GB2290996A

GB2290996A - Precision finishing process

Info

Publication number: GB2290996A
Application number: GB9513576A
Authority: GB
Inventors: Franz Bauer; Wolfgang Winkelmann; Richard Humpert
Original assignee: GRIESHABER MASCH
Current assignee: GRIESHABER MASCH
Priority date: 1994-07-06
Filing date: 1995-07-04
Publication date: 1996-01-17
Also published as: SE9502377D0; DE4423422A1; GB9513576D0; SE9502377L

Description

2290996 Title - Precision finishing process The invention relates to a

precision finishing process for the precision finishing of in particular rotationally symmetrical bodies with an undef ined generating line, in particular of rolling elements and bearing journals, the workpiece being driven in rotation in a uniform manner and the precision finishing tool or the workpiece being subjected to a high- frequency reciprocating movement of low amplitude parallel to the generating line.

This process serves, e.g. for the treatment of rolling elements or roller bearings and the bearing races thereof, but also for the precision finishing of journals, e.g. of a crankshaft or a camshaft and of cams. Rolling elements with a rectilinear cross-sectional shape or a straight generating line are, e.g. cylindrical or conical. They are consequently used in cylindrical roller bearings or tapered roller bearings. Disregarding normal tolerance variations, at least the theoretical course of the generating line is undefined and not straight after the grinding operation in both cases. The same thing also applies, e.g. to crankshaft journals which are usually circular cylindrical. The surface is finished by precision finishing in order to give it a straight course.

Roller bearing manufacturers or crankshaft and camshaft manufacturers often impose the requirement of a generating line differing slightly from an exactly straight generating line, as found, e.g. after superfinishing. Precision finishing can be, e.g. finishing with the aid of a finishing stone or finishing belt. Tools of this kind may be used in association with a hard pressure shell.

The aim of the invention is consequently to develop the process described at the outset in such a manner that at least in the 2 P12874 case of the precision finishing of in particular rotationally symmetrical bodies with a theoretically straight generating line or cross-sectional shape, a defined generating line differing from the theoretically straight generating line can be produced after the last operation or the precision finishing step in the finishing process, wherein the deviation is low in accordance with the usual removal during precision finishing, but is to be obtained in a reliable manner with the high accuracy usually found in precision finishing.

In order to solve this problem, it is proposed according to the invention that the process according to the pre-characterising part of claim 1 is characterised in that the precision finishing tool, which is narrow in relation to the length of the body or the surface to be treated, is additionally subjected to a low-frequency reciprocating movement, the amplitude corresponding essentially to the width of the surface to be treated or the length of the workpiece, and that the speed of the lowfrequency reciprocating movement varies over the length of the surface to be treated or the length of the body.

It is clear that a precisely defined removal rate can be achieved locally by precise fixing of the local speed of the low-frequency reciprocating movement, i.e. the speed at which the cutting tool moves relative to the workpiece as viewed in the longitudinal direction thereof. This means, e. g. that the removal at the end regions of a cylindrical roller can be greater than in the central region, resulting finally, e.g. in a logarithmic generating line. It is of course also conceivable for the removal to be greater in the central region or in quite specific regions as viewed in the longitudinal direction than in the other regions, so that any desired generating line can be produced. However, the ends of the journals or cams or of the rotationally symmetrical bodies are 3 P12874 preferably subjected to a greater removal rate than the central region or even than the greater part of the surface as viewed in the longitudinal direction. If there is a deviation from a pure cylindrical shape or a pure conical shape in this manner, this results, e.g. in the case of corresponding rolling elements, in better centring than that obtained in the case of a strictly straight generating line for the cylinder or cone. These workpieces therefore have a defined non-straight generating line in which grinding errors are moreover compensated for. Possible workpieces are, e.g. conical rollers, cylindrical rollers, spherical rollers and their inner and outer bearing races or bearing races in general, bearings and bearing journals of crankshafts, bearings and cams of camshafts, etc.. The process can therefore be used for the treatment of straight, axially parallel or oblique and curved generating lines, and in general for generating lines which are to have a defined course after treatment.

According to the pre-characterising part of claim 1, it is provided per se that the body or the workpiece executes a uniform rotational movement, although the invention can equally be used if the tool executes this movement instead. The same thing applies to the high-frequency linear reciprocating movement of low amplitude, which, although preferably executed by the tool, can also be effected by the workpiece. The same thing applies to the additional low-frequency linear reciprocating movement. However, it is preferably provided that the low-frequency and high-frequency movements are executed by the tool. This applies above all to the centreless treatment of rolling elements.

A development of the invention provides that the speed of the lowfrequency reciprocating movements at either end of the workpiece or of the workpiece surface to be treated is reduced compared to that in the central region of the workpiece or even 4 P12874 the greater part of the length of the workpiece. However, it should be borne in mind in this connection that the removal during finishing is extremely small, amounting to only a few pm. Consequently, the deviation from the mathematically straight line obtained cannot be seen with the naked eye. E.g. in the case of a cylindrical rolling element of the usual size f or roller bearings. the deviation at the point of the greater removal, i.e. in particular at either end, is, e.g. 3 pm. it can reach a maximum of approximately 30 or 40 pm, although these are high values seldom achieved in practice. If the length of a rolling element of this kind or of a bearing journal is, e.g. 30 mm, an associated finishing stone with a width of approximately 5 mm is selected. This ratio also applies essentially to rolling elements of other sizes.

Another embodiment of the process is clear from claim 3. if the cutting tool, e.g. the narrow finishing stone operates in the region of small diameter in the case of a conical rolling element, the speed of rotation of the body or of the workpiece will be lower than in the case of the cutting taking place at the thick end of the rolling element.

Rolling elements for roller bearings of the type described are generally ground in a centreless manner and are precision finished in the same way. The relevant process for centreless mounting with the aid of two supporting rollers which can be driven in the same direction is known per se. The inclined position allows the workpieces or rolling elements to be advanced in the axial direction at the same time as the rotation thereof. However, if, according to the invention, a low-frequency linear reciprocating movement also extending in the longitudinal direction or parallel to the axis of the rolling element is then also superimposed upon the rotational movement of the workpiece and the high-frequency linear reciprocating movement, the superposition according to claim 4 P12874 must be effected in such a manner that the relative movement towards the workpiece is substantially identical in any direction of the stroke of this low-frequency reciprocating movement.

In a development, it is provided that the diameter of the workpiece or the surface quality or the course of the generating line is measured during the treatment process. In addition, the cutting tool can be specifically controlled as a function of the measured value of the in- process measuring device, thereby improving the cutting quality. E.g. the measuring device can be provided with one or two sensing elements which bear against the surface of the workpiece to be treated in addition to the finishing stone or the grinding belt. The measuring device is moved either separately from or together with the tool (stone or belt).

The invention will now be described in more detail with reference to the accompanying drawings which illustrate the process according to the invention in highly diagrammatic form by way of the treatment of a bearing journal and in which:

Figure la is a detail, e.g. of a crankshaft the bearing journals of which are to be treated by means of the process according to the invention; Figure 1b is a detail from the inner race of a ball bearing; Figure 2 is a diagram of the speed range as a function of time, and Figure 3 is another diagram showing the removal over the height of the path.

The workpiece 1 with the journal 2 is rotated about the 6 P12874 geometric axis 4 in the direction of the arrow 3. It is treated by means of a tool 5, e.g. an adjustable finishing stone or finishing belt designed in the known manner. During finishing, the already good surface is further improved in order to ensure, on the one hand, good surface quality and, on the other hand, also excellent dimensional stability.

A high-f requency linear reciprocating movement 6 indicated by two short arrows pointing in opposite directions in Figure la is superimposed upon the rotational movement. In addition, according to the invention, another movement is superimposed upon the cutting tool, i.e. a low-frequency linear reciprocating movement 7. It is indicated by two long arrows also pointing in opposite directions. According to Figure la, both movements moreover extend parallel to the geometric axis 4 of the journal or in general of the rotationally symmetrical body or workpiece, which, deviating from Figure la, can also be a rolling element as used in roller bearings. However, the process according to the invention is preferably used for treating bearing journals and rotationally symmetrical bodies which have an approximately straight generating line or a rectilinear cross-sectional shape. This means that when carrying out the process according to the invention, there is a deviation f rom the strictly mathematically straight generating line in a precisely determined manner, e.g. in that the straight generating line extends in a slightly curved manner at either end by way of a reduction of the diameter in this region or these regions. However, the reduction in diameter is extremely small, approximately a few thousandths of a millimetre.

Figure lb shows the inner race of a ball bearing as the workpiece 1. The bearing race of a self-aligning ball bearing would also be conceivable. In this embodiment, the tool 5 describes a high-frequency circular reciprocating movement 8 7 P12874 about the centre point of the circular recess 9 in the inner race. A low.- frequency circular reciprocating movement 10 about the same centre point is then superimposed upon this reciprocating movement 9. The embodiment of Figure 1b consequently corresponds to the example of Figure la apart from the directions of the movements.

Referring now to the two diagrams of Figures 2 and 3, it is clear that a low-frequency reciprocating movement is superimposed upon the highfrequency reciprocating movement. This may be constant, resulting in a zig-zag curve. It can be deduced from Figure 3 that the removal rate is thus constant over the entire length of the journal or body. E.g. in the case of a cylindrical journal, its diameter remains constant over the entire length, with the given high accuracy and good surface quality.

If, on the other hand, the superposition speed is variable, i.e. if the central region of the journal is traversed relatively quickly, this results in a removal rate which, according to Figure 3, is quite low in the central region, but increases towards either end. The generating line or crosssectional shape of this body or workpiece therefore corresponds approximately to a logarithmic curve. The central region or the greater part of the length is generally preferably determined by a rectilinear generating line part.

8 P12874

Claims

1. Process for the precision finishing of in particular rotationally symmetrical bodies with an undefined generating line, e.g. of rolling elements and bearing journals or the like, the body being rotated and the body or the precision finishing tool being subjected to a high-f requency reciprocating movement of low amplitude parallel to the generating line, the precision finishing tool, which is narrow in relation to the length of the body or the surface to be treated, being additionally subjected to a low-frequency reciprocating movement, the amplitude of which corresponds essentially to the width of the surf ace to be treated or the length of the body, the speed of the low-frequency reciprocating movement varying over the length of the surf ace to be treated or the length of the body.

2. Process as claimed in claim 1, in which the speed of the low-frequency reciprocating movements at either end of the body or of the surf ace to be treated is reduced compared to that in the central region of the body.

3. Process as claimed in claim 1 or claim 2, in which the speed of the rotational movement is varied over the length of the body or of the surface to be treated, and in particular is reduced in regions of smaller diameter.

4. Process as claimed in any one of claims 1 to 3, in which the rolling elements are transported in the direction of their longitudinal axis in the longitudinal direction of two drivable supporting rollers in the centreless finishing process and the low-frequency reciprocating movement is applied parallel to the feed direction of the bodies, the relative movement of the 9 P12874 tool and the body during the reciprocating advance and return strokes beina adjustable, and in particular substantially identical.

5. Process as claimed in any one of the preceding claims, in which the tool is moved in a linear manner in a circular path or any desired path during the reciprocating movements.

6. Process as claimed in any one of the preceding claims, in which internal and/or external surfaces are treated.

7. Process as claimed in any one of the preceding claims, in which the surface quality, the course of the generating line and/or the diameter of the workpiece is/are measured during the treatment process.

8. Process as claimed in claim 7, in which the finishing stone or the finishing belt is controlled as a function of the measured value determined by an in-process measuring device.

9. Process for the precision finishing of bodies substantially as herein described with reference to and as illustrated in the accompanying drawings.