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  1. #1
    Join Date
    May 2005
    Posts
    27

    Problem with Z Axis creeping up

    I bought the CNC router that Andrew (coldfusion) built and and I've been having lots of fun with it. I plan to use it mostly for wood. I'm using Mach2, SheetCAM and MeshCAM. I'm cutting 2-1/2 D with SheetCAM fine, but when I try to do full 3D with MeshCAM from one side of the cut to the other the Z Axis creeps up (see image).

    Andrew said he did not have this problem so it must be something with my setup and not the mill. He sent me an .nc file from CNCone and I had the same problem with it so I do not think it is a MeshCAM issue. Andrew sent me his Mach2 XML config file and I'm using that.

    Any suggestions on how to troubleshoot this or what the problem might be?

    Lance
    Attached Thumbnails Attached Thumbnails ZCreep-2.jpg  

  2. #2
    Join Date
    Mar 2003
    Posts
    4826
    It could be a controller/motor issue.

    Full 3d is different than 2.5d because 3 axis linear interpolation has to be performed by the controller. There will be many more Z movements in the 3d file. If for any reason there is a step being lost as the Z lifts up, this will accumulate over time.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  3. #3
    The best thing to do would be to determine if it's a mechanical problem, or something on the electronics side. Take off the Z axis stepper and put a Sharpie mark on the shaft and motor casing. Then run a fairly long 3d program and see if those marks line up after it's all over.
    Proud owner of a Series II Bridgeport.

  4. #4
    Join Date
    May 2005
    Posts
    27

    electrical not mechanical

    Well, the good news is that it is electrical not mechanical. I tried the test Andrew suggested and the servo didn't return to zero. I expected this since I had disassembled the Z assembly to inspect it and and the ball joint, etc. were all rock solid.

    So is it lost steps in one direction? What's the best way to pin down the exact cause?

    Lance

  5. #5
    I had a great idea today. It may be the high/low active option on the step or direction controls in Mach2. If these aren't setup the right way, you'll lose one step pulse every time the motor changes direction. I don't know how you have them set right now, but try changing them and running the program again. I know other people have had this exact problem, so this is probably the issue.
    Proud owner of a Series II Bridgeport.

  6. #6
    Join Date
    May 2005
    Posts
    27

    Well, it had an effect...

    Going into Mach2...Config...Ports and Pins...Output Pins and reversing Z axis Step, Z axis Direction, or both caused the Z axis in my machine to creep down rather than up. This confirms that the issue is not with the machine itself but with the software.

    Any other ideas?

    Lance
    Attached Thumbnails Attached Thumbnails ZAxis-creep-Reversed.jpg  

  7. #7
    Join Date
    Mar 2003
    Posts
    35538
    Try increasing the pulse width and / or precharge.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  8. #8
    Join Date
    Mar 2003
    Posts
    35538
    That's Direction PreChange in Mach2, or Dir Pulse in Mach3. In the Motor Tuning window.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  9. #9
    Yeah, try increasing them to about 3ms, instead of one.
    Proud owner of a Series II Bridgeport.

  10. #10
    Join Date
    May 2005
    Posts
    27
    The PulseWidth and PreChange were set to 1 and 0. I set they both to 3, then 5, and finially 10 and made cuts at each setting. No change in behavior. The axis creep was still there.

    Lance

  11. #11
    Just to clarify, you did try all 4 possibilties with the active high/low options?
    Proud owner of a Series II Bridgeport.

  12. #12
    Join Date
    May 2005
    Posts
    27
    Yes. I started with active low on for both step and direction. Then I tried turning active low off for just step, then just direction and finially both. The only change I saw was that the Z axis creep would change directions (up or down).

    Lance

  13. #13
    Join Date
    May 2003
    Posts
    162
    I have to chime in here.....I had this problem, and it drove me nuts trying to locate the source of the problem.

    Go into CMOS and verify the mode of the LPT port, choose the appropriate mode(SPP,EPP,etc).

    In my case the solution was changing the LPT mode to ECP.

    I hope for your sake that fixes it, I was MORE than happy that happpened to be the source of my z axis creep problem.

    Justin

  14. #14
    Join Date
    Apr 2004
    Posts
    297

    Angry new router

    Hi all I,ve just change from a porter cable laminate trimmer to a porter cable 1-3/4 router,a lot bigger for sure .I played with the excelleration rates but can,t seem to find a sweet spot.I,ll run some code then after a short time I find the Z slowly plunging deeper.Also when I break from the program and go to jogg up or down it staggers,like it,s having a hard time.Checked ball screw,checked coupler slippage,checked end plates for top and bottom of Z assembly,still can,t figure it out,maybe too much weight? Cheers.

  15. #15
    Join Date
    Jan 2005
    Posts
    746
    The use of consant force spings would ideally help with the weight problem. They are the ones that look like a wound up ribbon or steel. Otherwise you can try springs, gas struts or a cable-pulley-counterweight arrangement. You don't want to completly balance things out, you want the router to be somewhat heavier to keep tension on the z-axis screw.
    If it's not nailed down, it's mine.
    If I can pry it loose, it's not nailed down.

  16. #16
    Join Date
    May 2005
    Posts
    27
    Wow, that's the same router I have! And just the same... all the mechanical stuff looks great. Try switching around the active low and or the motor reversal. Maybe if you can get it to slip up instead of down you have the same prioblem I do.

    Lance

  17. #17
    Join Date
    Apr 2004
    Posts
    297
    Thanks for the quick reply.Would it be posible that the motor is to weak even if it,s 160 0z,in.

  18. #18
    Join Date
    Jan 2005
    Posts
    746
    Is the motor direct coupled to the screw or geared/belted down. What's the pitch on the screw. Are you using linear bearings or slides.
    If it's not nailed down, it's mine.
    If I can pry it loose, it's not nailed down.

  19. #19
    Join Date
    Apr 2004
    Posts
    297
    It,s coupled with lovejoy coupler and the screw is 5/8 ball screw with thompson ball nut and hywin rail and bearings.

  20. #20
    Join Date
    Jan 2005
    Posts
    746
    Am I to assume that your ballscrew is a .2" pitch single lead.
    If it's not nailed down, it's mine.
    If I can pry it loose, it's not nailed down.

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