GB1599758A

GB1599758A - Measuring machine

Info

Publication number: GB1599758A
Application number: GB588377A
Authority: GB
Original assignee: LK TOOL CO Ltd
Current assignee: LK TOOL CO Ltd
Priority date: 1978-02-10
Filing date: 1978-02-10
Publication date: 1981-10-07

Description

(54) A MEASURING MACHINE (71) We, LK TOOL COMPANY LIMITED, a British Company of East Midlands Airport, Castle Donington, Derby, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a measuring machine incorporating a workpiece sensing probe.

Co-ordinate measuring machines are usually provided with a sensing probe which is mounted so as to be translatable along three mutually perpendicular axes. Such measuring machines are also usually provided with means adapted to indicate the distances which the probe translates along each of the axes. Within the probe there is provided a switching mechanism which is thrown upon contact of an arm on the probe with a workpiece. The throwing of this switching mechanism may, for instance, open a circuit which is linked to a latching circuit; the latching circuit being adapted to "freeze" the values indicated by the distance indicating means upon the throwing of the switching mechanism.Thus if the probe commences translation along its three axes of movement from a known datum position, the coordinates of the probe arm's point of contact with the workpiece may be derived from the "frozen" values indicated by the distance indicating means.

Such measuring machines suffer, however, from certain drawbacks which are associated with their workpiece sensing probes. Since the switching mechanism of such probes must be as light as possible in order to ensure that it is thrown almost immediately upon contact of the probe arm with the workpiece, it is not infrequently found that upon withdrawal of the probe arm from the workpiece, the switching mechanism does not reset itself.

Whilst this is not a great problem if the measuring machine is operated manually, it creates serious limitations if the machine is operated automatically.

Another limitation is one associated with the workpiece contacting arm of such probes.

Since some deflection of the probe arm, however slight, is almost inevitable before the switching mechanism is thrown, the coordinates indicated by the "frozen" distance indicating means are not exactly the same as that of the actual point of contact of the probe arm with the workpiece.

According to the present invention, there is provided a measuring machine comprising a workpiece supporting structure, a probe support adapted to translate along at least one axis, a workpiece sensing probe mounted on said support, measuring means adapted to measure the distance from a given datum along said at least one axis of said probe support and hence of said workpiece sensing probe, an electrical circuit which includes said workpiece sensing probe and a workpiece to be measured, said electrical circuit being adapted to maintain said workpiece sensing probe at a different electrical potential to said workpiece and to be completed by contact between said probe and said workpiece, and means adapted to detect the completion of said electrical circuit and to cause said measuring means to give an output indicative of the instantaneous distance of said probe from said datum along said at least one axis when said circuit is completed.

Preferably said workpiece sensing probe is provided with at least one arm for contacting a workpiece and support means for said arm, said arm being mounted on said support means in a given positional relationship thereto but adapted to be deflectable from said given positional relationship, means being provided whereby said arm is returned to said positional relationship with said support means after deflection.

A spherical member may be provided on either said arm or said support means with said receiving means on said support means or said arm respectively.

Preferably also said probe support is adapted to translate along three mutually perpendicular axes.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. I is a general view of a co-ordinate measuring machine provided with a workpiece sensing probe in accordance with the present invention; Fig. 2 is a partially sectioned side view of a workpiece sensing probe; Fig. 3 is a view on section line A-A of Fig. 2; Fig. 4 is a block diagram of part of an electrical circuit suitable for use with the coordinate measuring machine and workpiece sensing probe shown in Fig. 1; and Fig. 5 is a partially sectioned side view of an alternative workpiece sensing probe.

With reference to Fig. 1, a co-ordinate measuring machine generally indicated at 10 comprises a bed 11 upon which a gantry 12 is adapted to translate in directions indicated at 13. The gantry 12 is provided with a probe support 14 which is adapted to translate in further directions indicated at 15 and 16.

The directions indicated at 13, 15 and 16 are all mutually perpendicular. Thus the probe support 14 is capable of translation along three axes which are mutually perpendicular.

Each axis of movement of the probe support 14 is provided with a tape transducer (not shown) which, together with an associated multi-element reader (not shown), provides an electrical pulse output directly proportional to the distance moved by the probe support 14 along that axis from a datum position. The outputs from each of the tape transducers and associated multi-element readers are displayed on three respective digital displays 17a, 17b and 17c mounted on the gantry 12.

Co-ordinate measuring machines of the type described above are well known and in widespread use. Consequently a detailed description of the machine will not be given.

On a free or lower end of the probe support 14, there is provided a workpiece sensing probe 18. The probe 18, which can be seen more clearly in Fig. 2, comprises a generally cup-shaped, circular cross-section support structure 19 having a spigot 20 which is adapted to locate the probe on the probe support 14.

The probe 18 is part of an electrical circuit, indicated in general form in Fig. 4 which includes the workpiece 21 which is to be measured. It follows therefore that the workpiece 21 is electrically conductive. The probe 18 and workpiece 21 are connected in series with a high impedance polarising circuit 22 and a latching circuit 23. The circuit 22 is arranged such that the probe 18 is positively polarised to a value of six volts. The latching circuit 23 is in turn connected in series with a pulse counter 24 and digital display 17a.The pulse counter 24 is adapted to count the number of pulses P from one of the tape transducers and its associated multi-element reader and the digital display 1 7a is adapted to display the number of pulses counted as an indication of the distance travelled by the probe 18 from a datum position along one of its axes of movement. Although the circuit diagram shown in Fig. 4 is provided with one pulse counter 24 and one latching circuit 23 associated with the digital display 17a, it will be appreciated that other corresponding pulse counters and latching circuits are associated with the digital displays 17b and 17c.Each are similarly connected to the polarising circuit 22, workpiece 21 and probe 18 as well as to the appropriate tape transducer and multi-element readers and provide indications of the distance travelled by the probe 18 from the datum position along the remaining two axes of movement. Thus the digital displays 17a, 17b and 17c provide an indication of the position of the probe 18 relative to the datum position.

In operation, the probe 18 is manually moved into contact with the workpiece 21.

As soon as contact is made, the probe 18 is de-polarised and a voltage consequently applied to the latching circuit 23. This causes the latching circuit 23 to interrupt the signal from the counter 24 to the digital readout display 17a and effectively "freeze" the reading which was on the display 1 7a at the instant contact was made between the probe 18 and the workpiece 21. If a number of positions on the workpiece 21 are then contacted with the probe 18, it is possible to compute the actual dimensions of the workpiece 21.

The portion of the probe 18 which actually contacts the workpiece 21 is a sphere 25 positioned at the end of a straight arm 26 extending from the interior of the probe support structure 19. The other end of the arm 26 terminates in a second somewhat larger sphere 27. The second sphere 27 locates in a bore of a hollow, cylindrical member 28 provided within and positioned so as to be co-axial with the central axis of the support structure 19. An end plate 31 is attached to the open end of the support 19, and the probe arm 26 is provided with a flange 29 which itself has a flanged periphery 30 adapted to rest upon an inner surface of the end plate 31. A hole 32 is provided in the end plate 31 through which the probe arm 26 extends out ofthe probe support structure 19.

The flange 29 is biased into engagement with the end plate 31 by means of a compression spring 33. A rubber boot 34 positioned around the probe arm 26 prevents dirt etc.

from entering the probe support structure 19 through the hole 32.

The end plate 31, the hollow cylindrical member 28 and the flange 29 are arranged so that the probe arm 26 is normally co-axial with the probe support structure 19 and the spigot 20. However, if the probe arm 26 is subjected to a deflecting force, it will either pivot about its spherical end 27 and/or slide into the probe support structure 19. As soon as that deflecting force is removed, the compression spring 33 will urge the flange 29 back into full contact with the end plate 31, thereby returning the probe arm 26 to its original position i.e. co-axial with the probe support 19.

When the probe 18 is in use, the probe arm 26 remains co-axial with the probe support 19 until contact is made between the sphere 25 and the workpiece 21. As soon as that contact is made, the actual co-ordinates of the contact point relative to a datum position is indicated by the displays 17a, 17b and 17c.

It is likely that after contact is made between the sphere 25 and workpiece 21, the momentum of the probe 18 and its associated probe support 14 will carry probe 18 beyond its point of contact with the workpiece 21, thereby resulting in a deflecting force being applied to the probe arm 26. However the probe arm 26 will deflect in the manner described above, thereby avoiding probe damage. As soon as the probe 18 is moved away from the workpiece 21, the probe arm 26 will be returned to its original position, again in the manner described above in readiness for the taking of a further coordinate.

Since the arm 26 of the probe 18 is subjected to deflection forces after contact with the workpiece 21 has been made, the torsional stiffness of the probe arm 26 has no effect upon the accuracy of the probe 18.

The workpiece sensing probe 18a shown in Fig. 5 is similar in concept to that shown in Fig. 2 and for the sake of convenience, common integers are suffixed with the letter a. The probe 18a differs from probe 18 in that the flange 29a is provided with a straight hollow cylindrical portion 35 defining a bore which is adapted to receive the spherical end 36 of a location member 37 and to support the probe arm 26a. The location member 37 is fixed within the probe support structure 1 9a and positioned so that the central axis of the cylindrical portion 35 is maintained coaxial with the central axis of the support structure 19a. As in the case of the probe 18, the arrangement permits the probe arm 26a to be deflected from the position shown in Fig. 5, but ensures that it returns to that position when the deflection force is removed.

Throughout the specification, reference has been made to a datum position which is used for establishing the relative co-ordinates of the various points of contact between the probe 18 and workpiece 21. That datum position may be any position which is convenient to the particular workpiece being measured. Thus it may be a certain reference position on the bed 11. Alternatively it could be a particular co-ordinate on the workpiece 21 itself. In this case it could be convenient to zero the displays 17a, 17b and 17c upon contact of the probe 18 with the workpiece 21 at the particular co-ordinate in question.

Although the present invention has been described with reference to a co-ordinate measuring machine operated manually, it will be appreciated that it is particularly suited for use with automatic co-ordinate measuring machines. Such machines are provided with power drives for translating the probe support 14, and thus the probe 18 along the three mutually perpendicular axes, stopping when contact between the probe 18 and workpiece 21 is made, recording the coordinates of that contact point and then proceeding to further workpiece contact points. If such measuring machines are to operate completely automatically, it is highly desirable that the probe 18 is going to provide an electrical signal for the latching circuit 23 every time contact is made with the workpiece 21.Since this signal is provided by the completion of an electrical circuit which includes the probe 18 and workpiece 21, the possiblility of the failure of a signal being sent to the latching circuit 23 is low.

Various modifications may be made without departing from the invention.

WHAT WE CLAIM IS: 1. A measuring machine comprising a workpiece supporting structure, a probe support adapted to translate along at least one axis, a workpiece sensing probe mounted on said support, measuring means adapted to measure the distance from a given datum along said at least one axis of said probe support and hence of said workpiece sensing probe, an electrical circuit which includes said workpiece sensing probe and a workpiece to be measured, said electrical circuit being adapted to maintain said workpiece sensing probe at a different electrical potential to said workpiece and to be completed by contact between said probe and said workpiece, and means adapted to detect the completion of said electrical circuit and to cause said measuring means to give an output indicative of the instantaneous distance of said probe from said datum along said at least one axis when said circuit is completed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The flange 29 is biased into engagement with the end plate 31 by means of a compression spring 33. A rubber boot 34 positioned around the probe arm 26 prevents dirt etc. from entering the probe support structure 19 through the hole 32. The end plate 31, the hollow cylindrical member 28 and the flange 29 are arranged so that the probe arm 26 is normally co-axial with the probe support structure 19 and the spigot 20. However, if the probe arm 26 is subjected to a deflecting force, it will either pivot about its spherical end 27 and/or slide into the probe support structure 19. As soon as that deflecting force is removed, the compression spring 33 will urge the flange 29 back into full contact with the end plate 31, thereby returning the probe arm 26 to its original position i.e. co-axial with the probe support 19. When the probe 18 is in use, the probe arm 26 remains co-axial with the probe support 19 until contact is made between the sphere 25 and the workpiece 21. As soon as that contact is made, the actual co-ordinates of the contact point relative to a datum position is indicated by the displays 17a, 17b and 17c. It is likely that after contact is made between the sphere 25 and workpiece 21, the momentum of the probe 18 and its associated probe support 14 will carry probe 18 beyond its point of contact with the workpiece 21, thereby resulting in a deflecting force being applied to the probe arm 26. However the probe arm 26 will deflect in the manner described above, thereby avoiding probe damage. As soon as the probe 18 is moved away from the workpiece 21, the probe arm 26 will be returned to its original position, again in the manner described above in readiness for the taking of a further coordinate. Since the arm 26 of the probe 18 is subjected to deflection forces after contact with the workpiece 21 has been made, the torsional stiffness of the probe arm 26 has no effect upon the accuracy of the probe 18. The workpiece sensing probe 18a shown in Fig. 5 is similar in concept to that shown in Fig. 2 and for the sake of convenience, common integers are suffixed with the letter a. The probe 18a differs from probe 18 in that the flange 29a is provided with a straight hollow cylindrical portion 35 defining a bore which is adapted to receive the spherical end 36 of a location member 37 and to support the probe arm 26a. The location member 37 is fixed within the probe support structure 1 9a and positioned so that the central axis of the cylindrical portion 35 is maintained coaxial with the central axis of the support structure 19a. As in the case of the probe 18, the arrangement permits the probe arm 26a to be deflected from the position shown in Fig. 5, but ensures that it returns to that position when the deflection force is removed. Throughout the specification, reference has been made to a datum position which is used for establishing the relative co-ordinates of the various points of contact between the probe 18 and workpiece 21. That datum position may be any position which is convenient to the particular workpiece being measured. Thus it may be a certain reference position on the bed 11. Alternatively it could be a particular co-ordinate on the workpiece 21 itself. In this case it could be convenient to zero the displays 17a, 17b and 17c upon contact of the probe 18 with the workpiece 21 at the particular co-ordinate in question. Although the present invention has been described with reference to a co-ordinate measuring machine operated manually, it will be appreciated that it is particularly suited for use with automatic co-ordinate measuring machines. Such machines are provided with power drives for translating the probe support 14, and thus the probe 18 along the three mutually perpendicular axes, stopping when contact between the probe 18 and workpiece 21 is made, recording the coordinates of that contact point and then proceeding to further workpiece contact points. If such measuring machines are to operate completely automatically, it is highly desirable that the probe 18 is going to provide an electrical signal for the latching circuit 23 every time contact is made with the workpiece 21.Since this signal is provided by the completion of an electrical circuit which includes the probe 18 and workpiece 21, the possiblility of the failure of a signal being sent to the latching circuit 23 is low. Various modifications may be made without departing from the invention. WHAT WE CLAIM IS:

1. A measuring machine comprising a workpiece supporting structure, a probe support adapted to translate along at least one axis, a workpiece sensing probe mounted on said support, measuring means adapted to measure the distance from a given datum along said at least one axis of said probe support and hence of said workpiece sensing probe, an electrical circuit which includes said workpiece sensing probe and a workpiece to be measured, said electrical circuit being adapted to maintain said workpiece sensing probe at a different electrical potential to said workpiece and to be completed by contact between said probe and said workpiece, and means adapted to detect the completion of said electrical circuit and to cause said measuring means to give an output indicative of the instantaneous distance of said probe from said datum along said at least one axis when said circuit is completed.

2. A machine according to claim 1,

wherein the workpiece sensing probe comprises at least one arm for contacting a workpiece and support means for said arm, said arm being mounted on said support means in a given positional relationship thereto but adapted to be deflectable from said given positional relationship, means being provided whereby said arm is returned to said positional relationship with said support means after deflection.

3. A machine according to claim 2, wherein the connection between said arm and said support means, enabling deflection of said arm from said given positional relationship, is provided by a spherical member located in receiving means.

4. A machine according to claim 3, wherein said spherical member is provided on said arm and said receiving means is provided on said support means.

5. A machine according to claim 3, wherein said spherical member is provided on said support means and said receiving means is provided on said arm.

6. A probe according to any of claims 3 to 5, wherein said receiving means defines a bore within which said spherical member locates.

7. A probe according to claim 6, wherein said arm is provided with a flange with which the return means engages to bias the flange into contact with said support means, the flange being arranged such that when said arm is not deflected, said arm and said bore are in co-axial relationship.

8. A probe according to claim 7, wherein the return means comprises a tension spring.

9. A machine according to any of the preceding claims, wherein said probe support is adapted to translate along three mutually perpendicular axes.

10. A measuring machine substantially as hereinbefore described with reference to the accompanying drawings.