PROBE RADIUS COMPENSATION

• When the B-axis is tilted, compensating for the probe radius can get tricky. For example, if I’m probing a side of a rectangular block, I have to account for offsets in both X and Z (or other axes, depending on the tilt). This makes me wonder if switching to a spherical calibration disk might simplify things, as the sphere has constant radius compensation regardless of tilt.
• I’ve designed a potential DIY probe where the tool tip is connected to 24V, and the workpiece (or calibration disk) is connected to the CNC controller’s input port. When contact is made, the circuit closes, and the signal is detected. This seems similar to how commercial probes work but is much simpler.

Ahmed.emara wrote:

• my Questions are : 
  • 1- if i switched to spherical shape instead of rectangular one will it simplify the calibration process, especially when compensating for B-axis tilt?
    2- is there is a predefined probe radius compensation in linuxcnc which i can use and it take considaration a titled workpiece
    3- For a DIY probe using the method described above (24V signal on contact), will this provide sufficient accuracy and reliability for calibration purposes or should I invest in a commercial probe for the long term ?

• 1 - All commercial calibration artifacts I've seen are spheres
  • They are attached to the work surface, and the probing routines (DIY or purchased) probe the sphere at various locations - including A- and B-axis tilts.
  • Center of rotation for each rotary axis is calculated after the calibration ball is probed in multiple locations
  • Some machines then write the absolute value of the rotary axes CoR to non-volatile parameters for use by TCP.  All that's needed from that point is to re-probe a few locations after operation to check & adjust for thermal changes.
• 2 - A number of the GUI's available with LCNC have probing features/functions.  I believe all of them have some sort of probe tip diameter compensation adjustment.
  • I don't think any of the LCNC GUI's offer rotary axis calibration probing macros, however.
• 3 - This is not the way commercial probes work - they don't pass current through the probe and part back to the controller.
  • Probes have a number of internal mechanism designs, but they are essentially a switch.  When the probe stylus is touched/displaced from it's resting point, the switch triggers and the controller sees that state change as a 'touch.'
  • Higher-cost probes use internal strain gauges to detect stylus deflection.  Lower-cost probes use the 6-ball mechanism Andy mentioned in his response.  An internet search for 'CNC touch probe mechanism' or similar should help you visualize what's going on inside.
• If you don't want to DIY a probe, and you don't want to pay for a Blum or Renishaw, there are lower-cost probes (wired & wireless) available which provide quite decent results.
  • The nice thing about purchasing a higher-end probe is that they generally come with a suite of probing macros which can be installed on the controller.  I've never used a commercial probe with LCNC, so I don't know if commercial probing macros are compatible.