GB2033610A - Method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool - Google Patents

Abstract

In an arrangement for detecting changes in dimension of a tool holder or workpiece holder of a machine tool, the spindle bar 11 for example is set so that a predetermined point thereon which is unaffected by changes in dimension caused by thermal expansion and contraction is disposed at a preset machining co-ordinate on a datum axis parallel to the spindle axis 12. In order to detect and compensate for changes in dimension of the spindle bar 11 along the datum axis, the numerical control system is programmed on command to move the spindle bar 11 along the datum axis until an end surface 15 thereof is detected by a probe 16 of a detector 14, and the position of said predetermined point at which such detection takes place is measured. This measured position is then compared with the expected value and any difference is applied to the preset machining co-ordinate as a compensation. Changes in dimension of a tool holder relative to a workpiece holder in more than one direction may be accommodated. <IMAGE>

Description

SPECIFICATION Method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool This invention relates to a method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool, which changes occur due to thermal expansion, for example. The invention also relates to a machine tool in which this method is performed.

When a spindle of a machine tool rotates for a period of time, it tends to expand due to the rise in temperature which it experiences. The settings of the spindle relative to a workpiece holder then become inaccurate, resulting in a need to reset the datum co-ordinates of the spindle relative to the workpiece holder.

It is an object of the present invention to provide a method which enables such resetting of the datum co-ordinates to be performed in a simple manner.

According to the present invention, there is provided a method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool, the method comprising detecting when a first point on the holder is displaced by a known amount from a reference point along a datum axis, the position of the reference point on said axis being either predetermined or fixed relative to the other holder, and upon such detection measuring the displacement along said datum axis of a second point on the holder from the same reference point or a different reference point whose position on said axis is also either predetermined or fixed relative to the other holder, the first and second points being so chosen that their separation along said datum axis changes as the holder changes dimension in use.

In one particular example, the method comprises the steps of: (a) detecting when said first point is displaced by a predetermined amount from a predetermined reference point along said datum axis; (b) upon such detection, measuring the displacement of said second point from the same or a different predetermined reference point along said datum axis; (c) repeating steps (a) and (b) during a machining operation; and (d) adjusting the amount by which said second point is displaced from the respective predetermined reference point during the machining operation in accordance with any changes in the displacement measured in step (b).

By repeating steps (a) and (b) at various stages of the machining operation, any change in dimension of the holder will become manifest as a difference in the measured displacement of the second point from the respective reference point along the datum axis.

The position along the datum axis at which the second point is disposed during the actual machining operation can then be adjusted in accordance with the difference thus obtained, thereby compensating for the dimensional change in the holder. In this connection, it is assumed that the position of the first point along the datum axis is to be maintained constant during machining.

In another example, the datum axis forms one co-ordinate axis of the co-ordinate system, and the method comprises the steps of: (a) effecting relative movement between the tool and workpiece holders so as to vary their separation successively along each co-ordinate axis of said co-ordinate system; (b) during said relative movement along each co-ordinate axis, detecting when a respective first point on the holder and a reference point whose position on said axis is fixed relative to the other holder are at a predetermined separation along said axis; (c) upon such detection, evaluating the separation along said axis of a respective second point on the holder from the same reference point or a different reference point whose position on said axis is fixed relative to the other holder; (d) repeating steps (a) and (c) during a machining operation; and (e) adjusting the position which the respective second point occupies on each co-ordinate axis during the machining operation in accordance with any changes in the separation along said axis evaluated in step (c).

Where the tool holder includes a rotatable spindle bar, the co-ordinate system conveniently comprises rectangular Cartesian co-ordinate axes one of which is parallel to the rotation axis of the spindle bar. The co-ordinate system can however have other forms, such as a polar co-ordinate system.

Also according to the present invention, there is provided a machine tool comprising a tool holder and a workpiece holder one of which is subject to dimensional changes along a datum axis in use, detection means arranged to detect when a first point on said one of the holders is displaced by a predetermined amount along said datum axis from a reference point whose position on said datum axis is either predetermined or fixed relative to the other holder, and control means programmed on command to move said one of the holders along said datum axis and, upon said detection by the detection means, to measure the displacement along the datum axis of a second point on said one of the holders from the same reference point or a different reference point whose position on said datum axis is also either predetermined or fixed relative to the other holder, the first and second points being so chosen that their separation along said datum axis changes as said one of the holders changes dimension in use.

In one particular arrangement, the detection means is arranged to detect when said first point is displaced by a predetermined amount along the datum axis from a predetermined reference point, and the control means is arranged upon said detection by the detection means to measure the displacement of said second point along the datum axis from the same or a different reference point, the control means also being arranged to adjust the datum co-ordinates of said one of the holders in accordance with any changes in said measured displacement.

In another arrangement, the datum axis forms one co-ordinate axis of a co-ordinate system, means is operable to effect relative movement between the tool and workpiece holders along each co-ordinate axis of said co-ordinate system, the detection means is arranged for each co-ordinate axis to detect when a respective first point on said one of the holders and a reference point whose position on said axis is fixed relative to the other holder are at a predetermined separation along said axis, and the control means is programmed on command to effect relative movement between the tool and workpiece holders so as to vary their separation successively along each co-ordinate axis and, upon said detection by the detection means, to evaluate the separation along said axis of a respective second point on the holder from the same reference point ora different reference point whose position on said axis is fixed relative to the other holder, the control means also being programmed to reset the datum co-ordinates of the tool holder relative to the workpiece holder in accordance with any changes in each separation thus evaluated.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic side view of a tool holder of a machine tool in which changes in dimension thereof can be detected by one embodiment of a method according to the present invention; Figures2to 5are schematic side views of the tool holder at various stages in the performance of the detection method; Figure 6 is a perspective view of a machine tool in which the changes in dimension of a tool holder can be detected by a second embodiment of a method according to the present invention; Figure 7 is a plan view, partly in section, of a pair of probe units which form part of the machine tool shown in Figure 6; and Figure 8 is a side view, partly in section, of the probe units shown in Figure 7.

Referring first to Figure 1, the tool holder illustrated therein comprises a head 10 in which a spindle bar 11 is mounted for movement relative thereto along a datum axis 12. Such movement of the spindle bar 11 is controlled by a numerical control system (not shown). The spindle bar 11 is rotatable about an axis parallel to the datum axis 12, and is adapted to receive a tool (not shown) in a forward end 13 thereof for machining a workpiece held in a workpiece holder (not shown).

A detector 14 is mounted on the head 10 and is arranged to detect when a forward end surface 15 of the spindle bar 11 becomes aligned with a probe 16.

In one particular arrangement, the detector 14 operates on optical principles and is arranged to detect when a light beam emitted by the probe 16 is reflected by the spindle bar 11. In another arrangement, the detector 14 operates on magnetic principles and is arranged to respond inductively to the presence of the spindle bar 11 adjacent the probe 16.

The numerical control system is pre-programmed with a reference co-ordinate for the spindle bar 11 along the datum axis 12. When the spindle bar is set to this reference co-ordinate, a point thereon which is not affected by dimensional changes thereof occupies a predetermined and repeatable position relative to the head 10. In the particular embodiment illustrated, the reference co-ordinate corresponds to the position of the spindle bar in which tool changing is effected, shown in Figure 3. The numeral control system is also programmed with a machining co-ordinate, in which said point on the spindle bar is so located relative to the head 10 that the end 13 of the spindle bar (and therefore the tool mounted thereon) is disposed at a suitable position for machining of the workpiece to take place, as shown in Figure 2.

It will be manifest that any change in dimension of the spindle bar 11 along the datum axis 12 will cause the position of the tool relative to the workpiece to change, resulting in inaccuracies in the machining operation. Such dimensional changes can be expected to occur due to thermal expansion and contraction of the spindle bar as the machining operation progresses. In order to compensate for these dimensional changes, the numerical control system is programmed to perform the following sequence of operations on command.

(1) The spindle bar is rapidly retracted from its machining position (shown in Figure 2) to the tool change position (shown in Figure 3), for example at a rate of 3750mmimin.

(2) After reaching the tool change position, retraction of the spindle bar continues, but at a slower rate (for example, 700mmimin.). When the end surface 15 of the spindle bar passes the probe 16, a switch in the detector 14 opens and retraction of the spindle bar is arrested. There is however, some travel of the end surface 15 beyond the probe 16. The spindle bar is now disposed in the position shown in Figure 4.

(3) The spindle bar is advanced at a slow rate, for example 2.5mm/min., until the end surface 15 thereof comes into alignment with the probe 16, whereupon the aforementioned switch in the detector 14 closes and movement of the spindle bar is arrested.

The co-ordinate value along the axis 12 at which the spindle bar is stopped, i.e. the position of the aforementioned point on the spindle bar relative to the head 10, is then recorded. It will be manifest that the co-ordinate value thus recorded will vary as the dimension of the spindle bar along the axis 12 changes.

The numerical control system is commanded to perform this sequence of operations as a preliminary measure to the machining operation in order to calibrate the machine tool. Machining is then performed until an operator considers that the temperature of the spindle bar has risen sufficiently to require re-calibration. The above sequence of operations is then performed once again, with any difference in the recorded co-ordinate value being used to reset the machining co-ordinate to compensate for the thermal expansion of the spindle bar.

The embodiment described above deals with the detection of dimensional changes of the tool holder in a single direction only (i.e. parallel to the rotation axis of the spindle bar 11). An alternative embodi ment will now be described in which dimensional changes between the tool holder and the workpiece holder are detected in several different directions.

Referring now to Figure 6, the machine tool shown therein comprises a tool holder in the form of a spindle 110 which is mounted on a spindle holder 111 for rotation about a horizontal axis, and a workpiece holder (not shown) which is fixed to machine table 112. Relative movement can be effected between the spindle holder 111 and the table 112 in three directions which define respective axes of a rectangular Cartesian co-ordinate system.

The table 112 is movable horizontally relative to the spindle holder along an axis w parallel to the rotation axis of the spindle 110 and along an axis x at right angles to said rotation axis. The spindle holder 111 is movable relative to the table 112 along a vertical axis yand a horizontal axiszwhich is parallel to the spindle rotation axis. All such movements of the spindle holder 111 and the table 112 are performed under the guidance of a numerical control system (not shown).

Mounted on the machine table 112 in fixed relationship to the workpiece holder are two probe units 113 and 114. The probe unit 113 includes an optical probe Oi which is connected by way of a light guide 115 to a detection unit 116. The optical probe Oi is arranged to emit a beam of light in a direction parallel to thex axis, and the detection unit 116 is arranged to detect when this beam of light is reflected by the spindle 110. The probe unit 114 similarly includes an optical probe 02 which is connected by way of a light guide 117 to a detection unit 118. The optical probe 02 iS arranged to emit a beam of light in a direction parallel to the wandz axes, and the detection unit 118 is arranged to detect when the beam of light is reflected by the spindle.

Both of the detection units 116 and 118 are connected to the numerical control system.

In order to set the datum co-ordinates of the spindle relative to the workpiece holder, a set of machine datum co-ordinates are first fed into the numerical control system. These machine datum co-ordinates represent the desired position of the spindle relative to the workpiece holder for the particular machining operation, to be performed.

Upon command from an operator, the numeical control system then automatically performs the following sequence of operations.

(1) The spindle holder 111 and the table 112 are moved into a predetermined spatial relation, defined by respective calibration co-ordinates on the wand x axes for the table 112 and on the y and z axes for the spindle holder 111. These calibration co-ordinates are so arranged that the rotation axis of the spindle is in the same horizontal plane as the probes 01 and 02.

(2) While the table 112 is held stationary, the spindle holder 111 is moved so as to advance the spindle 110 along thez axis at a rate of approximately 2000mm/min. towards the workpiece holder, until a front face F of the spindle intercepts the light beam from the probe 01 at a position P1 (shown in broken line). In the event that the front face F overshoots the probe 0a, it is automatically returned by the numerical control system to the proper detection position.

The displacement of the spindle holder 111 from its calibration co-ordinate onthezaxis is then measured. If this measured displacement differs from a preset value, the difference is applied as a compensation to the machine datum co-ordinate for the z axis. This establishes the relationship between the w and z axis co-ordinates.

(3) The spindle holder 111 is held stationary while the table 112 is moved to the right (as viewed in Figure 1) along thex axis, again at approximately 2000mm/min., until a point L on the left-hand side of the spindle 110 intercepts the light beam from the probe 02. The spindle is then in a position P2 relative to the probe 02, as indicated in broken lines. The displacement of the table 112 from its calibration co-ordinate on the x axis is then measured and, if it differs from a preset value, the difference is applied as a compensation to the x axis machine datum co-ordinate. The relationship between thex andz axis co-ordinates is thus established.

(4) The table 112 is moved to the left along thex axis to return it to its calibration co-ordinate, the spindle head 11 is moved upwardly along the axis to move the spindle 110 above the probe 02, and the table 112 is moved back along thex axis until the rotation axis of the spindle is disposed vertically above the probe 02.

(5) The spindle holder 111 is moved downwardly along the y axis at approximately 2000 mm/min.

until a point B on the bottom of the spindle 110 intercepts the light beam from the probe 02. The spindle is then in a position P3 relative to the probe 02, as indicated in broken line. The displacement of the spindle holder from its calibration co-ordinate on the y axis is then measured and compared with a preset value. Any difference is applied as a compensation to the y axis machine datum co-ordinate. The relationship between the y axis and z axis coordinates is thus established.

The machining operation can then be carried out using the compensated machine datum coordinates.

During machining, the spindle 110 will tend to get hot, causing it to expand. The machine datum co-ordinate will be rendered inaccurate by such thermal expansion. However, these datum coordinates can be reset easily by commanding the numerical control system to perform the abovedescribed sequence of operations once again, each newly evaluated co-ordinate displacement being compared with the displacement previously measured.

The sequence of operations described above is only one example by which the method of the present invention can be put into practice. Other configurations of probes can be used, for example the probe 02 can be positioned vertically above or below the probe 01, and the sequence of operations effected by the numerical control system can be suitably modified.

Claims (18)

CLAIMS:

1. A method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool, the method comprising detecting when a first point on the holder is displaced by a known amount from a reference point along a datum axis, the position of the reference point on said axis being either predetermined or fixed relative to the other holder, and upon such detection measuring the displacement along said datum axis of a second point on the holder from the same reference point or a different reference point whose position on said axis is also either predetermined or fixed relative to the other holder, the first and second points being so chosen that their separation along said datum axis changes as the holder changes dimension in use.

2. A method as claimed in Claim 1, comprising the steps of: (a) detecting when said first point is displaced by a predetermind amount from a predetermined reference point along said datum axis; (b) upon such detection, measuring the displacement of said second point from the same or a different predetermined reference point along said datum axis; (c) repeating steps (a) and (b) during a machining operation; and (d) adjusting the amount by which said second point is displaced from the respective predetermined reference point during the machining operation in accordance with any changes in the displacement measured in step (b).

3. A method as claimed in Claim 2, wherein said holder is the tool holder, and said predetermined reference point corresponds to the position of said first point along the datum axis when the tool holder is in a tool change position.

4. A method as claimed in Claim 2 or 3, wherein the tool holder includes a rotatable spindle bar, and the datum axis is parallel to the rotation axis of the spindle bar.

5. A method as claimed in Claim 1, wherein the datum axis forms one co-ordinate axis of a coordinate system, and the method comprises the steps of: (a) effecting relative movement between the tool and workpiece holders so as to vary their separation successively along each co-ordinate axis of said co-ordinate system; (b) during said relative movement along each co-ordinate axis, detecting when a respective first point on the holder and a reference point whose position on said axis is fixed relative to the other holder are at a predetermined separation along said axis; (c) upon such detection, evaluating the separation along said axis of a respective second point on the holderfrom the same same reference point or a different reference point whose position on said axis is fixed relative to the other holder; (d) repeating steps (a) to (c) during a machining operation; and (e) adjusting the position which the respective second point occupies on each co-ordinate axis during the machining operation in accordance with any changes in the separation along said axis evaluated in step (c).

6. A method as claimed in Claim 5, wherein the tool holder includes a rotatable spindle bar, and the co-ordinate system comprises rectangular Cartesian co-ordinate axes one of which is parallel to the rotation axis of the spindle bar.

7. A method as claimed in any preceding claim, wherein said detection is performed by optical means.

8. A method as claimed in any one of Claims 1 to 6, wherein said detection is performed by magnetic means.

9. A machine tool comprising a tool holder and a workpiece holder one of which is subject to dimensional changes along a datum axis in use, detection means arranged to detect when a first point on said one of the holders is displaced by a predetermined amount along said datum axis from a reference point whose position on said datum axis is either predetermined or fixed relative to the other holder, and control means programmed on command to move said one of the holders along said datum axis and, upon said detection by the detection means, to measure the displacement along the datum axis of a second point on said one of the holders from the same reference point or a different reference point whose position on said datum axis is also either predetermined or fixed relative to the other holder, the first and second points being so chosen that their separation along said datum axis changes as said one of the holders changes dimension in use.

10. A machine tool as claimed in Claim 9, wherein the detection means is arranged to detect when said first point is displaced by a predetermined amount along the datum axis from a predetermined reference point, and the control means is arranged upon said detection by the detection means to measure the displacement of said second point along the datum axis from the same or a different reference point, the control means also being arranged to adjust the datum co-ordinates of said one of the holders in according with any changes in said measured displacement.

11. A machine tool as claimed in Claim 10, wherein the tool holder includes a rotatable spindle bar, and the datum axis is parallel to the rotation axis of the spindle bar.

12. A machine tool as claimed in Claim 10, wherein the datum axis forms one co-ordinate axis of a co-ordinate system, means is operable to effect relative movement between the tool and workpiece holders along each co-ordinate axis of said coordinate system, the detection means is arranged for each co-ordinate axis to detect when a respective first point on said one of the holders and a reference point whose position on said axis is fixed relative to the other holder are at a predetermined separation along said axis, and the control means is programmed on command to effect relative movement between the tool and workpiece holders so as to vary their separation successively along each coordinate axis and, upon said detection by the detection means, to evaluate the separation along said axis of a respective second point on the holder from the same reference point or a different reference point whose position on said axis is fixed relative to the other holder, the control means also being programmed to reset the datum co-ordinates of the tool holder relative to the workpiece holder in accordance with any changes in each separation thus evaluated.

13. A machine tool as claimed in Claim 12, wherein the tool holder includes a rotatable spindle bar, and the co-ordinate system comprises rectangular Cartesian co-ordinate axes one of which is parallel to the rotation axis of the spindle bar.

14. A machine tool as claimed in any one of Claims 10 to 13, wherein the detection means is composed of one or more optical detectors.

15. A machine tool as claimed in Claim 14, wherein the or each optical detector is arranged to detect when a leading surface of said one of the holders with respect to the datum axis or each co-ordinate axis intercepts a light beam.

16. A machine tool as claimed in any one of Claims 10 to 13, wherein the detection means is composed of a magnetic detector.

17. A method of detecting changes in dimension of a tool holder or a workpiece holder in a machine tool, the method being substantially as hereinbefore described with reference to Figures 1 to 5 or Figures 6 to 8 of the accompanying drawings.

18. A machine tool substantially as hereinbefore described with reference to Figures 1 to 5 or Figures 6 to 8 of the accompanying drawings.