Basic dimensions for true position
- Thread starter ckk33
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John Predmore
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Hole 60 is identified as datum B, and datum B's location is defined relative to datums A,D,E. Hole 63 is identified as datum C, and the datum location is defined relative to datums A,B,E. After all holes referencing A,D,E are machined, the part has to be re-fixtured twice, to locate first on hole B, and later on B and C, so the remaining holes can be machined.
Boxed dimensions are ideal in GD&T, and when used for locating the ideal position of features, have no tolerance. So in theory, there is no stack-up or variation between basic dimensions referenced to D and to B, or E and C. In practice, there will be a small difference in position between the ideal location and the actual location of the physical hole at datum B and C, but it is not practical on the engineering drawing to depict multiple witness lines for basic dimensions to different reference frames. The tolerance of position for the placement of any physical hole relative to an ideal position is given by the feature control frame or the general tolerance at number 83.
IMO the reason holes 60 and 63 are prioritized as datums B and C (rather than making B and C the plate edges now identified as D and E) is convention - the function of the finished part sets priority for identifying datums, rather than the chronology of manufacture.
Boxed dimensions are ideal in GD&T, and when used for locating the ideal position of features, have no tolerance. So in theory, there is no stack-up or variation between basic dimensions referenced to D and to B, or E and C. In practice, there will be a small difference in position between the ideal location and the actual location of the physical hole at datum B and C, but it is not practical on the engineering drawing to depict multiple witness lines for basic dimensions to different reference frames. The tolerance of position for the placement of any physical hole relative to an ideal position is given by the feature control frame or the general tolerance at number 83.
IMO the reason holes 60 and 63 are prioritized as datums B and C (rather than making B and C the plate edges now identified as D and E) is convention - the function of the finished part sets priority for identifying datums, rather than the chronology of manufacture.
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Kurt Smith
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If you are making this report from a CMM it should be able to sort your position just by noting the correct datums. If you are using conventional inspection tools you will have to find the locations of datums B and C first. Then incorporate any position error they have into the tolerance of the other features.
For finding the basic dimensions from B and C you can always just do the math. As in subtract dim 41 from dim 20 to get the spacing between the holes.
For finding the basic dimensions from B and C you can always just do the math. As in subtract dim 41 from dim 20 to get the spacing between the holes.
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