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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > Machining not perfect holes
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  1. #1
    Join Date
    Mar 2006
    Posts
    53

    Machining not perfect holes

    Hey All,
    I have a Sherline mill converted to CNC.
    I just installed A2Z's headstock riser.
    I have 2 of them on my machine. (an extra 4.2 inches of travel on the X axis.

    I machined these parts out of Delrin.
    Excuse the pictures, but kinda hard to see.
    My Circles/pockets are not perfectly round.

    I am running 700rpm, 15ipm.

    The roundness of the gear is fine, just the cam is out of round. Same with the bearing hole on the white piece.

    I use DeskCNC and VcarvePro.
    Anyone have any ideas?

    thanks,
    Grease
    Attached Thumbnails Attached Thumbnails fangear.jpg  

  2. #2
    Join Date
    Jun 2004
    Posts
    6618
    Looks like backlash to me.
    Lee

  3. #3
    Join Date
    Mar 2006
    Posts
    53
    Lee,
    Thanks for the quick reply...
    I will check into it.

    Do you think on the X, Y, axis?

    It would'nt have to do with the Z axis, would it?

  4. #4
    Join Date
    Mar 2008
    Posts
    3655
    How are you tramming your Z axis? With that much extension it would definitely tend to angle forward. Cut some straight 1" cuts on x and on y. Then measure to see if these are actually CUT exactly one inch long--or if they are off.

    CR.

  5. #5
    Join Date
    Mar 2006
    Posts
    53
    Yeah, that's another thought.
    I might take off one of the headstock risers.

    I will do the test today..
    Thanks for the help!

  6. #6
    Join Date
    Mar 2008
    Posts
    3655
    Those are two different things. Before you REMOVE the riser, (Or do the straight cut test) try to square up the column with the table. (tramming) You can get close with a good machinist square, but a dial indicator or nano tram is best.

    The straight cuts are to test the accuracy of CNC setup on x/y axis.

    For example: If X cut is exactly one inch, but Y cut is 1.20, holes will be out of round.

  7. #7
    Join Date
    Mar 2006
    Posts
    53
    Gotcha...

    So...if I have excess tramming, I would have to bracket my machine to the back wall?

    Gotcha..
    I am going to perform the line test now...
    I will check the backlash too!
    I will keep you posted.

  8. #8
    Join Date
    Mar 2008
    Posts
    3655
    Uh, NO. First you measure the column for alignment to X and Y. If not perfectly square to each then loosen the allen screw at the base of the column. Then you square up the column. Then you tighten the screw. Then you check the column alignment for square again and repeat until the column is square to x and y. This is called tramming.

    The Nano tram from LMS:

    http://littlemachineshop.com/product...2601&category=

    Makes this REALLY easy and quick to do.

    CR.

  9. #9
    Join Date
    Mar 2008
    Posts
    3655
    If your column is out of alignment, this can cause the problem. It can ALSO be caused by other things, But you have to rule out the Z first. Don't even THINK about doing the inch cut test until your Z column is known to be in perfect alignment.

    CR.

  10. #10
    Join Date
    Jun 2006
    Posts
    2512
    A serious question. How is it possible for out of Tram to cause a visibly out of round/square cut? By my calculation out of tram by .040" in 4" of Z travel will produce a 0.00008" out of round/square part. Am I missing something here?

    Phil

    Quote Originally Posted by Crevice Reamer View Post
    If your column is out of alignment, this can cause the problem. It can ALSO be caused by other things, But you have to rule out the Z first. Don't even THINK about doing the inch cut test until your Z column is known to be in perfect alignment.

    CR.

  11. #11
    Join Date
    Mar 2008
    Posts
    3655
    Are you familiar with a Sherline mill with 2 throat extensions? It can be a LOT farther out of tram than .040. My point is: Tram the column FIRST, then start looking for the out of round cause. Otherwise he will be chasing his tail.

    CR.

  12. #12
    Join Date
    Mar 2006
    Posts
    53
    Hey Guys...

    Figured it out..

    Just did some tests.
    Backlash it is!

    My 1" lines measure pretty dam close..

    I cut some small circles too, which came out perfect!
    After I adjusted my Backlash..

    Also, I upped my speed.
    The black delrin I had, was melting at the higher speeds.
    The white is fine though..


    Thanks everyone for your response and quikness!

    May this post help someone in the future!

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