Straightness tolerance - Effect on material boundary size

[Jacob Cheverie]

I have attached a very basic image of a part that I am dealing with. Datum feature C is a cylinder, datum feature D is a cylinder, and datum feature E is a cylinder. These three cylinders are mutually orthogonal.

(1) Datum feature D has a Straightness tolerance at MMC.
(2) Datum feature C has a Position tolerance at MMC with respect to datum feature D at MMB.
(3) Datum feature E has a Position tolerance at MMC with respect to primary datum feature C at MMB and secondary datum feature D at MMB.

I am trying to establish the boundary size of D at MMB for the third geometric tolerance as listed above and I am running into an issue. The boundary size of datum feature D would be its MMC size plus any applicable geometric tolerances to higher precedence datums (per Y14.5-2009 4.11.6/2018 7.11.6). In this case, the designer does not include a geometric tolerance on datum feature D with respect to datum feature C. They actually have it the other way as is mentioned in case (2).

My question is as follows: To determine the MMB size of datum feature D, am I allowed to use the Straightness tolerance to increase the boundary size even though the Straightness tolerance is not defined to be in effect while constrained to datum feature C at MMB? Would I be violating datum feature precedence by adding this tolerance to the boundary size?

Thank you.

 

[Jacob Cheverie]

To add on to this issue,

If I increase the size of the MMB of datum feature D by the straightness tolerance by using a custom boundary size, then, assuming that datum feature D is actually perfectly straight, wouldn't I be increasing the amount of positional error allowed on datum feature E by 1/2 of the straightness tolerance in either direction mutually perpendicular to both datum features?

Without the custom boundary size explicitly stated on the print, I do not think that I can make that call as it could lead to higher positional error, through additional datum displacement, than is called out on the print.

Here's the kicker - Functionally speaking, if I tried measuring the part on a gage with a true geometric counterpart at MMC size, I may fail parts just by not allowing the datum feature D to fit in it's simulator (if it is not straight). The simulator should be at MMC size + the straightness tolerance but measuring it this way to determine the Position of E could accept positional deviation that is not explicitly stated.

I am going back and forth here - I think the problem would be solved if datum features C and D were switched in their order of precedence but that is not something that will change and I am stuck with being tasked to measure these parts.

 

[Belanger]

Each tolerance on a drawing has to be met on its own terms.
So while the drawing you've been given is goofy (yes, they should switch those datums around), you still have to assess each one as a self-contained idea. If you measure and pass one callout, but that in turn causes another tolerance to fail, then it's a bad part. (Well, bad per their goofy tolerances.)

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