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Nook XPR Ballscrews

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  • Ray Livingston
    I finally had a chance today to play with the ballscrews I bought for the CNC conversion on my knee mill. I bought the Nook XPR screw stock, which is rolled,
    Message 1 of 1 , Feb 3, 2008
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      I finally had a chance today to play with the ballscrews I bought
      for the CNC conversion on my knee mill. I bought the Nook XPR screw
      stock, which is rolled, but higher precision than other rolled stock,
      with only 0.001"/foot maximum lead error, as opposed to the
      0.004"/foot for the more pedestrian screws, and 0.0005"/foot for the
      *very* expensive ground screws. The appearance is much more like
      ground stock, being smooth and a raw steel finish, as opposed to the
      more typical black oxide finish on most rolled screws. They are
      considerably smoother than standard rolled stock as well, even dry.
      This is really beautiful stuff, and very well worth the small
      additional cost.
      Since I'm fabricating my own double-nut assemblies, I've been
      agonizing for months over exactly how to provide the necessary pre-
      load. This is typically done with a Belleville spring between the
      two nuts, providing on the order of 150-300 pounds of opposing force
      between the nuts. However, after playing with the actual nuts and
      screws, I am going to eliminate the spring, and simply rigidly mount
      both nuts, rotating one of them against the screw to provide the pre-
      load. The screws are accurate enough that the motion is still very
      smooth and consistent over the full length of the screw, with
      absolutely *zero* backlash. I tested this by putting both nuts on a
      screw, tightening them against each other as tight as I could by
      hand, clamping the nuts in my vise, then using a dial indicator to
      attempt to measure the backlash in the screw. There was none, nada,
      zero, zip, zilch! There may be some very small penalty in terms of
      service life, but for a hobby machine, it'll never see enough service
      to come within even 50% of the theoretical service life anyway. I
      suspect contamination and corrosion will be far greater risks. And
      even if they do eventually fail, many years from now, new ones are
      not all that expensive (I paid $650 for all three screws - two 1" and
      one 5/8" - and all six nuts). Worst case maybe I have to replace
      them in 15 years. Who cares? This approach makes the fabrication of
      the double nut assemblies absolutely trivial.
      Nook did send me the wrong screw for the Z axis (0.250" lead
      instead of the 0.200" lead I ordered), but I'm sure they will
      straighten that out quickly. They've been very responsive so far.
      This whole conversion is going to be FAR easier, and FAR better,
      than the one I did on my mini-mill!

      Regards,
      Ray L.
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