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Reading through ASME Y14-5 I noticed a Note in para 2.6 "NOTE: When basic dimensions are used, there is no accumulation of tolerances. A geometric tolerance is required to create the tolerance zone. In this case, the style of dimensioning (chain, baseline, direct) is up to the discretion of the user. Locating features using directly toleranced dimensions is not recommended".

What does this mean? Aren't direct tolerances the best compared to to chain, or baseline? So why wouldn't someone want to locate a feature using a directly toleranced dimension, unless this has something to do with using a GD&T block (I'm self-learning GD&T at the moment, so I haven't gotten to those symbols yet).

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  • $\begingroup$ What exactly do you mean by "locate a feature?" If you mean "put it in the correct place when you make the part, or assemble a collection of parts", exactly where you measure from is important. It's not a matter of "best" in the mathematical sense. For example if you have assembled a "stack" of ten parts and the accumulated tolerance (within the specification!) is say 2mm from the original datum, when you add the next part to the "stack" you don't want to measure from the original datum and ignore than 2mm tolerance you have already accumulated. $\endgroup$ Commented Aug 6, 2021 at 2:55
  • $\begingroup$ They are saying use GD&T rather than a simple tolerance on a dimension - your "unless" statement is the key $\endgroup$ Commented Aug 6, 2021 at 3:06

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Note the phrase at the very beginning: "When basic dimensions are used..." So this paragraph isn't about regular size dimensions but onlybasic dimensions.

Basic dimensions aren't measured or inspected but are used as the theoretically perfect positions of features and/or other datums. So you essentially can't give them a tolerance, but you can tolerance the size and position of the feature (like a hole) at the location described by basic dimensions.

Example of Basic dimensions

Note the locations of the holes are basic, but the envelope dimensions of the part are directly toleranced as are the holes. Further there is geometric tolerancing on the holes to describe the allowable deviation from the theoretical perfect positioning set by the basic dimensions.

Source

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  • $\begingroup$ I see. Yea I missed "basic". So just to clear things up, basic dimensions are mostly used in conjunction with GD&T to help make more precise/accurate tolerance zones. I'm guessing it would not be practical (or not acceptable) to use something like a chain dimension alongside a positional GD&T tolerance. $\endgroup$ Commented Aug 7, 2021 at 5:44
  • $\begingroup$ Correct on both points. The only exception to the latter would be if your positional tolerance is greater than the sum of the tolerances in the dimension chain but that is unlikely to occur. $\endgroup$ Commented Aug 9, 2021 at 12:31

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