Strange behavior with Z Axis backing off a bit at each point

deejayspinz
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Strange behavior with Z Axis backing off a bit at each point

Post by deejayspinz » Mon Jul 29, 2013 8:19 pm

I have a point list (8 holes) MOP that I am drilling. Cutting into HPDE.  As the MOP progresses through each hole, the Z axis seems to be retracting a mm or so with each hole that it drills.  As it goes along, each hole gets shallower until it is milling in the air.  Any thoughts what may be going wrong.  The machine cuts the rest of the MOPs fine and can drill a single hole fine.   Machine setup has been the same for months and ran without issue. The gcode seems fine. I'm cutting at 1200mm/min. Using a tb6560 controller. Anyone experience this issue from a hardware standpoint?

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Re: Strange behavior with Z Axis backing off a bit at each p

Post by WillAdams » Mon Jul 29, 2013 9:38 pm

What plunge rate are you using?

Could you be losing steps going down?
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deejayspinz
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Re: Strange behavior with Z Axis backing off a bit at each p

Post by deejayspinz » Mon Jul 29, 2013 9:55 pm

Plunge rate is 300mm/min. Been like that since day 1. I suspect it is hardware related as I've noticed it on occasion doing outside profile cuts as well, so not just drilling holes. I checked the Z axis for stiffness, but am able to turn the screw with my finger when the power is off. I've got a Nema 17 driving a DW660 and IC's upgrade screw. Again, it's been this way for some time.

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Re: Strange behavior with Z Axis backing off a bit at each p

Post by Improbable Construct » Mon Jul 29, 2013 10:22 pm

Is it possible that the bit is sliding up into the collet?
If its not that it sounds like you are loosing steps plunging.
Run your g code in the air with the DW660 off see if the bit ends up the same height as it started.
if it doesnt, listen to the Z axis motor see if you can hear it miss a step.
There may be one spot that binds a little and is causing your problem.
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deejayspinz
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Re: Strange behavior with Z Axis backing off a bit at each p

Post by deejayspinz » Mon Jul 29, 2013 10:49 pm

Improbable Construct wrote:Is it possible that the bit is sliding up into the collet?
If its not that it sounds like you are loosing steps plunging.
Run your g code in the air with the DW660 off see if the bit ends up the same height as it started.
if it doesnt, listen to the Z axis motor see if you can hear it miss a step.
There may be one spot that binds a little and is causing your problem.

Great idea. Will try this.

deejayspinz
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Re: Strange behavior with Z Axis backing off a bit at each p

Post by deejayspinz » Thu Aug 01, 2013 12:03 pm

So, I tried IC's suggestion of cutting in the air and the Z is still retracting a small distance (1mm or so) with each drill hole it does. Also stripped out all MOPs and only have a 8 hole drilling MOP g-code and verified that all holes should drill to -5mm. First one hits near -4, then each subsequent one climbs up until it is not even drilling into the piece. It's almost as if the retract (3.0mm) is failing. Checked the bearings etc and the Z axis moves freely. I don't hear anything that would lead me to believe the Z axis is skipping.
Last edited by deejayspinz on Thu Aug 01, 2013 12:20 pm, edited 1 time in total.

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Re: Strange behavior with Z Axis backing off a bit at each p

Post by jasonharper » Thu Aug 01, 2013 12:17 pm

What does the actual G-code for these drill operations look like? CamBam, for example, will by default generate a "canned cycle" (G82, I think) for drilling, which is NOT supported by GRBL. I would expect that to result in no drilling at all, rather than the symptoms you're seeing, but maybe something similar is the explanation here.
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deejayspinz
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Re: Strange behavior with Z Axis backing off a bit at each p

Post by deejayspinz » Thu Aug 01, 2013 12:40 pm

I'll post the g-code tonight when I have access. I am not using GRBL. Using a Tb6560 controller and Mach3. Also, my setup has not changed in months. (Cambam + controller + Mach3). I'm planning to see if (somehow) my Z steps/per has changed in the config. It seems to be the most probable cause now that I think of it. Or my controller is starting to fail...

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Re: Strange behavior with Z Axis backing off a bit at each p

Post by nfortosis » Thu Aug 01, 2013 1:00 pm

One thing you might want to check is your seek rate. If I remember correctly cambam uses g1 to drill down and g0 to retract. If the feedrate for g0 is too high or your acceleration is too high, it can cause loss in steps. Try running code with both lower feed and seek rates and see what happens.

One other thing to try is to confirm that you are missing steps and not actually having code issues by watching to see if your safe height is continuing to increase/creep up along with your drill depth (it should be if you are missing steps).
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Re: Strange behavior with Z Axis backing off a bit at each p

Post by deejayspinz » Thu Aug 01, 2013 10:44 pm

Just had a brain fart, and recalled something. A few weeks ago I was doing some work on the machine and had to have the controller powered. I was doing some vacuuming and realized that I had pushed the keyboard which shifted over to the xbox controller and forced the Z down into the wood (motor was off). It was doing this for likely 30 seconds or so. I have a Nema 17 on there, so it was just stuck trying to drive into the wood. Starting to think that I may have caused an issue with the driver for that axis. However, I would expect that all my Z movements would have the same issue. i.e I can still do profile cuts and it cuts to the correct depth each time.

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