GB2150058A - A method for the manufacture of a toothed tool for precision machining gear wheels - Google Patents

Abstract

Toothed wheel-type and similar tools with CBN-coated tooth flanks, for precision machining the tooth flanks in particular of tempered gear wheels, are ground or lapped so that the CBN grain edges 33, which are thus produced in the CBN layer, all lie on the desired flank shape 32. In this way, improved surface qualities of the workpiece are achieved. <IMAGE>

Description

SPECIFICATION A method for the manufacture of a toothed tool for precision machining gear wheels The invention relates to a method for the manufacture of a tool, which is in the shape of a tooth wheel, toothed rack or worm, for precision machining gear tooth flanks in particular of tempered toothed gear wheels in which a coating of cubic boron nitride (CBN) is applied to the tooth flanks of a toothed metal body, the CBN particles being in one or more layers, and a tool produced according to this method.

For precision machining gear tooth flanks, in particular of tempered gear wheels, tools are known, in which a toothed body is covered on its flanks with diamond grains (German Offenlegungsschrift 2 306 780). In diamond tools, wear frequently occurs when grinding iron materials and is attributed to a chemical thermal reaction at the place of contact. For this reason in recent years tools covered with cubic boron nitride (CBN) have been introduced, which inter alia have proved reliable in rotary grinding, but not in precision machining of toothed wheels (VDI-Z 1 24 (1982) no. 17 P. 657).

When precision machining the tooth flanks of toothed wheels as is known, very high demands are made on the geometrical accuracy and the surface quality. Although CBN tools produce high performance in terms of material removed and are distinguished by long service life, they are bsically not suitable for producing surfaces with a low peak-tovalley height. The reason for this is on the one hand to be seen in the geometry of the CBN cutting edges, which are more pointed and sharper than those of corundum grains. The CBN grains therefore score the surface of the workpiece even in the case of small grain sizes. On the other hand, the considerably more rigid bonding of the CBN tools has the effect that even grains which project relatively far out of the bonded mass are still held therein.Both features, the cutting geometry and the cutting ability of grains which project to a large extent, together produce a relatively large peak-to-valley height of the surface of the workpiece. Qualitatively improved surfaces can only be obtained (and only under certain conditions) if additional movements are superposed on the feed movement of the tool relative to the workpiece, for example perpendicular to the feed movement. These additional movements are however to be avoided when precision machining toothed wheels in order to save machining time, i.e. preferably work is carried out according to the so-called plunge method, i.e. the tool effects, relative to the workpiece only movement reducing the distance between the axes.

The geometrical shape of a CBN grinding disc is normally produced by truing. In this way either a so-called diamond multi-grain dressing tool is moved backwards and forwards on the working surface of the grinding disc or a diamond or SiC-containing dressing roller is pressed against the working surface, there being a relative speed between the dressing roller and the grinding disc. This truing does not, however, produce any flat surfaces, because material removal at the grinding disc is only achieved by grain breaks.

Even the application of CBN grains of equal size onto a precisely manufactured tool body did not produce a satisfactory result, since the above features are naturally also present in this case. During truing the grains are then broken off and thus the number of cutting edges is decreased.

The object of the invention is to provide a toothed wheel-shaped CBN-coated tool and a method for the manufacture thereof, which does not have the above-mentioned disadvantages and which may also be used in the above-mentioned plunge method of machining, In order to achieve the object it is proposed according to the invention to produce the precise geometrical shape of the tool tooth flanks by grinding or lapping the CBN coating. In this way the CBN grains are not broken off as in the case of truing. but the shape of the individual grain is changed by grinding or lapping such that all the active grains project to the same extent out of the bonding and thus form a plane.

This change of shape can consist in the grains being smoothed or flattened so that together they form a type of "table moun tain", the edges of which have a similar effect to the cutting edges of a shaving wheel (claim 2).

The change of shape can also consist in that by choosing the corresponding parameters when grinding, for example the grain size of the grinding disc, the advance speed, the grinding pressure, breaks at the edges of the CBN grains are produced such that the newly produced edges are at least partially in the desired tool contour. Therefore the CBN grains are not broken off as such, as in the case of truing, but the grains are "reduced" With high mechanical stress the edges of CBN grains may break. Since the new edges are to a large extent in the desired tool contour, and do not project at all threfrom. they no longer have the disadvantageous properties as do the CBN grains before grinding (claim 3).

Changing the shape by breaks at the CBN grains is considerably easier and more even if, for coating, a microcrystalline CBN type is used, on whose grains microscopic breaks are produced by grinding (claim 4).

The invention also comprises a toothed wheel-shaped tool for precision machining the tooth flanks of gear wheels, which tool has a toothed body made of metal and a coating of cubic boron nitride (CBN) on its tooth flanks, the CBN grains being in one layer or many layers, the tool tooth flanks having ground or lapped surfaces.

In order to anchor the CBN coating in an improved manner the tool may also be formed with the feature of claim 7. Thus at the same time an improvement of the cutting ability at the points of transition from the grooves to the flank sections is achieved and good drainage of the coolant when using the tool is guaranteed.

The invention is described in the following with reference to exemplary embodiments shown in the accompanying drawings in which: Figure 1 shows a cylindrically-shaped, toothed wheel-type tool and a workpiece; Figure 2 shows a hyperboloidally-shaped, toothed wheel-type tool and workpiece; Figure 3 is a cross-section through a tool tooth on an enlarged scale; Figures 4 and 5 are in each case a longitudinal section (cutaway portion) of a CBNcoated tool tooth on an enlarged scale; Figures 6, 7 and 8 show the same cutaway portion, but with a ground or lapped coating; Figure 9 is a diagrammatic arrangement of the tool after resharpening; and Figure 10 is a diagrammatic longitudinal section through a tool tooth.

Figure 1 shows diagrammatically a toothed wheel-type tool 1 which, for the purpose of precision chip-removing machining, meshes in the toothing of a spur gear workpiece 2 and rolls therewith. The axes of the tool spindle 3 and workpiece spindle 4 cross, as in gear shaving and honing. The example shows a straight-toothed tool and a helically-toothed workpiece, but any combination is possible.

Instead of the toothed wheel-type tool 1 a toothed rack-type tool may also be used. The size of the crossed axes angle pl depends on the special machining operation and on whether in addition to the toothing to be machined there is a shoulder or the like limiting the angle. Machining may take place by advancing the tool axially relative to the workpice for example in the direction of the arrow 8. If the tool is wide enough, the longitudinal advance may be dispensed with.

In this case, the rolling body is to be formed in a concave and hyperboloidal manner, i.e.

with hollow crowned tooth flanks in the longitudinal direction,as is known for example from plunge shaving. For the drive of the rolling movement normally only one of the two ele ments, for example the tool, is to be driven, whereas the other element is entrained by way of the meshing toothing. The machine on which the tool 1 and the workpiece 2 are accommodated does not need to be illustrated and described here.

As a further form of tool to which the invention is applicable, Figure 2 shows a tool 11, which is hyperboloidal, similar to a globoidal worm. This tool 11 is mounted replaceably on a diagrammatically shown tool spindle 1 3. The clamping means and the drive for the tool spindle are known and are therefore not shown. The tool engages with a straighttoothed or helically-toothed workpiece 1 2.

The workpiece is clamped replaceably on a workpiece spindle 1 4. The tool spindle 1 3 is oblique with respect to the workpiece spindle 4 with a crossed axes angle y, which on the one hand should not be chosen so that it is too small to ensure adequate longitudinal sliding and which on the other hand should be considerably smaller than go. so that the workpiece can be machinedd in its entire width (from front face 1 6 to front face 17) by the plunge method. The effective width of the tool is indicated by the broken lines 1 9, 20.

Naturally with such a tool a helically-toothed workpiece may also be machined instead of a straight-toothed workpiece 1 2.

Figure 3 shows diagrammatically a section through a tooth of the tool 1, 11. The tooth consists of a metal body 5, onto which a coating 6 of CBN grains 7 and a binder 9, for example synthetic resin, bronze or the like are applied. The CBN grains 7 may be disposed in one or more layers, as is indicated on an enlarged scale in Figures 4 and 5.

The irregular surface 31, which thus occurs, does not form a geometrically precise flank shape and is therefore ground or lapped with a diamond grinding disc or with a diamond paste so as to obtain the geometrically precise flank shape 32. This produces a surface in which the CBN grains form "table mountains" as Figure 6 shows. The edges 33, lying in the region of the flank surface, of the CBN grains 7, which are now flattened off and project out of the binder 9, form the cuting edges which are effective in precision machining the workpiece 2, 1 2.

By using appropriate parameters during grinding, for example of the grain size of the grinding disc and/or the advance speed, use can be made of a characteristic which can be observed at a high mechanical load of CBN grains: the breaking off of the grain edges.

This produces new edges or peaks 34 on the grains, which are all in the desired flank surface 32 (Figure 7).

Figure 8 shows the same situation using a midrocrystalline boron nitride. Owing to its structure it tends only to produce microscopic breaks such that in this case a plurality of new edges 35 are produced which are at least partially in the desired flank surface 32.

Although the gear finishing tools coated with CBN have a satisfactory service life, at certain intervals a "resharpening" is however necessary, i.e. base material or binder between the CBN grains must be removed. As Figure 9 shows diagrammatically, the tool 1, 1 is rolled in mesh with a counterwheel 36, and electro-corundum or a similar abrasive is introduced at the same time into the tooth mesh by means of a carrier liquid (arrow 37).

It has proved advantageous to carry out the sharpening at the same crossed axes angle yy, which corresponds to the angle used when machining the workpiece 2, 1 2.

In Figures 6 to 8 only one layer of CBN grains 7 is shown in each case. The effect, which may be achieved using the described grinding or lapping, can naturally also be achieved in the case of CBN grains disposed in multiple layers.

In order to anchor the CBN layer in an improved manner the tooth flanks of the metal body may have one or more grooves 38 or similar recesses extending from the addendum to the dedendum. In this way, as shown in Figure 10, the grooves or recesses are also coated, but in this case the coating is not ground. An improvement of the cutting ability at the points of transition from the grooves to the flank portions and good drainage of the coolant when precision machining the workpiece 2, 1 2. result from the arrangement of the grooves or recesses.

Claims (7)

1. A method for the manufacture of a toothed tool for precision machining the tooth flanks of gear wheels, in which a coating of cubic boron nitride (CBN) is applied to the toothed flanks of a toothed metal body, the CBN particles being in one or more layers, and subsequently the precise geometrical shape of the tool tooth flanks is produced by grinding or lapping.

2. A method according to claim 1, characterised in that the CBN grains of the single CBN layer or the uppermost CBN layer are smoothed down by grinding or lapping and the resultant surfaces form the tool tooth flanks.

3. A method according to claim 1, characterised in that by grinding or lapping of the CBN grains of the single layer or the upper layer of CBN, breaks occur and the peaks and/or edges thus formed are in the surface of the tool tooth flanks.

4. A method according to claim 1, characterised by the use of a microcrystalline CBN type, such that by grinding or lapping of the CBN grains of the single or the uppermost CBN layer, microscopic breaks occur, the edges of which are at least partially in the surface of the tooth flanks.

5. A toothed tool for fi'ne machining of gear wheels, when made by the method claimed in any of claims 1-4.

6. A toothed wheel-shaped tool for precision machining the tooth flanks of gear wheels, which tool has a toothed body made of metal and a coating of cubic boron nitride (CBN) on its tooth flanks, the CBN grains being in one layer or many layers, the tool tooth flanks having ground or lapped surfaces.

7. A toothed tool according to claim 5 or 6, characterised in that its tooth flanks are interrupted by grooves or similar recesses, which extend from the addendum to the dedendum and have the CBN coating without being ground or lapped.