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Position Error

Richardbeyer87
Good Afternoon all,

I have a question that has been bothering me for a while, I posted a sample of the code. I am measuring the position of 7 cylinders that are pictured in the drawing, (i cannot post the actual drawing forgive the artistry). I align the part to the datum structure, and when I report the locations in the Position Xact measure the holes are varying in distance over the course of the 10" the cylinders run. When I measure the same dimensions with a height stand using my datum C as 0.00" they are all the same distance from the datum. Why am I seeing the error? Datum B and Datum C are not very square to eachother, and datum A about 0.002" perpendicular to both other datums. Is the combination of these errors causing this variation. They vary by about 0.007" in distance and the angle between datum B and C is 90.04°. Thanks For any Help.
Richard
  • I think Xact is showing you the 'worst' end of the cylinder, but if you are checking from one side only with the height stand, you will only see one end.
  • Thanks for the reply I will look at that and respond. I guess my main question is how does PC-Dmis look at the distance with datums that have some form error, and gow can I mitigate the issue.(commonly my machinists dont create perfect parts
  • If you want results from your CMM that correlate better with the surface plate, use Legacy Dimensions.

    There are two ways (Legacy and XACT Measure) to report stuff in PCD & each method varies in how it will compute certain dimensions.

    When you use a "Legacy" true position command, it does straight up true position math relative to your current alignment & workplane.

    XACT Measure computes true position per the latest revision of Y14.5M. That procedure tells PCD to do the true positon math you're familiar with but ALSO factor in datum shift as well as rotation. This is why you get slightly different numbers when you lay it out vs use XACT.

    If you...
    A) make sure your alignment is solid...
    B) Use that alignment (and correct workplane!!) to re-evaluate those hole positions....I'll bet you'll line up better.
  • Dan,
    Thank you for the response, I have a part coming to me in about 15 min I will give the legacy a try and see if it doesn't line up better with what I am seeing when I strap the part to my granite table on its specific datum. Rotation and datum shift are definitely playing a part in this case.
    Richard
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