Random Z-Axis plunge

[JAZZCNC]

I assumed that too, but it's usually going wrong right at the end of the job when the tool is being lifted right up out of the workpiece, moving slightly and then zooming back down again (when we are cutting in several passes - we still have to get our techniques right!) so it's when it moves a up and down a (relatively) long way in a short space of time that it goes a bit strange.

Thanks again!

Ahh ahh ok then you could well be overtuned or under powered on the Z axis like John said.

Either way if you have backed it off and it's working ok now then your sorted and happy days. . :)

[Jonathan]

I'm sure I posted in this thread after the original post...must have not sent :(

If the error is occurring it the same point in the G-code, which would appear to be the case, then it's likely to be a tuning problem. The error would be constant since it's the same command causing it each time. Assuming the Z-axis stalls at the same point then the error will be repeatable. Try just running the few lines of G-code round where it stalls and see if it happens again.

I very much doubt it would be backlash since errors due to backlash are not cumulative and 10mm of backlash would be rather silly!

A couple of weeks ago I had a similar problem in that my machine wouldn't get past a certain line of code in a spot drilling pass. There didn't seem to be anything special about it, just a rapid move in X and Y. However after noticing that both X and Y would stall every time the machine got to that line I tried just running the same line and line before on it's own - sure enough it still stalled. Naturally checked all the wiring and the usual stuff, which made no difference. Reduced the rapid feeds on X and Y which worked only if I went to about 60% of the normal speed which is a bit pathetic. Tried running the code zeroed at different points and it still stalled so couldn't have been a mechanical fault.
In the end I decided to switch to LinuxCNC and a couple of hours later the machine was running just fine at full speed.

[gomako]

Thanks again for all the info.

I don't think it was a specific line in the G-Code. When I was messing about with it trying to diagnose the problem, I made a small square cut-out with some depth clearance, bridges etc. Then I'd load the G-Code into the control program (NCStudio in my case), run the code. When it finished (usually with no problem), I'd manually move the bit and reset the origin and just run the code again so it would make the cuts again on another part of the scrap board and more often than not, but randomly, I would have the crazy plunging problem. It was roughly 10mm, not exactly 10mm every time too.

I couldn't re-create it and looking at the G-Code, it didn't look like there was anything obviously wrong ( I am a php programmer so am familiar with what code should look like ) there.

@Jonathan, could you tell me what backlash is, and what causes it/what to look for? Also, do I have to use the control program that came with the router, or can I use anything (like LinuxCNC) to control the machine? Is there a machine profile or something like that to set up the control program to work with the router? I don't really like NCStudio, and I think it's pretty buggy.

[Jonathan]

@Jonathan, could you tell me what backlash is, and what causes it/what to look for?

Backlash is lost motion when the motor driving an axis changes direction. For the majority of machines there is some clearance between the nut (ball-nut or trapezoidal) and the screw, so when the screw changes direction the nut wont move until it has moved the distance of this clearance. Better explanation here:

http://www.machinetoolhelp.com/Repai..._backlash.html

If backlash is significant you'll for instance see it if you cut out a circle - at the limits of travel (so 4 points) there would be a slight blip. However on a ballscrew driven machine generally the backlash is so small (<0.05mm) that unless you're cutting metals it wont be noticeboard.

Also, do I have to use the control program that came with the router, or can I use anything (like LinuxCNC) to control the machine? Is there a machine profile or something like that to set up the control program to work with the router? I don't really like NCStudio, and I think it's pretty buggy.

It looks like NCStudio uses a PCI card to control the machine:

http://www.erel-cnc.de/html-uk-cnc/software-uk.html

So assuming that's what your machine uses, unless there are drivers for it for Mach3/LinuxCNC, which seems unlikely, I doubt you can use any other software. That's unless you remove the card and change to a parallel port system, but that's downgrading if anything. The PCI card should be better.