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Thread: Shopbot never cut flat

  1. #1
    Join Date
    Dec 2015
    Posts
    24

    Default Shopbot never cut flat

    I have a problem that I cannot seem to figure out, as it doesnt make any sense. My machine has been doing this since i first purchased it new, just never mattered that much because of the way I use it, but I am wanting to do some 3d stuff so i need to get it taken care of.

    Machine is a PRS standard (96x48), purchased late 2015 new. It cuts downhill as it goes down the y axis. But what I dont understand is, I can resurface the spoilboard perfectly flat, then switch in a different bit and zero near somewhere close to the end lets say x12 y12. If i run the spindle and move it down toward the end, it gradually starts cutting into the board more and more to im guessing .020" or so, maybe less. Any ideas? I have checked and rechecked level of the table as well as tightness of bolts and the hardwood surface under the vacuum plenum.

  2. #2
    Join Date
    Dec 2005
    Location
    Pro Signs, Coal CIty IL
    Posts
    260

    Default

    Does the Z axis drift? I don't have a clue if that could be the problem, just a thought. Maybe mark the Z axis rails to see if that axis actually moves down as the gantry travels? May not be the problem, but at least you could cross it off as a cause. I had an issue where the Z axis, while doing a series of up & down moves during a job, would go down slightly less every time it cycled up and down. After several moves it would no longer even touch the substrate. I thought I had a bad stepper, turned out to be a bad USB cable.

  3. #3
    Join Date
    Oct 2009
    Location
    Elgin Illinois
    Posts
    652

    Default

    Hello Clay. I had/have a somewhat similar problem. Though I bought my PRT alpha used.

    I can resurface the table, then install a V-bit, and zero it at any point of the table, and move the bit around the table, and find that the tip of the bit does not remain the same distance from the table top...…. I posted on this a long time ago, and talked to tech support. I got all kinds of helpful suggestions from the forum, and tried out everything anyone could come up with. Someone suggested finally that my Z bushing may be worn, and upon removal, it was in fact worn.

    And I started to re-align my Z axis after putting in the new Z bushing (I replaced all bushings while I was at it). But I never got back to it because I got caught up in making a big 3D model, that turned out to have big problems...… so I am not firing on any cylinders, so to speak.

    Maybe by parking my machine, I am leaving it where I HOPE it will finally work after I finish the re-alignment.... better false hope than no hope.... Sorry to be of no help, other than to point out odd things like what you have going on, may not be easy to fix.

    Good luck, and unlike me, press on! Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  4. #4
    Join Date
    Jan 2004
    Location
    iBILD Solutions - Southern NJ
    Posts
    7,809

    Default

    Clay - How is the sacrificial (top layer) board attached to your machine? Is it glued to the grid/plenum (if so what glue) or is it screwed down?

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  5. #5
    Join Date
    Aug 2011
    Location
    Wilkesboro, NC
    Posts
    79

    Default

    It may be your surfacing pass and not your second cut.

    Could a dull surfacing bit cause push itself up as it cut, or push the table material down?

    When I'm problem solving, I try not to take anything for granted and start at the beginning. I'm just making a suggestion, not speaking from a much experience.

  6. #6
    Join Date
    Dec 2015
    Posts
    24

    Default

    Quote Originally Posted by pro70z28 View Post
    Does the Z axis drift? I don't have a clue if that could be the problem, just a thought. Maybe mark the Z axis rails to see if that axis actually moves down as the gantry travels? May not be the problem, but at least you could cross it off as a cause. I had an issue where the Z axis, while doing a series of up & down moves during a job, would go down slightly less every time it cycled up and down. After several moves it would no longer even touch the substrate. I thought I had a bad stepper, turned out to be a bad USB cable.
    how/where would i mark them and look for the drift? like compare the gantry to the rail?

  7. #7
    Join Date
    Dec 2015
    Posts
    24

    Default

    Quote Originally Posted by Chuck Keysor View Post
    Hello Clay. I had/have a somewhat similar problem. Though I bought my PRT alpha used.

    I can resurface the table, then install a V-bit, and zero it at any point of the table, and move the bit around the table, and find that the tip of the bit does not remain the same distance from the table top...…. I posted on this a long time ago, and talked to tech support. I got all kinds of helpful suggestions from the forum, and tried out everything anyone could come up with. Someone suggested finally that my Z bushing may be worn, and upon removal, it was in fact worn.

    And I started to re-align my Z axis after putting in the new Z bushing (I replaced all bushings while I was at it). But I never got back to it because I got caught up in making a big 3D model, that turned out to have big problems...… so I am not firing on any cylinders, so to speak.

    Maybe by parking my machine, I am leaving it where I HOPE it will finally work after I finish the re-alignment.... better false hope than no hope.... Sorry to be of no help, other than to point out odd things like what you have going on, may not be easy to fix.

    Good luck, and unlike me, press on! Chuck
    what and where is the Z bushing? doubtful since it was doing it brand new but worth a look!

    Quote Originally Posted by Brady Watson View Post
    Clay - How is the sacrificial (top layer) board attached to your machine? Is it glued to the grid/plenum (if so what glue) or is it screwed down?

    -B
    It is glued using Titebond 3, applied with a roller.

    Quote Originally Posted by Red F View Post
    It may be your surfacing pass and not your second cut.

    Could a dull surfacing bit cause push itself up as it cut, or push the table material down?

    When I'm problem solving, I try not to take anything for granted and start at the beginning. I'm just making a suggestion, not speaking from a much experience.
    Its possible, I need a new bit anyway so we will see, but like I was sayin its done this since day one basically

  8. #8
    Join Date
    Jan 2004
    Location
    iBILD Solutions - Southern NJ
    Posts
    7,809

    Default

    Clay,
    Get yourself a dial indicator. They are like $25-30 with a magnetic base from any number of places. Stick the indicator base on the YZ car and indicate the spoilboard. See if you can measure the deviation reliably in the areas you describe.

    I would also put the indicator on the YZ car and measure along the bottom edge of the gantry beam and makes sure it isn't going up & down. I would also indicate/measure the length of your table and see if there are any dips etc.

    There is no other tool more useful than a dial indicator for times when you can't find where the problem is.

    Keep in mind that when your bleeder board starts to get thin, it can warp internally and pop up in the middle - plus other weird things. I'd say once it gets below 5/16" thick, it's game over. Titebond as good as it is can still fail on MDF if there is a significant amount of movement from atmospheric conditions. The glue itself doesn't fail, the MDF does - it will literally pull away and leave jagged break points.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  9. #9
    Join Date
    Dec 2008
    Location
    Diamond Lake, WA
    Posts
    1,547

    Default

    Quote Originally Posted by ClayM325 View Post
    I have a problem that I cannot seem to figure out, as it doesnt make any sense. My machine has been doing this since i first purchased it new, just never mattered that much because of the way I use it, but I am wanting to do some 3d stuff so i need to get it taken care of.

    Machine is a PRS standard (96x48), purchased late 2015 new. It cuts downhill as it goes down the y axis. But what I dont understand is, I can resurface the spoilboard perfectly flat, then switch in a different bit and zero near somewhere close to the end lets say x12 y12. If i run the spindle and move it down toward the end, it gradually starts cutting into the board more and more to im guessing .020" or so, maybe less. Any ideas? I have checked and rechecked level of the table as well as tightness of bolts and the hardwood surface under the vacuum plenum.
    Do you have large humidity swings in your area over the course of the day? (your info doesn't indicate where you are) This can cause the spoil board to swell and shrink. Gary Campbell described to me some of the issues he dealt with when his shop was in FL.
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

  10. #10
    Join Date
    Dec 2007
    Posts
    2,255

    Default

    Are you using vac hold down

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