrest. The biasing assembly may comprise a resilient component such as a spring for example.

The upper body portion of the workrest may include a rigid support structure which carries the workpiece engaging components and the rigid support structure is pivotably mounted on the workrest for rotation about a pivotal axis which is parallel to the longitudinal machine reference axis.

The rigid support structure may then be pivoted relative to the remainder of the upper body portion so as to bring the workrest into or out of engagement with a portion of a workpiece. A drive may be provided in the grinding machine which is operable to pivot the rigid support structure about the pivotal axis. The drive may form part of the workrest.

io The pivotal axis of the rigid support structure may be located between the workpiece engaging components and the portion of the rigid support structure acted on by the drive, in a lateral direction relative to the longitudinal machine reference axis. Exertion of a downward force on the rigid support structure may therefore act to raise the workpiece engaging components.

Instead of a pivotal mechanism, a drive mechanism for the workrest may be provided which moves the workpiece engaging components along a linear axis to bring them into or out of engagement with the workpiece.

The grinding machine may include a displacement sensor configured to measure a vertical displacement of a first journal bearing of a workpiece due to runout during a complete rotation thereof, generate the runout signal, and send the runout signal to the controller. The displacement sensor may comprise a metrology probe.

The sensor may be configured to measure a vertical or horizontal displacement of the first journal bearing of the workpiece (or displacement in another direction lying in a plane perpendicular to the central longitudinal axis of the first journal bearing) due to runout during a complete rotation thereof It may be preferable to measure the horizontal displacement. In some embodiments, the sensor may be configured to measure both the vertical and the horizontal displacements of the first journal bearing of the workpiece due to runout during a complete rotation thereof and generate respective vertical and horizontal runout signals.

The grinding machine may include a dimensional sensor configured to measure a dimension of the first journal bearing, such as its diameter, and generate a dimensional signal indicative of the sensed dimension.

The displacement sensor and dimensional sensor may be embodied by the same sensor assembly.

The two end supports of the grinding machine may be configured to engage with respective end journal bearings of a workpiece, with the first journal bearing located o between the end journal bearings. Alternatively or additionally, the end supports may be configured to engage with other features formed on at or near the ends of the workpiece.

The present invention further provides a method of grinding a workpiece with a grinding machine, the workpiece comprising two opposite ends and a first journal bearing between the ends, the first journal bearing having a central longitudinal axis, and the grinding machine comprising a machine base, two end supports carried by the machine base for supporting respective ends of the workpiece and allowing rotation of the workpiece about a longitudinal machine reference axis, and a workrest carried by the machine base for supporting the first journal bearing, the workrest comprising workpiece engaging components for engaging the workpiece, and a drive for changing the height of the workpiece engaging components above the machine base, wherein the method comprises the steps of receiving a runout signal in a controller of the grinding machine, wherein the runout signal is related to displacement of the first journal bearing of the workpiece due to runout during rotation thereof; outputting a drive signal from the controller to the drive in response to the runout signal; and raising the workpiece engaging components with the drive in response to the drive signal until the midpoint of the vertical displacement of the central longitudinal axis of the first journal bearing due to runout is at substantially the same height as the longitudinal machine reference axis.

After the raising step, the method may include a step of measuring runout of part of a workpiece (such as a journal bearing) in the horizontal direction during rotation of the workpiece whilst the workpiece is supported by the workrest, with the workrest permitting horizontal motion of the workpiece.

Preferably, the method includes, after the raising step, a first grinding step of grinding a second journal bearing on the workpiece whilst the workpiece is rotated by the grinding machine and the first journal bearing is supported by the workrest, wherein the first journal bearing is allowed by the workrest to oscillate in a horizontal io direction perpendicular to the longitudinal machine reference axis as the workpiece rotates.

The first journal bearing may be substantially unconstrained by the workrest in a horizontal direction perpendicular to the longitudinal machine reference axis.

The first journal bearing may thereby be allowed to move freely back and forth in the horizontal direction during grinding of the second journal bearing. This accommodates the horizontal oscillation due to runout. The workrest may include a slide or guideway which is unlocked during the grinding process to permit this horizontal oscillating motion.

In this way, fewer preliminary grind operations may be needed, thereby reducing the time taken to find the natural centre of rotation of the shaft.

The method may allow a more precise grinding operation to be carried out on the journal bearings earlier in the grinding process, therefore reducing the overall grind time.

After the first grinding step, a second grinding step may be carried out to grind the 3o first journal bearing and/or other journal bearings on the crankshaft whilst supporting the second journal bearing with the/or another workrest. The accuracy of the surface formed on the second journal bearing during the first grinding step (which may be a "semi-finishing grind") means that the workpiece may be constrained by a workrest in engagement with the second journal bearing with minimised runout effects.

To facilitate oscillation of the workrest, the workrest may comprise an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components, and a slide or guideway which enables the upper body portion to oscillate in the horizontal direction.

The present invention also provides a method of operating a grinding machine, the to grinding machine comprising a machine base, two end supports carried by the machine base for supporting respective ends of a workpiece and allowing rotation of a workpiece about a longitudinal machine reference axis, and a workrest carried by the machine base between the two end supports, the workrest comprising workpiece engaging components for engaging a part of a workpiece which is located between the ends of the workpiece, an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components, and a slide which enables the upper body portion to move in a horizontal direction relative to the lower body portion, wherein the method comprises a step of rotating a workpiece supported by the two end supports and the workrest about the longitudinal machine reference axis whilst the upper body portion of the workrest oscillates in the horizontal direction relative to the lower body portion of the workrest to accommodate runout of the part of the workpiece.

The method may include further steps of measuring with a sensor displacement in the horizontal direction of a part of the workpiece due to runout during rotation thereof and generating with the sensor a runout signal related to the measured displacement(s).

3o Thus, the method may include a step of measuring runout of part of a workpiece (such as a journal bearing) in the horizontal direction during rotation of the workpiece whilst the workpiece is supported by the workrest, with the workrest permitting horizontal motion of the workpiece due to runout.

The data provided by the runout signal relating to the displacement in the horizontal direction of a part of the workpiece due to runout during rotation thereof may be stored in electronic memory in association with data recording rotational positions of the workpiece corresponding to the displacement measurements. This information may then be utilised in a further step of grinding a or the part of the workpiece in order to reduce the runout of the part.

Brief description of the drawings

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein: Figures 1 to 3 are diagrams illustrating the effects of runout and sag on a workpiece; Figures 4 and 5 are side and plan views, respectively, of a crankshaft grinding machine; Figure 6 is a diagram illustrating a workpiece constrained in a manner described herein; and Figures 7 and 8 are side and perspective views of a workrest according to an embodiment of the invention.

Detailed description of the drawings

Figures 1 to 3 illustrate the effects of runout and sag on a journal bearing located towards the centre of a workpiece mounted via its ends in a grinding machine. It will be appreciated that these effects are exaggerated in Figures 1 to 3 and 6 for the purposes of illustration.

The circle 2 represents the circumference of a journal bearing. The bearing has a central longitudinal axis 4. The bearing is part of a crankshaft. The grinding machine is configured to rotate a workpiece mounted therein about a longitudinal machine reference axis 6. Ideally, the central longitudinal axis 4 of the workpiece and the longitudinal machine reference axis 6 are coincident. In that event, circle 2 will be in the location indicated by dashed circle 8. However, due to runout, the workpiece axis 4 may be displaced from the machine reference axis in a transverse direction at the journal bearing by a distance "x". As a result, when the workpiece is rotated by the machine about axis 6, the workpiece axis 4 orbits around the machine axis 6 in an eccentric manner along orbital path 14. The cross-sectional area swept by the workpiece as a result is indicated by a runout envelope 10.

Figure 2 indicates the position of the journal bearing 2 after rotation anticlockwise through 90 degrees about the longitudinal machine reference axis 6.

Sag of the workpiece under gravity introduces further errors in its positioning as is illustrated in Figure 3. As a result of sag, the journal bearing 2 rotates about a displaced axis 12, which is below the longitudinal machine reference axis 6 by a distance "s". The shaft runout envelope 10 is centred on the displaced axis 12 rather than the longitudinal machine reference axis 6.

An example of a grinding machine embodying the present invention is shown in Figures 4 and 5. The grinding machine 22 has a machine bed 24. The machine bed carries a wheel mount in the form of a wheelhead 26. This includes a drive spindle for rotating a grinding wheel 28 about an axis of rotation 30. A drive mechanism (not shown) is provided for moving the wheelhead 26 along a linear machine axis parallel to the reference axis X. A headstock 32 is also carried by the machine bed 24. A workpiece mount 34 is mounted on the headstock 32. The workpiece mount includes a drive spindle operable to rotate the tool mount about a longitudinal machine reference axis 6 which io is parallel to the linear reference axis Z. The drive spindle is operable to rotate the headstock about axis in either direction of rotation (marked "C" in Figure 4). Axis 6 is perpendicular to linear reference axis X. A crankshaft 44 is supported by the workpiece mount 34 and a footstock 36, which is in turn carried by a footstock support 37.

The machine tool includes a controller 38 which is communicatively coupled to the wheel mount drive spindle, the workpiece mount drive spindle and the drive mechanism for moving the wheelhead. The controller has an input 40 for a signal carrying input data related to vertical displacement of a journal bearing of the workpiece due to runout during a complete rotation of the workpiece. The controller also includes a processor and memory. The processor serves to run instructions io loaded into the memory and receive data via input 40.

The controller may be a number of control units and/or the processor may be embodied by a plurality of processors, a multi-processor core or some other type of processor, depending on the particular implementation. The memory is any piece of hardware that is capable of storing information, such as, for example, without limitation, data, program code in functional form, and other suitable information either on a temporary basis or a permanent basis. The memory may be, for example, a random access memory or any other suitable volatile or non-volatile storage device.

In operation of the grinding machine, the grinding wheel 28 is fed in towards the crankshaft 44 along direction X. It will be appreciated that relative movement between the workpiece and the grinding wheel along direction X may also be achieved by moving headstock 32 and tailstock 36 relative to the machine bed in addition or instead of movement of the wheel mount 26. The grinding wheel is also movable relative to the workpiece along a machine axis parallel to the reference axis Z, perpendicular to the reference axis X. Again, this relative motion may instead, or additionally, be achieved by movement of the headstock 32 and tailstock 36 along a machine axis parallel to the reference Z relative to the machine bed.

io The crankshaft 44 mounted in the grinding machine includes journal bearings 46, 48, 50, 52 and 54. A workrest 56 is carried by the machine bed 24 and located beneath journal bearing 50. The workrest includes a rigid support structure 58 carrying workpiece engaging components (not visible in Figure 5) for contacting the journal bearing 50.

A metrology probe 60 is carried by the wheelhead 26. The diameter, vertical displacement and/or horizontal displacement of a journal bearing of the crankshaft 44 may be measured by the probe by resting a suitable end effector such as a symmetrical or asymmetrical vee (not shown) on an upper surface of the bearing during rotation thereof Vertical displacement of the upper surface of the journal bearing 2 in Figure 3 due to runout is marked as displacement "d" by way of illustration. There will be an equal vertical displacement "d" of the central longitudinal axis 4 of the journal bearing. It can be seen that the midpoint of displacement d' corresponds to the axis 12 about which the central longitudinal axis 4 of the journal bearing orbits. It is therefore is possible to determine the amount of runout of the journal bearing by monitoring the variation in measured height of the top of the bearing above the machine base using probe 60 as the workpiece rotates. The difference "s" between the height of axis 12 and the height of the longitudinal machine reference axis 6 above the machine base is the vertical displacement of the journal bearing in Figure 3 due to sag.

The controller of the grinding machine is operable to determine the displacement due to sag, s, and output a drive signal to workrest 56 causing rigid support structure 58 to raise up the journal bearing by the vertical displacement s. The workrest will substantially constrain the journal bearing 2 so as to remove the effects of sag in the vertical direction.

As the journal bearing is raised by the workrest, compensating for the gravitational sag, the vertical displacement due to runout monitored by the height gauge or probe is monitored and the journal bearing is raised until the centre of the vertical 3o displacement of the journal centreline as monitored by the height gauge is substantially the same as the height of the longitudinal machine reference axis above a machine base. The actuator is then locked at this position to maintain a substantially constant upward force on the journal bearing and hold the vertical position of the bearing substantially constant.

The workrest 56 is configured so as to allow oscillation of a journal bearing supported by it in the horizontal direction. This has the result of constraining motion of the journal bearing as it is rotated as shown in Figure 6. The height of the journal bearing axis 4 above the machine base is now the same as that of the longitudinal machine reference axis. Where the runout of the journal bearing is substantially less than the gravitational sag, the journal bearing may be free to oscillate in a horizontal direction only due to runout, along path 70. The shaft runout envelope 72 has a somewhat reduced area in comparison to envelope 10 shown in Figures 1 to 3.

By way of example, the gravitational sag of a relatively low stiffness, 10 cylinder crankshaft might be around 0.7mm for example. It may require a force of more than is 4000N to remove this degree of deflection. Once the crankshaft has been lifted using the workrest as described herein so that the height variation due to runout is centred on the longitudinal machine reference axis, and the workrest locked at that height, the runout of the shaft might cause a motion in a range of between of 0.2 and 0.5mm for example.

With a journal bearing supported in this manner to counteract gravitational sag, whilst the workrest allows the bearing to move freely in the horizontal direction, it has been found that an accurate measurement of runout can be obtained. The magnitude and phase of the runout may be measured by a sensor which generates runout signals in response thereto which carry measurement data for recordal in electronic memory.

The stored measurement data may then be retrieved for use in controlling a subsequent grinding operation. For example, a grinding operation may be carried out which deliberately adds "throw" to a part such as a journal bearing to displace the finished bearing surface radially with respect to the longitudinal machine reference axis having regard to the stored measurement data. The finished bearing surface may then no longer exhibit runout and instead be in alignment with the central longitudinal axis of the workpiece.

With a journal bearing supported in a workrest which counteracts the effects of gravitational sag, at least a semi-finishing operation can be performed on adjacent journal bearings. The semi-finished adjacent journal bearings can then be constrained by workrests during grinding of other journal bearings on the crankshaft with minimised runout along the length of the shaft. Thus, workrests can be applied to semi-finished journal bearings earlier in a grind cycle, thereby optimising the overall Grind time.

A workrest 56 embodying the invention is shown in Figures 7 and 8. The workrest includes front and rear support rollers 80 and 82 which act as workpiece engaging components. They are shown in contact with a centre main journal bearing 50. The journal bearing may be 150mm in diameter for example. The rollers are disposed below the central longitudinal axis 4 of the journal bearing. In the configuration is illustrated, they are located asymmetrically with respect to a vertical plane including this axis, to minimise the risk of fouling the grinding wheel and wheelfeed assembly. The rollers are rotatably mounted on a roller carrier 84, which is in turn mounted on a tilting support arm 86. The roller carrier 84 and tilting support arm 86 together form a rigid support structure 58 for the rollers.

The tilting support arm 86 is mounted on a supporting structure 88. The tilting support arm is rotatable relative to the supporting structure 88 about a pivot 90, around a pivotal axis parallel to the longitudinal machine reference axis. The pivoting motion is controlled by a drive in the form of a linear actuator 92. The housing 96 of the actuator is mounted on the supporting structure 88, whilst the distal end of a push rod 94 of the actuator is pivotably coupled to the tilting support arm 86.

The actuator may be electrically operated. The actuator may be in the form of an electrically driven ball screw actuator for example. In other embodiments, it may be hydraulically actuated.

As the distal end of the push rod 94 is moved downwardly by the actuator, this pivots the rigid support structure about pivot 90, causing the front and rear support rollers 80, 82 to rise upwardly, exerting an upward force on the journal bearing 50.

An upper body portion 98 of the workrest comprising the supporting structure 88 and rigid support structure 86 is slidably mounted on a lower body portion 100. The lower body portion comprises a support block 102 carrying a linear slide 104, which in turn carries the upper body portion 98.

io The slide may be a rolling element slide for example. It preferably exhibits very low friction to ensure that there is only minimal resistance to horizontal motion of the supported journal bearing 50. The slide permits motion along a horizontal axis perpendicular to the longitudinal machine reference axis.

A profiled lower portion 106 of the support block and a clamp member 108 are shaped for engagement with a dovetail-shaped groove in the machine base. This allows the workrest to be moved parallel to the longitudinal machine reference axis to a desired position before being locked in place using the clamp member. To do so, the clamp member 108 may be urged against the side of the dovetail groove using clamp actuation arm 110. The arm is pivotably coupled to one end of the clamp member. A mid-portion of the clamp member is pivotably coupled to the support block 102 whilst an end of the clamp member opposite to the point coupled to the clamp arm protrudes below the support block for engagement with the dovetail groove.

The supporting structure 88 is coupled to the support block 100 by one or more retract springs 112. They are provided to bias the upper body portion of the workrest towards an end of the slide 104 furthest from the grinding wheel. This ensures that when the workrest is not in engagement with a workpiece, it is urged away from the working region, for the purposes of safety.

It will be appreciated that references herein to perpendicular or parallel relative orientations are to be interpreted as defining substantially perpendicular or parallel relationships between components within practical tolerances. Similarly, references herein to horizontal or vertical directions are to be interpreted as concerning substantially horizontal or vertical directions within practical tolerances.

Claims

Claims 1. A method of operating a grinding machine, the grinding machine comprising a machine base, two end supports carried by the machine base for supporting respective ends of a workpiece and allowing rotation of a workpiece about a longitudinal machine reference axis, and a workrest carried by the machine base between the two end supports, the workrest comprising workpiece engaging components for engaging a workpiece, an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components, and a slide which to enables the upper body portion to oscillate in a horizontal direction relative to the lower body portion, wherein the method comprises the step of rotating the workpiece about the longitudinal machine reference axis whilst the upper body portion of the workrest oscillates in the horizontal direction.is 2. A method of grinding a workpiece with a grinding machine, the workpiece comprising two opposite ends and a first journal bearing between the ends, the first journal bearing having a central longitudinal axis, and the grinding machine comprising a machine base, two end supports carried by the machine base for supporting respective ends of the workpiece and allowing rotation of the workpiece about a longitudinal machine reference axis, and a workrest carried by the machine base for supporting the first journal bearing, the workrest comprising workpiece engaging components for engaging the workpiece, and a drive for changing the height of the workpiece engaging components above the machine base, wherein the method comprises the steps of receiving a runout signal in a controller of the grinding machine, wherein the runout signal is related to displacement of the first journal bearing of the workpiece due to runout during rotation thereof outputting a drive signal from the controller to the drive in response to the runout signal; and raising the workpiece engaging components with the drive in response to the drive signal until the midpoint of the vertical displacement of the central longitudinal axis of the first journal bearing due to runout is at substantially the same height as the longitudinal machine reference axis.3. A method of claim 2 including, after the raising step, a first grinding step of grinding a second journal bearing on the workpiece whilst the workpiece is rotated by the grinding machine and the first journal bearing is supported by the workrest, wherein the first journal bearing is allowed by the workrest to oscillate in a horizontal direction perpendicular to the longitudinal machine reference axis as the workpiece rotates.4. A method of claim 3, including after the first grinding step, a second grinding o step of grinding the first journal bearing whilst supporting the second journal bearing 5. A method of any of claims 2 to 4, wherein the workrest comprises: an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components; and a slide which enables the upper body portion to oscillate in the horizontal direction.6. A method of claim 1 or claim 5 including a further step of measuring with a sensor displacement in the horizontal direction of a part of the workpiece due to runout during rotation thereof and generating a runout signal related to the measured displacement.7. A method of any preceding claim including a further step of grinding a part of the workpiece with reference to the runout signal in order to reduce the runout of the part.8. A grinding machine configured to carry out the method of any preceding claim.9. A grinding machine for grinding a workpiece, the workpiece comprising two opposite ends and a first journal bearing between the ends, the first journal bearing having a central longitudinal axis, wherein the grinding machine comprises: a machine base; two end supports carried by the machine base for supporting respective ends of a workpiece and allowing rotation of the workpiece about a longitudinal machine reference axis; a workrest carried by the machine base for supporting the first journal bearing, the workrest comprising workpiece engaging components for engaging a workpiece, and a drive for changing the height of the workpiece engaging components above the machine base; and a controller configured to: to receive a runout signal related to displacement of a first journal bearing of a workpiece due to runout during rotation thereof; and output a drive signal to the drive in response to the runout signal, wherein the drive signal causes the drive to raise the workpiece engaging components until the midpoint of the vertical displacement of the central longitudinal axis of the first is journal bearing due to runout is at substantially the same height as the longitudinal machine reference axis.10. A grinding machine of claim 9, wherein the workrest comprises: an upper body portion carried by a lower body portion, with the upper body portion including the workpiece engaging components; and a slide which enables the upper body portion to move relative to the lower body along a horizontal reference axis perpendicular to the longitudinal machine reference axis.11. A grinding machine of claim 10, wherein the slide comprises a rolling element slide.12. A grinding machine of claim 10 or claim 11, wherein the workrest includes a biasing assembly configured to exert a biasing force on the upper body portion along 3o the horizontal reference axis.13. A grinding machine of any of claims 10 to 12, wherein the upper body portion of the workrest includes a rigid support structure which carries the workpiece engaging components, and the rigid support structure is pivotably mounted on the workrest for rotation about a pivotal axis which is parallel to the longitudinal machine reference axis.14. A grinding machine of claim 13 including a drive operable to pivot the rigid support structure about the pivotal axis.15. A grinding machine of any of claims 9 to 14, including a sensor configured to measure displacement of a first journal bearing of a workpiece due to runout during a to complete rotation thereof, generate the runout signal, and send the runout signal to the controller.