GB2155372A - Improving or forming curved tooth surfaces - Google Patents

Abstract

A method of improving or generating continuous curved tooth surfaces comprises a slow grinding step between a workpiece and a tool (16, 18) both of which rotate under power in a timed or geared relationship in a conjugate manner. The tool may be a normal mating workpiece which has at least a portion of its conjugate mating part being made undersized by a predetermined amount and being integrally coated with a grinding type of abrasive material having a thickness related to the predetermined amount to serve as the grinding member. <IMAGE>

Description

SPECIFICATION Improving or forming curved tooth surfaces.

The gear manufacturing industry has perennially been faced with the problems of producing gears with good tooth surface accuracy, eliminating machining scallops and other tooth irregularities such as pressure angle errors and lead or spiral angle errors, as well as face runout of skew axis face type gearing. These problems are often due to improper hobbing, improper handling or unusual distortion in heat treating which can result in poor profile, lead, as well as, nicks and burrs. There has been no economical way to rework these hardened gears. Attempts have been made to use plastic hones impregnated with abrasive and tungsten carbide hones have also been utilized. But, the plastic hones, being generally soft, would follow the form already on the gear and since they did not truly remove metal they gave no correction of lead or profile and did not remove nicks.In fact, the pressure of lead errors or nicks often caused the teeth of the plastic hones to be chipped or broken out resulting in such plastic hones being used primarily as a finishing tool in the removal of the surface oxides.

It should also be noted that in most equipment such as a lap, a hone, a shaver, and a burnisher only one workpiece is driven and the other is free wheeling. Grinding wheels generally only work at high speeds on a single surface at a time and must be readjusted for other surfaces. Additionally, many of these older methods require intermittent indexing of the gear to bring each tooth into confrontation with the tool as opposed to generating a continuous conjugate action.

According to a first aspect of this invention a method of improving continuous generated tooth surface accuracies and finishes in parts having a curved shape includes a slow grinding step in which a pair of workpieces both rotate in a timed or geared relationship under power and feed in a conjugate manner, one of the workpieces having at least a portion of its mating parts serving as the grinding means.

According to a second aspect of this invention a system for producing continuous generated tooth surfaces on curved shaped parts includes a machine tool, at least a pair of powered spindles operating in a timed or geared relationship with one another, means for fixing a workpiece on each of the spindles, means for adjusting the spindles and the workpieces carried on them so that each of the workpieces rotates in a timed relationship under power in a conjugate manner relative to the other workpiece, one of the workpieces having at least a substantial portion of its conjugate mating parts including means serving as a slow grinding means.

The present invention may be used for overcoming and correcting the machining or forming errors created in the course of manufacturing parts having continuous generated tooth surfaces created directly or indirectly by hobbing, milling, molding, cold or hot forming, etc. It may also be used to remove tooth errors and blank distortions created by a heat treating step and to remove tooth and face runout errors.

Preferably the one of the workpieces that serves as the grinding means is a normal conjugate mating workpiece that has a portion of it made undersized by a predetermined amount and the portion is coated with a grinding type of abrasive material having a thickness related to the predetermined amount of undersize.

Another advantage of the present invention is that grinding of predetermined shapes on the tips and roots of teeth takes place at the same time as the flanks are ground. Further grinding takes place at lower surface speeds than presently thought possible whilst still achieving a desirable finish.

The grinding is applicable to generated teeth having either equal or unequal pressure angles.

Particular examples of methods and systems in accordance with this invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a machine tool; Figure 2 is a fragmentary partially sectioned elevation through a form of face type gearing; Figure 3 is an elevation of another form of face type gearing; Figures 4a and 4b are plans of face type curved tooth coupling members which can be used to provide rotational coupling between a power source and a driven shaft.

Figure 5 is a schematic representation of a curved tooth coupling shown in Figures 4a and 4b in coaxial confrontation; Figure 6 is a fragmentary cross-section through a gear tooth coated with grinding material; Figure 7 is a side elevation of a pinion of the type that is coated and used for treating the teeth of a face type gear; and, Figure 8 is a fragmentary cross-section through a pinion member coated with grinding material and having unequal pressure angles.

The machine embodiment shown in Figure 1 is merely exemplary of the type of machine which can be utilized in the present invention which basically is a method for improving continuous generated (vis-a-vis indexing) tooth surface accuracies, finishes, elimination of machining scallops and other tooth irregularities such as pressure angle errors, lead or spiral angle errors as well as reducing or eliminating face runout of curved tooth face gearing and other similar curve shaped parts. This is accomplished by means of grinding in a timed or geared relationship under power and in a conjugate manner on a machine 10 of the type shown in Figure 1.In the illustrated embodiment the gear 12 will be acted upon by a pinion or worm 14 which is coated or plated with a grinding type of abrasive material such as a boron (one type of boron being sold under the trademark BORAZON); diamond grit; carbides; or other materials. Preferably, the abrasive material utilized should have a hardness equal to or in excess of 65Rc.

In the course of manufacturing parts by either hobbing, milling, molding, cold or hot forming, etc.

there are machining or forming errors which occur that create tooth inaccuracies and coarse finishes.

Unfortunately subsequent heat treating often causes further tooth errors and blank distortions.

To correct for these distortions a special grinding tool, similar to the roughing tool or tool used to make the gear or to make the mold or die (if it is a cold or hot formed gear), is used to finish the tooth form. This special tool has a non-interrupted helical thread form (either cylindrical, conical or hourglass style) without axial gashes of the type normally found in the forming tool or hob. The grinding portions of the tool are coated or plated with an abrasive material. This tool is then phased with the existing gear teeth and both are rotated under power with the part to maintain its conjugate relationship and thereby remove the tooth and face runout errors.Such a method is ideally adapted for use with a skew axis face type gearing of the type shown in Figure 2, which was disclosed in US-A2696125 wherein the pinion 16 is a constant lead on a tapered surface that is mated with the curved teeth on the face gear 18. Another form of curved gear tooth 20 as seen in Figure 3 is illustrated in US-A-2954704 where the pinion 22 is cylindrical in nature and has a constant lead. Basically a pinion 22a of the type shown in Figure 7 can similarly be used with a gear 20, as seen in Figure 3 and is re lated to the configuration of the hob (not shown) that is utilized for forming the face gear 20 except that the pinions 22 and 22a have continuous tooth forms that are not interrupted in the fashion of the hob or roughing tool.Such a conjugate pinion is re-worked or formed undersize to a predetermined amount t as seen in Figure 6 and with the reduced side, end and root surfaces 24 serving to carry an abrasive material 26 that has a thickness approxi mately equal to t" to thereby insure the conjugate action between the two parts. It will be appreciated that the tool 14 is generally a toughened or hard ened rigid device that serves to carry the abrasive material 26. Similarly, this method can be applied to devices wherein the gear has straight teeth and pinions that have variable leads, i.e. as shown in US-A-2896467; HYPOID gearing, and spiral bevel gears having varying leads or alternatively differ ent relative speeds of rotation in off centre relationship to insure a sliding action to permit grinding.

As was previously pointed out, Figure 6 shows a tooth form of equal pressure angles while the de vice 14a in Figure 8 shows a tooth surface 24a hav ing unequal pressure angles on opposite flanks of the tooth. In Figures 6 and 8 the coating 26 and 26a is shown to cover the tips of the teeth as well as the roots. It is totally possible that only the flanks of the tooth might be coated, however, the coating of the tips and the roots of the workpiece do permit it to grind predetermined shapes for the tips and roots of the teeth at the same time it grinds the flanks.

Application of this technique can be made to the forming tool for the devices shown in Figures 4a, 4b and 5. Such a tool, not shown, generally is a hob in the case of cut teeth. Such coupling devices are intended for applications requiring a high degree of accuracy during use and the elimination of the problems mentioned hereinbefore are extremely desirable to insure full tooth contact for coupling accuracy of the devices. The broad geometry and teachings of such a coupling member can be found in US-A-4307797.

This method of tooth correction, as has been mentioned above, is applicable to teeth with equal or unequal pressure angles so long as the teeth are generated by conjugate action. It permits grinding at a lower surface speed which can be controlled or adjusted to obtain the proper finish on the gear faces. The machines used to carry out the present invention have utilized a worm shaft speed which translates into a relative sliding velocity of the gear to pinion with linear speeds generally in the range of 45-500 ft/minute (13-150 m/ minute). Normally, grinding speeds are in excess of 2,000 ft/minute (600 m/minute).

It will be recognized that depending on the type of material used to form the gear, an abrasive coating tool of the type contemplated could be used for forming teeth and finishing them in a one step operation which would eliminate the roughing stage, i.e., this may be appropriate for fine, small, shallow depth teeth. Admittedly such a process in larger sizes would be longer in time but might affect the grain structure due to heat generation.

Conversely the method can also be applied to correct errors in the worm member, such as 16 and 22, by coating the gear member, 18 and 20, respectively, with an abrasive coating and then following the same timed or geared relationship under power in a conjugate manner to improve the tooth surface of the pinion or worm.

Claims (15)

1. A method of improving continuous generated tooth surface accuracies and finishes in parts having a curved shape including a slow grinding step in which a pair of workpieces both rotate in a timed or geared relationship under power and feed in a conjugate manner, one of the workpieces having at least a portion of its mating parts serving as the grinding means.

2. A method according to claim 1, wherein the one of the workpieces that serves as the grinding means is a normal conjugate mating workpiece that has a portion of it made undersized by a predetermined amount and the portion is coated with a grinding type of abrasive material having a thick ness related to the predetermined amount of undersize.

3. A method according to claim 1 or 2, wherein the one of the workpieces is a continuous helically threaded pinion arranged for conjugate mating action with a gear member.

4. A method according to claim 1 or 2, wherein the one of the workpieces is a gear member ar ranged for conjugate mating action with a helically threaded pinion.

5. A method according to claims 3 or 4, wherein the gear member is a face type curved tooth member.

6. A method according to any one of the preceding claims, wherein the one workpiece serving as the grinding means is used to form and finish teeth in one step on the other workpiece.

7. A system for producing continuous generated tooth surfaces on curved shaped parts including a machine tool, at least a pair of powered spindles operating in a timed or geared relationship with one another, means for fixing a workpiece on each of the spindles, means for adjusting the spindles and the workpieces carried on them so that each of the workpieces rotates in a timed relationship under power in a conjugate manner relative to the other workpiece, one of the workpieces having at least a substantial portion of its conjugate mating parts including means serving as a slow grinding means.

8. A system according to claim 7, wherein the one of the workpieces that serves as the slow grinding means is a normal conjugate mating workpiece that has a portion of it made undersized by a predetermined amount and the portion is integrally coated with a grinding type of abrasive material having a thickness related to the predetermined amount of undersize.

9. A method or system according to any one of the preceding claims, wherein the abrasive material has a hardness in excess of 65Rc.

10. A method or system according to claim 9, wherein the abrasive material is diamond, or is a boron material, or is a carbide material.

11. A method or system according to any one of the preceding claims, wherein the grinding takes place at lower linear surface speeds than normal grinding.

12 A method or system according to claim 11, wherein the surface speeds are generally less than 500 feet per minute (150/mimin).

13. A method or system according to claim 11 or 12, wherein the surface speeds are generally not less than 45 feet per minute (13 m/min).

14. A method of improving or generating curved tooth surfaces substantially as described with reference to the accompanying drawings.

15. A system for improving or generating curved tooth surfaces substantially as described with reference to the accompanying drawings.