I have a bit of a problem that's seemed to come from nowhere.

Running MACH3, and a HobbyCNC controller and 125 oz. steppers. Machine is homebrew but has been working well with no problems.

I was cutting a block of text  and was 6 letters in when suddenly the Z dropped what looked about -0.5" and went through the work.  Letter was a B and it looked as though the X&Y were taking the bit to the lower-left corner of the letter.

Reset everything to 0 and tried again - this time it happened at another place in the sign. Same symptoms - Z dropped about -.5". Note sure if the machine meant to do this or it suddenly lost enough steps to put it that far behind.

I was running slow. Have run considerably faster with no step loss. Cables seem tight, etc.

So - I have the type of problem I HATE to try to troubleshoot: Random and intermittent. It looks as though the problem is limited to the Z axis - since it gouged in a "logical" spot when it was cutting the letter. Not like the X & Y went off to the middle of nowhere and gouged there.

I have a spare controller board and will install and test again - but just thought I would ask for any suggestions.

Help

I have no idea what the cause might be, as you clearly realise odd happenings are difficult to trace.

The reason for the post if to ask Art if we might have a "black box" in later editions of Mach3 - say something that recorded the last 5 secs of output from Mach 3. We should then be able to see which axis are moving and in what direction, and with a little interpolation, we could even get the speed. This would make trouble shooting so much easier.

In this case - is it Mach3? is the z stepper wire picking up stray signals? or is the driver card doing its own thing. To be able to eliminate one possibliity would make it so much simpler.

Hey Jerry,
   Can you post your code and show at what line it dipped? It might help to move your z up a good bit and run the program in air and watch the dros. Do they change when the z dips or does the z just dip?

Brett

Hmmmm...
When I returned home after the gouge the DRO was agreed with the spindle - I THINK...not sure. I remember slapping at E switches, turning of the spindle motor, spilling my coffee and spouting some words that would make Blackbeard blush...I do think it was off though which would lead me to think it was the code.....

I'll try running the same code in air to see what happened. Good suggestion..

Also - on another forum someone suggested that I  check my vac hose. He says his will cause his machine to cough if it contacts the frame. Static I guess...

stand by - and thanks for the help thus far.

Ok - ran the file again and it did it again - sorta...

The file I was running is attached.  As before it ran fine until I got to the "B" - then at or about the same spot the spindle made a hard 45 degree angle to the lower - left corner of the letter. I think this was correct - but the problem was that it did not lift the spindle and the bit cut through the letter. This happened at or about line 3030. It might have been a few lines before or after because I was swatting the E-switch as quickly as I could but my reaction times might be a little slow.

I took a breath and let the file continue - it ran a short distance and at on or about line 3050 it dove deep - much deeper than the -.250" that it was supposed to be at. I checked the DRO and it said +0.531. Not sure why the DRO was off but it looks as though it was 180 degrees off - or thereabouts.

The file was created in Vectric's "V-Carve Pro" ver 4.  This is the newer version.

Can more knowledgable than I please take a look at the file or suggest why the DRo wouold show a number opposite to what the spindle would actually be? And remember it's happened twice in the same spot with the same code.


Thanks -

Jerry, I looked at some of the code, it looks good. I have sent you a personal message.

Brett

I think it is time to run the file with the spindle off and no vacuum... see if you have the problem and if you don't we will know where to start looking

If you need help mail me off list and I will see what I can do to help you as you are a local guy

Thanks
Brian
 

MIGHT have found the trouble.

Ran the file a few times letting it "trace" through the existing kerf that it cut before.

The second time it dipped at the same place as the first.

The third time it dipped before it got to the initial gagging point - but I was watching (and listening) closely and heard a quick "bzzzz" just before the bit dived down. Hit the STOP switch and looked at the DRO for Z. Again - it showed +.5, but the bit was obviously in negative territory.

I think the "bzzz" sound was the Z motor's shaft spinning inside the coupling until it's gets a bite.  I filed flats on the X and Y shafts for the setscrew to push against - but not sure if I did the Z.

I'll find out tonight when I do an autopsy. I'm betting I'll find the Z shaft chewed up from spinning against the setscrew - and if so I'll smooth it out and file a flat on it.

Odd that it happened a couple times in the same spot - but maybe the coordinated movement of all axis caused a bit of a bind there, at least enough to break the coupling loose for a few spins.

Hopefully this'll fix it.

Might be better to set your Z height above the workpiece before doing any testing, certainly when you have had known issues with problems, saves having mechanical damage done to your equipment. CNC work requires a certain level of confidence, that is, you having confidence with the machine and that it will, in fact, do EXACTLY as you programmed it to do. When that confidence is not there, you have to play the safety routine to protect the machine.
       Dave.

Yep - loose Z coupling.

It would apparently spin a few turns under heavy load - but didn't apear loose on quick examination. Set screws were tight, but when I removed the couplings I saw that they had cut slight threads in their opposing surfaces.

This was the only joint where I didn't grind flats on both the leadscrew and motor shaft (forgot  )

So I added the flats - dripped on some threadlocker, tightened everything up and everything is fine now here in Downeast Maine.

Moral(s) of the story:
Things can be loose on the inside even though they seem tight on the outside.

Setscrews can't be relied upon unless they push against a flat - so if you don't have flats ground on your shafts you should consider adding them. An abrasive wheel on your Dremel makes quick work of this.

Thanks for all the help!

Jerry In (partly cloudy and temps in 70s) Maine