#### Calibrating a C Axis Lathe

These lathes have a C Axis control. That is, the macros for these lathes can rotate the spindle and can read back the current rotation angle. A C Axis is defined on the Physical tab page of the MachineDefinition dialog of the CncServer.exe application as shown below.

Physical tab of the MachineDefinition dialog

See the Creating and Modifying Machines PC-DMIS NC topic for a more detailed description of each of the MachineDefinition tab pages.

The Hysteresis column has been added for lathes in PC-DMIS v2011 MR1 so spindle rotation can be avoided until a point deviates in the Y coordinate from zero by a user-defined amount. The Physical tab on the MachineDefinition dialog shown above describes a Fanuc lathe using inch units will only rotate the spindle for points that are at least 0.01 inch away from Y=0.

Groove Calibration

PC-DMIS NC has been adapted by building on dialogs found in PC-DMIS Vision in order to obtain a groove calibration where both inside and outside diameters are probed. An example of a calibration groove is shown below.

Groove Calibration Example

Select Edit | Preferences | Rotary Table Setup and ensure the Single rotary table option is selected and clear the offsets if calibrating from scratch as shown below.

Rotary Table Setup Dialog Box

Select or create a tip using the Probe Utilities dialog:

Probe Utilities Dialog Box

Click on the Edit button to view or edit the probe data. Click the OK button to accept any changes and return to the previous dialog box or click the Cancel button to return without saving.

Edit Probe Data Dialog Box

Click on the Setup button to view, edit and/or set the probe setup values as necessary. Click the OK button to accept any changes and return to the previous dialog box or click the Cancel button to return without saving.

Probe Setup Dialog Box

Click on the Measure button on the PC-DMIS Probe Utilities dialog and select a ring tool as shown below.

Measure Probe Dialog Box

Click on the Edit Tool... button to update the groove parameters or to create a new tool setup. The Edit Lathe Tool dialog box appears:

Edit Lathe Tool Dialog Box

This will measure 4mm deep into a groove at Z=162.173mm with an outside circle of diameter 179.867mm, an inside circle of diameter 205.149mm, a plane with an offset equal to the prehit distance beyond the 205.149 circle. After completion, click the OK button to return to the Measure Probe dialog then click on the Measure button to create the calibration part program in the Edit window.

Note: Calibration on a ring instead of a groove is done by making the Outside circle diameter larger than the Inside circle diameter.

View the path lines and check the path in the Graphics Display Window shown below.

Display of Groove Calibration Path Lines

This calibration ignores any offset due to the probe tip being offset in the Y axis. It is robust in that it allows recalibration using the current values and in practice has given similar results to the following calibration which adds a further small circle to compensate for the Y offset.

Edit Tool Dialog Box

This will create a measure 6mm deep into a groove at Z=162.173mm with an outside circle of diameter 179.867mm, an inside circle of diameter 205.149mm, a plane with offset equal to the prehit distance beyond the 205.149 circle, and finally an optional inside circle of diameter 61.204. This latter circle is used if the probe Y offset is required. After completion, click on the OK button to return to the Measure Probe dialog. Click on the Measure button to create the calibration part program in the Edit window. View the path lines and check the path in the Graphics Display Window shown below.

Display of Groove Calibration with Optional Inside Circle

Create the CNC program by selecting Operation | CNC Programming | Create CNC Programming.

Select CNC Machine Dialog Box

The CNC Program file will display as shown below:

CNC Calibration Program Output File

Note: The default is now for non-circular moves inside the groove since circular motion is naturally achieved by the lathe.

Copy the calibration program to the controller and run it to produce a Journal file as shown below.

Note: This journal file has been adapted from data collected during testing of V43B by copying over the file, machine and feature number identification from the .mpf file generated by PC-DMIS v2010.

CNC Journal Output File

Verify that the probe calibrations have been updated by bringing up the Probe Utilities dialog box.

Updated Probe Utilities Dialog Box

Click the Edit... button to see the updated details.

For comparison, the image below on the left is an example of the calibration that uses all three circles, on the right an example of the difference seen when using just the two circles.

Comparison of Edit Probe Data Dialog Boxes

Note: The same Journal file data has been adapted as before to be used in both cases

Navigate to the Rotary Table Setup menu option from Edit | Preferences | Rotary Table Setup to see the updates to the probe tip offset from the rotation center:

Updated Rotary Table Setup Dialog Box

It is instructive to dimension the calibration program in PC-DMIS to produce a report. Shown below, the deviations come in red as the nominals supplied are so far from true for this calibration artifact.

PC-DMIS Calibration Report

A sanity check can be done by turning off Marking the Probe Calibration feature in the part program, turning off the Ignore Rotary Table feature and re-running the Journal file as if running a normal measurement part program to check if the results are near perfect as shown below.

Updated PC-DMIS Report

Note: In practice, no discernible improvement in measurement accuracy has so far been seen by including the small circle and thus creating a Y offset figure in the rotary table settings and bearing in mind the caveat mentioned above it is recommended this option be ignored unless absolutely needed.

Vector Calibration

2D Vector Calibration has been added to the Groove calibration to compensate for out of roundness of the tip ball. Groove calibration only measures at three points on the tip so this may be insufficient. Vector Calibration measures a number of points around a circular profile.

As an example, to use the profile of the torus described above, add the diameter of the circle in the Edit Tool dialog in the highlighted position as follows and add the X axis offset of the circle center in the last edit box in the dialog.

Edit Tool Dialog Box for Vector Calibration

The generated part program is shown below.

Generated Part Program for Vector Calibration

This will create a calibration program as before but with the addition of a profile measurement which is used to create a look-up-table of vector versus tip radius.

Vector Calibration

Create the program by selecting Operation | CNC Programming | Create CNC Programming.

Copy the calibration program to the controller and run it to produce a Journal file as shown below.

Vector Calibration Journal Output File

Verify that the probe calibrations have been updated by opening the Probe Utilities Dialog Box.

Probe Utilities Dialog Box after Vector Calibration

Click on the Edit... button to view the new settings after Vector Calibration has been completed.

Edit Probe Data Dialog Box after Vector Calibration

Navigate to the Rotary Table Setup Dialog Box by clicking on Edit | Preferences | Rotary Table Setup from the main menu to see the update to the probe tip offset from the rotation center.

Rotary Table Setup Dialog Box after Vector Calibration

If the second vector calibration has been done following the simpler groove or ring calibration, you should see no significant difference to the values in either of the above dialogs since the addition of the second vector calibration option only calibrates the profile of the probe tip.

Note: When running saved Journal files to calibrate and recalibrate with different options selected, it is important to make sure the tool dialog matches too. If for example a simple groove calibration part program (with no second vector calibration) and the corresponding Journal file is run but the tool dialog retained the values for the second vector circle, then a bad calibration will result. This scenario does not apply when creating fresh calibrations on a machine as the tool dialog will always match where a fresh calibration program is created.

It is instructive to dimension the calibration program in PC-DMIS to produce a report. In this case, we are also interested in the circularity figure for the profile measurement.

PC-DMIS Report with Dimensioned Calibration Program

Including a graphical view of the Circularity is also instructive as shown below.

Graphical View of Circularity

As before, a sanity check can be done by turning off marking the probe calibration feature in the part program, turning off the Ignore Rotary Table feature and re-running the Journal file as if running a normal measurement part program to check if the results are near perfect.

Updated PC-DMIS Report after Vector Calibration

As shown below, the Circularity figure is near perfect.

Updated Graphical View after Vector Calibration