Z Honing issue on Tool Change

Mostly Printed CNC - MPCNC Advice
1

I’ve finally got my MPCNC back up and running after discovering that that a one of the filaments I originally used was faulty. (Every single printed part had become brittle, most of which had huge numbers of cracks throughout)

I run end stops + LCD + joystick.

A very strange thing happened today in 2 separate jobs. The z-honing failed to descend (it was as if it had touched the z-probe, when in reality it was no where near it). What is weird is that it did this on the final tool change only, all previous tool changes had worked as expected.

here is my tool change code section that failed;
;No. 73: Cutout
;Change tool: 1/8Alum

G0 X-20.0000 Y-20.0000 F3000
G1 Z25 F500 ;Raise z axis by 25mm
M0 Spindle OFF
M18 Z ; disable z stepper motor
M0 1/8Alum Attach
M0 position z-probe
G28 Z ; Auto Home z axis
G1 Z5 F500 ;Raise z axis by 5mm
M0 Remove probe & Start Router

Here is earlier one that worked perfectly;
;No. 71: Cutout
;Change tool: Alum 1/4"

G0 X-20.0000 Y-20.0000 F3000
G1 Z25 F500 ;Raise z axis by 25mm
M0 Spindle OFF
M18 Z ; disable z stepper motor
M0 Alum 1/4" Attach
M0 position z-probe
G28 Z ; Auto Home z axis
G1 Z5 F500 ;Raise z axis by 5mm
M0 Remove probe & Start Router

(I’m not accounting for the z-probe thickness as I’m cutting aluminum and making a connection with the z-probe elsewhere on the material)

I was thinking of adding a 2nd z-hone to try give it a 2nd chance to do it correctly. But perhaps someone has seen this behavior before?

2

My guess is it is triggered even though it hasn’t touched the plate. Here are some things you can try:

  • Try to reproduce the problem while checking with M119. It makes testing a lot easier and you can see what is happening much easier.
  • Try unplugging the router. The router may be grounding the bit, causing it to short to grounf withou touching the plate
  • Try swapping the ground and S pins. Connecting the bit to ground and the plate to S.
3

I had a similar issue with my old board.

Setup wise, I had ground (-) connected to the alligator clip that went to the tool, and signal (S) connected to the touch plate. Increasingly often, homing would stop before the tool touched the plate, particularly if I were also touching the plate.

The problem turned out to be that the pullup resistor on my board was failing, which left the (S) pin floating instead of pulled up to logic high.

My solution was to connect a 470 ohm resistor between the 5V (+) and signal (S) pins. (Up to 2k ohms would be fine.) Basically, I used all 3 pins on the Z- connector with a 3 pin connector. (+) connected to one end of the 470 ohm resistor, (S) connected to the other end of the resistor, and the wire leading to the touch plate, (-) connected to the wire leading to the alligator clip.

To check, keep running M119 (check endstops) while testing various conditions that should not trigger the endstops.

While typing, it occurs to me that you mention that you’re cutting aluminum. Aluminum chips on the bed might be grounding out the work, or chips in the spindle might have been grounding the tool tip. Chips anywhere on the control board could wreak havoc pretty easily, too, but you’d likely get really bizzare results from that, so probably not that.

4

Thanks, I’ll need to setup a computer in the workshop to test for all your suggestions. (I’m only setup to use the SDcard usually)

I’ll get back to you with my findings.

5

I had the tool change code do 2 x Z-homes which seemed to be a band-aid fix.

I also did some upgrades to add relay control to a dedicated spindle/vacuum power outlet and at the same time upgraded the firmware to the latest. On the first run the initial z-home crashed into the work-piece for a split sec, before recovering and probing correctly for the 2nd touch.

Dupont issues I suspect yet again (I have them all secured with hot glue)… I find 1 has partially popped out of the plastic sheath!

I can see I’m going to have to rewire the whole thing and eliminate the runs that required joins.