Recently I discovered a great program called AutoLeveller which allows you to probe the surface and adjust the G-code for the uneven thickness of the stock. Its primary application is making printed circuit boards. Since PCBs have a conductive copper layer, you can simply use a V-cutter or another sharp tool as a probe. But what if you want to engrave a non-conductive material?
I looked at some commercially available and DIY probes, and most of them revolve around the same old idea of three ball bearings 120 degrees apart. The problem with this approach is that you need to have a pretty strong return spring in order to have a reliable electrical contact - which means the probe tip exerts noticeable pressure that can deform thin stock material and invalidate the measurements. Also, for applications like engraving you don't really need 3-axis probing - all you need to measure is the surface height.
So I decided to make my own probe using a different approach. Z-axis only, least amount of mechanical parts, minimum force, no electrical contacts. My probe consists of just two main components - a sewing machine needle and a photo interrupter. Plus, of course, the case that holds them together, the 1/8" shank and the electrical cable.
I just put the probe together and haven't had a chance to play with it much, apart from some basic repeatability testing. It appears that I'm seeing the repeatability of under 0.01mm (including the slack in my machine's Z-axis) - which is perfectly adequate for my purposes.
I think I will let pictures tell the rest of the story. Feel free to ask any questions and make suggestions.
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