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Surface Plate Pad Positions

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  • Surface Plate Pad Positions

    I know I've seen this before on this board, but can't find it. Where should the pads under a surface plate be located?

  • #2
    The support points should be marked on the underside of the plate. The number of these and their position depends on the size of the plate. If they are not marked, the best positions are probably the Airy points. See:

    https://en.wikipedia.org/wiki/Airy_points

    for a brief introduction.
    Bill

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    • #3
      Here is the federal spec on surface plates, including how to support them:

      http://www.tru-stone.com/pdf/FedSpecGGG-P-463c.pdf

      In brief, there are 3 support points, as you probably know, laid out as an isosceles triangle. The top point is on the longer centerline of the plate and 1/5 to 1/4 of the length of the plate from the end. The two base points are 1/5 to 1/4 the width from the sides and 1/5 to 1/4 the length from the end.

      "Airy points" are applicable only for supporting a beam, I think.
      ----------
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      • #4
        Thanks, guys. Exactly what I needed to know. My back-of-the-envelope calculations came out at 22.3% of the length -- midway between 1/4 and 1/5. Not so bad!

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