View Full Version : probing on all metal machine

05-03-2014, 11:23 PM
Question about probing (X edge, Y edge, hole centres finding etc.) on all-metal machines.

With the recent upgrade of my cnc machine to all metal parts, the tool and the workpiece are always electrically connected. This means I can no longer use croc clips on the tool and workpiece together with Mach3 2010 screenset (which is excellent by the way, best few pounds ever spent on this hobby!) to probe to the workpiece edges.

My previous machine used 2 layers of 18mm ply for the gantry sides so was electrically isolated. The probe was only active when the tool touched the work so Mach3 could set the edge locations.

To try and get around this I wrapped a sheet of lining paper around the part before tightening it up in the vice, but there was enough conduction through the paper for the probe to be active all the time. I took the part out of the vice and it had not ripped through the paper.

I have thought above putting glass fibre sheets (the type for making circuit boards) under the vice, plus a mica washer under the bolt head to electrically isolate the vice.

Or I could try and make up a plastic spacer of a very accurately known thickness, with a piece of metal over the top and a wire/clip to the metal, then hold the plastic side of that against the edge I want to probe to find the edge. Then type in the thickness of that part into Mach3 to get the true edge position.

I can't be the first to have come across this - what is everyone else doing to probe the edges of the workpiece? I don't want to go back to the paper-trap method which was laborious to say the least.


06-03-2014, 12:17 AM

I have the same situation - good conductivity throughout the machine (tool to bed).

For Z axis tool height I use a piece of copper faced PCB board which is wired to the probe input on my control board and a crocodile clip on the z-axis for the other end of the circuit.

For edge finding etc. I had to bite the bullet and buy a ruby tipped probe off of ebay, which was fairly pricey with import etc, but I needed it in a hurry to do some sub-frame mounts for a customer (after a couple of abortive attempts at trying to make my own - I nearly got there but couldn't get the bl**dy bearings to solder to the PCB permanently). Since I've had the probe its been great although I do have to go into config in MACH3 and change the probe to enable low if using this rather than the tool height which is the other way round.

Of course now I want an auto changer to switch the probe in and out and cutters, but I have other stuff to fit first ;)

06-03-2014, 12:33 AM
I just use a piece of single sided copper PCB in a long thin strip. It's stable material and Mics up accurate enough for most work.

I use Shrink wrap the stuff used for wrapping pallets for isolating from bed it's wide and strong. Suppose Cling film could work.?

06-03-2014, 01:29 PM
Thanks Washout and Jazz, some good suggestions there.

Copper faced PCB board seems like the best solution all round - I don't want to keep changing the bit for a probe etc.