The invention relates to a method for producing a hob for machining a face gear, said hob being regrindable through grinding of the cutting faces, and said hob comprising a disc-shaped element having an axis of rotation and on the outer periphery a number of cutter tooth rows which are evenly distributed over the periphery, and which are each bounded by a cutting face at the front side and by a clearance face at the outside, the clearance faces near the cutting faces forming a profile which consists of one or more teeth and extends essentially helically over the outer periphery of the disc-shaped element, each cross- section of the helical profile being derived from an imaginary gear of infinitesimal thickness, the centre point (profile centre) of which lies in a centre plane situated at right angles to the axis of rotation of the tool, and the helical profile having a lead which is such that on a complete revolution of the disc-shaped element about the axis of rotation the imaginary gear rotates about the profile centre over one or more tooth pitches, and for each cutter tooth row a distance (b) from the centre to the axis of rotation of the hob becomes smaller when moving the cross-section of the profile from the cutting face towards the rear side of the cutter tooth row.

Such a method is known, from WO 92/09395. With the known method the clearance face is machined with a grinding disc in which case there is in principle point contact between the grinding disc and the clearance face. This means that the grinding disc always has a pointed or circular contact face with the clearance face, due to the shape of the clearance face and the required accuracy the area of this contact being very small, for example approximately 0.2 mm ² . The machining of the clearance face by means of this pointed or circular contact is carried out for all points of the clearance face of the hob which come into contact with a workpiece. This means that the production of the hob is very time-consuming, and thus

expensive.

The object of the invention is to provide a method by means of which hobs for machining face gears can be produced more quickly and therefore more cheaply, and in which the clearance face is machined in such a way that the profile which is obtained after grinding of the cutting face, and with which the face gears are machined, deviates little if at all from the theoretically correct profile.

This object is achieved according to the invention through the fact that in the method mentioned in the preamble the clearance face is machined by a rotating profiled grinding disc which is moved away from or towards the axis of rotation while the hob is rotating about the axis of rotation during machining of the clearance face, and is rotated simultaneously about an axis of rotation going through the profile centre.

The clearance face in this case is machined with a profile grinding disc which is in contact with the full tooth height of a cutter tooth during the machining, with the result that all tooth flanks facing the same side have the same profile at right angles to the clearance face. The machined clearance faces of the cutter tooth are machined in one go over the full height of the tooth, with the result that considerable machining time and cost saving is achieved. This saving can amount to a reduction in the machining time to less than 0.1 of the time required in the case of the known process.

The saving in machining time can be at the cost of inadmissible deviations from the theoretically correct tool shape. This correct tool shape is determined, inter alia, by the profile of the cutting edge obtained after the grinding of the cutting face. It is possible for the deviations occurring to be too great to allow the produc¬ tion of a face gear which meets the required quality standards.

Another object of the invention is to avoid these deviations.

This is achieved by the method according to claim 3.

The deviations in the cutting edge are reduced by these measures, in such a way that those deviations are smaller than the deviations occurring through other causes.

The invention also relates to a hob for machining a face gear which can mesh with a cylindrical pinion, said hob being provided, inter alia, with a number of cutter tooth rows, each of which is bounded by a cutting face and a clearance face, the clearance face being provided with a helical profile, and is characterized in that each side of the clearance face is machined in such a way that each cross-section at right angles to the helical profile is identical.

The invention is explained below with reference to a drawing. Figure 1 shows a hob according to the prior art, in side view.

Figure 2 shows the outer periphery of the hob of Figure 1.

Figure 3 shows the hob along section III-III of Figure 1.

Figure 4 shows the machining of the clearance face of the hob with a profiled grinding disc.

Figure 5 shows section V-V of Figure 4.

Figure 6 shows a diagrammatic view of Figure 4. Figure 7 shows the movements of the hob and the grinding disc relative to each other.

In the figures the same parts are as far as possible provided with the same reference numbers.

Figures 1, 2 and 3 show a hob 1 with which a face gear 7 can be machined. In this case the hob 1 is provided with a number of cutter tooth rows 8, each of which has a cutting face 3 and a clearance face 2. There is a gash 11 between two cutter tooth rows 8. The hob 1 rotates about an axis of rotation 4 and moves along the face gear 7 during the machining. The shape of a cutting edge 10 of a cutter tooth 9 is derived from a pinion 6, the shape and number of teeth of which correspond to a working pinion which meshes with the face gear to be machined by the hob 1. The pinion 6 has a centre point 5, which lies in a centre plane 17 at

right angles to the axis of rotation 4, a distance (b) from the centre point 5 to axis of rotation 4, starting at the cutting face 3, becoming smaller for each cutter tooth row 8 on rotation of the hob 1. On rotation of the hob 1 through a full revolution about its axis of rotation 4 each cutter tooth 9 rotates through an angle ψ about the centre point 5, the angle φ being equal to one or more times 360° divided by the number of teeth of the pinion 6. This rotation produces a single-thread or multiple-thread hob in which the cutter teeth 9 are positioned, as it were, helically with a lead angle γ over the outer periphery of the hob.

For making face gears of great accuracy it is necessary for the cutting edges of the hob to be very accurate. This means that the cutting faces 3 have to be distributed accurately over the periphery of hob 1. This is carried out on the grinders designed for the purpose, in a manner corresponding to that used for sharpening hobs used in the machining of cylindrical involute gears. The cutting face 3 is usually designed as a face at right angles to the helix direction, which is somewhat helical. It is however also possible that the cutting face is a flat face.

The accuracy of the cutting edge 10 is also largely determined by the accuracy with which the clearance faces 2, which are complex in shape, are machined. Since the hob material is hardened, these faces are generally ground, and the grinding disc can be designed in the known manner as a profiled disc to which wear-resistant granules, for example of cubic boron nitride (CBN) or diamond, are applied. Also a ceramic grinding stone may be used, which is frequently sized by dressing.

Figures 4 and 5 show the machining according to the invention of the clearance face 2 of the cutter tooth row 8. The cutter tooth row 8 shown is a part of the hob 1, which corresponds to the hob shown in Figures 1 - 3 and comprises a large number of such cutter tooth rows, for example 9 to for example 30, two adjoining cutter tooth rows 8 being separated by the gash 11. The hob 1 has an outer periphery 15 and a radius (r) lying in a plane at

right angles to an axis of rotation 16 corresponding to axis of rotation 4 in Figure 1.

The cutting edges 10 of the hob 1 have a profile which is derived from the pinion 6, and which can mesh with the face gears made with the hob. These cutting edges 10 arise as an intersecting line between the cutting face 3 and the clearance face 2, the clearance face 2 and the cutting face 3 acquiring their shape through, for example, grinding. The distance from the clearance face 2 to the axis of rotation 16 of the hob 1 decreases as the distance from the cutting face 3 increases. This ensures that after the face gear has been in contact with the cutting edge 10 of the hob the clearance face 2 does not come into contact again with the face gear to be machined. The decrease in this distance can be achieved in various ways, but there is always an angle β between clearance face 2 and the outer diameter 15 which has the same value over the full width of the cutting face 3, while the angle β can vary over the length of the cutter tooth row 8. However, the most usual embodiment is that this angle β is also constant over the length of the cutter tooth row 8, with the result that the clearance face 2 becomes spiral at right angles to the axis of rotation 16. The angle β must be sufficiently large to allow sufficient clearance between the clearance face 2 and the face gears to be produced over the full tooth length of the face gear, and thus at all pressure angles of the face gear. The determining factor here is the smallest pressure angle of the face gear made with the hob. If the smallest pressure angle is, for example, 10 degrees, a clearance angle β of more than 12 degrees is needed in order to achieve sufficient clearance between workpiece and tool.

The cutter tooth row 8 is machined in a plane 18 by a grinding disc 12, which is provided with wear-resistant granules at the position of a machining profile 13. The grinding disc 12 has an axis of rotation 20 (not shown in Figure 4) , on which a centre point 21 lies in the plane through the centre of the machining profile 13.

The angle β is provided in the clearance face 2 by moving the centre point 21 of the grinding disc 12 towards the axis of rotation 16 in a direction B on rotation of the hob about its axis of rotation 16 in a direction A. At constant speed of rotation of the hob 1 this movement in direction B must be equally large for all teeth 9 in one machining plane 18, so that the angle β always has one value in one machining plane 18 over the width of the hob. In practice, this is achieved when machining the cutter tooth row 8 by making the movement towards the axis of rotation 16 always take place identically each time for each cutter tooth row 8, i.e. after each gash 11. When machining second and subsequent teeth of the cutter tooth row 8 the movement is therefore also always identical in the direction B and the angle β has the same value in each machining plane 18.

In the machining plane 18 lies the centre point 5 of the section of the helical profile, and at right angles to said machining plane 18 there is an axis of rotation 22 through the centre point 5. While the hob 1 rotating about axis of rotation 16 is being machined with grinding disc 12, the grinding disc 12 moves in the direction B towards the axis of rotation 16, while the grinding disc 12 rotates simultaneously about the axis of rotation 22, with the result that the teeth 9 are machined in the helix direction.

The centre point 21 of the grinding profile 13 of the grinding disc 12 lies at a distance (a) from the machining plane 18. This distance (a) varies during the rotation of the grinding disc 12 about the axis of rotation 22.

Figure 5 shows the machining of the clearance face 2 by the grinding disc 12 in section V-V of Figure 4. It can be seen here that in the section shown, which generally corresponds to the section of the cutting face 3, the cutter tooth row 8 has a shape which corresponds to the shape of the pinion 6 shown in Figure 3, which can mesh with the face gear produced by the hob. The centre point 5 and the teeth 9 of the pinion are visible, and the shape of

the outer periphery 15 of the hob 1 corresponds to that of the pinion 6.

Both in Figure 4 and in Figure 5 the grinding disc 12 is shown in two machining positions, namely during machining of the tooth 9 placed in the centre plane 17 and during machining of the tooth placed most towards the side (in practice this tooth will generally not be machined, since this tooth usually does not mesh with a face gear to be machined) . If section V-V is examined at various points on one cutter tooth row 8 and on various cutter tooth rows, the teeth 9 rotate about the centre point 5, and the centre point 5 moves away from and towards the axis of rotation 16 of the hob at successive positions of the section. Through rotation about the centre.point 5, the teeth 9 acquire a lead with a lead angle 7, and through movement towards the axis of rotation 16, the clearance angle β is obtained. During the machining of the clearance face 2 and the rotation of the hob 1 about the axis of rotation 16 the grinding disc 12 moves in such a way that the contact face between grinding disc and hob moves away from and towards the axis of rotation 16 and rotates about the centre point 5 which is moving away from and towards the axis of rotation 16 of the hob. When the grinding disc 12 is positioned approximately in the centre plane 17, the grinding disc 12 is positioned approximately in the direction of the lead angle γ of the teeth 9 of the hob and moves in such a way that the axis of rotation 20 of the grinding disc 12 lies in a normal plane 19, which normal plane 19 lies at right angles to the clearance face 2 at the position of the machining plane 18.

If the grinding disc 12 is in such a position that it is machining, for example, a cutter tooth 9, as shown in Figure 5, the axis of rotation 20 of the grinding disc 12 then forms approximately an angle β with the machining plane 18.

In order to produce good and accurate face gears in each section V-V over the length of the cutter tooth row 8 the intersecting line of the cutting face 3 and the

clearance face 2 of the cutter tooth row 8 must correspond to the profile of the pinion meshing with the face gear. However, the clearance face is not machined in the plane of this section V-V, but in the contact plane 19 which lies at right angles to the clearance face 2 and passes through the axis of rotation 20 of the grinding disc 12. The result of this is that the theoretically correct shape of the profile 13 of the grinding disc 12 varies with the place where the clearance face 2 is being machined, and that theoretically different grinding discs are always needed.

An approach which is found satisfactory in practice is that in which the shape of the grinding profile 13 is calculated for the position in which said profile makes the tooth space 30 in the theoretically correct manner, if the latter is placed in the centre plane 17 of the hob 1. The shape of the grinding profile is different here for the left side and the right side of the tooth space, through the fact that the lead angle plays a role in the calculation. Such a calculation can be carried out by the known computing techniques, using the known theoretical data. These calculations are comparable to the calculation of grinding discs for the hobs of cylindrical involute gears. Figure 6 shows a diagrammatic section which corresponds to Figure 5, with an arbitrary number of left flanks of the cutter teeth 9, such as are possible in such a section. A minimum pressure angle a^ of a face gear to be produced gives a contact line 23 for the contact between hob and face gear, and a maximum pressure angle α,, _^ of a face gear to be produced gives a contact line 24 for the contact between hob and face gear. The broken line indicates a maximum height 25 of the teeth of the face gears to be produced. The hatching indicates an active working area 26 of the left flanks of the hob. The double line indicates an active part 27 of the left flank, while the thin line indicates a non-active part 28.

During the machining of the clearance face 2 it is necessary for the cutting profile for the active part 27 of the flank to have minimal deviations from the theoretically

- 9 - correct profile. If the grinding disc 12 is made to rotate about the axis of rotation 22 (see Figure 4) which corresponds to the centre point 5 of the profile derived from the pinion 6, the teeth are ground in the correct profile. However, deviations arise for teeth which are not in the centre plane 17, for the above-described reasons.

In practice, it has been found possible to machine the active part of the flank with very slight deviations by giving the grinding disc another movement in addition to the rotation about the axis of rotation 22. In most cases a movement parallel to the axis of rotation 16 of the hob is found to be sufficient. The movement is calculated by means of an optimization method, and it is found that in most cases the correction for the machining of each tooth can be held constant over the length of a cutter tooth row, and that the accuracy of the cutting edges is extremely high. If corrections are made properly, the maximum deviation occurring as a result of the method described above is less than 3 micrometres (μm) in the case of a hob with the modulus value of 3 mm.

Figure 7 shows the section of the hob, with the various movements which the grinding disc 12 rotating about the axis of rotation 20 makes during the machining of the clearance faces 2, while the hob rotates about its axis of rotation 16. During the machining of the cutter tooth row 8 and the rotation of the hob 1 about its axis of rotation 16 the grinding disc 12 makes an intermittent movement B towards the axis of rotation 16 of the hob 1, and during the transfer to the next cutter tooth row, passing through the gash 11, the grinding disc 12 always moves back to its starting point in a direction opposite to B. In addition, during the machining of the cutter tooth row 8 the grinding disc rotates 12 in the direction of arrow C about the centre point 5 of the pinion at the position of the machining plane 18, with the result that the lead angle 7 is produced in the outer periphery. As a result of this lead angle 7, the grinding disc 12 must also be placed in this angle 7 through rotation about an axis 31 which lies in the machining plane 18 and is parallel to the centre

- 10 - plane 17. In order to correct the active flank for the abovementioned deviations, a further correction must be made in a direction D. Other corrections in the position of the grinding disc can also be made if necessary. A further correction of the active flank 1 can also take place by making the grinding disc 12 to carry out a small oscillating movement, for example about the centre of the active flank 27. The feed movement of the grinding is slow and the amplitude of the oscillation is very small, so that the quality and the speed of the grinding process is hardly influenced. If necessary the shape of the machining profile 13 is adapted to this oscillation. The size of the oscillating movement and the position of the oscillation point is the same for all grinding positions. They can however also vary.

In general the adaptations described before will not be necessary for producing a hob for face gears which can mesh with spur pinions. In the case wherein the pinion meshing with the face gear has helical teeth, the curvatures of the clearance face 2 will be stronger and the movements of the grinding stone 12 will be more complex. However, this has no influence on the machining time of the hob 1, so that the cost price of the hob is hardly influenced.