Method for cutting a gear, gear-cutting tool and gear-cutting machine

Abstract

Method for cutting a gear (4) from a metal workpiece (2), in which a tooth flank, still having an oversize compared with its predefined final geometry, of the gear is, in one or more cutting passes in cutting engagement with one or more cutting tools (10) fed thereto, hard-fine finished with a geometrically undefined cutting edge made of cutting grains incorporated in a binder matrix, in order to produce a reflective property, existing in the final geometry, of its surface, wherein, in a cutting pass of a cutting tool (10b), both an elastically resilient mounting of the cutting grains set by its binder matrix acts on this surface property, and a cutting reduction of the oversize by at least 2 m at the tooth flank is realized by a compressive preload, set via the infeed of the cutting tool, to which the cutting engagement is subjected; as well as a gear-cutting tool and a machine tool for this purpose.

Claims

1. Method for cutting a gear (4) from a metal workpiece (2) in which a tooth flank, still having an oversize compared with its predefined final geometry, of the gear (4) is, in one or more cutting passes in cutting engagement with one or more cutting tools (10) fed thereto, hard-fine finished with a geometrically undefined cutting edge made of cutting grains incorporated in a binder matrix, in order to produce a reflective property, existing in the final geometry, of the surface of the tooth flank, characterized in that, in a cutting pass of a cutting tool (10b), both an elastically resilient mounting of the cutting grains set by its binder matrix acts on the surface property of the tooth flank, and a cutting reduction of the oversize by at least 2 m at the tooth flank is realized by a compressive preload, set via the infeed of the cutting tool, to which the cutting engagement is subjected, wherein the compressive preload is effected by a radial overfeed of at least 20 m and/or at most 100 m corresponding as a reference-related, negative oversize beyond the final geometry.

2. Method according to claim 1, wherein, in the cutting pass of the cutting tool (10b), more than 3 m and/or less than 12 m oversize is removed.

3. Method according to claim 1 wherein, for the modulus of elasticity of the cutting tool (10b), a value, measured in GPa, of less than 20.0 and/or greater than 10.0 is used.

4. Method according to claim 1 wherein the cutting grains of the cutting pass of cutting tool (10b) have a Knoop hardness in N/mm.sup.2 of more than 23,000.

5. Method according to claim 1 wherein the cutting speed (v.sub.c) measured in m/s of the cutting pass of cutting tool (10b) is greater than 42 and/or is less than 80.

6. Method according to claim 1 wherein the grain sizes of the cutting grains are within a range of greater than 5.5 m and/or the grain size is less than 18 m.

7. Method according to claim 1 wherein an average roughness depth (R.sub.z) indicated in m of the surface after the cutting pass of cutting tool (10b) is less than 1.2.

8. Method of claim 1 comprising continuous generating grinding.

9. Method according to claim 1 wherein the cutting tool (10b) includes a material selection for the binder matrix as a plastic material or a rubber material.

10. Method of claim 9 wherein the material selection for the binder matrix comprises polyurethane.

11. Method according to claim 1 wherein said cutting pass of cutting tool (10b) is a subsequent cutting pass immediately preceded by a cutting operation which reduces the oversize by a total of more than 30 m in one or more cutting passes.

12. Method according to claim 11, wherein the prior cutting comprises at most two cutting passes.

Description

(1) Further features, details, and advantages of the invention will be apparent from the following description with reference to the accompanying figures, of which

(2) FIG. 1 schematically shows a generating grinding machine,

(3) FIG. 2 schematically shows an axial section through a worm-shaped tool, and

(4) FIG. 3a, b shows Abbott curves of two gears to be cut.

(5) An exemplary embodiment of the invention is further explained below for continuous generating grinding. A rolling grinding machine which can be used for this purpose has, in terms of the workpiece, a workpiece spindle or table spindle 80, mounted in a machine bed 40, on which a workpiece wheel 2 is clamped which is already pre-toothed and has a gear 4. The type of the workpiece wheel 2 and the gear 4 is not limited any further; a cylindrical wheel is shown, but the invention is not restricted thereto, and other types of gears could also be cut. In the case of undulating workpieces, a tailstock (not shown in FIG. 1) can be provided for this purpose.

(6) In terms of the tool, a carriage assembly is provided which holds and can position the cutting tool 10 in the form of a grinding worm. Specifically, as in FIG. 1, three linear movement axes X, Y, and Z could be provided which can adjust the relative position of the tool 10 with respect to the workpiece wheel 2 by CNC-controlled machine axis movements from a controller 99.

(7) A linear carriage 50 is provided for a radial (X) feed movement. The latter carries a vertical carriage 60 for a machine axis Z running parallel to the workpiece axis of rotation C. Another carrier 70 is rotatably arranged in relation to the vertical carriage 60; the swiveling takes place in this case and the X-axis direction (swiveling axis A). The carrier 70 also includes a tangential carriage (Y-axis) with which the tool 10 can be displaced along the tool axis of rotation. In the situation shown in FIG. 1 with pivot angle A=0, the tool axis of rotation runs in the Y direction of the rectangular coordinate system X, Y, Z shown in FIG. 1. The axis of rotation of the tool about its own axis is identified as the axis of rotation with B, and the workpiece axis of rotation is identified as the axis of rotation with C.

(8) As far as described, the design can therefore be a known structure of a generating grinding machine, and correspondingly, other constructive designs and machine axis distributions can be alternatively provided.

(9) The worm-shaped tool 10 shown schematically in FIG. 2 is a combination tool with two coaxial sections 10a and 10b that are connected in a rotationally-fixed manner in this exemplary embodiment. In this exemplary embodiment, section 10a is a grinding worm designed for the roughening of gears in a continuous generating grinding methodin particular, with ceramic bondingas is also well known from the prior art. In this exemplary embodiment, the other section 10b is also a worm-shaped tool that can be, and is here, similar to section 10a-particularly with respect to the worm parameters. The width of 10b corresponds to a minimum of the engagement width between tool 10 and gearwheel 4, and a maximum of half the grinding wheel width (10a=10b)frequently within a range of 60 mm.

(10) In this exemplary embodiment, the modulus of elasticity of section 10b is 13.6 GPa when measured alone. In addition, in the present exemplary embodiment, the binding matrix is formed from a polyurethane material, and the abrasive grains are of green silicon carbide. As will be explained in more detail below, the section 10b of the screw 10 serves to bring a roughened gear 4, without an intermediate finishing pass (which is usually carried out, for example, with section 10a of the worm 10) to remove material, to the final geometry with the desired surface properties.

(11) For this purpose, the oversize relative to the final geometry is first removed in one or more roughening passes up to a residual oversize of, for example, 6 m in this embodiment. This residual oversize is therefore quite lower than is usually left during roughening for a subsequent finishing pass (typically, the finishing process is responsible for a material removal of approximately 20 m). Subsequently, the worm section 10b is used without carrying out a finishing pass, in order to remove the remaining residual oversize to the final geometry in this exemplary embodiment in only one cutting pass. With a higher radial (X) infeed than required to achieve the final geometry by means of a non-resilient tool such as section 10a, the worm section 10b is brought into a cutting engagement of continuous generating grinding with the gear 4. Due to the combination of the worm section 10b, which is designed to be resilient in this way, with the preload set to a negative oversize, material is nevertheless removed only down to final geometry, because of the resilient nature of the tool section 10b, simultaneously with a high surface quality. In addition, due to the cutting tool that is still harder, compared to pure polishing, significantly more material removal results in comparison to conventional polishing.

(12) Two specific exemplary embodiments are described below.

(13) In a first embodiment, a gearwheel with a 3 mm module, 22 teeth, and an engagement angle of 20, which is helically toothed with a helix angle of 20, is ground. The gearing width in this embodiment is 34 mm. The outer diameter and root circle diameter of the exemplary embodiment are 76.77 mm and 62.50 mm, respectively. The pre-cutting dimension before hard-fine finishing as a diametrical, two-ball dimension with a measuring ball diameter of 6 mm is 81.150 mm, and the final dimension corresponding to the final geometry is 80.605 mm, resulting in an oversize per flank of 125 m.

(14) The grinding tool is a two-component tool as shown in FIG. 2a three-start grinding worm with an engagement angle of 20 in this embodiment.

(15) The cutting speed was set to 50 m/s, and the cutting strategy is one with three grinding strokes, of which two roughening strokes with worm region 10a and a final pass with worm region 10b were used.

(16) In this specific exemplary embodiment, the radial infeed was initially 0.237 mm in the first roughening pass, and then 0.179 mm in the second roughening pass. The radial infeed set for the machine in the third cutting pass is still 0.070 mm. The nominal material removal rate Q.sub.w in [mm.sup.3/s] in the order of the strokes was 65.5; 43.7, and 43.4; at an infeed also in that order, 0.273; 0.241, and finally 0.846 mm per workpiece revolution. Relatively speaking, the infeed is therefore significantly higher in the last cutting pass than in the preceding roughening passes. This therefore allows overall short cutting times and, correspondingly, significantly better cycle times than conventional methods in which, after the roughening, another finishing pass is first interposed.

(17) Measurements of the surface quality of the gear 4 machined in this way were carried out using a Hommel-Etamic Turbowave V 7.60 (probe TKU 300, measuring range 400 m, scanning distance Lt 4.80 mm, speed (Vt) 0.5 mm/s, recording 24,000 measured values with a filter P-R-W profile according to ISO 11562 with L.sub.c (cut off) of 0.800 mm with L.sub.c/L.sub.s:OFF probe:r=5 m/90).

(18) The curve shown in FIG. 3a (right flank) was determined for the material section R-profile (Abbott curve); surface characteristic values were R.sub.a=0.09 m, R.sub.z=0.71 m.

(19) In a second exemplary embodiment, a two-stroke strategy was usedone stroke each with worm sections 10a and 10b. The gear data in this case were a 1.275 mm module with 36 teeth, engagement angle of 18, helix angle of 22 with a gearing width of 16.6 mm, outer diameter of 51.98 mm, and root circle diameter of 45.40 mm.

(20) Here, a pre-cutting dimension (in diametrical, two-ball dimension M.sub.dk with measuring ball diameter of 2.5 mm) of 53.803 mm was removed to a finished dimension of 53.275 mm (corresponds to an oversize per flank of 0.099 mm). The tool used here was a five-start grinding worm with an angle of engagement of 18; the cutting speed was unchanged compared to the first exemplary embodiment.

(21) Other process parameters set in strokes 1 and 2 were a machine axis setting for the radial infeed of 0.306 mm and 0.065 mm, respectively, and the feed per workpiece revolution was 0.227 and 0.948 mm, respectively, with a nominal material removal rate of 45 or 40 mm.sup.3/s.

(22) The tooth system processed in this way was also measured, with changed measurement parameters with regard to the sensing distance of 1.50 mm and speed of 0.15 mm/s and L.sub.c 0.250 mm.

(23) In this way, a value of 0.07 m was determined for R.sub.a, and a value of 0.51 m for R.sub.Z. The Abbott curve, again for the right flank, is shown in FIG. 3b. In addition, a core roughness depth R.sub.k of 0.22 m and reduced tip height of 0.08 m and a reduced groove depth R.sub.vk of 0.11 m were determined.

(24) In both exemplary embodiments, the user-desired surface qualities could therefore be achieved and even exceeded, with nevertheless favorable cutting times by dispensing with the usual finishing pass by the pre-cutting tool, preferably with a ceramic bond (worm section 10a).

(25) Moreover, the invention is not limited to the exemplary embodiments explicitly portrayed in the preceding description. Rather, the individual features of the foregoing description and of the claims below may, individually and in combination, be essential to the implementation of the invention in its various embodiments.

(26) Although the invention has been described in more detail in specific embodiments of continuous generating grinding, the process mechanisms and features described in the introduction can also be applied to other hard-fine finishing methods of gears.

(27) The tools for roughening and cutting with resilient mounting and a compressive preload do not have to be realized by a combination tool as shown in FIG. 2; separate tools could also be used which are clamped together in a grinding head, or they could also be provided in separate grinding heads, just as other grinding machine configurations can be used as shown in FIG. 1.