Clarification on Position with Bonus Tolerance - Feature Control Frame

G

GRS&S

#1
I have a feature control frame of

True position | 2.0 | A | B | C M (where C is Ø 8.0 ± 0.1)

It is my contention that the 2.0 applies when the diameter of the hole is at it's minimum size of Ø 7.9. Bonus tolerance would be added as the Hole Ø increases. Therefore, if the Ø measured at Ø 8.1, I would be able to apply the most "bonus" to the exisiting position of 2.0.

Any feedback?
 
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Cari Spears

Super Moderator
Staff member
Super Moderator
#2
That is correct; you would end up with positional tolerance of 2.2 if the hole diameter is 8.1 (LMC)

Where MMC is specified, the stated positional tolerance is increased an amount equal to the actual size departure from MMC.
 

Jim Wynne

Staff member
Admin
#3
GRS&S said:
I have a feature control frame of

True position | 2.0 | A | B | C M (where C is Ø 8.0 ± 0.1)

It is my contention that the 2.0 applies when the diameter of the hole is at it's minimum size of Ø 7.9. Bonus tolerance would be added as the Hole Ø increases. Therefore, if the Ø measured at Ø 8.1, I would be able to apply the most "bonus" to the exisiting position of 2.0.

Any feedback?
Here's a useful calculator for determing bonus tolerance in situations such as the one you describe Bonus Tolerance Calculator

In answer to your question, see the "**note" on the linked page. (Short answer: the datum features with the MMC modifier may move off-center by 1/2 of the bonus tolerance.)
 
G

GRS&S

#4
Thanks guys, I knew the answer... I just wanted to have it verified so it can be presented to s/o who wants to argue otherwise :thanks:
 

Jim Wynne

Staff member
Admin
#5
GRS&S said:
Thanks guys, I knew the answer... I just wanted to have it verified so it can be presented to s/o who wants to argue otherwise :thanks:
You're welcome. Your best weapon in those kinds of battles is the standard itself. You should never preach chapter and verse unless there's a bible handy:D .
 
D

Dave Dunn

#6
GRS&S said:
I have a feature control frame of

True position | 2.0 | A | B | C M (where C is Ø 8.0 ± 0.1)

It is my contention that the 2.0 applies when the diameter of the hole is at it's minimum size of Ø 7.9. Bonus tolerance would be added as the Hole Ø increases. Therefore, if the Ø measured at Ø 8.1, I would be able to apply the most "bonus" to the exisiting position of 2.0.

Any feedback?
I'm don't consider myself a GD&T expert, so take my comments with a grain of salt, but this is my understanding of it:

When the MMC modifier is applied to a datum, it allows what's termed "datum shift" which is similar but not exactly the same as bonus tolerance gained from MMC on the feature itself. In this case, the MMC condition is noted on the tertiary datum. Now, not knowing what the actual datum features are, I can only give example of how it should be used.

Assume for example that the A datum is a plane, B datum is a wall perpendicular to A, and C datum is your 8.1 diameter. The part would be staged on the A datum feature which would restrict pitch, roll, and translation in X. It would then be aligned to datum B which would further restrict yaw and translation in X (or Y if aligned to Y). Finally the part would be restricted in Y by the C datum, removing the last degree of freedom.

Noting that the MMC condition is allowed for the C datum only, the part could be shifted to the extents of the C datum feature size, but must remain restricted in translation or rotation by the A and B datums.

It gets more complicated if there are multiple features that are to be evaluated using the same datum structure, as by GD&T rules, features controlled by the same datum structure with the same modifiers are to be evaluated simultaneously. This means that the datum shift must apply to all these features in the same direction and by the same amount, i.e. a position error of -.1 in the X direction and another of +.1 in the X could not both be accomodated by shifting the part in one direction or the other. Two features with positional error in the same direction could be acceptable assuming the datum structure allows shift in that direction.
 

Miner

Forum Moderator
Staff member
Admin
#7
Dave is correct. Depending on the geometry of the part, and what features are defined as A and B datums, the bonus tolerance may or may not be available to both axes. If the bonus tolerance was applied to the feature itself, it would be available in both axes.
 
C

Coleman Donnelly

#8
Re: Clarification on Position w/bonus tolerance - Feature control frame

Hello,

Maybe I am just not searching the forum correctly, but this was the clossest topic I could find to my question which is surprising...

I fully understand MMC and how it relates the the positional tollerance of a feature. My question is how do I validate a measurement system that uses bonus tolerance?

Typically the stance tends to be use worst case where bonus would be 0 and use only the positional tolerance within the control frame since I can not run a gage R&R when the tolerance is a different value for every measurement...

So...

What do I do when my control frame is built to [email protected]? Which I would argue is more often than not a better application of the tolerance band than to split the tolerance in a nonfunctional way causing opportunity to reject functional product.

Anyone?
 
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