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  1. #1
    Join Date
    Apr 2015
    Posts
    23

    Z Axis slipping deeper

    This problem is going to drive me to drink.. Mmmmm Beer!!!

    While doing complex engraving/relief cuts the Z axis will slowly cut deeper and deeper. I checked the ball screw and there is minimal backlash. Checked the velocity settings and thats all fine, checked all the bearings and they seem to be as tight as they were when i first preloaded them. Checked everything with a dial gauge and and all looks good.

    As far as I can tell there seem to be no missing steps and there is little to no load on the machine when its engraving due to the fact im using a 0.01mm V bit.

    The collet is tight and there is no slippage at all in the tool.

    I am kind of at my whits end trying to work out where the problem may be and coming up with things to check so if anyone has any ideas I would love to hear them.

    Regards
    B

  2. #2
    Join Date
    Jun 2011
    Posts
    692

    Re: Z Axis slipping deeper

    What are you using for stepper drivers? On a cheap board I had years ago, one of the axes was missing the first step after a direction change, only in one direction. So the axis very slowly drifted in one direction. Could test if it's that or something similar by just running the axis up and down a few hundred times and see if it returns to its original location.

  3. #3
    Join Date
    Apr 2015
    Posts
    23

    Re: Z Axis slipping deeper

    Quote Originally Posted by skrubol View Post
    What are you using for stepper drivers? On a cheap board I had years ago, one of the axes was missing the first step after a direction change, only in one direction. So the axis very slowly drifted in one direction. Could test if it's that or something similar by just running the axis up and down a few hundred times and see if it returns to its original location.
    I am using a 4 axis board with the TB6560 chip's. It has been working well for years and just to be sure that there was not a fault in the Z i swapped the motor over to the A axis only to get the same results. I will however pull the motor out and run it with no load for a few hundred repetitions and see if it returns to the center point so as to eliminate that.

    Last night I started to strip the unit down and noticed that there was a bit of play in the gib, If i applied pressure just in front of the spindle to pull it forward and up a little I was able to move the ball screw with my fingers to make it go up and down. if however i let it rest it would go down but i could not get enough pressure to make it go back up. So i have a feeling this may very well be where the issue is. There is just enough resistance that is causing the motor to occasionally miss a step on a retraction move. Over 20'000 moves it ends up around 0.8mm deep. not ideal when trying to make a die casting mold with a 0.1mm bit and 0.03mm passes.

    Thanks for the idea @skrubol. tracing a fault like that must have given you the same headaches. I have been seriously contemplating replacing all the motors and drivers with a closed loop system or adding encoders to the back end.

  4. #4
    Join Date
    Apr 2015
    Posts
    23

    Re: Z Axis slipping deeper

    @skrubol It looks like you were on to something, I machined a new gib from Acetal just to ensure that there was very little resistance. that did not solve the problem.
    Then removed the ball nut mounting from the head and stuck the dial gauge on that while it ran up and down and there was still an issue.
    then removed the stepper, put a black mark on the zero point of the shaft and body and still there is an issue.
    So either the stepper motor has a problem, The driver has gone faulty (I connected the stepper to another axis and got the same results)
    Or there is some deeper issue in the settings that I have inadvertently messed up.

    Or there is a PC parallel port issue causing the problem.

  5. #5
    Join Date
    Apr 2015
    Posts
    23

    Re: Z Axis slipping deeper

    The machine is all back together and now has another controller, it seems this was defiantly the issue and appears that on the change of direction from a + move to a - move it was loosing steps. At some stage I may strip down the faulty controller and see if i can locate where the problem is. I also think that the next upgrade is going to be to build a new controller system that has independent drivers for each motor and possibly add encoders to the steppers to create a kind of closed loop system.

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