I trying to use the touchoff function in Mach4.  I have everything set up and it probes correctly but the results are wrong and I am very confused as to why. I have the backoff set to .1 and the diameter set to .1 and I think after the touch of the Y- position it should read -.15. that would be the -.1 backoff and half the .1 diameter. Is that correct?
Attached is the result after the operation. The zero position is way into the part.
It shows the backoff position as -.3482
How is that possible? What am I missing?
Oh I am using the ESS if that makes a difference.

After hours of reading thru the code for the touch module I found that the calulation for setting the offset uses the touchplate height in the x and y touchoffs. Why x and y use a height setting I do not comprehend.
Only problem now is if the diameter is set to .1 the backoff position should be -.15 and it now set it at -.1482. I'm not sure where the .0018 difference is from.

Hi, I will try to help by sharing my experience so far. I have been using the Touch off routine for mostly Z Ref using a 15mm brass plate. The diameter of the tool has no bearing on Z Ref of course but Touch Plate Height does obviously. I have in some cases had to re-do the routine more than a few times to get it to work. I am also running the ESS by the way. It should not be a factor. Mach4 is just waiting for a message from the ESS "probe input equals true"
I cannot comment on the weird numbers you are getting, but I would make a habit of using 0.1" for instance rather than just .1"
Anyway you seem surprised that Mach4 uses the Touch Plate Height value in solving the offset. Well that makes sense, if you consider that all touch plates have a precise thickness of conductive material between the probe and the part regardless of axis.
 Until recently I have rarely used edge finding and just do Z Ref in my wood creations. X and Y ref Zero are G54 as temporary jig and most of my material begins as square blanks.
I find the code is a little fussy and sometimes I lose my patience during the second or third time for the offset to register. I haven't voiced my beef about this flaky behaviour yet and won't till I know for sure it is not a "me" problem.
For doing a Z Ref I will position the tool approx 5mm (0.125") from the Touch Plate using the pendant or MPG and start from there. It is so slow. But lots of time to think of what to make for dinner. When successful the new offset is posted along with 0.1mm retraction that allows the brass touch plate to be removed without scuffing the endmill.
In the case where I have done x/y edge finding to date, I have used aluminum tape. Really neat solution IMHO it's cheap, and sticks to anything. It was a wood carving, so not demanding ultra high accuracy. In that case I changed the value in the Probe Plate Height variable to 0.04mm
 to reflect the thickness of the tape and inserted value for the tool diameter and in the event where the contact of the probe just stops the feed you (we) have the option of manually inputting the obvious value. Just a temp fix till you get a better handle on the weird glitch you have there.
You change Plate Height (or thickness) to match whatever edge finding accessory used. Personally I find that you really have to keep an eye on the endmill you are dialing in to make sure you don't measure the space between the flutes.  I bought one of those high priced XYZ plates and shelved it because the stated thickness was not the same for X and Y. I will have to pull it out and try milling it to spec once I have metal work holding capabilities upgraded.
Next on the list of upgrades is a 3D edge finder and the use of the full featured probe tab in Mach4.


Regards, Chris

AH-HA.. Thats why its adding in the Z height touch plate.. Can you point me to that, please. I want to remove it from mine. Drives me nuts to have to zero that out when probing x and y....