Measuring methods for use on machine tools

• US 20080184579A1
• Filed: 03/31/2008
• Published: 08/07/2008
• Est. Priority Date: 09/24/2003
• Status: Active Grant

• Current Assignee: Renishaw PLC
• Original Assignee: Renishaw PLC
• Inventors: McFarland, Geoffrey
• Granted Patent: US 7,523,561 B2
• Time in Patent Office: Days
• US Class Current: 33/551
• CPC Class Codes: G01B 21/04 by measuring coordinates of points
• CPC Class Codes: G05B 19/19 characterised by positioning or contouring control systems, e.g. to control position from one programmed point to another or to control movement along a programmed continuous path
• CPC Class Codes: G05B 2219/36503 Adapt program to real coordinates, software orientation
• CPC Class Codes: G05B 2219/37097 Marker on workpiece to detect reference position
• CPC Class Codes: G05B 2219/50049 Control machine as function of position, angle of workpiece
• CPC Class Codes: G05B 2219/50063 Probe, measure, verify workpiece, feedback measured values

Abstract

On a machine tool, a program receives data from a scanning or analogue probe, measuring a feature of a workpiece. This data is combined with assumed machine position data during the scanning movement. This avoids having to break into the servo feedback loop to get actual measured machine position data. The assumed machine position data can be derived from a part program which controls the scanning movement. Several ways are described for compensating for errors between the assumed machine position values and the actual values.

Claims

1. A method of measuring workpieces on a machine tool, comprising the steps of:

causing relative movement between an analogue or scanning probe and a workpiece, along a pre-defined path comprising nominal positions of the probe relative to the workpiece, under control of a part program;

during the movement, taking measurements of a position of a surface of the workpiece relative to the probe, using an output of the probe; and

combining said measurements from the output of the probe with assumed data derived from said part program.

2. A method according to claim 1, wherein a result of said combining step gives a dimension of the workpiece.

3. A method according to claim 1, wherein the assumed data derived from said part program comprises at least one target or nominal probe output reading.

4. A method according to claim 1, wherein a result of said combining step gives an error value for a dimension of the workpiece.

5. A method according to claim 1, including a step of compensating for a difference between the assumed data derived from the part program and an actual position of the probe relative to the workpiece.

6. A method according to claim 5, wherein the compensating step uses data from an artifact of known dimensions and form.

7. A method according to claim 6, wherein the artifact has similar dimensions to a feature of the workpiece being measured.

8. A method according to claim 6, wherein the artifact has a dissimilar dimension to the feature being measured, or is measured at a dissimilar speed, and the compensation includes interpolation to correct for such dissimilar dimension or speed.

9. A method according to claim 5, wherein the compensating step uses data from reference points measured on the surface of the workpiece.

10. A method according to claim 5, wherein the compensating step uses an error map determined by comparing scans of the surface of the workpiece at slow and fast speeds.

11. A controller or probe interface for a machine tool, programmed to perform a method according to claim 1.

12. A controller or probe interface for a machine tool, programmed to perform a method according to claim 3.

13. A method of measuring workpieces on a machine tool, comprising the steps of:

causing relative movement between an analogue or scanning probe and a workpiece, along a pre-defined path comprising nominal assumed first positions of the probe;

during the movement, taking measurements of a workpiece surface relative to the probe, using an output of the probe; and

combining said measurements from the output of the probe with data derived from the assumed first positions of the probe.

14. A method according to claim 13, wherein a result of said combining step gives a dimension of the workpiece.

15. A method according to claim 13, wherein data derived from the pre-defined path comprises at least one target or nominal probe output reading.

16. A method according to claim 13, wherein a result of said combining step gives an error value for a dimension of the workpiece.

17. A method according to claim 13, including a step of compensating for a difference between a nominal assumed position in the pre-defined path and an actual position of the probe relative to the workpiece.

18. A method according to claim 17, wherein the compensating step uses data from an artifact of known dimensions and form.

19. A method according to claim 18, wherein the artifact has similar dimensions to the feature of the workpiece being measured.

20. A method according to claim 18, wherein the artifact has a dissimilar dimension to the feature being measured, or is measured at a dissimilar speed, and the compensation includes interpolation to correct for such dissimilar dimension or speed.

21. A method according to claim 17, wherein the compensating step uses data from reference points measured on the workpiece surface.

22. A method according to claim 17, wherein the compensating step uses an error map determined by comparing scans of the workpiece surface at slow and fast speeds.

23. A controller or probe interface for a machine tool, programmed to perform a method according to claim 13.

24. A method according to claim 13, wherein:

the causing relative movement is under control of a part program; and

the assumed first positions of the probe are derived from the part program.