PCNC1100 Runaway Z malfunction under power after machine shut off

Mill
1

Hello Tormach Forum. This is my first post.

I have a PCNC 1100 Series 3 running PathPilot. Last night, while checking tram and level, the mill suddenly lost power, although the PC stayed on. I had stepped on the power cord right when it happened, so I’m not sure whether that was coincidence or if there was an issue with the plug itself.

After shutting down the PC/controller, I unplugged the machine and tightened all terminal/wire connections inside the plug. Nothing looked obviously wrong, but the strain relief was loose, so I corrected that by wrapping an additional layer of insulation around the cable to tighten it up.

Next, I removed the access cover for the Z-axis ballscrew/coupler area at the top. I used channel-lock pliers with heat shrink tubing on the jaws to avoid damaging the coupler and manually moved the Z-axis to a safe position by turning the coupler with the pliers.

After reconnecting power and booting the control PC, I powered up the machine. Immediately, the Z-axis started moving downward toward the table and made a new growling/grinding-type noise. I still had the tram tool installed, and it looked like a crash was imminent, so I hit the E-stop.

I rebooted the system and tried again. Same behavior. This time I kept my hand on the E-stop because the Z-axis was already very close to the table.

I used the pliers again to manually move the Z-axis back to a safe position, then disconnected power completely and called it a night.

Today, I tried again and the exact same thing happened: as soon as the machine powers up, the Z-axis immediately starts driving downward with the same growling noise.

I’ve checked the visible connections and don’t see anything obvious. Since Tormach is closed today (Saturday), I’ll open a support ticket when they reopen, but I was hoping someone here might recognize this issue. Is this a known problem that can be reset/recovered from, or does this sound more like a hardware failure (something in the panel)?

I was hoping to finish the tram work and get the machine running again over the Memorial Day weekend, but this definitely threw a wrench into things. Seems like something needs to be reset or a hardware failure to me and not something simple like a stuck pendant or keyboard button, etc.

Any insight would be greatly appreciated.

2

I have an 1100mx so I’m not the expert here. But are you waiting for the controller to boot all the way and then resetting the e-stop? I can’t imagine the program is restarting after shutdown and moving down. Maybe check to make sure the limit switch isn’t shorted. If it is it could think it’s already at the top and is moving away to open the switch. I think your 1100 has a Z axis brake, that could be the grinding. Also not sure but on my1100mx the spindle nose can’t actually touch the table even if it’s all the way down. The travel ends like a half inch above the table. You could let it go all the way down and see if it stops. I’m really just guessing at this point. Also the fuses on these mills can cause strange behaviors so check those. I would check them. It could also be a board problem. Hopefully not. Probably no help but maybe.

3

FWIW the pcnc and 1100mx are vastly different electrically.

I haven’t worked on our troubleshot the PCNC but this sounds like an electrical issue. Are you absolutely sure that the wires you cleaned and tightened are back where you found them?

In PP, is the z DRO counting down while the axis is moving? If so then I would unplug the keyboard and jog shuttle and check again.

4

It sounds like a Z-axis brake issue. Check over everything you touched and look at schematics for that subsystem if you are anxious to continue trying to get things going over the holiday.

Page 224: did you uncouple the manual Z-axis brake to move the Z-axis manually and is it back in the correct position?

5

Resolved. Thank you all for the replies and help. A tip of my hat to everyone who pointed me in the right direction- especially @Davie

I took out the multimeter and started with the low-hanging fruit. First, I should mention the machine was plugged into a Parkworld 15’ extension cord. With the cover removed from the machine-side male plug, I clamped my meter leads to both hots and was pleased to see 239V. I then started wiggling the wiring and noticed it would momentarily show “OL” before returning to 239V. By twisting both plugs slightly, I was able to reproduce the issue consistently.

After disconnecting the plug and inspecting the receptacle contacts, I noticed one side of the clasp was missing on one of the hot legs. The other hot still had both clasps intact, which made the problem immediately obvious. Any movement of the cord caused just enough twist to create a momentary open circuit.

I made a Lowe’s run and built a new extension cord, which resolved the power loss issue. However, the immediate Z-axis movement would still be there. I followed @Davie ‘s advice to disconnect all input devices from the controller. After rebooting, taking it out of E-stop, and pressing Start, voilà — no more immediate Z movement toward the table.

Everything is now working correctly. I’m convinced the noise I was hearing from the Z-axis was the brake engaged under power/ runaway Z. It’s completely quiet now, and I’m happy to report all is well.

Now I can finally get back to addressing the .0025" over 6" tram project I was about to embark on.

6

Thanks for the update!

I’m happy to help and glad you are moving forward again