WO1991011760A1

WO1991011760A1 - Machine tool apparatus

Info

Publication number: WO1991011760A1
Application number: PCT/GB1991/000112
Authority: WO
Inventors: Malcolm Thorneycroft; Colin Wyatt
Original assignee: Matrix Churchill Limited
Priority date: 1990-01-27
Filing date: 1991-01-25
Publication date: 1991-08-08
Also published as: GB9001906D0

Abstract

In machine tool apparatus non-circular profiles are cut on workpieces by rotating the workpiece about its axis and locating a cutting tool and toolholder for displacement parallel to and radially of said axis during cutting. Movement of the tool in the radial direction is according to the angular position of the workpiece about the axis of rotation. Such radial movement may be achieved by a cam and camfollower arrangement, the cam drive being controlled according to the position of the workpiece about its axis.

Description

Machine Tool Apparatus

This invention relates to machine tool apparatus which includes a rotating component or workpiece which is worked by a tool, for example lathes and turning machines.

When the finished workpiece does not have a profile which is circular about the axis of rotation, for example elliptical, eccentric and cam forms; other forms which include threads and protrusions, various methods have been used to produce these forms. These have included electrical and hydraulic copying techniques. It has been found difficult to adopt direct numerical techniques to provide such forms at an adequate speed and because of limitations in response time of currently available drives.

An object of the invention is to provide a machine tool which is capable of producing a wide variety of workpiece profiles at increased feed rates and speeds.

According to the invention machine tool apparatus includes a workpiece holder rotatable about an axis, a tool for operating on the workpiece, drive means for rotating the holder about its axis and drive means for positioning the tool at a variable position adjacent the workpiece whereby the tool can be moved at a predetermined rate in the direction of said axis and in a direction radially of said axis, the drive means being controlled by control means so that said radial movement is a radially reciprocal movement according to the angular position of the workpiece about the axis of rotation of the workpiece and the position of the tool in the direction of said axis of rotation.

The control means preferably includes programming means whereby the reciprocal movement of the tool is determined, at least in part, by the angular position of the workpiece about its axis of rotation. The movement may also be determined by the speed of rotation of the workpiece holder. The reciprocal movement is preferably generated by cam means, conveniently a mechanical cam and follower arrangement, whereby rotation of the cam at the desired speed causes the follower, attached to the tool holder, to move reciprocally thereby moving the tool.

In one arrangement the tool holder is mounted on slide means to move reciprocally transversely relative to the axis of rotation of the workpiece, the tool holder moving via the slide means in response to the interengagement of the cam and cam follower.

In another arrangement the cam follower is associated directly with the mounting for the tool without the intermediary of a slide. The latter arrangement is particularly appropriate for use with turret mounted tools such as used in CNC machines by which different tools can be indexed to the operative position.

Instead of using a mechanical cam and cam follower arrangement it may be possible to achieve the desired reciprocation using a drive motor with variable displacement through a rotary to linear conversion mechanism such as a screw or a crank.

Further features of the invention will appear from the following description of embodiments of the invention given by way of example only and with reference to the drawings, in which:-

Fig. 1 is a schematic diagram of machine tool apparatus according to the invention,

Figs. 2a and 2b are end views of workpiece profiles which the machine tool can produce,

Fig. 3 is a side view of an alternative arrangement of tool holder to that of Fig. 1, and Fig. 4 is an enlarged view of part of the holder of Fig. 3.

Referring to the drawings and firstly to Fig. 1 a machine tool includes a base A on which the various parts are mounted. A headstock 3 mounted on the base A carries a spindle 16 on which a workpiece 4 to be machined is carried.

The spindle 16 is driven by a drive motor 1 through a transmission 2 consisting of belts or gears and the spindle drives a differential mechanical gearbox 5 through a belt 17.

An electronic encoder 7 and a servo motor 6 are provided so that rotation of the servo motor 6 provides a phase shift of the encoder 7 relative to the speed of the main spindle 16. Thus the servo motor 6 acts as part of an angular position control system using an encoder 18 on the gearbox 5 as a means of velocity and control feedback.

Signals from the encoder 7 are fed to an electronic 'gearbox' 30 by which the pulses or other signal information can be altered by, for example, a multiple or division effected by the gearbox 30.

A tool 19 for cutting the workpiece 4 is mounted on a tool post or turret 20 which in this embodiment, is carried on a sub-slide member 21 which is located and slides along cylindrical guides 22 which are hydrostatically lubricated. The guides 22 are rigidly attached to a main slide member 12, although shown separately in Fig. 1 for clarity.

The main slide member 12 is slidable along guides 15 for the initial positioning of the tool 19 radially of the workpiece prior to operation by means of a ball screw 13 driven by a servo motor 14 having an encoder feedback 28. The position of the slide member 12 determines the nominal diameter to be given to the workpiece 4. The sub-slide member 21 is reciprocally moved along the guides 22 by cam 24 or cam 24A and cam follower 23 or 23A carried on arm 23B and driven by a servo motor 26 having an encoder 27. Drive from the motor 26 to the cams 24 and 24A is through a transmission 25.

The cams 24 and 24A are carried on a common shaft 25A and the cams 24 and 24A are slidable along the shaft 25A by, for example, a splined arrangement so that one or other of the cams 24 and 24A is in driving engagement with its associated follower 23 or 23A. By the use of two or more cams with different cam profiles different shapes can be generated without the need for the operator to change the cam profile.

Operation of the cam 24 or 24A and follower 23 or 23A causes the sub-slide member 21 and hence the tool 19 to reciprocate in a direction radially of the axis of rotation of the workpiece 4. The nature of the reciprocating movement is determined in part by the cam 24, 24A profile but also by the control signals to the motor 26. Thus the speed of reciprocation and position of the motor 26 compared with the angular position of the spindle 16 is controlled by processing he signal pulses from the encoder 7 and the encoder 27 as modified in the electronic gearbox 30.

The cam profiles 24, 24A can, of course, also be changed according to the cutting operation to be performed.

The position of the tool 19 longitudinally of the axis of the spindle 3 is changed by movement of a carriage 11 to which the slideways 15 are rigidly attached. The carriage 11 moves in said longitudinal direction supported on slideways 8 carried directly on the bed A. A servo motor 9 effects this movement through a ballscrew 10 and the motor 9 carries an encoder 29.

Figs. 2a. and 2b gives examples of profiles which may be obtained with the machine tool. In Fig. 2a there is shown in end view a workpiece having a part-cylindrical surface B and a protrusion or land C.

In effecting the cutting of the profile of Fig. 2a the workpiece is rotated about its central axis and the tool 19 maintains a constant position in relation to the axis as it cuts the surface B. As the workpiece approaches an angular position at which the protrusion C is to be formed the tool 19 is withdrawn radially outwards of the axis by operation of the cam arrangement 23 and 24 until it reaches the diameter of the outer edge of the protrusion C. The rate of withdrawal of the tool relative to the speed of rotation of the workpiece about the spindle axis determines the slope of the leading side surface of the protrusion C.

The tool 19 then maintains a constant position in relation to the axis of rotation for a small angular movement of the workpiece until the position of the downslope of the protrusion is reached. At this point the tool is moved radially inwardly of the axis of rotation to resume its position in relation to the position of the surface B.

It will be seen that more complicated profiles may be produced, for example having multiple or different protrusions. Moreover the shape and/or angular orientation of the protrusions may be changed in the axial direction of the workpiece, for example to produce helical or spiral forms about the workpiece.

In Fig. 2b there is shown another example of a profile which may be formed. In this case the workpiece is formed with two curvilinear surfaces D and E on the workpiece. Fig. 2b is one typical section through this workpiece.

To form the surface D the tool is moved as described for Fig. 2a. However, as the tool progresses axially along the component, the phase relationship between the spindle and cam is changed by rotation of motor 6. To form surface E, the direction of rotation of the main spindle is changed whilst retaining the same direction of rotation for the cam. In this case tool 19 would be changed by means of the turret for a tool suitable of cutting in the opposite direction. This technique of reversing also allows the section C of the Fig. 2a profile to have steeper angles to avoid tool interference.

The machine tool is also capable of forming other profiles. For example the tool may form high lead threads such as rope threads, ballscrews and helical pump components. Such threads may be multi-start threads made in one machining operation and finishing at a blind shoulder. In conventional techniques such as copying, thread whirling and thread chasing each thread may have to be completed individually and/or the thread must extend substantially all along the workpiece and not be capable of finishing at a shoulder.

The machine tool is also capable of forming taper threads i.e. a thread form in which the thread diameter changes lengthwise of the workpiece.

Transposition of the reciprocal motion to the axial direction can allow profiles to be made on the faces of components.

Amongst the other wide variety of profiles which may be produced are polygon profiles. Where n is the number of sides of the polygon the cam is rotated at n times the spindle speed. Due to limits on the acceleration rate of the tool the corners of the polygon may be rounded. In this case with an even number of faces two passes of the tool may be necessary, one pass cutting one set of faces and the next pass cutting the other alternating set of faces to achieve sharp edges.

It will be appreciated that the various motors driving the machine tool components have to be accurately coordinated and monitored to achieve the results described. Moreover the operation of the motors must be pre-programmed according to the desired profiles to be cut. This is achieved by programming means (not shown) which employs conventional CNC technology.

Part of these functions are achieved by the encoders and by the servo motors coupled to the programmer which energises and controls the operations of the motors in the necessary sequence and at the desired rates.

Referring now to Figs. 3 and 4 there is shown an alternative arrangement for mounting and operating the tool 19. The Fig. 3 arrangement is primarily intended for use in machine tool apparatus in which a number of tools are mounted on a turret and the turret can be indexed through a number of positions in each of which a tool can be employed.

Figs. 3 and 4 show only the tool mounting and drive arrangement, the remainder of the apparatus being as shown and described in relation to Fig. 1 and Figs. 2a and 2b.

A turret 20 is directly mounted on the toolslide 12 of Fig. 1. The turret 20 includes an indexing mechanism including a tool carrying disc 35 which can be rotated through several stations to present a different tool 39 in the working position.

A tool carrier body 36 is carried on the disc 35 and carries a shaft 42 mounted in journal bearings (not shown) . The shaft 42 is coupled through a coupling 32 to a motor 33 when in the operating position of the tool 39. Drive from the motor 33 is transmitted to the shaft 42 through a belt or gear drive 34.

The motor 33 is switched into synchronous running mode by connection to the electronic gearbox 30 (Fig. 1) with an encoder 31 providing the feed back means.

In some arrangements the releasable coupling 32 is omitted and a plurality of drive shafts are provided one for each tool and driven from a main shaft passing through the centre of the turret. Such an arrangement can also be adapted to operate as described.

Inside the tool carrier body 36 rigidly attached to the disc 35 is a cam 37 and cam follower 40 (Fig. 4) to provide a reciprocating motion to the tool 39. The cam 37 is, as shown, of the kind having spring load onto the cam. Alternatively two rollers may be provided which entrap the cam.

Thus it will be seen that dependent on the cam profile and motor speed and the location of the turret in relation to the workpiece the tool 39 will reciprocate in a variable manner relative to the workpiece to effect the desired cut profile on the workpiece. The arrangement of Fig. 3 may have the advantage compared with that of Fig. 1 of not only being applicable to indexable turrets but also of the reciprocating parts having less inertia. Thus higher acceleration may be possible and less stresses are set up.

As a further alternative (not shown) it may be possible to effect the reciprocating movement of the tool in other ways, for example without using a cam and cam follower arrangement. Thus reciprocation may be achieved by coupling a motor with variable rates of rotation to a suitable transmission whereby the variable rotary motion of the motor is converted to reciprocating linear motion, such as by using a screw or crank arrangement.

It has been found that the invention provides means whereby non-round components of complex shape can be produced efficiently and quickly compared with prior methods. In general conventional machine CNC functions provide the control of feed rates, speeds, geometry and size of offsets. The use of the electronic gearbox provides the additional facility of varying the ratio between the main spindle drive and the cam operation using a numerical input. This facility provides a ready means for reproducing the same shape two or more times around a workpiece.

If the shape around the component is to adopt a different angular position along the workpiece, such as in a thread form, there is introduced a varying phase shift in the synchronous link between the main spindle 16 and the cam drive 26. This can be achieved by the addition or subtraction of pulses to the pulse train connecting the two drives or by mechanically phase shifting the encoder running on the main spindle which encoder provides the synchronising pulse train. A mechanical differential gearbox 5 provides the means of performing this function in the illustrated embodiment.

Another feature of the invention is that standard tooling can usually be used to produce the desired profile and the profile may be produced internally as well as externally of workpieces.

In order to effect small corrections in the cut profile of the workpiece to allow for dynamic effects, deflections etc. a change in the angular orientation of the cam in relation to the workpiece or phase change can be created momentarily between the cam and the workpiece to effectively advance or retard the profile. This is achieved by injecting a digital offset into the electronic gearbox 30 at discrete intervals. Alternatively an alternating offset is injected to the drive motor 14 amplifier to bodily move the carriage 12 via the ball screw 13 and thus impose a secondary waveform on the cam actuated motor.

Claims

1. Machine tool apparatus which comprises a workpiece holder rotatable about an axis, a tool for operating on the workpiece, drive means for rotating the holder about its axis and drive means for positioning the tool at a variable position adjacent the workpiece whereby the tool can be moved at a predetermined rate in the direction of said axis and in a direction radially of said axis, the drive means being controlled by control means so that said radial movement is a radially reciprocating movement according to the angular position of the workpiece about the axis of rotation of the workpiece and the position of the tool in the direction of said axis of rotation.

2. Apparatus according to Claim 1 wherein the control means includes programming means whereby the reciprocating movement of the tool is determined, in part, by the angular position of the workpiece about its axis of rotation and in part the speed of rotation of the workpiece holder.

3. Apparatus according to Claim 1 or 2 wherein the reciprocating movement is generated by cam means whereby rotation of a cam causes a follower associated with the tool holder to move reciprocally radially of the axis of rotation of the workpiece holder.

4. Apparatus according to Claim 3 wherein the cam means includes at least two cam profiles selectively engageable in driving connection with the drive means for the tool through associated cam follower means to obtain different reciprocating movements of the tool.

5. Apparatus according to Claim 3 or 4 wherein the tool holder is mounted on slide means to move reciprocally transversely relative to the axis of rotation of the workpiece, the tool holder moving via the slide means in response to the interengagment of the cam and cam follower.

6. Apparatus according to Claim 3 or 4 wherein the cam follower is associated directly with a mounting for the tool without the intermediary of a slide.

7. Apparatus according to Claim 1 or 2 wherein the reciprocating movement is generated using a drive motor with variable displacement through a rotary to linear conversion mechanism.

8. Apparatus according to any one of the previous claims wherein the drive means for the holder and the drive means for the tool are operatively connected so that the speed and operation of one determines the speed and operation of the other.

9. Apparatus according to Claim 8 wherein the drive means for the holder and the drive means for the tool are each arranged to feed signals related to their speeds and positions to control means, the control means being adjustable to vary the relative speeds of said drive means according to their positions.

10. Apparatus according to any one of the preceding claims wherein the angular orientation of the selected reciprocating movement of the tool is adjustable by controlling the drive means for the tool.

11. A method of cutting a profile on a rotating workpiece wherein the workpiece is rotated about an axis, a cutting tool is located in the path of the workpiece to cut the desired profile on the workpiece, the cutting tool being movable relative to the workpiece in directions parallel to the axis of the workpiece and radially thereof and is controlled in said movements so that the position of said tool in said radial direction is changeable during each rotation of the workpiece about its axis whereby non circular profiles are cut in the workpiece.

12. A method according to Claim 11 wherein the variation in radial position of the tool in relation to the workpiece is varied in the axial direction of the workpiece.