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Author Topic: More Z-axis difficulties...  (Read 3576 times)

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Offline Chet

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More Z-axis difficulties...
« on: April 14, 2012, 01:23:41 PM »
It's been an interesting week. The computer I was using to run Mach3 and my
HobbyCNC driver board had a motherboard and power supply failure. Was able to
get a used HP COMPAQ DC7 in its place... Everything computer-wise seems to be
back in order.
HOWEVER I am running into Z-axis slippage(?) that I can't seem to troubleshoot:
My foam patterns take roughly 5-1/2 hours with both primary rough and secondary
finish cuts.
The primary always seems to run fine, but at variable places along the secondary
finish cut, the Z-axis looses its place and plunges the cutter twice as deep
into the pattern.
I did notice a 'stall' on the Z-axis when I did a fast move with Page UP and
Shift keys just now...
This may be indicating an intermittent problem with the motor itself?
I've already lowered my motor tuning lower than earlier working speeds.
In fact I recently had to replace my X-axis motor (425oz bipolar) for slippage
problems...
But I thought these motors were supposed to be pretty hardy...
More info...
I did a REF HOME for machine coordinates before starting the pattern.
I found my start point on the pattern and zero'd the X, Y, & Z coordinates, but
then went to the Machine Coord to see what that reading was: X .4133, Y .3504
and Z -0.6799
After the rough cut, the machine was supposed to relocate to the original X 0, Y
0 start point with the Z being at 1.5 for the tool change...
But when I brought the Z axis down to 0 to check the Machine Coord now read
-2.0334, instead of -0.6799
So is it motor slippage, or something else going on????
Not sure how to proceed...
So I decided to try just the finish cuts and eliminate the tool change.
But even that failed... each X-axis pass had the Z-axis plunging deeper...
(Yes, the Z-axis is rigid to the motor and table...)
Thanks for any insights...
Re: More Z-axis difficulties...
« Reply #1 on: April 14, 2012, 02:10:51 PM »
turn the spindle on and let the machine sit there, put a dti on the z axis and leave for 20 mins. If after 20 mins the z axis has moved it's a noise issue.

Offline Chet

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Re: More Z-axis difficulties...
« Reply #2 on: April 14, 2012, 03:56:27 PM »
I'd earlier upped the debounce to 1000...
Ran the DTI on Z-axis with just the router, and no movement during the 20 minute test.
Added the vac to the combo, and still no movement after an additional 20 minutes.
The test appears to be a great way to check for noise on the line, though...
Thanks for the suggestion.
Re: More Z-axis difficulties...
« Reply #3 on: October 21, 2012, 06:39:32 PM »
was anyone able to come up with a solution for this problem? I seem to be having the same issue and being fairly new the the cnc world im far beyond lost at this point. I have found that my z axis loses its zero and will plunge into the material farther than the original z limit was set. Ive seen a few other people on here have had the same issue but nobody has posted anything saying they found and resolved the issue. Im only have the problem on large cuts where the machine time is around an hour or more. Can anyone think of anything? Also, if someone does reply please dont be afraid of dumbing down the answer as low as possible. I promise no feelings will be hurt.
Re: More Z-axis difficulties...
« Reply #4 on: October 21, 2012, 06:53:44 PM »
do you have any other programs running on the computer, antivirus, internet (either offline or online),
Re: More Z-axis difficulties...
« Reply #5 on: October 21, 2012, 07:53:30 PM »
Not that im aware of but I will double check just to be sure. Can we think of anything else that may cause the problem?

Offline RICH

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Re: More Z-axis difficulties...
« Reply #6 on: October 21, 2012, 09:46:55 PM »
A stepper can lose position and when it does it can be a small amount, a large amount , or just go off into the wind with a mind of it's own.

I am guessing that the problem is random. One could play around and deliberately add a few lines of code, maybe a number of dwells in the code so one can see if they could pinpoint something in particular that causes the problem or if it is progressive.

My generalization would be that in doing 3d milling the axis can't do the requested move in the time provided and position is lost, maybe once or can also be numerous times at a small amount. I think that it would be more pronounced for
finish cuts since the movements are smaller.

Try modifying your velocity and acceleration settings in motor tuning....50% less and see how it goes.
RICH

Re: More Z-axis difficulties...
« Reply #7 on: October 22, 2012, 02:02:23 AM »
OK, so i set my z velocity and acceleration to 50% the original value and it looks as tho that may have prevented the sudden and random plunge into the material. But now it has a gradual and steady plunge through out the entire cut. I generaly only cut very small items usually between 3"x3" and 6"x6" but have started to broaden my horizens a bit and have moved up to larger cuts. The project im working on now is still only 24x24 but I have it tiled to 12x12 pieces. The design is a 2.5d decorative piece that starts out cutting .01" below the surface as to not leave any flat spots with a max plunge of .2". By the time i get to the end of the first 12x12 tile my max plunge is still .2" but my z zero went from .01" to .04". When i tell the machine to return to zero mach tells me im at zero but the tip of the bit is .04" deeper than whats its suppose to be. The x and y are spot on and I never have any issues with them. I did forget to mention that a while back my stepper driver went a little weird on me and out of nowhere my z started getting very twitchy and would make funny noises when jogging. I slaved the A to the Z and have been running it that way for the past few months without problems until now when i started doing larger cuts. Thanks Rich for the advice earlier! Im hoping you might have a few more tricks up your sleeve for me!