Z axis skipping/going too deep.

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Z axis skipping/going too deep.

Post by SlickShoez » Sun Jun 01, 2014 9:26 pm

Hi there!

Quick question for you all, On some jobs, my z axis seems to gradually get deeper and deeper as I run a job. X and Y stay on target, but my Z axis eventually drops so low by the end of the job it goes through the material and the job is ruined. Typically it only happens on really fine finishing paths of a 2.5D design. I think I may have crossthreaded/damaged my delrin nut, because when I look down the nut it looks "gappy" between a couple threads. Would that potentially cause the problem?

Thanks for any info and assistance.


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Re: Z axis skipping/going too deep.

Post by WillAdams » Sun Jun 01, 2014 10:34 pm

While it doesn't help, I can't see it causing problems if it's only a few of the many threads.

More likely is that you're losing steps when trying to lift the spindle --- I'd check to see if your acceleration is too high, or current too low, see http://www.shapeoko.com/wiki/index.php/ ... or_Current and the following section.

Might be that your Z-axis is over-heating --- may need to add a fan and/or heat sinks.
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Re: Z axis skipping/going too deep.

Post by Auarhau » Mon Jun 02, 2014 7:46 am

Is your end mill secure enough in the collet? I had spiral upcut end mills pull them self out slightly, causing the Z to plunge deeper then it should.
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Re: Z axis skipping/going too deep.

Post by SlickShoez » Wed Jun 04, 2014 3:01 am

So check this out. I cleaned up my machine, checked pulleys, wheels, and also got a fan and pointed it directly at grbl board. After that I tried re running the job and the missed/added steps happen in what seems to be the same exact spot! Any ideas? I've attached pics below of the two jobs. I still need to check current and acceleration, but I thought I'd post this in case it might point to any other cause.



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Re: Z axis skipping/going too deep.

Post by RobCee » Wed Jun 04, 2014 9:08 am

This may be due to missing steps while raising the Z-Axis. It happens when you ask the Z-Axis to move up too fast and it jams, possibly only very slightly on the way. This happens because the load on the Z motor when raising the axis is significantly higher than when lowering it, due to the weight of the axis and spindle acting against the stepper motor instead of with it, which is what happens when it is moving down. Because the stepper motors have no feedback to their controller, it doesn't know that they have not raised as far as expected. When it lowers the Z-Axis again, it moves deeper than desired.

There are a number of things that you can do to resolve this problem:
  • The simplest one is to slow down any Z movements that are going up with an associated feed rate command (e.g. G91 G1 Z3 F50, would move your Z-Axis up by 3 units at 50 units/min)
    As Will says, check your current, feed and acceleration settings - they can impact the potential to jam significantly
    Double check the tightness of your V-Wheels, are they tighter than necessary?
    Is your lead screw running straight and true?
    When the motor is disconnected, is it easy to wind the Z-Axis up and down
You should be able to see if there is a problem by raising/lowering the spindle by around 40mm a few times and measuring before and after moving it. If you do this without a bit in the spindle it is easier to get a good range of movement - the longer the better. You may actually be able to hear it binding if you are not running the spindle/vacuum at the same time. I found that with mine, I had to lower the acceleration and the maximum feed speed before I had a reliable Z-Axis movement.

Once the next GRBL release is out, this will become simpler, as it will allow you to control the default speeds in each axis rather than combined as they are today.

Good luck with it! It is definitely worth the effort and perseverance!
There are probably more things to check, but your first
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Re: Z axis skipping/going too deep.

Post by seigenblues » Fri Jun 06, 2014 6:28 pm

If it happens at the same spot repeatedly, i suspect the g-code. What did you use for your CAM toolchain?
I know makercam can create individual gcode lines that are too long for GRBL's buffer. (e.g., it specifies the position with waaaaay too many digits)
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