RepRap: Blog

Detecting when the probe contacts the surface is not easy. When it gets there, it just stops. This makes determining bed levelling difficult to do by just probing at it. Even using an electrical probe has problems, as that relies on a perfectly flat contact surface and a perfectly clean one at that, with no oxide build-up anywhere and minimal electrical field effects. This never happens for me.

So I wondered if it might be possible to make a mechanical surface probe. I have a DTI gauge on my lathe which is notionally accurate to 10 microns, after all. I came up with a "Scott-Russell" flexure system with a flag sticking out of it. This is basically a flag on a very unstable toggle joint. I made several sizes: 

The sensitivity to motion less than 10 microns seems to be roughly comparable to that of the DTI, but the flexures require far less force to actuate. By just watching for the flag to start to move, you can fairly easily pick up a 10μm tip motion with the naked eye. 

These examples are printed on a Prusa XL, but it would be interesting to try to print a smaller one on a μRepRap. Doing that with an improperly calibrated μRepRap might not be possible, but if I can keep the delicate parts arranged in one direction then you only have to calibrate reasonably accurately along one axis for a relatively short distance. The theory is that once printed, one of these could be stuck to a probe tip, and then - when observed through a microscope - be used to level the Stage and determine the height of printed structures accurately.

Lots of if's and but's, dubious rigidity of the UV resin,a desire for the thing not to swivel sideways on contact, concern over separating the thing from the Stage etc. Might work, might not. Won't know until we try, will we? 

 

["Do You Have A Flag?" reference is to a comedy sketch by  Eddie Izzard, go look it up.]