I used the 24v ground from the power supply. Wiring isnt the issue, since I tried running probe commands with nothing wired up and still get the instant probe failed error. Thats fine though, I’m looking to upgrade from the birthday cake stack anyways. Probably gonna go leadshine MX3660 or TinyG.
So after some more fiddling and taking the entire machine apart; it seems that the touch probe works, but is just extremely finicky. If I flip the pin via $6 command, then soft reset (in UGS) then jog a bit, the G38.2 command works provided I keep the ends of the probe away from the electronics/xcarve. Also if I get the probing fails warning at any point in testing I have to flip $6=1, jog a bit and soft reset, then $6=0, jog a bit, soft reset and it will work for 1 probe. I’m almost 100% positive its some sort of RF/EMI because when I try to use the probe close to the machine/hooked up to the bit, it will instantly fail or probe for half a second then finish (nowhere near the other lead). I’m looking for a filter cap now to see if I can’t get rid of some EMI. I haven’t tried shielded cable because the only cable I have is very thick with a low strand count making it difficult to bend. I might stop at home depot and pick up some 2wire security camera shielded cable.
I am finally getting around to wiring up my touch plate since it is a rainy Saturday here in Kennesaw.
Attaching one side of the probe to the A5 pin was easy, but where is an easy place to attach the other side to ground?
The two common grounds on the Arduino have the pins from the Grbl shield sticking into them so it would be difficult to attach there, My first instinct is to use the power ground connector on the Arduino but reading above it seems that may not be a good idea.
Is there a ground pin on the Grbl shield I can attach to?
Mine goes to the arduino ground.
I just use the ground at the Z-axis home switch. 
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When you say the ground at the Zaxis do you mean the main power input connector (24V) sitting beside the Z connector?
No, the ground wire on the home switch. I use an alligator-clip jumper wire from there to the tool. Works great.
OK I have the the touch probe wired to A5 and ground. But when I send the command
M30;G38.2 Z-.5 F1;G92 Z.5;G0 z.25
the z axis does not move, and it seem to lock the Grbl (not able to jog) until I do a soft reset and $X
Seems like the G38 command should start the spindle moving down regardless. Is there a parameter I need to set to enable the touch probe logic? I am running Grbl .9i
just send this first:
G38.2 Z-.5 F1
Then after it moves and touches and stops send:
G92 Z.25
Assuming your plate is 0.25 thick
OK, I entered the G38.2 Z-.5 F1 portion of the command line all by itself
and the Z still not move, same result as before I had to do a soft rest and $X to unlock before anything would move again
The G92 Z.5;G0 z.25 portion of the command works great,
Are you using UGS?
Yes, sorry I should have mentioned that.
It just seems to me that the G38 command will start the movement regardless of the condition of the touch probe, unless it is just grounded out before the command is sent. To test that I disconnected the ground wire from the Arduino so there was no way I was pulling A5 to ground. Re-entered the G38 command again and still no movement.
Try inverting the probe pin $6=1 then try again.
Tried setting $6=1, same results as before.
I am starting to think my old way of setting the Z0 worked pretty good.
Are you jogging in milimeters? Verify you have everything set to inches.
It might be moving .5 mm and stopping
I just did the same command with jogging in mm and I get the same eror you get
Good thought. I will try that when I get home.