[lerman]

Don't forget that there are requirements other than the low force requirement. My requirements list looks something like:

1 -- It must have a mount that "self centers" in three dimensions.

2 -- It must detect displacement in three dimensions.

3 -- It must permit a large motion (so that it won't break when the machine overshoots) in three dimensions. It should be possible to overshoot and break a probe without destroying the probe body. It must have replaceable probes so that when it crashes, the probe can be replaced.

4 -- It must be repeatable.

5 -- It must apply a minimal force to the object being probed (not required for all applications).

6 -- It must give repeatable measurements, independent of temperature, barometric pressure, humidity, etc. It shouldn't wear significantly in use (ruby tips are inexpensive and generally used).

7 -- The measuring probe must be concentric with the spindle axis; either machined that way or adjustable. (Adjustable is probably better so that cheap probes can be interchanged.)

Whether mechanical contacts (rods and balls), strain gauges, piezo for sensors, optical, or other means of sensing is used, the above requirements still exist. I don't think it likely that the simple rod and ball kinematic mount where the rods and balls complete the circuit can be beat for my application.

Ken