My new/second-hand CNC mill came with a touch probe. Basically the usual simple design with three pins at 120deg resting in pairs of balls making electical contact. Any movement and at least one contact will be broken. The probe (after breaking the original) is a 5mm ball turned on the end of a 4mm or so stem, screwed into the block holding the pins. There is a fairly strong spring keeping the pins in contact with the balls. When I first had it, there was poor contact between the pins and the contacts but all has been scrubbed with a glass fibre scratch brush and the contact is now reliable. Not exactly Renishaw but at least if I break a probe I can afford to replace it (as I can make it myself). There are quite a few similar designs around on the Internet.

I've just been trying to recalibrate with my latest replacement probe. The plastic plate holding the contacts rests on three small screws which allows the probe tip to be centred (with quite a lot of effort - it's a fiddly job). However, the best I've been able to achieve is a repeatabiity of around 0.04mm or so. I'm happy to describe how I calibrated it, but my real question is whether anyone having a similar device could comment on repeatability? I have a feeling that the spring is a bit too strong but before I play with that I wanted to compare notes. Thoughts?