huangyhg

where in asme 14.5 does it state we can add & subtract basic
where in asme 14.5 does it state we can add & subtract basic dim's. without tol. accumulation? i know we can, i just can not find a direct statment in the std. (venders say no).
thanks all,
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i might be confused by the question, but if you add or subtract basic dimensions you would just get a ref dimension.
the reason it is safe to add and subtract basic dimensions is because there is no applied tolerance and they are not affected by physical nonconformities. the dimensions are absolute with respect to the idealized model or drawing. it is not until a tolerance frame is applied to a feature that any variance is introduced.
i thought like you, that i had seen a statement like that somewhere in the standard but after looking again i do not think that it is stated directly.
there are definitions and explanations that when considered in concert with one another support the outcome of the statement.
1.3 definitions > 1.3.9 dimension, basic 鈥?used to describe the theoretically exact...
1.4 fundamental rules (a)鈥?(or indirectly in the case of basic dimensions), 鈥?br />5.2 positional tolerancing (a) 鈥?(theoretically exact) 鈥?br />5.2.2 application to base line and chain dimensioning 鈥?for positional tolerancing, unlike plus and minus tolerancing 鈥?
i'll just chime in to say that i am in agreement with paul and tick. you can add and subtract basic dimensions because they are theoretically exact. if you have a feature control frame that calls out a tolerance wrt a datum that is not directly tied to the feature, as long as you can get from the feature to the datum by adding or subtracting other basic dimensions, you are okay.
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i do not think there is a mention anywhere in asme y14.5m-1994 about adding and subtracting basic dimensions. why should there be? the standard explains what they mean, and you can apply this to your arithmetic.
true position tolerances and profile tolerances usually relate each feature to common datums. it is bad practise to show feature to feature dimensions on such a drawing, but the tolerances are from the datums, regardless. your addition and subtraction of the basic dimensions is exact. tolerances are not accounted for.
if you have a bunch of holes located to true position, the distance between any two holes varies by the true position tolerance. this can be verified by simple geometry.
jhg
where is it stated anywhere that you are allowed to add and subtract any dimension? yet it is (un)common sense that you can, as long as tolerances are considered. this is a necessity of design. to do so with basic dimensions doesn't even require that, as they are absolute, as thetick points out. being able to add and subtract basic dimensions was one of the very first things that i learned about gd&t, and not being able to can make for a much more complex drawing.
it sounds as if your vendors want to get out of a situation that they find uncomfortable, and are grabbing at straws.
if the above is true, then a basic dimensions could be repeated without making them reference, just like datum symbols can be repeated without making them reference.
gary, i will be 72 on october 10th using gdt since 1953.
"if the above is true, then a basic dimensions could be repeated without making them reference"
i don't see the connection. i think that would still be considered double dimensioning, whether it is a basic dimension or a toleranced dimension. it would only serve to obfuscate the drawing if it weren't a reference dimension, datum or not.
ewh
we had a magnified detail on a printed circuit board where the basic dimensions showed up again and we could have made them reference for double dimensioning, or not.
i would have made them reference. it would make the drawing just a little more clear.

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