GB2416725A

GB2416725A - Superfinishing device and method

Info

Publication number: GB2416725A
Application number: GB0514124A
Authority: GB
Inventors: Richard Bonachera; Raymond Millot
Original assignee: Procedes et Machines Speciales Ste
Current assignee: Procedes et Machines Speciales Ste
Priority date: 2004-07-08
Filing date: 2005-07-08
Publication date: 2006-02-08
Anticipated expiration: 2025-07-08
Also published as: ES2296455A1; GB0514124D0; DE202005010666U1; FR2872726A1; ITMI20051227A1; ES2296455B1; FR2872726B1; GB2416725B

Abstract

A device for superfinishing a rotating cylindrical workpiece 2 comprises a body 4, a grinding belt 3 and at least three jaws 8-10 radially movable relative to the body 4. Each jaw 8-10 has at least one pad 14, 15 & 18-21 fixed to it that comprises elements made of a natural mineral having a Shore hardness of at least 80. The pads 14, 15 & 18-21 may be made of stone or a mixture of quartz particles and a synthetic material such as epoxy resin. The pads 14, 15 & 18-21 press the belt 3 onto the workpiece 2. Pads 14, 15 & 18-21 can be formed into the desired shape by placing new unfinished pads on the jaws 8-10 and shaping them by rotating a former with an abrasive surface and a diameter equal to the desired diameter of the workpieces. The device can be used to finish internal combustion engine cylinder jackets and combustion engine crankshafts.

Description

241 6725 Superfinishing device and associated method The present invention

relates to a device for superfinishing the surface of a workplace and an associated method.

The present invention relates more particularly to a device for superfinishing a surface of a workpiece, particularly a cylindrical piece, comprising a grinding band intended to come into contact against the workpiece, and jaws comprising at least one pressure pad capable of keeping the grinding band against the said piece.

Currently, particularly in the motor vehicle industry, there is a search to obtain pieces having a high surface quality by eliminating as much as possible, via superfinishing devices, the shape defects resulting from machining operations carried out to obtain the said pieces. Specifically, as an indication, the cylinder jackets of an internal combustion engine for a motor vehicle having undergone a superfinishing operation can be used to reduce the combustion engine's consumption by approximately a quarter, and to reduce emission of soot particles into the atmosphere.

Furthermore, motor vehicle manufacturers are induced to lower the roughness of combustion engine crankshafts so as to be able to use powerful motors without generating possible phenomena of oscillations of the bearings supporting the crankshaft.

Through patent application US-A-2001/0041523, a superfinishing device is known comprising a body, a grinding band, and three elements forming jaws mounted mobile relative to the body. Each jaw comprises at least one pad, made of rigid synthetic material of the "Courbahne" type, shaped so as to exert a pressure force on the band in order to keep it pressed against the surface to be machined. The pad, of generally cylindrical shape and extending substantially along the workpiece, comes into lineic (linear) contact with the band. Although being relatively effective, such a superfinishing device comprising a pad made of a synthetic material of the "Courbahne" type cannot be used to obtain pieces having a required particularly smooth surface state, notably of the crankshafts intended to be used with powerful engines.

Also known, through US patent 4 682 444, is a superfinishing device comprising a body, a grinding band intended to be pressed against a workpiece, and two upper and lower jaws radially mobile relative to the body. Each jaw comprises four pads made out of a material conventionally used for such applications, for example stone. Here, the pads are mounted with a certain tilting capacity relative to the jaws. In other words, the pads are mounted floating relative to the jaws. Such a device also has the disadvantage of not being usable to obtain pieces having a very high quality surface.

It is therefore desirable to remedy these disadvantages.

It is also desirable to provide a superfinishing device that can be used to diamond grind the pads directly on the said device before carrying out the superfinishing operation.

Accordingly, the device for superfinishing a surface of a workpiece, particularly a cylindrical piece, comprises a body, a grinding band intended to come into contact against the workpiece and elements forming jaws movable relative to the body, each jaw comprising at least one pressure pad capable of exerting a radial pressure force on the band (that is, substantially in the radial direction of the workplace if cylindrical - 3 and substantially towards the centre of the workpiece) so as to keep the said band in contact with the workpiece. The device comprises at least three jaws on each of which the pad is fixedly mounted, the said pad comprising elements made of natural mineral material and also having a Shore D hardness of at least 80.

Such a device fitted with pressure pads comprising a Shore D hardness of at least 80 can be used to considerably reduce the defects caused by the machining operations that precede the superfinishing operation.

For example it becomes possible to reduce and remove, particularly effectively, the defects in the machined pieces, particularly the defects of circularity.

Specifically, a device comprising a pad having a Shore D hardness of at least 80 can be used to improve the finishing of the surface to be machined by the elimination of small-dimension facets that cannot be eliminated by means of the usual pads. It thus becomes possible, with such a device, to substantially improve the roughness of the machined pieces relative to conventional superfinishing devices.

With such a device, it is also possible to be able to grind, directly on the said device, the profile of the pads in order to obtain pads having a profile suitable for that of the workpiece. In other words, the use of pads comprising at least one rigid element having a Shore D hardness of at least 80, and the design of a device comprising three jaws and pads mounted fixedly on the said jaws make it possible to carry out a machining of the pads, directly on the said device, so that the contact surface of the pads matches the shape of the workpiece.

Specifically, it becomes possible to mount beforehand on the device unfinished pads, for example of generally rectangular shape, and to mount between the jaws a specimen piece having a nominal diameter substantially - 4 equal to that of the workplace and comprising an abrasive outer surface, for example furnished with diamonds, coming directly into contact with the pads in order to obtain a calibration of the pads during the successive rotations of the specimen piece.

Such a calibration of the pads directly on the superfinishing device is achievable thanks to the use of pads, having a sufficiently high hardness, mounted fixedly relative to the jaws, and thanks to the use of three jaws. Such an initial calibration of the pads is in effect not achievable on a device comprising two jaws furnished with pads mounted tillable relative to the jaws, nor for a device comprising pads that do not have sufficiently high hardness characteristics. In other words, the combination of a device comprising three jaws fitted with fixed pads with the use of pads having a Shore D hardness of at least 80 can be used to dispense with the operation of diamonding the pads on a specific machine, thus substantially reducing the production costs by making it possible to dispense with the machine specially dedicated to the calibration of the pads.

Preferably, the pad has a Shore D hardness lying between 82 and 90.

The pad may also comprise an element made of synthetic material.

In one embodiment, the pad comprises a cylindrical pressure surface, the pressure surface and the band being in lineic (linear) contact in a direction substantially parallel to an axis of the workpiece.

The device has the advantage of comprising pads of generally cylindrical shape extending substantially along the workplace and being in lineic contact with the grinding band. Such a device has the advantage of being able to be adapted to workplaces of different diameters without requiring a change of jaws or pads.

In another embodiment, the pressure surface of the pad is concave, and matches the shape of the workpiece. In other words, the pad comprises a substantially thinner central portion than the side portions. Such pads can be used to obtain a lesser degree of abrasion of the workpiece at the centre of the pad relative to the sides of the pad so as to obtain less machining in the central zone of the pad and thus produce a machined piece having a substantially convex shape.

Advantageously, a jaw comprises a single pad made in one piece with the said jaw.

Preferably, at least one jaw comprises two pads spaced circumferentially apart from one another.

The present invention also relates to a method of superfinishing a surface of a workpiece, particularly of a substantially cylindrical workpiece, in which the piece is rotated, jaws are moved radially towards the piece so as to come to press, via pressure pads, a grinding band on the workpiece. At least three jaws are moved radially, the pads, fixed relative to the jaws, comprising elements made of a natural mineral material and having a Shore D hardness of at least 80.

In one embodiment of the method, before the machining of the piece, the pads are machined according to a profile substantially equivalent to that of the workpiece by mounting a diamond grinding piece having a diameter substantially equal to the workpiece and comprising an abrasive outer surface.

The present invention and its advantages will be better understood on studying the detailed description of the - 6 - embodiments taken as non-limiting examples and illustrated in the appended drawings, in which: - Figure 1 is a partial view of a superfinishing device according to a first embodiment of the invention, - Figure 2 is a view in section along the axis II-II of the superfinishing device of Figure 1, - Figure 3 is a partial view of a superfinishing device according to a second embodiment of the invention, and - Figure 4 is a view in section along the axis IV-IV of the superfinishing device of Figure 3.

As illustrated in Figure 1, the superfinishing device, given reference number 1 in its entirety, comes to press a grinding band 3 against a cylindrical workpiece 2 with the axis 2a. The piece 2 may be rotated, in known manner, by an arm (not shown) conventionally used in such devices. The grinding band 3, made of synthetic material, for example polyester, comprises an abrasive surface 3a intended to be in contact with the piece 2.

The superfinishing device 1 comprises a body 4, inside which are mounted three support elements, given reference numbers 5 to 7, comprising respectively a jaw 8, 9 and 10.

The support elements 5 to 7 are circumferentially regularly distributed around the piece 2, and are radially movable, for example by cylinders (not shown), in the direction of the piece 2, in three directions which converge substantially at the axis 2a of the said piece 2. The cylinders are suitable for allowing the three jaws 8 to 10 to apply a substantially equal pressure on the grinding band 3.

The jaws with reference numbers 8 to 10, identical to one another, are mounted respectively on the support - 7 elements 5 to 7 by means of attachment elements 11 to 13.

The jaw 8 comprises a stepped bore 8a extending radially between an outer surface 8c and an inner surface 8b of the said jaw 8, the outer surface 8c coming into contact with the body 4. Inner surface here is intended to mean a surface facing the piece 2, and outer surface is intended to mean a surface facing the body 4.

Inside the larger diameter portion of the stepped bore 8a, the attachment element 11 is mounted. The attachment element 11 protrudes relative to the outer surface 8c so that it can be mounted inside a radial bore 5a of the support element 5, the said bore 5a being in the same radial plane as the bore 8a. The attachment element 11 comprises balls lla that are mobile and capable of interacting with an annular recess 5b of the bore 5a to attach the jaw 8 relative to the support element 5 and prevent any radial movement of the said jaw 8 relative to the support element 5.

The attachment element 11 also comprises a spring (not shown) that can be compressed by means of a rod (not shown) passing through the reduced diameter portion of the stepped bore 8a so as to release the balls lla in order to allow the jaw 8 to be removed from the support element 5. The attachment element 11 allows a rapid removal and a rapid mounting of the jaw 8 onto the support element 5. Naturally, it can also be envisaged to provide other attachment elements used to provide the link between the jaw 8 and the support element 5, for example screws, or else to produce the jaw 8 and the support element 5 in a single piece.

The jaw 8, having an axial dimension substantially equal to that of the support element 5 (Figure 2), - 8 comprises two pads 14, 15, sleevejointed inside the housings 16, 17. In other words, the pads 14, 15 are mounted in fixed manner relative to the jaw 8.

The pads 14, 15 here are symmetrical with one another relative to a radial plane passing through the centre of the stepped bore 8a. The pads 14, 15 are spaced circumferentially apart from one another.

A portion of the pads 14, 15 protrudes radially, towards the piece 2, relative to the inner surface 8b of the jaw 8. The pads 14, 15 have here an axial dimension substantially equal to the axial dimension of the jaw 8 (Figure 2). The pads 14, 15 comprise respectively a surface of contact 14a, 15a with the grinding band 3, the said surfaces 14a, 15a substantially matching the shape of the outer surface 2b of the piece 2. The pads 14, 15 are shaped to come to press against the outer cylindrical surface 2b of the piece 2 and have, in cross section, a C-shaped profile open in the direction of the piece 2. In other words, the pads 14, 15 comprise a central portion having a radial dimension that is reduced relative to the radial dimension of the side edges of the pads 14, 15 so as to obtain a slightly convex machined surface.

The pads 14, 15 are made of a material having a Shore D hardness greater than 80. Advantageously, the pads 14, have a Shore D hardness lying between 82 and 90.

The pads 14, 15 may advantageously be made of a natural mineral material, for example stone, or else of a rigid synthetic material comprising elements made of natural material, particularly quartz. Advantageously, the pads are made of a synthetic material, for example an epoxy resin comprising particles of natural quartz, such as a synthetic resin of the "Belzona" type. Naturally, it is also possible to use different pads 14, 15 having a 9 - Shore D hardness greater than 80, for example pads obtained via an electrolytic deposition method.

In a similar manner, the jaws 9, 10 comprise respectively pads 18 and 19, 20 and 21, of the same type as the pads 14, 15 previously described, furnished respectively with surfaces of contact 18a and 19a, 20a and 21a with the grinding band 3 and made of a material identical to that of the pads 14, 15.

The pads 14, 15 and 18 to 21, disposed in one and the same axial plane, are used to press the grinding band 3 against the outer surface 2b of the piece 2 so as to come to machine the said surface 2b, the grinding band 3 surrounding the piece 2 and here passing between the pads 17 and 18.

In the embodiment illustrated, the pads 14, 15 and 18 to 21 are dimensioned and mounted so as to obtain a regular angular space between two successive pads. The pads 14, 15, and 18 to 21 here extend substantially over an angular sector lying between 40 and 50 .

Naturally, it may also be envisaged to provide pads 14, and 18 to 21 having a reduced circumferential dimension so as to obtain a greater angular space between two pads of different jaws relative to the angular space separating two pads of one and the same jaw.

In a variant embodiment, it may also be envisaged to provide jaws 8 to 10 each comprising two pads of generally cylindrical shape extending substantially along the piece 2 and comprising an axis substantially parallel to the axis 2a of the piece 2, the pads being spaced circumferentially apart from one another and coming into lineic contact with the grinding band. Such an embodiment has the advantage of allowing the device to be adaptable for a large number of workplace diameters. -

The superfinishing device 1, thanks to three jaws fitted with fixed pads, allows an isostatic mounting of the piece 2. Thus the device 1, with the isostatic mounting of the piece 2 and the use of pads having Shore D hardness characteristics of at least 80, can be used to carry out the machining and calibration of unfinished pads, for example of generally rectangular shape (produced in dashed lines in the figure), having a nominal diameter substantially equal to that of the workpiece and comprising an abrasive outer surface so as to obtain pads shaped for the cylindrical surface 2b of the workplace 2 later.

The embodiment illustrated in Figure 3 differs from the preceding embodiment in that each jaw 8, 9, 10 comprises respectively a single pad with reference number 22, 23, 24. The pad 22, 23, 24 is made in one piece with the jaw 8, 9, 10. The pads 22, 23, 24 here extend substantially over an angular sector lying between 100 and 110 and have an axial dimension substantially equal to those of the jaws 8 to 10 (Figure 4).

In a manner similar to the embodiment, the pads 22, 23, 24 comprise respectively a surface of contact 22a, 23a, 24a with the grinding band 3 that matches the shape of the outer cylindrical surface of the piece 2.

The pads 22, 23, 24, made of a material having a Shore D hardness greater than 80, may advantageously be made of the same type of material as the pads 14, 15 and 18 to 21 (Figure 1) described in the previous embodiment.

Irrespective of the embodiment of the invention, the superfinishing device offers many advantages, including that of obtaining a particularly satisfactory surface state of the machined pieces, but also a possible use of the device as a machine allowing the calibration of the pads thus avoiding the use of a specific machine that generates additional production and maintenance costs. - 12

Claims

1. Device for superfinishing a surface of a workpiece, comprising a body, a grinding band intended to come into contact with the workplace and jaws radially movable relative to the body, each jaw comprising at least one pressure pad capable of exerting a pressure force towards the centre of the workplace on the band so as to keep the band in contact with the workpiece, characterized in that the device comprises at least three jaws each having a pad fixedly mounted thereon, the pad comprising elements made of natural mineral material and also having a Shore D hardness of at least 80.

2. Device according to Claim 1, characterized in that the pad has a Shore D hardness between 82 and 90.

3. Device according to Claim 1 or 2, characterized in that the pad comprises an element made of synthetic material.

4. Device according to any of Claims 1 to 3, characterized in that the pad comprises a cylindrical pressure surface, the pressure surface and the band being in lineic contact in a direction substantially parallel to an axis of the workpiece.

5. Device according to any of Claims 1 to 3, characterized in that the pressure surface of the pad is concave, and matches the shape of the workpiece.

6. Device according to any of the preceding claims, characterized in that at least one of the jaws comprises a single pad made in one piece with the said jaw.

7. Device according to any of Claims 1 to 5, characterized in that at least one jaw comprises at - 13 least two pads spaced circumferentially apart from one another.

8. Method of superfinishing a surface of a cylindrical workpiece, in which the piece is rotated, and jaws are moved radially towards the piece so as to come to press, via pressure pads, a grinding band on the workpiece, characterized in that at least three jaws are moved radially, the pads, fixed relative to the jaws, comprising elements made of a natural mineral material and having a Shore D hardness of at least 80.

9. Method according to Claim 8, characterized in that before the machining of the piece, the pads are machined according to a profile substantially equivalent to that of the workpiece by mounting a diamond grinding piece having a diameter substantially equal to the workplace and comprising an abrasive outer surface.

10. A device or a method substantially according to either of the embodiments shown in the drawings and/or

described in the description.