Y-axis home occasionally resets when jogging

Hi all!

I’ve had this happen several times. I’ll start jogging the machine with the keyboard, or be in the middle of jogging with the keyboard (bluetooth keyboard), and all of a sudden the y has lost it’s position. Here’s a screen shot of Mach 4 when it happens. The real position is in the front, but the machine suddenly thought it was at the max y, as shown in the DRO (set to machine coordinates).

What in the world could cause this to happen? It has me really worried.

I got this sort of problem when I first built my AVID… It eventually turned out to be a loose connection inside the electronics boxes.

Are you a member of the AVID Facebook group? Here’s a link to my post about it:

Loose connectors

There were several of them so I went through both boxes and checked them all. I’ve had no similar problems since then. (All the other problems have been my fault :))

Awesome, thanks! I will look into it and see. I am on the FB group - I was going to post there if I didn’t here back from anyone.

So, your specific issue was the power cord being loose while the spindle was running, which generated RF and caused the problem. Right? And the loose connection was inside the spindle box.

Currently when I have the problem I’m just jogging without the spindle running, so hopefully that can’t cause any RF issues.


Everything in both boxes can cause a lot of noise. The VFD can for sure, but the steppers are driven by high current square wave pulses, and there are very large switching power supplies in there too. They are all designed to reduce the effects, but if things are not properly connected it could be a problem. I’m not saying this is your problem, just that pretty much everything in a CNC is a high noise device.

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Thanks Jim - that is a good point! I’ll definitely be poking around inside the boxes.


The thing I don’t understand: isn’t this a software problem? Something is telling Mach that the y-home location has changed. I wonder where that happens, and if I could back-trace it from that location.

I’ve been doing some testing:

  1. Turned off the VFD control box; doesn’t help
  2. Made sure everything was tight (it is)
  3. Rapid jog with my wired keyboard: no problem
  4. Rapid jog with my bluetooth keyboard: At some point it resets the home position.

What in the world could cause that to happen?

I can reproduce this on a wired keyboard; it is just harder to make happen due to no latency. I think it is a software bug in Mach 4. Could someone else try to reproduce it? @jjneeb or @Eric

Keyboard jogging turned on.


  1. Home Machine (front/left)
  2. Shift-Up arrow for a short bit to jog towards Y+ (away from you)
  3. Shift-Down arrow, and immediately after the jog starts hit the up arrow, while still holding down shift and the down arrow. As soon as you add in the up arrow it will reset the home position, and jogging stops because it is now at a soft limit.

It might take a few tries…but this happens nearly every time for me. It doesn’t happen when doing it in the opposite direction.

I can make a video if you all want to see it in action.

Yep, I can reproduce it too. Happens about 80% of the time. Seems to be the specific combination of a rapid negative jog and then hitting the positive jog at the same time. Most people don’t hold down the opposite jog direction arrows at the same time, so they wouldn’t notice. What were you doing that you ran across this? :stuck_out_tongue_winking_eye:

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Awesome! It’s good to know I’m not crazy. I was just rapid jogging back and forth to see how the machine moves. Second time it happened I was demo’ing the rapid rates to someone. That got me worried…so I had to figure out what was happening.

The work around is easy:
Me: “Doc it hurts when I do this!”
Doc: “Well, don’t do that!”

Ya, its not something you would normally want to do…but also not something it should do.

I’ll try it myself when I get the chance tomorrow. Thanks for finding this one @corbin

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I think this topic is addressed on the Mach4 forum. I remember reading about the Windows Event subscription was purposely limited to certain key combos. Same with multi-point touch (which is why I was there).

I would look it up for you but then I would read the other stuff on that forum and start screaming at people again :grimacing:

Hey Eric - any word on reproducing this? Someone on the Facebook forums said they were having issues with their home axis getting reset. I pointed them to this post, and sure enough…they realized they were hitting this bug.

FYI, the post: Avid CNC Users Group | Facebook


I have this one on my radar but we haven’t had a chance to try and work on it internally. We’re at a trade show now but we will be able trig
Into this next week.

Are you around next week? I’d love to chat with you about this and a few other things.

Yup, I’ll be around next week in/out of my shop. Feel free to get ahold of me anytime!