DIY digitising probe

Hi all,

Whilst waiting impatiently for my SC420/2 to be delivered, I have been filling in the time making a digitising probe. It's based on the familiar 6 balls and 3 rods principle used by many commercial units. The pcb was machined from 1.6mm FR4 using my mini-lathe and mini-mill and accomplished in an hour what the SC420 would have completed in a few minutes.

Gold plated balls and rods started life as body piercing barbells. I assumed that the gold would solder easily to the pcb with part of the rod left in place to position the ball accurately. How wrong I was!! No matter what flux I used,the gold refused to tin (I tried arax, fluxite, solder paste - all to no avail) - very weird to say the least.

Eventually I replaced the rod with some 16G copper wire screwed into each ball instead. The wire tinnned OK using resin cored solder, then the modified balls were soldered to the pcb via the copper wire. Not an ideal solution, but at least good electrical connection was made.

Three gold plated rods were a force fit into a delrin carrier, this having been drilled at exact 120 degree intervals. An M3 tapped hole in the bottom of the carrier supports a Renishaw 2mm ruby ball probe, and a spring on top ensures that positive contact is maintained between rods and balls. Continuity around the 6 balls and 3 rods is broken when the probe is disturbed.

The probe body was machined from 1.5" aluminium bar, the top cap and shaft from mild steel. Adjusting screws allow the body of the probe to be moved relative to the top cap to set the probe tip concentric with the mounting shaft.

Initial crude measurements show a repeatability of about .0002" (5 microns), and a concetricity of about the same size as far as I can tell. Once I have my Stepcraft complete and its HF500 spindle mounted, then an 8mm ER11 collet will carry the digitiser and allow final adjustments to be made in situ.

I'll post some photos once I've figured out how to do it - advice welcome!!