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symmetric/assymmetric tolerances, preferences or rules?

symmetric/assymmetric tolerances, preferences or rules?

symmetric/assymmetric tolerances, preferences or rules?

(OP)
Hello all,
The subject is an old favourite; how to best model a 3D part
a) mid tolerance + symmetric toleranses (ie diam 30 +-0.1mm)
b) max material condition (i.e. diam 30.1+0-0.2)
c) nominal dimension and any tolerance acc. to my own best practice (for example diam 29 +1.1 +0.9)

Using 3D models for all engineering design work nowadays, I see no reason why not to always use a) above.
- errors are supposed to be manufactured on both on plus & minus side, right? We model the part as its most likely dimension (30)
- NC manufacturing benifits, right? Or do we prefer MMC?
- tolerance stackup & analysis do not need to recalculate assymmetric to symemtric before number crunching
- mass propertiy calculations are more reliable due to the in-the-middle is modelled.
-

Of course, exceptions to the rule exist, where certain tolerances are given by manufacturing methods, press fits etc.

I always ask my designers "why?" if they use assymmetrical tolerances (If I see no reason for it). Especially when using simplified drawings + STEP-file for manufacturing, sending a part modelled with assymmetric tolerances, leaves a lot ow work for the machinist to compensate for)

The question is really;

Are there good reasons, or only opinions, in doing it in one way or another?
(The other question is, am I a pain in the a** asking "why" all the time to my designers?)

/Sebastian, Saab Dynamics (not affiliated with the unfortunate car manufacturer)

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