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Thread: CNC sometimes has a mind of its’ own.

  1. #1
    Join Date
    Apr 2011
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    near Atlanta, Georgia
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    Default CNC sometimes has a mind of its’ own.

    Over the past few weeks I’ve experienced a strange situation intermittently on 3 different jobs. I always start with a climb cut then finish with a conventional cut to clean it all up. The run goes fine until the last few inches on the FINAL pass. Sometimes the Bot suddenly takes a turn back towards the X Axis along the Y axis. When this happens. My X zero point is changed by seemingly the distance amount between the original cut and the ‘new’ cut. It seems to be coming a more frequent occurrence. Any ideas on where to look for an issue?

    Facts:
    1) Older (~2006) PRTAlpha 4x8 Control software 3.6.46 Control board p/n 001003-01 v0.6
    2) Windows XP computer - has never been connected to internet, all extraneous stuff shut down. Nothing else is ever run on the box EVER.
    3) No comm errors or any other errors or power cliches
    4) When jogged home, X home is now changed by the amount of the ‘detour’. Y is fine as is Z.
    5) Shut down PC and control box and turned off power at the electric disconnect box, then powered back up normally.
    6) Neither the spoil board or plywood is moving. Usually use vacuum hold down but started screwing down to make sure nothing moves.

    Details:
    2 pics attached.

    1) When it occurs, it is ALWAYS in the last few inches of the final conventional pass. (I usually do it in one pass but even changed to 3 passes. Then it ran first 2 fine, opps’ed on last pass, same place.)
    2) All 3 jobs have been run several times over months/years with no previous issues
    3) Recreated the tool path. Original tool paths were created in VCarve 7.5. Recreated paths Vcarve 9.0. Interestingly file size changed from 205 KB to 72 KB.
    4) Changed 0,0 point from actual 0,0 to 0,24. Ran 5 panels and all was fine (almost.) Check image 2. All 5 had a slight deviation as seen withing the green circle though on this job I can live with that.
    5) Other day made 3 runs - all fine, next one messed up, next 7 ran fine. Nothing changed.
    6) Material - Baltic Birch ply, 18mm with .189 mm DS bit (.189 because I’m creating some .25" bolt holes and don’t want to start a fire again) , 12 mm with .25 DS

    7) An interesting observation a couple of weeks ago - was cutting a similar job as this but larger circles. Was cutting 2 pieces - 2x1 matrix. Again this was on final pass. The first one deviated on the X=0 side of the circle by about .45" I didn’t get to the machine instantaneously so the bot had moved over to the second piece for the final pass. It was cutting the final conventional pass there CORRECTLY until it go to the last few inches at the end, then also opps’ed just like the first piece. Very interesting. I would have expected it to cut the final pass on the second one off by .45". Was not so.

    Sorry this is so long, but full of details. Any ideas on what is going on and how to fix it?
    Image 1.jpg Image 2.jpg

  2. #2
    Join Date
    Dec 2000
    Location
    Thorp, WI
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    Default

    Grab on to the bottom of the spindle and pull/push back and forth in the X direction. Is there a lot of play in the lower rollors allowing the Z axis to be pulled into the cut at that point. Climb passes will pull to the outside of the part and when you come around with a conventional cut at full depth, it may all of a sudden get a full depth bite right there and pull the Z axis into the cut/part. At that point, it goes into Alpha mode and tries to recover, but ends up off track by the time it finishes.
    Scott

    If guns kill people, I guess pencils misspell words, cars drive drunk and spoons make people fat.

    "Those who hammer their guns into plows, will plow for those who do not" - Thomas Jefferson


  3. #3
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    Apr 2011
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    near Atlanta, Georgia
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    Quote Originally Posted by srwtlc View Post
    Grab on to the bottom of the spindle and pull/push back and forth in the X direction. Is there a lot of play in the lower rollors allowing the Z axis to be pulled into the cut at that point. Climb passes will pull to the outside of the part and when you come around with a conventional cut at full depth, it may all of a sudden get a full depth bite right there and pull the Z axis into the cut/part. At that point, it goes into Alpha mode and tries to recover, but ends up off track by the time it finishes.
    Opps, I left out a fact I see, I use a Porter-Cable 7518 but the same concept should apply. I'll check it out. My only thought on this - the deviation is ALWAYS at the end of the literal very last pass a few inches from where it started. Even when I changed it to NOT make a full depth pass at once but to make several passes it STILL deviated on the literal last pass perhaps 2 seconds before finish.
    Thanks for your valuable input.

    Just checked it. Seems solid as a rock in both X and Y.

  4. #4
    Join Date
    Dec 2007
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    Did you use a Vectric product to create the file if so how does it look when you run the preview. It may also be a static problem or even a broken wire.

  5. #5
    Join Date
    Apr 2011
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    Quote Originally Posted by jerry_stanek View Post
    Did you use a Vectric product to create the file if so how does it look when you run the preview. It may also be a static problem or even a broken wire.
    3) Recreated the tool path. Original tool paths were created in VCarve 7.5. Recreated paths Vcarve 9.0.

    Weather was very humid recently - I'm in southeast USA.
    When it happens it is predicable to be in the last few inches of the final pass and ONLY there.

    Tomorrow if it happens I will note the line number to see if it is consistent.
    Thanks for the input.

  6. #6
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    Run an air cut, no material.
    Scott

    If guns kill people, I guess pencils misspell words, cars drive drunk and spoons make people fat.

    "Those who hammer their guns into plows, will plow for those who do not" - Thomas Jefferson


  7. #7
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    near Atlanta, Georgia
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    Update - mostly good news. Cut 22 circles today - all perfect except one with very slight deviation in the expected place. 22 more tomorrow of this, then 50 more of a similar part.

    However - interestingly, between the 3rd from last and 2nd to last, somehow the Bot decided my X zero point changed about .45 inches. Lower left corner WAS 0,0. That point changed to -.45,0. I did nothing but change the plywood blank. Gremlins at work.

    Only deference between today and Friday/Saturday - I cleaned up the saw dust that the cable was dragging over, then started cutting after zeroing. That potentially could have created some static but really weird the deviation was ALWAYS at the same place.

  8. #8
    Join Date
    Mar 2013
    Location
    Memphis TN
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    I'm curious about how you've grounded your machine? Do you have a dedicated grounding rod? Are you grounding every moving part separately? Is your electrical source grounded to the same rod as your machine uses? These machines and their dust collectors can generate a large amount of static, especially if the weather is warm and dry. Careful grounding can eliminate a lot of "gremlins".
    ShopBot Details:
    PRS Alpha 96x60x12
    4hp Spindle
    12" indexer
    Aspire
    Rhino

  9. #9
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    Apr 2007
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    Marquette, MI
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    From your pictures and descriptions you are losing or gaining steps. You say it always happens in one location. What is happening at that location? Is there a mechanical bind? Does the dust hose come in contact with the machine? Does a connector or wire loose connection at that point? Since it ALWAYS happens at the same position, it should be easy to diagnose.
    Gary Campbell
    CNC Training & Technology
    GCnC411(at)gmail(dot)com
    The Ultimate Woodworking Machine
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  10. #10
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    Quote Originally Posted by coryatjohn View Post
    I'm curious about how you've grounded your machine? Do you have a dedicated grounding rod? Are you grounding every moving part separately? Is your electrical source grounded to the same rod as your machine uses? These machines and their dust collectors can generate a large amount of static, especially if the weather is warm and dry. Careful grounding can eliminate a lot of "gremlins".
    Sadly (and embarrassingly) I have not done anything along those lines. Bought it used 5 years ago and set it up as the previous and original owner (Built cabinets) had it set up. Neither of us have put a dust collector it. I've seriously considered it but it has the 2 router heads and no room for a hose. Also it is located in a tight space and ceiling is barely 8' Early on I read about grounding and started looking into it. As time went on, surprisingly, never encountered issues. All excuses I know. Also considered getting echain but haven't.

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