GB2072069A

GB2072069A - Method and machine for finishing a worm screw thread

Info

Publication number: GB2072069A
Application number: GB8109183A
Authority: GB
Original assignee: Equipements Automobiles Marchal SA
Current assignee: Equipements Automobiles Marchal SA
Priority date: 1980-03-25
Filing date: 1981-03-24
Publication date: 1981-09-30
Also published as: GB2072069B; DE3111501A1; JPS56152523A; FR2479053B1; FR2479053A1

Abstract

A method of roller finishing a worm screw thread involves tightly holding a worm screw having pre-cut threads therein in engagement between a finishing roller and a counter member, itself preferably a similar finishing roller, the finishing rollers having a thread formation which conforms to the thread formation already cut on the workpiece, and the workpiece and the finishing roller(s) being each separately driven for synchronous rotation so that there is no slip between the engaging surfaces of the finishing rollers and the workpiece. A multi-spindle lathe, having one station for cutting the threads, may be used for the rolling.

Description

SPECIFICATION Method and machine for finishing a worm screw thread The object of the present invention is a roll-over finishing method for a worm screw thread and more particularly, a method for finishing a worm screw having a single-start thread or a multi-start thread.

It is generally known that roll-over finishing operation makes it possible to improve the surface condition (i.e. finish) of the components subjected thereto and to increase the surface strength of the metal from which the components are made. Using this technique, a component of a complementary shape is caused to roll, and possibly to slide, on the surface to be finished and bears on the said surface with a relatively high pressure. During this operation, surface defects due to machining are more or less crushed and the surface condition is thus improved.

In the particular case of worm gear threads, the roll-over operation is effected using finishing rollers comprising gear threads which are complementary to those whose surface condition are to be improved.

Using a first technique, the workpiece carrying the thread to be finished is tightly held between two finishing rollers whose axes are disposed parallel to, and on either side of, the axis of the worm screw thread to be finished. At least one of the two finishing rollers is caused to rotate whilst the workpiece to be finished is left to rotate freely so the rotation of the said workpiece is induced by cooperation with the driven roller or rollers of the device. In this technique, the finishing rollers have threads inclined in relation to their axes, i.e. helical gear threads.

According to a different technique, the roll-over finishing operation is effected by means of one roller whose axis is inclined in relation to the axis of the worm screw having the thread to be finished, and the bearing force of the finishing roller is compensated for by the reaction of a cylindrical supporting casing on which the periphery of the workpiece bears. In this technique, the finishing roller is caused to rotate and the workpiece is freely rotatably mounted, so that it is subjected to induced rotation deriving from the cooperation of the roll-over wheel with the worm screw thread to be finished.

In all cases, the induced rotation of the finishing roller or rollers on the one hand, and of the workpiece to be finished on the other hand, is always effected by only one of these types of members, and generally by the finishing roller or rollers.

In the present state of technology this roll-over technique of worm screw threads is known to produce relatively unsatisfactory results because the induced rotation of the worm screw thread to be finished by the finishing roller is always effected with a certain sliding action which creates a slight shift between the finishing rollers and the workpiece thread to be finished, so that if the period of roller cooperation is to be extensive there will be a displacement of the workpiece in relation to the finishing rollers or a deformation of the members engaging each other.Moreover, the fact must be taken into account that the finishing operation produces a slight plastic deformation which can only increase the relative sliding of the pieces engaging each other, and that the helices of the thread tips carried by the finishing rollers cooperate with the helices of the thread roots (dedenda) carried by the workpiece whilst they do not have exactly the same crosssection because one is below (i.e. the dedendum) and the other above (i.e. the addendum) the pitch circle on the basis of which the cooperation between the finishing roller and workpiece is calculated.

The object of the present invention is to provide a method for effecting a roll-over finishing operation on a work screw thread, whereby the operating conditions are greatly improved. It has, in fact, been found according to the invention that a very satisfactory result can be obtained in practice if the finishing roller or rollers, on the one hand, and the workpiece to be finished, on the other hand, are driven to rotate simultaneously, the induced rotations of course being effected synchronously and with indexing of the angular position between the worm screw workpiece and the finishing rollers.

This technique may be easily used when the cutting of a worm screw gear thread is effected on a multispindle lathe, and when provision is made on the same lathe for a station where the roll-over finishing operation on the workpiece can be effected after cutting of the worm screw thread has been completed. In fact, on such a multispindle lathe it is possibleto index with ease the angular position of, on the one hand, the already cut workpiece and, on the other hand, the roll-over finishing rollers, the whole assembly being driven synchronously by the same kinematic drive train included in the multi-spindle lathe.

More specifically, therefore, a first aspect of the present invention provides a roll-over finishing method for a worm screw thread formed on a cut cylindrical workpiece, such method comprising: tightly holding the workpiece to be finished between a rotating finishing roller defining a complementary worm thread and at least one counter member; and causing said workpiece to rotate synchronously with the roll-over finishing roller.

A second aspect of the present invention provides a roll-over finishing machine for finishing a worm screw thread formed on a cut cylindrical workpiece, said machine comprising a finishing roller carrying a gear thread complementary to that of the worm screw workpiece to be finished; at least one counter member; means for tightly holding a said workpiece to be finished between said finishing roller and said counter member; and a kinematic drive train effective to drive said workpiece and said finishing roller to rotate synchronously.

The invention also provides a workpiece having a worm screw thread whose surface finish has been imparted by the method of the first aspect or the machine of the second aspect.

The workpiece to be finished may be tightly held between several identical finishing rollers regularly distributed around the axis of the workpiece on axes parallel to the axis of the workpiece, said several finishing rollers comprising the first mentioned fin ishing roller and at least one further finishing roller serving as said at least one counter member.

Said several finishing rollers preferably comprise said first mentioned finishing roller and a single said further finishing roller tightly held on either side of the workpiece to be finished, the axes of said first and further finishing rollers and the axis of the workpiece being coplanar. Said first and further finishing rollers are tightened by means of a jack. Said first and further finishing rollers are mounted at the ends of two compass legs. All of said first and further finishing rollers are driven to rotate synchronously.

Said first and further finishing rollers are driven by the kinematic drive train of a multispindle lathe which has at least one working station effective to perform the cutting of the worm screw thread on said workpiece to be finished, the angular position ing of at least said first finishing roller in relation to the surface to be cut being ensured by the said kinematic drive train at the same time as imparting synchronous drive thereto. Said workpiece to be cut is a worm screw with a gear thread having one or several thread starts.

It has been found that by making use of the method according to the invention, an extremely satisfactory surface finish was obtained by the finishing operation on the worm screw treated and without encountering any operational incidents. The finishing rolling operation may be effected by causing the finishing rollers to roll over the workpiece to be finished for a very long time, i.e. in excess of a few seconds, without producing any incident since the workpiece subjected to finishing is synchronously driven atthe same time asthefinishing rollers but not merely by contact with them; there cannot be any sliding between the rollers and the workpiece to be finished because their drives are simultaneous and separate.

In orderto make the invention more readily understood there will now be described a preferred mode of implementation, given by way of a purely illustrative and non-restrictive example.

In this example, the workpiece to be finished is a double-threaded worm screw cut, by means of a multispindle lathe, in a cylindrical rod 8 mm in diameter and made of medium hard steel. The pitch of each thread is 5.2 mm and therefore the axial pitch separating two successive helices, measured along a generatrix of the worm screw, is 2.6 mm. The depth of the thread is 1.8 mm. This worm screw is obtained by cutting on a multispindle lathe in forty successive cutting passes.

One of the working stations of the multispindle lathe performs the roller finishing operation and makes use of the method according to the invention using one finishing roller as the finishing roller means and a further finishing roller as the counter member. The worm screw subjected to finishing is tightly held between the two identical finishing rol lers arranged at the ends of two compass legs. The compass legs are manoeuvred, by a hydraulic jack, around their axis of articulation and, during the roll over operation, the two finishing rollers are pressed one against the other, with a force of 500 Newtons, on either side of the worm screw to be finished. The axes of the two finishing rollers are parallel to the axis of the worm screw to be finished, and these three axes are coplanar in the position in which the finishing operation is effected.Each of the two finishing rollers has an external diameter of 34 mm and carries on its side surface acting on the worm screw a worm thread complementary in relation to the thread cut on the worm screw workpiece. One of the spindles of the multispindle lathe drives the worm screw workpiece subjected to finishing, and the lathe also drives each of the two finishing rollers, rotational synchronism being ensured by the internal kinematic drive train of the multispindle lathe. Since the worm screw workpiece subjected to finishing is cut on the multispindle lathe which also performs the roll-over finishing operation, the relative angular position of the worm screw workpiece and the two finishing rollers may be ensured by the same kinematic drive train which rotationally drives the finishing rollers.

It has been found that in these conditions, the roll-over finishing action may be maintained for more than 3 seconds without noticing any operational incidents, and that a surface finish giving a residual roughness of less than 0.2 microns on the gear thread of the worm screw workpiece is thus obtained.

Claims

1. A roll-over finishing method for a worm screw thread formed on a cut cylindrical workpiece, such method comprising: tightly holding the workpiece to be finished between a rotating finishing roller defining a complementary worm thread and at least one counter member; and causing said workpiece to rotate synchronously with the roll-over finishing roller.

2. A method according to claim 1, wherein said counter member comprises at least one further finishing roller.

3. A method according to claim 2, wherein said workpiece is tightly held between several identical said finishing rollers regularly distributed around the axis of said workpiece, the axes of said finishing rollers being parallel to the axis of the workpiece to be finished, and the said several finishing rollers comprising the first mentioned and further finishing rollers.

4. A method according to claim 2, wherein there is one said further finishing roller, and wherein the first mentioned and further finishing rollers are tightly held on either side of the workpiece with their axes coplanar with the axis of rotation of the workpiece.

5. A method according to claim 4, wherein said two finishing rollers are tightened by means of a jack.

6. A method according to claim 5, wherein said two finishing rollers are mounted at the ends of two compass legs.

7. A method according to any one of claims 2 to 6, including synchronously rotationally driving the first mentioned finishing roller and the or each said further finishing roller bearing on the workpiece.

8. A method according to any one of claims 2 to 7, wherein the first mentioned finishing roller and the at least one further finishing roller are driven to rotate by the kinematic drive train of a multispindle lathe, and the angular positioning of the first mentioned and further finishing rollers in relation to the workpiece is ensured by the said kinematic drive train simultaneously with imparting of said rotational drive thereto, said multispindle lathe having at least one working station operable to cut gear threads on the workpiece.

9. A method according to any one of claims 1 to 8, wherein said workpiece is a worm screw having a gear thread comprising at least one thread start.

10. A roll-over finishing method for a worm screw thread, such method being according to claim 1 and substantially as hereinbefore described.

11. A roll-over finishing machine for finishing a worm screw th read formed on a cut cylindrical workpiece, said machine comprising a finishing roller carrying a gear thread complementary to that of the worm screw workpiece to be finished; at least one counter member; means for tightly holding a said workpiece to be finished between said finishing roller and said counter member; and a kinematic drive train effective to drive said workpiece and said finishing roller to rotate synchronously.

12. A machine according to claim 11, wherein said at least one counter member comprises at least one further finishing roller.

13. A machine according to claim 12, wherein there is one said further finishing roller, and the first mentioned and further finishing rollers are carried by compass legs.

14. A machine according to claim 11, 12 or 13, comprising a multispindle lathe having at least one station for cutting the worm screw gear thread on a cylindrical workpiece.

15. A roll-over finishing machine substantially as hereinbefore described.

16. Aworkpiece having a worm screw thread whose surface finish has been imparted by the method of any one of claims 1 to 10 or using the machine of any one of claims 11 to 15.