Z homing prob

Hi everyone,


I have MPCNC set using dual endstops, they work fine.
To my understanding I need to find zero level of Z axis every time I print due to changing bit, moving Z
between session and so on. So I need to activate Z homing, get Z moving down until the bit touches the plate - then it knows material top or bed bottom or whatever refference point from which it can find where to start routing form.
So, I click Z home and Z axis moves 1-2 mm down then stops and moves another way just like it did touch the probe plate. But it didn’t. It happens when working from touchscreen and from pronterface/repetier.

Did you face such prob?

Is Z set for normally open or closed in the firmware? It needs to be normally open, all the others are normally closed.

It is normally open. When I touch bit by plate then m119 shows triggered.

Z stops it’s traveling down without circuit closing.


As a matter of fact, once it worked fine - moved down until a proper touch. But no more since that single time. Reboot doesn’t help.

Sounds like it’s getting some interference from something then. Probe wire isn’t close to any ac wires is it?

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Probe’s wire is laid without an order through other wires. I will try to isolate it and report, thanks for suggestion )


I found that it’s interference with my body. The fun part is that I don’t even need to touch.

Touching either bit or probe results in machine thinking that bit has touched the probe. However, M119 will not show “triggered” state.

Then, if I even APPROACH hand OR metal rod held by hand (even isolated one) to any part of probe’s wire to the distance of 1 inch - same result.

So I put probe, clear away and use non-metal rod to hold the probe in place, that’s enough for me.

Now I gotta understand why Z does not move up to some safe height after bit touching probe.

Just add it in with your homing sequence, G01 Z10 F400

Well, that would work but I’m operating with LCD screen without PC connected so cannot send commands on the fly. Except maybe saving homing gcode commands in files on SD.


Do you know how to compensate for probe’s thickness? Assuming that probe is 5 mm thick, machine gotta understand somehow that actual zero is 5mm lower than it felt.

G92 Z5

This g92 command should be called each time after homing and is meant to tell machine that it would need to move additional 5mm downwards before starting a session, right?

Edit. Oh I get it. It’s telling machine that it’s currently 5mm above the level and should be called after each homing. Thanks.

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Right, it tells the machine when the stop is triggered, it’s 5 mm above 0.

Thanks to all!

And the problem came back. Now even when I clear off and move away from the machine - it sporadically feels that it reached Z zero without actually touching anything. Taking wire out of RAMPS helps to prevent false triggering.

So far I don’t have any ideas of what is going on and how to solve it. I didn’t change anything since last successful operation.

I don’t have anything grounded, maybe problem is there?

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The following comment in “recent” Markin sample configuration files could give you an answer:


  • Endstop Noise Filter
  • Enable this option if endstops falsely trigger due to noise.
  • NOTE: Enabling this feature means adds an error of +/-0.2mm, so homing
  • will end up at a slightly different position on each G28. This will also
  • reduce accuracy of some bed probes.
  • For mechanical switches, the better approach to reduce noise is to install
  • a 100 nanofarads ceramic capacitor in parallel with the switch, making it
  • essentially noise-proof without sacrificing accuracy.
  • This option also increases MCU load when endstops or the probe are enabled.
  • So this is not recommended. USE AT YOUR OWN RISK.
  • (This feature is not required for common micro-switches mounted on PCBs
  • based on the Makerbot design, since they already include the 100nF capacitor.)


If you can, I’d suggest finding out if you can enable the noise filter for your configuration (and recompile), if not, trying and putting a capacitor as suggested.

You can also try swapping what wire si touching what, one is ground and one is signal, so use teh signal on teh other side.


For example if the signal is on the spindle it can very easily pick up noise.

Solution with capacitor worked like a charm, thanks!

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