GB2203079A

GB2203079A - Gear cutting

Info

Publication number: GB2203079A
Application number: GB08708411A
Authority: GB
Inventors: Bernard Harold Barber; Steven Walter Ashmore
Original assignee: Davy McKee AG
Current assignee: Davy McKee AG
Priority date: 1987-04-08
Filing date: 1987-04-08
Publication date: 1988-10-12
Also published as: GB8708411D0

Abstract

An internal gear cutting machine comprises: a hobbing machine having a hobbing slide, and an internal gear cutting attachment firmly secured to the hob slide. The attachment has a generally L-shaped body (14), and one leg of the L-shaped body is mounted on the hobbing machine. A rotatable gear cutting tool (17) carried on the other leg of the L-shaped body, and a motor (21) mounted on the body is operatively connected to the gear cutting tool (17) through a drive train (13, 12a, 16, 16). <IMAGE>

Description

Gear Cutting.

The present invention relates to the cutting of internal gears.

Conventionally long faced interna] gears with involute spl ines have been produced by slotting and short faced internal gears have been cut using planer type machine tools. The accuracy available using either of these methods is limited, typical pitching errors being of the order of 0.002 to 0.003 inches, tooth to tooth. There is a demand for internal gears with smaller pitching error, particularly as external gears can be produced to a much higher accuracy than internal gears. This is in part due to high accuracy external gear cutting machines being available, but al so because it is possible to grind external gears on computercontrolled grinding machines to improve even only moderately accurately cut gears.Such grinding techniques cannot be employed on internal gears, so the accuracy of the gear cutting process becomes crucially important. Also, short faced internal gears are now required in materials which are too hard for the planer type machine tools.

As mentioned above highly accurate machines, known as hobbing machines, are available for cutting external gears. A hob cutter is carried on a mechanism which is highly accurately postionable and moveable in three dimensions. A gear blank is held horizontally on a rotatable table and, while the table is held stationary, the hob cutter is brought into a cutting position. The hob rotates to cut the teeth vertically, that is axially along the gear blank, or at an angle to the axis of the gear.

The gear blank is subsequently rotated step by step, a process called indexing, and the cutting process is repeated so that teeth are cut about the whole circumference of the gear blank.

These machines rely on the accuracy of the hob positioning mechani sm and the accuracy of indexing of the rotatable table. By varying the described procedure different forms of external gears can be cut, all with high accuracy.

The present invention came about in attempts to employ a hobbing machine of the type set out above in internal gear cutting.

Initial attempts were not at all successful. The external gear hob cutter was replaced with an attachment including a gear cutter, at the end of the downward projecting end of an L-shaped arm. The gear cutter was connected to the existing hob cutter drive through a chain drive linkage. There was considerable chatter in the drive which meant the cutter was inaccurate in operation, thus negating the whole object of using the highly accurate hobbing machine. The chain drive was replaced by a gear system but that had severe backlash problems which again resulted in inaccurate operation. It was in this context that the present invention was made.

The present invention provides, in one aspect, an attachment for a hobbing machine or the like, for cutting internal gears comprising: a generally L-shaped body; means at the end of one leg of the L-shaped body for mounting the attachment on the hobbing machine or the like; a rotatable gear cutting tool carried on the other leg of the L-shaped body; a motor mounted on the body, having an output shaft operatively connected to the gear cutting tool through a drive train.

By providing an independent motor unit on an attachment for cutting internal gears, which can be secured to the hob slide of hobbing machine, all the problems of using the drive of the hobbing machine itself are avoided. It is thus possible to employ the accurate positioning mechanism of the hobbing machine in the cutting of internal gears.

The invention provides, in a second aspect, an internal gear cutting machine comprising: a hobbing machine having a bobbing slide; and an internal gear attachment firmly secured to the hob slide and comprising: a generally L-shaped body; means at the end of one leg of the L-shaped body for mounting the attachment on the hobbing machine or the like; a rotatable gear cutting tool carried on the other leg of the L-shaped body; a motor mounted on the body, having an output shaft operatively connected to the gear cutting tool through a drive train.

It will be appreciated that attachment will normally be demountable so that the hobbing machine can be used conventionally.

Preferably the rotatable cutter is mounted adjacent the end of the L-shaped body, and also it is preferred that the drive interconnecting the cutter and motor comprises a shaft connected at one end by a flexible coupling to the motor, and through another flexible coupling at its other end to a worm which engages with a gear, which itself is connected by a short shaft directly to the rotatable gear cutter. By keeping the drive simple and direct, possible backlash effects are minimised.

A preferred embodiment of the invention will now be descr i bed with reference to the accompanying drawings, wherein: Figure 1 is a schematic view of a convention external gear hobbing machine tool, Figure 2 is a plan view of the hobbing machine of Figure 1, Figure 3 is a schematic view of hobbing machine incorporating an internal gear cutting attachment in accordance with an embodiment of the invention, Figure 4 is a plan view of the machine of Figure 3, Figure 5 is a more detailed side elevation of a machine tool attachment in accordance with the preferred embodiment of the present invention, Figure 6 is an end elevation of the attachment shown in Figure 5, Figure 7 is a sectional end elevation of the cutting head of the attachment taken on the line B-B of Figure 8, and Figure 8 is a sectional plan view taken on line A-A of Figure 7.

Referring to Figures 1 and 2, a conventional hobbing machine has a bed 1 which carries a moveable main column 2 and machine table 3. The position of the main column 2 with respect to the table is adjustable by means of hydraulic jacks 4. The main column 2 carries hob slide 5 which can be moved up and down, and which in turn carries a machine head 6 including an arbor 7 on which is carried on hob cutter 8. As workpiece 9 is clamped to machine table 3, which is rotatable under an indexing controlling system.

The range of movements of a conventional hobbing machine are shown in Figures 1 and 2. The main column 2 is capable of transverse movement. The hob slide 5 is capable of up and down movement, the table 3 onto which the workpiece 9 is clamped is capable of rotational movement, and the hob cutter 8 is mounted on the arbor 9 for longitudinal movement and is driven to be rotated.

An attachment according to a preferred embodiment of the invention is shown attached to hobbing machine in Figures 3 and 4.

The parts of the hobbing machine in common with Figure 1 and 2 have like numerals. The major difference is in the replacement of the hob cutter 8 with the attachment 11 according to the preferred embodiment.

Referring now to Figures 6 to 8, the attachment of the embodiment consists of, a generally L-shaped body 14 on which is mounted a motor 21. At the end of one arm of the body 14 a cutter 17 is rotatably mounted on shaft 19 in bearings 22. An output shaft 23 of motor 21 is connected through flexible coupling 12a to drive shaft 13 and that shaft to a worm 15 through flexible coupling 12b. Worm 15 is supported by bearings 18 in end portion 20 of the body 14 and meshes with a worm wheel 16 mounted on the cutter shaft 19.

The range of movements of the external gear hobber adapted for use as an internal gear hobber by the addition of the present invention is shown in Figures 3 and 4. The movements of the hobbing machine parts are as before, with the attachment replacing the hob cutter and retaining the range of movement of the hob cutter. The operation of the table 3, carrying workpiece 9 now an internal gear - is not affected in any way.

Various features wi]l be apparent: the up and down movements of the hob slide 2 allows the machining of the length of the internal gear teeth, the in and out movement of the machine column relative to the table 3 allows the machining of the depth of the gear teeth, and the rotational movement of the table 3 allows indexing of the machine from one tooth to the next and control the pitching.

The motor is a variable speed AC motor 21 typically rated at 5.5 KW at 1500 r.p.m.. This motor 21 is wired into the conventional gear hobbing machine control circuits.

Claims

Claims:

1. An attachment for a hobbing machine or the like, for cutting internal gears comprising: a generally L-shaped body; means at the end of one leg of the L-shaped body for mounting the attachment on the hobbing machine or the like; a rotatable gear cutting tool carried on the other leg of the L-shaped body; a motor mounted on the body, having an output shaft operatively connected to the gear cutting tool through a drive train.

2. An internal gear cutting machine comprising: a hobbing machine having a hobbing slide; and an internal gear cutting attachment firmly secured to the hob slide and comprising: a generally L-shaped body; means at the end of one leg of the L-shaped body for mounting the attachment on the hobbing machine or the like; a rotatable gear cutting tool carried on the other leg of the L-shaped body; a motor mounted on the body, having an output shaft operatively connected to the gear cutting tool through a drive train.

3. A machine as claimed in claim 2, wherein the attachment is demountable so that the hobbing machine can be used conventionally.

4. A machine as claimed in claim 2 or 3, wherein the the rotatable cutter is mounted adjacent the end of the Lshaped body.

5. A machine as claimed in claim 2, wherein the drive interconnecting the cutter and motor comprises a shaft connected at one end by a flexible coupling to the motor, and through another flexible coupling at its other end to a worm which engages with a gear, which itself is connected by a short shaft directly to the rotatable gear cutter.

6. An attachment for a hobbing machine or the like substantially as herein described, and with reference to the accompanying drawings.

7. An internal gear cutting machine substantially as herein described, and with reference to the accompanying drawings.