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Thread: How do I calculate a position with only a width of a spline and angle between splines

  1. #1
    Associate Engineer
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    How do I calculate a position with only a width of a spline and angle between splines

    I have a positional callout on a drawing for some splines. These 24 splines are supposed to be 15 dg apart from each other, a thickness of 5.18 and the center is supposed to be 2.59. the true position is 0.5 (then to the three datum's). I've got my alignment, what I need is the formula for calculating this kind of true position. I am using a cosmos program on a CMM. I've googled it and the closest I've come is some kind of calculation with the thickness and angle. (I am sorry I can not upload a drawing due to confidentiality reasons, picture the spokes on a bicycle).

  2. #2
    Administrator Kelly Bramble's Avatar
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    You can sketch the spline only specification and post here.... Splines are not a national security secret.

    I think ~ you're trying to measure the position of each individual spline feature of size relative to the datum’s.

    There's not a conversion formulae. You would establish your 3D zero from the datum’s and then measure the derived center plane of each spline feature. Measure at many opposed surface locations of each spline to establish the as-built center plane of the spline and compare the measurements to a symmetrical tolerance boundary oriented and located relative to the datum zero.

  3. #3
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    My security settings will not let me upload anything from here... I created my 3D center, my alignment, a line on each side of the spline, a bisecting line going in-between the two lines, but not sure where to go from here. It is calling out the angle of 15 dg from the center line of one spline to the next, and the thickness of each spline, with the position 24X (TP symbol) 0.5 (E)(F)(G). Reading within the Geometrics IIIm book, it states that this is what is needed, but not stating exactly how to calculate it. I am pretty well versed in regular Cartesian TP, and in TP in polar mode (angle with radius), but I am not certain with this particular type.

  4. #4
    Technical Fellow
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    Quote Originally Posted by looneytn View Post
    It is calling out the angle of 15 dg from the center line of one spline to the next
    That sounds like a very risky approach as any error will be propagated to each successive tooth. For a start, the spline angular displacement needs to be determined as unit of the full 360 degrees.

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