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| GD&T -- Dimensioning Datum Targets |  | ||
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Posted by: CV ® 09/08/2004, 14:20:20 Author Profile Mail author Edit |
I am confused about dimensioning Datum Targets. ASME Y14.5M-1994 Figure 4-30 shows how to dimension datum targets. First you have to locate the three Datum A target points to create Datum A. You find those points by measuring off of what will be Datum’s B & C (i think). But how do you measure off of Datum’s B & C if you have not created Datum’s B & C yet? You can’t create those datum’s, which have to be perpendicular to Datum A, until you have created Datum A. Going in Circles, Chris |
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Posted by: daves ® 09/11/2004, 14:52:32 Author Profile Mail author Edit |
It's the other way around - you locate three points in space to create datum A, then locate the two points of B relative to the points you chose for A, then C from A and B. |
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Posted by: CV ® 09/13/2004, 11:01:45 Author Profile Mail author Edit |
You mean you are basically creating a jig that you set the part in? And that jig is made to those dimensions shown locating the datums. You are not measureing off of the parts surfaces and then positioning the part, correct?
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Posted by: daves ® 09/21/2004, 23:43:02 Author Profile Mail author Edit |
That's correct. With a CMM you have to fit the math jig to the part without seeing the jig. It can take several iterations before points of the math jig lie on the part surface. With an actual jig part placement is more obvious, though jig construction errors need to be considered when evaluating tolerance zones that depend on the jig. Obviously, if the jig is less accurate than the tolerance zones, there will be little value in using it. |
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Posted by: Cragyon ®  09/08/2004, 14:31:13 Author Profile Mail author Edit |
Chris, The basic dimensions that you see (dimension enclosed within a rectangle) indicate the location of the datum target points by way of standard tooling tolerances. These dimensions could have been limit tolerances such as +/- .015 or something. Read paragraph 4.6.2 "Datum Target Dimensions" within ASME Y14.5M 1994. A good book which is easy to read and understand GD&T is "Geometric Boundaries - The Interpretation and Application of Geometric Dimensioning and Tolerancing". |
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