nOOb question on probe zero height

It’s probably been asked before but I’m going to ask so I get a clear understanding of it.

I have a dog river tools touch probe. Every single project I always cut completely out of whatever stock is laid down (therefore using my wasteboard as an actual wasteboard.

That said, obviously in settings there are options to set zero height to top or bottom

I always select bottom and cut a hair below (sometimes more) into the stock bottom of whatever the thickness the piece is (therefore cutting into the wasteboard ever so slightly) mostly cause my bed sinks slightly lower in the middle of my SO3 XXL.

I digress, when or why would you need to set the zero height to top?


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Short description about zeroing on stock top vs stock bottom:


  • zeroing on stock top is more convenient (and precise) when you do not cut all the way through. The perfect example is v-carving: you want your Z zero to be spot on at the top surface of your stock, so zeroing at stock bottom would likely induce some Z error.
  • on the opposite, zeroing on stock bottom as you do allows to precisely control the max depth of a profile cut. Some folks (including yours truly) like to keep their wasteboard pristine and not overcut, and zeroing at stock bottom is a way to ensure this and be tolerant to slight error/variations in stock thickness.

You may want to search the forum for “XXL wasteboard sag”, for a few ways to address this (basically add a threaded insert and screw underneath the center of your bed, and adjust it to compensate the sag)


My so3 xxl had a sag in the middle. I put a block of 3/4 plywood in the middle. I eventually built a torsion box and removed the leveling feet and put the xxl on a piece of 3/4 rigid foam and no more sag.

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I’ve removed the leveling feet (a long time coming - so the damage has been done) I’ve read that people take the feet off and use rigid foam underneath to help? I don’t know about the effectiveness of that but I guess I could see it helping?

Building a torsion box is out right now. (great idea) I don’t really have the time with the holidays coming up. Right now it’s on a flat file cabinet (very sturdy and super heavy on top of a box with two drawers on castors that lock the wheel and the swivel) it doesn’t move at all; and because the flat file cabinet isn’t big enough the machine sits on top of a 3/4 piece of plywood that is just wider than the flat file cabinet.

Thank you very much for that info and the direct link. That makes way more sense now. Guess I’ve been doing it incorrectly for quite some time! lol.

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