GB2149144A

GB2149144A - Machine tools

Info

Publication number: GB2149144A
Application number: GB08422664A
Authority: GB
Inventors: Roger Harvey Slee
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1983-09-21
Filing date: 1984-09-07
Publication date: 1985-06-05
Also published as: DE3434175C2; JPS6085802A; CA1232657A; US4612833A; KR850002792A; IT8422743A0; GB8422664D0; IT8422743A1; FR2551999A1; GB2149144B; DE3434175A1; IT1176738B; GB8325294D0

Description

1 GB 2 149 144A 1

SPECIFICATION

Machine tools The invention relates to machine tools and in particular to machine tools of the kind corn prising a rotatable tool head which carries a plurality of tools for performing sequential machining steps on a generally stationary workpiece in a single pass. Such machine tools are commonly known as---cat-head machines.

Such machine tools have a number of ad vantages. They are able to perform rough turning and finishing operations on a work piece in a single pass and so increase con siderably the rate of production of finished workpieces. In addition, because the work piece is usually stationary, the finished work piece can be very rapidly removed and re placed by a fresh workpiece.

There have been various proposals for vary ing the relative positions of the tools and the workpiece in order to produce non-cylindrical workpieces. In general, these have comprised 90 stationarily arranged cam surfaces arranged remote from the toot head and followed by a cam follower whose movement is translated by a mechanical linkage into relative move ments see, for example, French Patent Specifi- 95 cation No. 1,516,951 and U.S. Patent Specifi cation No.2,720,806.

It is a disadvantage of such cat-head ma chines that they cannot machine workpieces to complex non-uniform shapes where com plex tool movements within a revolution of the cat-head are required. This is of particular significance where the workpiece is an alumi nium or aluminium alloy blank for forming a piston for an internal combustion engine. It has recently become apparent that the effici ency of operation of such pistons, and of the internal combustion engines which contain them, can be improved substantially if the piston is shaped to improve its lubrication characteristics. For example, it has been found that having non-uniform surface features can produce considerable improvements in per formance.

According to the invention, there is provided a machine tool in which a rotating tool machines a workpiece to a profile which requires tool movements within a revolution thereof, and comprising a rotatable tool head which carries at least one tool for performing a machining step on a workpiece in a single pass, an electrical actuator for effecting relative movement of said at least one tool and the workpiece during a revolution of the rota- tion of the tool head in a machining pass to allow variation of the machined workpiece profile during each said revolution, and an electronic control system for generating electrical control signals which are representative of a required workpiece profile variation and which are fed to the actuator for effecting the required relative tool and workpiece movements.

The following is a more detailed description of some embodiments of the invention, by way of example, reference being made to the accompanying drawings, in which:

Figure 1 is a schematic view of a machine tool, and Figure 2 is a cross-section through an alternative form of tool head for the machine tool of Figure 1.

The machine tool is of the---cat-head- type and is for machining piston blanks to have a required non-uniform profile with non-uniform surface features in order to improve its performance when installed in an internal combustion engine. These features may, for example, be of the kind described in British Patent Specification No.2,104,188.

Referring now to Figure 1, the machine tool comprises a---cat-head- type tool head 10 of generally cylindrical shape open at one end and having two tools 11, 12 carried on its interior surface. The tools 11, 12 are so arranged that the tool 11 closest to the open end is a rough turning tool while the other tool is a finishing tool, so that the two tools 11, 12 machine the workpiece in sequence in a single pass.

It will be appreciated that only a single tool may be provided or three or four or more tools may be provided. It will also be appreciated that where there are two or more tools, the sequential machining effect may be achieved by circumferential spacing of the tools in addition or as an alternative to the axial spacing of the tools.

The two tools 11, 12 are so mounted in the tool head 10 that they are movable in a radial direction. For this purpose, a pivoted bar 13 engages the outer ends of the tools 11, 12 and is engaged by a jacking screw 14 of a servo motor actuator 15 whose control is described in more detail below.

The tool head 10 is mounted for rotation on a shaft 16 which is connected to the output shaft 18 of a drive motor 19 by a pivot 17 normal to the axis of rotation of the tool head 10. An actuator 32 is provided for swinging the tool head 10 about the pivot 17. The tool head 10 and the drive motor 19 are in turn mounted on a feed slide 20 movable in a direction parallel to the axis of rotation of the tool head by a motor 21 for feeding the tools 11, 12 in a machining pass at a required feed rate.

The piston blank 22, forming the workpiece, is carried in a chuck 23 which is connected via a pivot 24 to a chucking head 25. The pivot 24 allows relative pivotal movement between the chuck 23 and the chucking head 25 about an axis normal to the axis of rotation of the tool head 10 and under control of an actuator 33. The chucking head 25 is 2 GB2149144A 2 carried on a slide 26 movable in a direction parallel to the axis of rotation of the tool head by a motor 27 to bring the workpiece 22 into a datum position for the commencement of machining.

The machine is controlled by a computer control system. The system includes a computer 28 which operates in the following way. The computer 28 is fed with input digital profile data which defines the required radius of the piston blank 22 at angular intervals around the periphery of the workpiece and at a number of axial locations along the workpiece, in order to provide a required work- piece profile. In practice, the required radius is generally specified as drops from a datum radius at, for example, 5 intervals around the workpiece at each axial position of the workpiece for which such a sequence of data is given say, 0. 1 mm or 1 mm. Of course, if the workpiece is symmetrical about a plane including the workpiece axis or about two such planes mutually at right angles, and drops only 180' or 90' respectively are required.

The computer 28 produces from this data digital signals representative of the required relative positions of the tool and the workpiece at any instant. These are held in the computer and are outputted on receipt of the digital position signals from the angular encoder 29 and the feed encoder 35.

The output of the computer 28 is a sequence of digital signals representative of instantaneous required tool positions to achieve the required workpiece profile. These are received by a control system 31 which produces a control signal which is fed to the servo motor 15 via slip rings 30 carried on the output shaft of the drive motor 19. The servo motor 15 moves the jacking screw 14 in accordance with these signals and thus, via the bar 13, produces the required movements of the tools 11, 12.

Additionally or alternatively, the control sys- tem 31 may feed signals to the actuators 32, 33 for pivoting the tool head shaft 16 and the chuck 28 about their respective pivotal axes 17, 24. The angle of pivoting and the duration of pivoting are chosen so as to provide the required workpiece profile. In general, the shaft 16 will be pivoted in order to provide larger dimensional variations in the shape of the workpiece 22, while the tools 11, 12 will be actuated in order to provide smaller dimen- sional variations in the shape of the workpiece 22. For example, the actuators 32, 33 may be used to produce on the workpiece required variations in ovality along the piston or a required taper or barrel.

It will be appreciated that the tool head 10 125 may rotate at 50 revolutions or more per second. At this speed, the relative positions of the tools 11, 12 and the workpiece 22, must be changed within microseconds in order to achieve the required workpiece profile. For 130 this reason, it is important for the tools to be of low mass and for the servo motor 15 to be able to react very rapidly indeed to control signals. Where the tool head is pivoted, it will also be appreciated that the tool head will be designed to have minimum inertia. This is also true of the chuck 23 where the work piece 22 and chuck 23 are pivoted about the pivot axis 24.

It will be appreciated that the conversion of the control signal into tool movement may be achieved in a number of alternative ways by any form of transducer which converts an electrical signal into mechanical movement.

For example, piezo-electrical or magnetostric- tive effects may be used or electromagnetic effects may be used such as linear motors and solenoids which have the tool holder as a movable armature.

An alternative form of tool head 10 is shown in Figure 2. In the embodiment, the too[ head 10 is provided with three tools 32, 33, 34 angularly spaced around the tool head 10. The tools 32, 33, 34 are mounted for movement in a radial direction in accordance with the control signals. This movement may be effected by any suitable actuator, such as solenoids or linear motors, of which the tool holders form respective movable armatures.

In addition, the control signal may be fed to the tool head in ways other than via the slip rings 30. For example, they may be conveyed by radio waves or ultrasonic waves or by light signals. In the last mentioned case, the sig- nals may be conveyed to the tool head by a fibre optics system. Alternatively, at least a part of the computer control system may be provided on the too[ head so that the computer control system rotates with the tool head 10. Of course, not all the computer control system may need be so mounted, there may only be provided on the tool head a store for storing future required tool positions and for outputting a control signal in accor- dance with the stored signals.

It will also be appreciated that in certain circumstances, it may only be necessary to control the pivoting of the workpiece or the pivoting of the tool head 10, or the radial movement of the tools 11, 12 in order to produce a required profile.

Claims

1. A machine tool in which a rotating tool machines a workpiece to a profile which requires tool movements within a revolution thereof, and comprising a rotatable tool head which carries at least one tool for performing a machining step on a workpiece in a single pass, an electrical actuator for effecting relative movement of said at least one tool and the workpiece during a revolution of the rotation of the tool head in a machining pass to allow variation of the machined workpiece profile during each said revolution, and an 3 GB2149144A 3 electronic control system for generating electrical control signals which are representative of a required workpiece profile variation and which are fed to the actuator for effecting the required relative tool and workpiece movements.

2. A machine tool according to claim 1, wherein said at least one tool carried on the tool head is movable in a radial direction relative to the axis of tool head rotation, the actuator being carried on the tool head and acting directly on said movable tool, the control system signals being applied to said actuator to move the tool radially during a revolu- tion of said tool head to machine the required profile.

3. A machine tool according to claim 2, wherein the control system is stationary and wherein the actuator receives the control sig- nals by way of a slip ring system arranged between a stationary part of the machine tool and the too[ head.

4. A machine tool according to claim 2 or claim 3, wherein the actuator comprises a servo motor which has an output jacking screw which moves in accordance with said electrical control signals to effect radial movement of said at least one tool.

5. A machine tool according to any one of claims 2 to 4, wherein two tools are provided on the tool head and are spaced axially along the tool head for performing sequential machining steps on the workpiece in a single pass, a bar being provided on the tool head and pivoted for movement by the actuator, the bar contacting the two tools so that both tools are moved radially by the bar on actuation of the actuator by the electrical control signals.

6. A machine tool according to claim 1 or claim 2, wherein the actuator produces a force which changes the tool position and which is generated in accordance with a magnetic field applied to the actuator, an electromagnet being provided around the tool head for converting said control signal into said magnetic field for controlling the tool position.

7. A machine tool according to any one of claims 1 to 3, wherein the actuator comprises a piezo-electric transducer for converting the control signal into corresponding mechanical movements of the tool.

8. A machine tool according to any one of claims 1 to 7, wherein the control system is a digital control system which receives data defining a required workpiece profile and which produces a sequence of digital signals defining required variations of the relative positions of the workpiece and the tool or tools during each revolution of the tool head.

9. A machine tool according to claim 8, wherein an angular encoder is provided for producing digital signals representative of the angular position of the tool head within a revolution thereof, a feed encoder also being provided for producing digital signals representative of the relative axial positions of the workpiece and the tool, the control system receiving the encoder signals and producing the digital signals in accordance therewith.

10. A machine tool according to any one of claims 1 to 9, wherein the workpiece is mounted for pivotal movement about an axis normal to the axis of rotation of the tool head, the or a further actuator pivoting the workpiece about said axis during the machining pass and in accordance with control signals provided by said electronic control system.

11. A machine tool according to any one of claims 1 to 9, wherein the tool head is mounted for pivotal movement about an axis normal to the axis of rotation thereof, the or a further actuator pivoting said tool head about said axis during the machining pass and in accordance with control signals provided by said electronic control system.

12. A machine tool substantially as hereinbefore described with reference to Figure 1 or to Figure 1 as modified by Figure 2 of the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.