GB2110125A

GB2110125A - Machine tool straightness calibration

Info

Publication number: GB2110125A
Application number: GB08135109A
Authority: GB
Inventors: Ian Douglas Gooch; John Malcolm Morrison
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1981-11-20
Filing date: 1981-11-20
Publication date: 1983-06-15
Also published as: DE3241344A1; GB2110125B

Abstract

Apparatus for correcting straightness errors in a machine tool having a first member (10) movable nominally along a first axis (12) on guideways carried by a second member (11) includes a tensioned filament (13) supported at its ends by the second member and extending at a known angle to the first axis (12). A sensor (14) is provided, carried by the first member (10), and operable to co-operate with the tensioned filament (13) to indicate the sense and magnitude of any movement of the first member (10) along a second axis (16) at an angle to the length of the filament (13). Preferably the filament (13) is parallel to the first axis (12) and perpendicular to the second axis (16). The sensor may include a light source and two light sensitive devices; the source and devices being disposed on opposite sides of the filament and providing an output signal to circuit means indicating an error movement of the first member. <IMAGE>

Description

SPECIFICATION Apparatus for correcting straightness errors in a machine tool This invention relates to apparatus for correcting straightness errors in a machine tool. The accuracy of any form of machine tool, whether for metal working, plotting digitising or inspection, depends to a large extent on the straightness of the guideways along which various parts of the machine move. If, for example, the X-axis guideways are slightly curved, then measurements about the Y-axis will contain errors. The errors may be built into the machine during manufacture, or may be due to wear and other causes arising through use of the machine.If the errors are built-in, then it is possible to calibrate the machine so that the errors may be corrected continuously, but this requires complex electronic circuitry, and cannot be done if the errors develop during use unless the machine is re-calibrated at intervals.

It is an object of the invention to provide apparatus for correcting straightness errors in a machine tool.

According to the present invention there is provided apparatus for correcting straightness errors in a machine tool having a first member movable nominally along a first axis on guideways carried by a second member, which includes a tensioned filament supported at both ends by the second member and extending along an axis at a known angle to said first axis, and a sensor carried by the first member for movement therewith and operable to co-operate with the tensioned filament to indicate the sense and magnitude of any movement of the first member along a second axis at an angle to the length of the filament.

Preferably the axis of the tensioned filament is parallel to said first axis and perpendicular to said second axis.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic plan view of a machine tool: Figure 2 is a plan view showing one form of sensor; Figure 3 is a side view of the sensor of Fig.

2; Figure 4 illustrates the operation of the sensor of Figs. 2 and 3 Figure 5 is a schematic diagram of a circuit for use with the sensor of Figs. 2 and 3; and Figures 6 and 7 are plan and side views respectively of an alternative form of sensor.

Referring now to Fig. 1, this shows a plan view of part of a typical machine tool. A first member, in the form of a carriage 10, is movable on guideways carried by a second member 11, forming the bed of the machine.

The movement of the carriage is nominally along a first axis 12, parallel to the guideways. The bed 11 of the machine supports a tensioned filament 13 which is arranged to extend parallel to the first axis 12.

The carriage 10 supports a sensor 14, which will be described in detail later.

In the machine tool illustrates in Fig. 1 the carriage 10 itself has guideways on which a further movable member 15 moves, nominally along a second axis 16. The carriage 10 supports a second tensioned filament 17, whilst the member 15 supports a second sensor 18.

Since both sensors co-operate with the corresponding tensioned filament in the same way, further description will be limited to the operation of sensor 14 and filament 13.

The tensioned filament may be a wire stretched between two points or a plastic filament. It is perhaps easier to obtain a distortion-free plastics filament.

Figs. 2 and 3 show plan and side views of one possible form of the sensor, showing also the tensioned filament. The sensor 14 comprises a light source 21 and a pair of lightsensitive devices 22 and 23. Between the light source 21 and the devices 22 and 23 passes the tensioned filament 13. The devices 22 and 23 are arranged along a line perpendicuiar to the length of the filament 13 and close together, so that in a datum condition the filament is symmetrically located with respect to the two light-sensitive devices. In this embodiment of the sensor, where only small variations in the straightness of the machine tool guideways are anticipated, the filament may be of a thickness equal to or even greater than the active area of each light-sensitive device. In the datum position, therefore the filament obscures an equal part of the active area of each device.Straightness errors will cause effective movement of the filament across the sensor, thus obscuring more of the active area of one or other of the sensors. If the two sensors, which will conveniently be photodiodes, are connected in parallel with opposite polarity across a virtual earth amplifier with feedback, then a discriminator-like characteristic is produced such as is shown in Fig. 4. This diagram shows amplifier output voltage plotted against displacement d of the filament from its datum position. In the datum position equal areas of each photodiode are unobscured, and hence the resulting output voltage is zero. Only that part of the curve shown in full line is useful; any attempt to use the dotted parts of the output characteristic will result in an ambiguous output.

Fig. 5 shows one possible circuit in which the sensor may be used. This shows the virtual earth amplifier A with the two photodiodes D1 and D2 connected across its input.

The output of the amplifier is digitised by an analogue-to-digital converter AD and thus represented at any instant by an N-bit num ber. This number is applied to a latch LT which stores the number which exists at any particular sampling time. It is convenient to take the samples each time that the carriage of the machine moves a certain predetermined distance along the X-axis, remembering that the deviation being measured is in the Y-axis.

The deviation so determined may be applied to the existing measuring circuit output. If, for example, a counter holds the value of the Yaxis movement from a datum position, then the output from the latch may simply be added to the value in that counter. The sense of the addition will depend upon the sense of the deviation. As already stated, exactly the same technique is used to measure X-axis deviation.

In the above embodiment the two photodiodes are arranged perpendicular to the length of the filament. There may be occasions when this cannot be done, and when the diodes have to be arranged along a line inclined to the length of the filament. In such a case it will be necessary to apply a correction to the output of the amplifier A to enable it to represent the true deviation. Such a correction may readily be applied by using a randomaccess memory to which the output of the latch LT is applied. This memory will respond to a particularly input to deliver an ouput representing the corrected value.

If the deviation is expected to be greater than can be accommodated by using two diodes, then an array of photodiodes may be used. Such an arrangement is shown schematically in plan and elevation in Figs. 6 and 7 respectively. The array of photodiodes is arranged in a line, inclined at whatever angle is necessary to accommodate the expected deviation. The circuitry will require to be modified to take account of the number of photodiodes used in order to distinguish between the outputs of the different photodiodes.

Other types of sensor could be used and these may be light sensitive or use other sensing tenchiques.

Claims

1. Apparatus for correcting straightness errors in a machine tool having a first member movable nominally along a first axis on guideways carried by a second member, which includes a tensioned filament supported at both ends by the second member and extending along an axis at a known angle to said first axis, and a sensor carried by the first member for movement therewith and operable to co-operate with the tensioned filament to indicate the sense and magnitude of any movement of the first member along a second axis at an angle to the length of the filament.

2. Apparatus as claimed in Claim 1 in which the axis of the tensioned filament is parallel to said first axis and perpendicular to said second axis.

3. Apparatus as claimed in either of Claims 1 or 2 in which the tensioned filament comprises a wire.

4. Apparatus as claimed in either of Clams 1 or 2 in which the tensioned filament comprises a plastic filament.

5. Apparatus as claimed in any one of Claims 1 to 4 in which sensor includes a light source and at least two light-sensitive deivces operable to detect the position of the filament along said second axis.

6. Apparatus as claimed in any one of the preceding claims which includes circuit means responsive to the output of the sensor to determine the extent of movement of the first member along said second axis.

7. Apparatus for correcting straightness errors in a machine tool substantially as herein described with reference to the accompanying drawings.

8. A machine tool having at least one axis of movement fitted with apparatus for correcting straightness errors as claimed in any one of the preceding claims.