True position without diamter

[jeannev]

There is some disagreement among engineers at my work in the calculation of true position without a diameter call-out. Given an open ended datum notch with the following tolerances:
Width .100 +/-.002
Drop from base of slot to datum surface is 1.267+/-.005
[TRUE POSITION|.006|E|F] (with basic dimensioning to datum points on datums E & F)

When calculating the true position without a diametric symbol is it necessary to double the positional measurement?

ie. if the position checks .0084 from nominal is this a true position of .0084 (.0024 O/T), or is it a true position of .0168 (.0108 O/T)?

Jeanne

 

[Belanger]

The diameter symbol or lack of a diameter symbol merely indicates the shape of the tolerance zone: whether it's 360º or not. Your callout yields a tolerance zone of two parallel planes .006 apart, and centered around the perfect location.

Thus, the feature in question can only be .003 from nominal, in either direction. That's where the .006 comes from.

If a diameter symbol were included, then it would be allowed .003 from nominal, in any radial direction (.006 total diameter tolerance). Of course for your situation the diameter wouldn't make sense. But the numbers work the same, only the shape is different.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[jeannev]

Your explanation makes complete sense. Thank you for your help. I couldn't quite figure out from ASME GD&T why there was sometimes a diameter call-out and other times not.

Jeanne

 

[KENAT]

It's because some types of feature the zone is a rectangle not a circle. Circle is pretty much for screws in holes or similar.

For other applications a rectangular zone can make sense.

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(probably not aimed specifically at you)
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[dingy2]

A diametrical tolerance zone should only be used on features of size that are round and not on slots or rectangular notches. How many times have I seen a DTZ on a slot? Quite a few but it is not applicable since one would lose orientation.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

Quite right, Dave. Time to slap the knuckles of those designers.

Jim Sykes, P.Eng, GDTP-S
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