Odd behavior Z touch plate

I was setting up a Z touch off plate today. Have it wired to signal and ground min z endstop, alligator clip on endmill, then other wire connected to a small piece of sheet metal. I have an aluminum plate (24"x24") attached to my spoil board. Now the odd thing is when I go to home z axis it raise then goes down a little and stops, doesn’t go all the way to my touch plate and not showing triggered with m114. So I’m not sure why it’s stopping. If I try it on mdf area works fine. Something with a large aluminum mass causing the problem?

What if you reverse the polarity?

Thanks Ryan,
I tried that and the same result. Also double checked to make sure my aluminum plate wasn’t ground to anything.

I had the same behavior several months ago and chased it down, eventually, to a loose connection for the z-axis stepper on the ramps board. I ended up replacing the connector on the end of the stepper wiring because the original connector had a very loose fit.

Thanks BT, I double checked all connections, everything checks out good. If I put thin cardboard betwent my touch off plate and my aluminum plate I’m milling, works fine. Must be picking up interference somewhere. I’m going to see if I can track it down or I might just rubber coat the bottom of my touch off plate. Not a big deal, just a nice to have.

Works on MDF but not aluminum… Hmmm.

The Arduino has a pull up resistor, right? I think that’s configurable in the software. You could connect a resistor, anything 1kohm - 20kohm between the signal and Vcc and see if that helps.

Do you have a voltmeter? If it’s set up correctly, the signal pin should be 5V and the other should be 0V until they touch, then they should both be 0V. That’s compared to the Arduino ground.

Also, the aluminum is isolated completely from any wiring except through the touch plate, right? Same with the drill bit, Right? If you suspect the bit, you can unplug the router.

It is very likely that your plate is picking up noise.
You can try to add a capacitor, to smooth the values. It worked for me on an other project where I had a touch probe connected who was acting weird.

The suggestion made by Jeffeb is valid too, you may want to try both separately and, if you still have the issue, at the same time. Basically, your aluminum plate is acting like an big antenna, picking up noise frome everywhere and making some little spikes. a bigger resistor will make the trigger level higher, a capacitor will smooth the spikes.