MMB calculation

[Viper555]

I was hoping someone could help me with the MMB concept. I understand VC and RC, but the respecting datum precedence for MMB has me a little confused and I don't feel like the standard does a great job in explaining this.

Scenario 1: Suppose you have a block with a call out of pin #1 with datum reference of position of .5 when established to A, B, C all at RMB. Another pin on the part defined with position of .3 and as Datum Feature C (pin#2) whose FCF is A, B(M).

For external features, would you just add the positional tolerance from pin 1 (.5) to MMC of pin #2 to get MMB and not consider the .3 from pin #2?

Scenario 2: What if pin #1 was now at A, B(M),C. Does that change the calculation?

Scenario 3 - you have multiple FOS callouts with DRF A, B(M). How do you select which features get incorporated into the calculation for MMB? Just the largest (most impactful) that would encompass all others?

Thanks in advance.

 

[Burunduk]

The MMB of a datum featute is simply it's worst case for assembly boundary accounting for the controls that apply to that datum feature relative to the preceding datum features listed in the feature control frame that references it. Most commonly it's the VC of the datum feature. For example, if there is some geometric control that references A,B(M), most chances are that datum feature B is controlled at MMC relative to A. Then the VC of datum feature B in the A datum reference frame is the MMB of datum feature B where it is used as a secondary reference after A.

 

[Viper555]

It's where I see examples with tertiary that gets me tripped up. I've also seen examples of just primary at MMB which yields the MMC.

Does the MMB tertiary calculation affected by a feature that has secondary at MMB and same primary vs secondary RFB and same primary?

 

[Burunduk]

[A,B(M)] and [A,B(M),C] yield the same MMB. That MMB is based on the limits of size of datum feature B and its control relative to datum A, which precedes it for both A,B(M) and A,B(M),C. Had it been A,C,B(M) the MMB would be based on the limits of size of datum feature B and its control relative to A,C.

 

[Viper555]

[A,B(M)] and [A,B(M),C] yield the same MMB. That MMB is based on the limits of size of datum feature B and its control relative to datum A, which precedes it for both A,B(M) and A,B(M),C. Had it been A,C,B(M) the MMB would be based on the limits of size of datum feature B and its control relative to A,C.

Would it be correct to say that only orientation controls (including location and profile) dictate the MMB calculation along with the size?

If there are multiple tolerance controls to the higher order of precedence, would you just use the one that has the largest effect?

For instance when calculating MMB of C for a feature with A,B,C(M), would you take into account features controlled only by A,B, or would a feature with A, B(M) also be considered?

If there was a perpendicularity tolerance of .5 on a feature with A,B and another perpendicularity tolerance of .7 on another feature with A,B, would it just be MMC+.7, or MMC+.5, or something entirely different (external feature)?

 

[Burunduk]

It's just the controls applied to that specific datum feature (how this datum feature is "qualified" relative to the higher precedence datums) that matter.