Field of the disclosure

The present disclosure relates to a workrest for a grinding machine, and more particularly to such a workrest which is suitable for use in centreless grinding.

Background to the disclosure

Thin, annular workpieces such as bearings may contain significant internal stresses. They may be clamped during a machining process using a chuck. However, once the component is released from the chuck after machining, the internal stresses may cause the workpiece to deform quite severely.

In centreless grinding, a side face of the workpiece is pulled against a faceplate, usually using magnetic forces, and then the workpiece is rotated at a set distance from the centre of rotation of faceplate. Shoes are used to support the workpiece at the required offset position by engaging with an outer circumferential surface of the workpiece. The shoes exert forces on the workpiece which act in directions parallel to the plane of the faceplate. A pair of shoes may be used to support the workpiece, with one shoe located near the grinding wheel and the other on the opposite side of the workpiece to the grinding wheel.

A workpiece holding assembly for centreless grinding is disclosed in WO2021/094756 (filed by the present applicant).

The shoes may be provided with a pivoting arrangement to allow the shoes to rock relative to their support about a pivot which is parallel to the rotational axis of the faceplate.

Summary of the disclosure The present disclosure provides a workrest for a centreless grinding machine, the workrest comprising: a mount for mounting on a grinding machine; and a shoe holder for carrying a shoe to be engaged with an outer circumferential surface of a workpiece during grinding, wherein the shoe holder is able to rotate relative to the mount about at least two non-parallel rotational axes.

The workrest provides at least two rotational degrees of freedom for the shoe holder, thereby enabling the workrest to accommodate different variations in the profile of a workpiece and/or any misalignment of the shoe holder or shoe. This reduces the likelihood of a workpiece only engaging a shoe at an edge, which may lead to scoring of the surface of the workpiece.

Preferably, the shoe holder is able to rotate relative to the mount about three mutually perpendicular rotational axes. This greater range of motion of the shoe holder relative to the mount enables the workrest to accommodate a greater variety of misalignments between the workpiece surface and the mount whilst keeping the shoe aligned with surface.

In a preferred example, the shoe holder is able to rotate relative to the mount in all directions. In this case, workrest may be able to facilitate rotation of the shoe holder relative to its mount in any direction, including twisting motions.

The shoe holder may have a convex engagement surface which defines part of the surface of a sphere, with the mount having a complementary concave engagement surface. Engagement surfaces of this form facilitate greater freedom of movement between the shoe holder and its mount relative to a single pivot. This enables the workrest to accommodate a wider variety of variations in the workpiece surface. Also, this freedom of motion of the shoe holder means that the manufacturing tolerances for the workrest components can be significantly greater. The shoe holder may comprise a shoe support for receiving one or more shoes and a coupling arrangement for coupling the shoe support to the mount, with the coupling arrangement enabling rotation of the shoe support relative to the mount.

In some examples, the amount of resistance to movement of the shoe holder relative to the mount is adjustable. As a result, the stiffness associated with rotation of the shoe holder relative to the mount can be adjusted to suit the requirements of a particular machining process.

The shoe holder may be held against the mount by a biasing force which is adjustable.

The workrest may include a retainer for holding the shoe holder against the mount.

The retainer may have a concave engagement surface, and the shoe holder may have a complementary convex engagement surface which defines part of the surface of a sphere.

In preferred examples, the distance between the retainer and the mount is adjustable. It is then possible to adjust the force exerted on the shoe holder by the retainer by altering the distance between the retainer and the mount.

The retainer may be urged away from the mount by a biasing arrangement. The biasing arrangement may comprise at least one resilient biasing member. The biasing member may be in the form of a Belleville washer, for example.

The present disclosure also provides a grinding machine including a workrest as described herein, which in preferred examples is a centreless grinding machine.

Brief description of the drawings

A known workrest configuration and examples of the present disclosure will now be described with reference to the accompanying schematic drawings, wherein: Figure 1 is a perspective view of a known workrest including pivotable shoes, for use in a centreless grinding machine;

Figures 2 to 4 are diagrams illustrating misalignments of a workrest with a workpiece; Figure 5 is a cross-sectional view of the workrest shown in Figure 1;

Figure 6 is a perspective view of a workrest according to an example of the present disclosure;

Figures 7 and 8 are cross-sectional views of the workrest shown in Figure 6;

Figure 9 is an enlarged perspective view of components of the workrest shown in Figure 6; and

Figures 10 to 13 show the workrest of Figure 6 in combination with workpieces at different orientations relative to the workrest.

Detailed description

Figure 1 shows a known workrest configuration 2, in which shoes 6 are pivotable about a single rotational axis. The workrest comprises a mount 4 to be attached in use to a grinding machine. A pair of shoes 6 is carried by a shoe holder 8. The shoe holder is rotatably coupled to the mount by a pivot pin 10. This enables the shoe holder to rotate relative to the mount about a rotational axis 12. To ensure that the shoe holder is constrained so as to rotate as intended about axis 12, the engagement surfaces of the mount, pivot pin and the shoe holder need to be machined to a high degree of accuracy.

The inventor has realised that a workrest of this form often results in the shoes being slightly misaligned with the surface workpiece engaged by the shoes. Although the pivoting motion is able to accommodate some variations in the profile presented by the workpiece to the workrest, in some cases, there may be other variations in the workpiece profile which caused workpiece to only engage with an edge of a shoe. This may lead to formation of score marks on the surface of the workpiece.

Some potential variations in the workpiece profile from the desired profile are illustrated in Figures 2 to 4. Each of these Figures schematically shows a workrest in contact with the workpiece, in side view in a direction perpendicular to the rotational axis 12 of the shoe holder 8. Each workpiece is annular and has a central axis extending parallel to the rotational axis 12.

Each of Figures 2 to 4 shows a side view and two further partial side views at successively increased levels of enlargement.

In Figure 2, the face 14 of the shoe 6 for engaging the workpiece 20 lies in a plane which is at 90° to a reference plane 16 extending perpendicular to the rotational axis 12 . The outer circumferential surface 18 of the workpiece however deviates slightly from the expected orientation and subtends an angle of 90.5° with the reference plane 16. As a result, only an upper edge portion 14a of the face 14 of shoe 6 is in contact with the workpiece and a gap 19 exists between a lower edge portion 14b of the face 14 and the workpiece 20.

In Figures 3 and 4, the shoe 6’ of shoe holder 8’ is configured to present a face 14’ for engaging workpiece 22, with the face lying in a plane which is at 65° to reference plane 16. In Figure 3, the outer circumferential surface 18’ of the workpiece 22 subtends an angle of 65.5° with the reference plane 16 and in Figure 4 surface 18’ of workpiece 24 subtends an angle of 64.5° with the reference plane. This leads to formation of a gap 30 between the lower edge portion of the shoe 6’ in Figure 3 and the workpiece 22, and a gap 32 between an upper edge portion of the shoe 6’ in Figure 4 and the workpiece 24.

In examples of each of Figures 2 to 4, it can be seen that slight misalignment between the engagement face of the shoe and the workpiece results in only a small area of contact between the shoe and the workpiece at one edge of the shoe. This is likely to lead to the shoe damaging the surface of the workpiece.

A further disadvantage associated with the known pivotable shoe configuration shown in Figure 1 is that forces exerted on the shoe during grinding act on a central portion of the pin, whereas these forces are resisted by the shoe holder at the outer ends of the pin. This means that the forces acting on the pin have a shearing effect, increasing the risk of deformation of the pin leading to misalignment of the shoe. This is illustrated by Figure 5.

In Figure 5, arrow 40 schematically indicates the distribution of forces acting on the shoe through the workrest. A force exerted on the shoe 6 by a workpiece acts via the shoe holder 8 on a central portion 10a of the pivot pin 10. This force is then transmitted in turn from the pivot pin into the body of the mount 4 via the outer ends 10b and 10c of the pivot pin. This causes substantial shearing forces to be experienced by the pivot pin which are concentrated in the region is marked by lines 42 and 44 in Figure 5.

Figure 6 shows a workrest 50 according to an example of the present disclosure and longitudinal cross-sectional views are shown in Figures 7 and 8. Workrest 50 comprises a mount which includes a support block 52 for attaching to a support 53 (see Figure 8) using a pair of fasteners 54. The mount also includes a ball carrier 56 which is attached to the support block 52 by fasteners 58 and 60 (visible in Figures 7 and 8).

Workrest 50 further includes a shoe holder embodied by a shoe support 62 which is mounted in a slot 64 cut into a spherical ball 66. The shoe support is attached to the ball by a fastening bolt 68. A pair of shoes 70 and 72 is mounted on the shoe support 60. The shoes may be fixed in position on the shoe support using an adhesive for example. The ball carrier 56 defines an engagement surface 74 for contacting the ball 66.

A retainer 76 surrounds the ball 66 and is fastened onto the outer end of the ball carrier 56 by four bolts which are not visible in the drawings. The bolts are inserted axially into the assembly via respective openings 84 in the front face of the retainer 76. The retainer defines an inwardly facing engagement surface 78 for engaging with the ball 66. The diameter of an outer portion of engagement surface 78 is less than the diameter of ball 66. Accordingly, the retainer acts to retain the ball in position against the engagement surface 74 of the ball carrier. The engagement surfaces 74, 78 of the ball carrier and the retainer are machined to be complementary to the outer surface of the ball 66. Accordingly, the ball is able to rotate in all directions, that is, about any axis passing through the centre of the ball, from a central orientation in which a centre line 80 of the shoe holder is aligned with the longitudinal axis 82 of the mount.

Figure 8 illustrates how forces exerted on a workrest according to an example of the present disclosure are distributed in contrast to the force distribution in the prior workrest illustrated in Figure 5. Figure 8, the force distribution is indicated schematically by arrow 90. A force exerted on the shoe 6 by a workpiece acts via the shoe support 62 onto the ball 66. This force is then transmitted from the ball into the ball carrier 56 via the engaged portions of the respective engagement surfaces 74 and 78. The engaged portions are highlighted by lines 92 and 94 in Figure 8. In contrast to the configuration of Figure 5, it can be seen in Figure 8 that these forces are transferred in line with the direction of the forces acting on the workrest over a large surface area of direct contact between two solid bodies. This provides a substantially more robust configuration, with greater stiffness along the direction of the forces acting on the workrest.

Figure 9 shows the workrest of Figures 6 to 8 with the retainer 76 removed. In this view three of four Belleville washers 92 are visible. In the assembled workrest, these washers are located between the ball carrier 56 and the retainer 76. The washers are equally circumferentially spaced around the engagement surface 74 of the ball carrier between planar transverse surfaces of the ball carrier and retainer.

When the retainer is fastened onto the ball carrier, it is spaced from the front face of the ball carrier by the washers, leading to the formation of a gap 86 therebetween (indicated in Figures 7 and 8). The washers act to urge the retainer away from the ball carrier. By tightening the bolts which fasten the retainer to the ball carrier it is possible to increase the clamping forces (and therefore the frictional forces) exerted on the ball by the retainer and ball carrier. In this way, the resistance to movement of the ball 66 relative to its mount can be adjusted. Preferably, the ball 66 is formed of hardened material with a high precision spherical surface. For example, a commercially available ceramic ball may be suitable. The complementary spherical surfaces of the retainer and ball carrier may also be precisely manufactured. This may be facilitated by machining the spherical surfaces of each component simultaneously, using a suitable spacer to correspond to the gap formed by the washers in the complete assembly.

In a preferred example, a coating may be applied to the spherical engagement surfaces of the retainer and ball carrier to reduce friction and increase wear resistance. This coating may be a Diamond-Like Carbon Coating (DLC) for example. Selection of suitable materials for the ball and the corresponding engagement surfaces may make the hardness of these components higher than that of the material being removed by the grinding process.

It is desirable for the face of the or each shoe to be precisely machined to lie in a specified plane. Electrical discharge machining (EDM) may be used for example, once the workrest has been assembled.

Figures 10 to 13 illustrate accommodation of different workpiece-to-workrest mount orientations using an example of a workrest according to the present disclosure. Different alignments of the workpiece as shown in Figures 11 to 13 are exaggerated for the purposes of illustration.

Figure 10 shows an annular workpiece 96 in engagement with the workrest 50. The workpiece is orientated with its central axis 98 in the plane of the drawing and perpendicular to the longitudinal axis 82 of the mount. The workpiece has a cylindrical outer surface. In this idealised configuration, centre line 80 of the shoe holder is aligned with the longitudinal axis 82 of the mount and the ball is not rotated about the centre line 80 from its centred position.

Figure 11 illustrates a scenario in which workpiece 96 presents an outer circumferential surface to the workrest which is tilted relative to the longitudinal axis of the mount such that the central axis of the workpiece lies in the plane of the drawing, but is no longer perpendicular to the longitudinal axis of the mount. Due to the freedom of movement of the ball relative to its mount, the workrest is able to alter the angle at which the shoes are presented to the workpiece in order to accommodate this variation.

In Figure 12, the workpiece is viewed in a direction parallel to its central axis 98. This central axis has been displaced downwardly in the drawing relative to the longitudinal axis 82 of the mount. The workrest is nevertheless able to maintain its shoes in contact with the outer circumferential surface of the workpiece by rotation of the ball in a direction perpendicular to the rotation shown in Figure 11.

A further example is shown in Figure 13, in which the central axis 98 of the workpiece 96 has been tilted out of the plane of the drawing. This has been accommodated by the workrest by rotation of its ball about the longitudinal axis 82 of the mount. Thus, Figures 11 to 13 illustrate rotation of the ball about three mutually perpendicular axes so as to demonstrate its freedom of motion.

It will be appreciated that references herein to perpendicular or parallel relative orientations and the like are to be interpreted as defining perpendicular or parallel relationships between components within practical tolerances.