[CitizenOfDreams]

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.

Attachment 338150 Attachment 338152 Attachment 338154 Attachment 338156 Attachment 338158 Attachment 338160

[RCaffin]

Cute, very cute!
Springs?
What sort of flag on the needle are you using?
And does it need a stylus? :-)

Cheers
Roger

[sajurcaju]

What keeps the needle from falling out if you raise Z?
This looks like a huge improvement over the three bearing approach.

Steve

[CitizenOfDreams]

A sewing machine needle has a thick shank on top, that's what's keeping it from falling out.

Attachment 339038

There is no spring, no flag, and no stylus - just the needle. The weight of the needle pulls it down. The shank blocks the light beam. The needle point replaces the fancy ruby ball found in $$$$ Renishaw probes. I did grind the top edge of the shank flat as accurate as I could so it would cross the beam with a straight edge.

I will say without false modesty that I really liked how this little project turned out. I do think that as a DIY single axis probe my design is better than what I have seen so far.

[manofmanyhats]

This is pretty brilliant. Such a simple and elegant solution.

[RCaffin]

Ah, but the ruby balls look so ... ruby! And you can MYOG them as well. See:
http://www.cnczone.com/forums/calibr...ml#post1956342

But very simple and elegant - and inexpensive.

Cheers
Roger

[CitizenOfDreams]

Thanks for the kind words!

Update: just broke the needle (rammed the probe hard into the stock due to an error in the script). No damage to the rest of the probe, so it seems to be pretty crash-resistant.

I replaced the needle with a heavier one (Singer size 18/110). I also took measures to keep the needle from rotating (which improved repeatability).

The bore for the needle is the only critical part of the design. It needs to be just the right size so the needle could slide up and down freely yet without wobbling. I shall get myself a set of precision drill bits...

[LeeWay]

You might consider making the holder out of delrin or UHMW. Both are pretty slick and impact resistant.
Looks pretty cool though.

[CitizenOfDreams]

The advantage of polycarbonate (which is what I'm using) is that it's easy to glue. With delrin or polyethylene I would have to come up with a different assembly procedure.

[LeeWay]

Yep.
You would even have a tough time of it with epoxies designed for plastics. They both sort of repel any stickyness.

[CitizenOfDreams]

Update: I made a second version of the poor man's probe. Mechanically it's the same, but I used a cheaper photo interrupter (20 cents on eBay) that did not have any built-in circuitry. So I added an external board with a 74HC14 chip (Schmitt trigger/inverter).

[CitizenOfDreams]

Seven years later, I'm still using my Poor Man's Probe. :wave:

Here is the current version: