True Position Deviation

[[Me...]]

Anybody ever had deviation not showing in true positions?
I have MMC on, but I find it hard to believe there is zero deviation in X and Y on more than 10 parts.
TIA
-Dave

[[St...]]

Hi Dave,
It could be the alignment or the measurement strategy, for starters. Can you upload screenshots of those?

[[Me...]]

[

ForumTP.pdf

[[Za...]]

set your Position Characteristic use the same alignment as your feature & see what numbers you get. could help point you towards an issue

[[Br...]]

It looks like it's the Datum shift from Datum feature B that's making it perfect. When you apply MMC to a datum feature in your Datum reference frame, it allows the DRF to shift and optimize the location of the toleranced feature. This is legitimate, but be aware there is a simultaneous requirement with Y14.5. If there are other features with the same Datum Reference frame and same boundary conditions, then they must be evaluated simultaneously. In other words, The DRF isn't allowed shift to a different location for each individual feature, they must be evaluated simultaneously.

[[Me...]]

So what you're saying Brett is multiple features check as a best fit bore pattern?

[[Br...]]

You got it! But also any other Position or Profile tolerance on your drawing with the same Datum Features referenced in the Datum Reference frame, in the same order, and with the same boundary conditions need to be evaluated at the same time as well. The best fit bore pattern actually will let you even put features with different tolerances into it. Make sure to use the "View Tolerances" option for the best fit.

[[Me...]]

Brett,
got it, I appreciate your help.
There is no skew, only two datums, plane and circle for origins, so rotation is allowed as well as translation?

[[Br...]]

Yes, both are allowed. the "translation" will only shift the amount of displacement allowed based on the size of Datum Feature B and its respective size tolerance. But rotation is completely up for grabs since it hasn't been constrained in the datum reference frame. Just check the boxes and Calypso will take care of the rest. you can also look at the plot of your best fit bore pattern to see where datum B shifted.

[[An...]]

Simultaneous or separate,that also depends on further dimensionings
See attached.

Contribution_01_10_2019.pdf

[[Aa...]]

A word of caution about terms:
The term "bonus tolerance" isn't found in the standard. That's a term that's often used in textbooks and training courses for what the standard calls "additional tolerance" for situations where position or orientation tolerances are applied with material condition modifiers (and interpreted according to axis interpretation). Additional tolerance is only applicable where the modifier is applied directly. The additional tolerance is available ONLY from the FCF-controlled feature moving away from its size limit, NOT from a datum feature moving away from its material boundary. So the description "Datum B-bonus" is very misleading, and it should instead say "The same 'Datum B-shift'".

The idea that the datum shift must be the same for all 8 bores is because of simultaneous requirement. (There's no other way to support that with the standard.) So to say that you don't need simultaneous requirement because the datum shift is the same is self-defeating.