GB2069142A

GB2069142A - Measuring workpiece dimensions

Info

Publication number: GB2069142A
Application number: GB8102235A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-01-31
Filing date: 1981-01-26
Publication date: 1981-08-19
Also published as: GB2069142B

Abstract

A numerically controlled machine tool has a tool 12 on a support 13 for machining surface 11A of a workpiece 11, and for sensing the surface for measuring a dimension 43 between it and a datum surface 29. The tool is first moved to a position 44 clear of the surface 11A and an electric circuit 31 between the tool and the workpiece is switched on. The tool is then moved toward the surface 11A by a control system 17 which reads tool position by counter 39. When the tool touches the workpiece the circuit produces a signal 32 to read the content 43A of counter 39 into computer 37. This determines the difference between the actual and demanded dimension of the surface 11A and corrects the tool position accordingly. Alternatively, the signal 32 is produced by a piezo-electric crystal in the tool 12 to sense force when the tool engages the workpiece. Or the tool 12 is allowed to slide on the support, and the extent of the sliding used to measure the difference between actual and demanded dimensions. <IMAGE>

Description

SPECIFICATION Method of and apparatus for measuring distance in numerically controlled machine tools This invention relates to a method of and apparatus for measuring distance in numerically controlled machine tools.

It is known in numerically controlled machine tools to carry out both machining operations and measuring operations on a workpiece held in the machine. For the purpose of the measuring operation, the known machine includes a probe having a stylus whereby to sense a selected surface of the workpiece. The probe outputs a signal response to such sensing. The signal is communicated to a numerical position control system of the machine thereby enabling that system to determine the position of the probed surface and thus the dimension of that surface in relation to a predetermined datum. When the probe is to be taken into use it is necessary to remove the last used tool from the operative position and introduce the probe in the position previously occupied by the tool.For example, if the tool and the probe are mounted at respective stations of a multi-station turret, the latter has to be indexed to bring the probe into the operative position. If the machine has an automatic tool change mechanism which moves the tools between the tool holder of the machine and a magazine of such tools, the tool has to be returned to magazine and the probe has to be moved from the magazine to the tool holder before the measuring operation can be carried out. The need to exchange the tool for the probe and vice versa can be a source of delay, and is otherwise disadvantageous, especially in the case where the measuring operation has to be carried out between successive machining operations.

Further, different surfaces of the same workpiece often have different shapes or orientations, requiring differently-shaped tools and correspondingly different styli for sensing of those surfaces. This, in turn, may require different probes to be held in said turret or said magazine.

The invention as claimed in the claims hereto overcomes or reduces the above disadvantages in that it uses the tool both for the machining and for the sensing function. In other words, the probe is dispensed with. This has the special advantage that inasmuch as the tool is necessarily shaped in accordance with the shape or orientation of the surface concerned, it is often ideally suited for performing the sensing function and the need to hold probes with different styli, or a single probe with a stylus movable to different orientations, is generally avoided.

Examples of the invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a plan view of a numerically controlled lathe, Fig. 2 is an end elevation of Fig. 1, Fig. 3, continued in Fig. 3A, is a flow diagram relating to Figs. 1 and 2, Fig. 4 is a view similar to Fig. 2 but showing a modification, Figs 5 and 6 are views similar to Figs 1 and 2 but show a further modification.

Referring to Figs. 1 and 2, the lathe has a holder or chuck 10 supporting a workpiece 11 to be machined at a cylindrical surface 11 A thereof by a cutting tool 12 having a cutting point 12A. The tool is secured to a support 1 3 mounted on a first slide 1 5 adapted to be moved transversely to the axis, 14, of the workpiece by a motor 16. A digital control system 1 7 provides a signal 1 8 for driving the motor. A position sensor 19 provides a feedback signal 20 for closed loop control of the position of the slide 1 5.

The slide 1 5 is supported for its said movement on a second slide 21 in turn supported for movement in the direction of the axis 14 on a bed 25. The slide 21 is adapted to be moved by a closed loop position control means (not shown) analagous to the means 1 6, 1 8, 1 9, 20 associated with the slide 1 5. A datuming probe 26 has a sensing member 27 supported in a rest position by a housing 28 secured to the bed 25. The member 27 has a surface 29 lying in a vertical plane through the axis 14 or in some other selected datum plane.

If the slides 1 5, 21 are moved to engage the cutting point 1 2A with the surface 29, the probe 26 outputs a signal 30, signifying that the cutting point is at a datum or zero position.

Wires defining an electrical circuit 31 are connected between the tool 12 and the chuck 10 such that the circuit 31 is established when the cutting point 1 2A engages the workpiece 11, thereby giving rise to a pulse signal 32. The connection of the circuit 31 to the chuck 10 is shown symbolically by a slip ring 33. The tool itself is supported by insulating plates 22 to avoid a short circuit through the slide 1 5 and the remainder of the machine to the chuck 10.

The control system 1 7 is arranged to read the position of the point 1 2A as zero when the point 1 2A is situated at the axis 14.

Consequently, when the slide 1 5 is operated to move the tool 12 toward the cylindrical surface 11 A of the workpiece, the position of the point 1 2A at the instant of the signal 32 is a measure of the radius of the surface 11 A.

It will be clear that the making or breaking of electrical contact between the tool and the workpiece constitutes an interaction which produces the signal 32 by varying the electrical resistance in the circuit 31.

The control system 1 7 comprises a tape 34 containing coded digital information defining predetermined positions for the cutting point 12A.

The latter information is conveyed by a tape reader 36 through to a digital computer 37 to a register 38. The feedback signal 20 is connected to a counter 39 whose content defines the instantaneous position of the cutting point 12A, the counter having previously been set to zero by the signal 30 by engagement of the point 1 2A with the datuming probe 26. The register 38 and counter 39 are connected to a comparator 40 having an output defining the instantaneous difference between the contents of the register 38 and the counter 39.The output of the comparator 40 constitutes the signal 1 8 which, as mentioned, is connected to drive the motor 1 6 to position the slide 1 5. A corresponding arrangement (not shown) is provided for positioning the slide 21 in accordance with corresponding information defined on the tape 34.

The signal 32 is connected through gates 41 (only one shown) to feed the instantaneous content of the counter 39 to a register 42 whose content, defined by a signal 43B, is adapted to be connected to the computer. Referring to the flow diagram Fig. 3, the computer is programmed to sequentially perform the following steps:- S001: Output a signal 48 connected to advance the tape 34. This presents to the tape reader 36 a code 35A defining the finished dimension 35 of the workpiece.

S002: Read and store the code 35A.

S003: Output the code 35A as signal 35B to the register 38 thereby initiating movement of the point 1 2A toward the workpiece. It is assumed that the point 1 2A is initially well clear of the workpiece.

S004: Read the signal 18 and decide when the movement is complete i.e. when the signal is zero.

At this stage the workpiece does not have the dimensions 35 but has a dimension 43 because of deflection of the tool or of the workpiece.

S005: Output the signal 48 to further advance the tape. This presents to the reader 36 a code 44A defining a "pull-back" position 44 of the point 1 2A clear of the workpiece by an amount known to be greater than said deflection.

S006: Read and store the code 44A.

S007: Output the code 44A as signal 448 to the register 38 thereby to move the point 1 2A to the position 44.

S008: Output a signal 46 to a gate 47 to connect the signal 32 to the gate 41 and read the instantaneous content, denoted 43A, of the counter 39 into the register 42.

S009: Read the content signal 43B of the register 42.

S010: Form the sum 358 -- (438 -- 358) = 458 wherein 4318 - 35B constitutes said deflection and 45B defines a dimension 45 to which the tool has to be moved for the workpiece to attain the dimension 35.

S011: Output the value 45B to the register 38 to effect the final sizing of the workpiece.

S012: Output signal 448 to return to dimension 44.

It will be appreciated that mechanical contact between the point 1 2A and the workpiece to establish electrical contact is, initially, not sufficient to transmit significant mechanical load between the tool and the workpiece. Therefore there is no significant deflection to falsify the measurement of the position of the workpiece surface.

Instead of relying on electro-mechanical contact with the workpiece for establishing the circuit 31 , the presence of the point 1 2A may be sensed on attaining a predetermined spacing from the surface by any known means such as apparatus for sesning a change in electrical capacitance between the point 1 2A and the surface, or apparatus for sensing the break-down of electrical resistance across a predetermined air gap between the point 1 2A and the surface. In such cases of sensing the surface at a distance therefrom, the computer is programmed to take account of that distance when determining the deflection at step S010 of the programme.

The programme described is part of a longer programme covering both machining and measuring phases in the operation of the machine.

In the example described the measuring phase succeeds the machining phase and is intended to take account of deflection during the machining phase. Measuring may of course also take place before machining, e.g. to establish the dimensions of a blank, in which case the tool may be merely moved into engagement with the workpiece to establish the signal 43B, i.e. the actual dimension of the workpiece, e.g. with a view to deciding a safe depth of cut for the first machining operation.

In a modification (Fig. 4) a tool 100 comprises a shank 101 and a cutting element 102 having the form of a small plate secured to the shank by a screw 103. A piezo-electric crystal 104, placed between the element 102 and the shank 101 is held in compression by the force of the screw 1 03.

When a cutting force F1 is applied to the cutting point, denoted 102A, the load applied to the crystal increases above that applied by the screw 103 and the crystal generates a corresponding potential in conductors defining an electrical circuit 105 which includes an amplifier 106 and trigger circuit 107 to produce a pulse signal 108 corresponding to the signal 32 described with reference to Figs. 1 to 3.

Since it is desired not to apply a cutting force between the tool and the workpiece but merely to sense initial engagement between the latter, a very sensitive piezo is used and the amplifier 106 and circuit 107 are selected to respond to forces sufficiently small to signify engagement between the point and the workpiece but not sufficiently large to represent a significant deflection of the tool or of the workpiece.

In this example the sensing interaction is such that a change of force due to mechanical contact is sensed. In other words, the electrical signal 108 is produced during the movement of the tool when the latter engages the workpiece and interacts therewith to produce the force F1 which acts on the crystal 104 to give rise to the signal 108.

It is equally possible to produce the signal 108 when the tool is disengaged from the workpiece at the end of a cutting operation and the cutting force on the tool ceases. In that case the amplifier 106 and trigger circuit 107 produce the signal when crystal 104 outputs a potential during the reduction in the force F1 toward zero as the tool disengages from the workpiece. The crystal potential, which rises during the change in the force toward zero, falls to zero when the force ceases and is therefore a definite indication of the point of disengagement of the tool from the workpiece.

Instead of being situated directly below the element 102, the crystal 104 may be situated in a suitable location between the tool 100 and the support 13.

In a further modification (Figs. 5, 6) a tool 200 is supported on the support 13 for motion in a direction Y transverse to the direction X of the axis 14. During such motion the tool is guided against displacement in the direction X by a pair of leaf springs 201 and against displacement in a direction Z transverse to the directions X, Y by the support 13 and by the head 202 of a clamp bolt 203. The tool 200 is biased toward the workpiece 11 by a light spring 204 which holds the tool against a stop 204A. The clamp bolt is connected to a cam 205 acting against an under-surface 206 of the support 13 to hold the tool against displacement in the direction Y under a component force FY acting on the tool when cutting. The clamp bolt is releasable by a hydraulic actuator 207 connected to rotate the cam.The actuator is controlled by a valve 208 operated electromagnectically by a signal 209B derived from a code 209A on the tape 34 together with the code 44A. The tool has an extension being a movable core 211 of a differential transformer 210, having an output signal 12 which is set to be zero when the tool rests against the stop 204.

The operation of the tool is similar to that described with reference to Figs.1,2 in that the tool is withdrawn to the dimension 44 and then fed into engagement with the workpiece.

However, in the present example the clamp bolt is released by the signal 209B when the tool is at the dimension 44 so that when the tool is fed back to the dimension 35, the tool slides on the support 1 3 from the instant at which the tool engages the workpiece at the dimension 43. As the support moves to bring the tool notionally to the position 35, the resulting displacement between the tool and support is indicated by the transformer 210 and is a direct indication of the amount by which the tool position must be corrected to produce the dimension 35. The signal 212 is taken through a digitizer 213 to produce a signal 21118 connected to be read by the computer in a way corresponding to the reading of the signal 43B at step S009 of the programme. But in the present example the steps S009, S010 are replaced by the following: S209: Read signal 21118 S210: Form the sum 35B211 B = 45B Instead of the transformer 210, the arrangement shown in Figs. 5, 6 may comprise a pair of electrical contacts 220, 221 defined respectively on the tool 200 and the stop 204.

Conductors defining a circuit 223 are connected to the contacts 220, 221, and when the tool 200 is moved against the spring and the contacts 220, 221 become separated, the state of the circuit changes to produce a signal 222 connected and used in the same way as the signal 32 described with reference to Figs. 1 to 3.

Claims

1. A method of measuring distance in a numerically controlled machine tool having two members being respectively a tool support and a work holder, means for moving one of the members relative to the other for machining a workpiece held in the work holder by means of a tool mounted on the tool support or for sensing a surface of the workpiece by a sensor mounted on the tool support, means for continually measuring a distance between the movable member and a datum for determining the position of the tool relative to the datum during said machining or determining the position of the surface during said sensing; the method comprising using said tool for said machining during a machining operation and for said sensing during a sensing operation.

2. The method according to claim 1 comprising, for the purpose of said sensing, producing between the tool and the surface an interaction having a parameter which varies according to the position of the tool relative to the surface, producing a signal indicative of a variation of the parameter, and reading said measuring means responsive to said signal having a magnitude related to a predetermined position of the tool relative to the surface.

3. The method according to claim 2 wherein said signal is a step signal and the measuring means is read in response to said step signal.

4. The method according to claim 2 wherein said signal is a continually variable signal, and the measuring means is read in response to the latter signal attaining a predetermined magnitude.

5. The method according to any one of the preceding claims wherein said signal is produced responsive to the tool making or breaking engagement with the surface.

6. The method according to claim 5 wherein said parameter is a measure of a change of electrical resistance between the tool and the surface.

7. The method according to claim 5 wherein said parameter is a measure of a change of a force applied between the tool and the surface.

8. The method according to any one of the preceding claims comprising using said tool for a machining operation prior to using the same tool for sensing a said surface produced by said machining operation.

9. The method according to any one of claims 1 to 7 comprising using said tool for a said sensing of a said surface prior to using the tool for machining the surface so sensed.

1 0. Apparatus for measuring distance in a numerically controlled machine tool, having two members being respectively a tool support and a work holder, means for moving one of the members relative to the other for machining a workpiece held in the work holder by means of a tool mounted on the tool support, the tool having a cutting point, the workpiece having a surface machined or to be machined by the tool, means for continually measuring a distance between the movable member and a datum for determining the position of the cutting point relative to the datum during a machining operation; the apparatus comprising means for producing between the cutting point and said surface an interaction having a parameter which varies according to the position of the cutting point relative to the surface, means for producing a signal indicative of a variation of said parameter, and means for reading said measuring means responsive to said signal having a magnitude related to a predetermined position of the tool relative to the surface.

11. Apparatus according to claim 10 comprising means defining an electric circuit, said parameter being related to the state of the circuit, and the circuit being arranged for said state to vary responsive to the cutting point making or breaking engagement with the surface thereby to produce said signal.

12. Apparatus according to claim 11 wherein the tool and the workpiece are connected in said circuit to vary the resistance thereof when the cutting point makes or breaks contact with the workpiece.

13. Apparatus according to claim 10 comprising means for supporting the tool on said tool support for movement relative thereto in response to a force exerted on the tool by contact between the cutting point and the surface, means for releasably locking the tool against a said movement, and means for producing said signal responsive to a said movement.

14. Apparatus according to claim 13 comprising means for producing said signal responsive to said movement commencing on engagement between the cutting point and the surface.

1 5. Apparatus according to claim 13 comprising means for producing said signal responsive to said movement ending on disengagement of the cutting point and the surface.

16. Apparatus according to claim 13 comprising means responsive to said movement for producing a said signal varying according to the extent of said movement, and means for reading said measuring means responsive to the signal attaining a predetermined magnitude.

17. Apparatus according to claim 10 comprising means arranged between said cutting point and tool support for sensing strain due to a force applied between the tool and the workpiece, and means for producing said signal responsive to a change in said strain due to occurance of said force on engagement between the cutting point and the surface or due to cessation of said force on disengagement between the cutting point and the surface.

18. Apparatus according to claim 1 7 said means for sensing strain comprising a piezoelectric crystal.

19. Apparatus according to claim 18 wherein said tool comprises an element defining said cutting edge and a body to which the element is secured, and wherein said crystal is arranged between the element and the body.

20. Apparatus for measuring distance in a numerically controlled machine tool substantially as described herein with reference to the accompanying drawings.