Bonus Tolerance Question, MMC on a Datum 4

[cfordyce05]

I have looked through ASME Y14.5-2009 and maybe I am blind, but I can't find what I'm looking for. Attached is an example drawing.

How is the true position of the .056 hole affected by the MMC on datum B? The hole has a .001 tolerance zone at MMC, so I can get a little bonus tolerance if I make the hole larger than MMC (to .056), correct? If the hole ends up outside of the bonus tolerance, does the MMC on datum B give me more bonus to keep the hole in tolerance?

 

[dgallup]

Yes, and since the tolerance on datum B is larger, this has the most bonus.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Belanger]

This would be covered in paragraphs 4.11.5 through 4.11.9 in the 2009 standard. It can sometimes be rather tricky. You already have a handle on the bonus from the hole that's being positioned (as you say, making it at .056 will gain the full bonus there).

But because of the M modifier (properly called MMB, since it's slightly different than MMC), there may be even more position tolerance available. You have to look at ALL of the variation on datum feature B, not just its size. The total variation on B could be .016 (.006 size tolerance and .01 perpendicularity). The standard calls this extra .016 datum shift. They do that to distinguish it from "bonus," which is technically only the fudge factor on the small hole (the .002).

The bonus tolerance makes the position tolerance grow. The datum shift doesn't really make it grow, but allows it to shift or "play" as if you had the part on a gage fixture and it jiggled a little. But in this case it's safe to add bonus and shift to the given position tolerance and treat it as if it were one grand total.



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

http://www.gdtseminars.com

 

[pmarc]

Look at fig. 7-18. I belive this is something you are looking for.

 

[cfordyce05]

So the .056 hole doesn't shift relative to the shifted datum B? With that scenario, the .056 hole still has a .001 position tolerance with .002 bonus tolerance.

 

[Belanger]

Well, your last post might be correct but we have to clarify the terms. The .056 hole can move within a .003 position tolerance relative to the perfect datum B (I get this from the .001 stated tol + .002 bonus tol). But the extra "M" after the B says that the real, actual datum hole (which we call datum "feature" B) could shift within a .016 zone from the perfect datum B, if the real hole were at .114 and perfectly perpendicular.

That might sound weird, because I'm saying that the hole we call datum B might deviate from the actual, true datum B. There's the rub. (Think of a physical gage fixture that we drop this part onto. A gage pin of .104 would be used to simulate the true datum, which we call MMB.)

Now put it all together: if the .056 hole moves within .003 relative to the perfect datum, and the actual hole that the datum is derived from moves within .016 from the perfect datum, it's like saying the distance between the two actual holes could be within a total range of .019 (translate: max = .1675 and min = .1485 axis to axis). Again, these numbers are only true if both holes are made at their smallest size and are exactly 90º to A.

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

http://www.gdtseminars.com

 

[Belanger]

Correction to my last sentence: these numbers are only true if both holes are made at the biggest size and are exactly perpendicular to A.

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

http://www.gdtseminars.com

 

[dingy2]

I was about to make a comment here until I reviewed the perpendicularity tolerance on datum B. Is it really .010 or is there an error and the tolerance should be .001? Something is not correct especially since the perpendicularity tolernace is in 2 decimal places while all the other tolerances are in 3 decimal.

Dave D.

http://www.qmsi.ca

 

[cfordyce05]

The drawing is "correct" with the .010 perpendicularity tolerance. The folks who created this drawing are known for their GD&T funny business.

 

[pmarc]

cfordyce05,

I hope the explanations given by the others are clear for you, because I would like to add that presence of (M) modifier following datum B within positional FCF's would have no impact on any relationship between .056 hole and other features which are controlled by positional tolerance with relation to the same datum reference frame A|B(M).

 

[cfordyce05]

It is clear now. Thank you all.

 

[SeasonLee]

I would rather talk further more about the MMC on a datum than to create a new thread.

As you can see on the OP, there is a material condition modifier (MMC) on the secondary reference datum B, all above are talking about the datum shift and its calculation, but no one asked “why” and “when” a MMC is needed on the FCF.

Quoted from a GD&T book: When a cylinder, hole (or FOS) is used as a datum feature, any FCF that reference the datum feature must include the material condition MMC or LMC,…

I just want to know “why” and “when” MMC is needed on the FCF. Thanks for the forum provided this learning environment.

SeasonLee

 

[Belanger]

SeasonLee, the book is wrong. If it said that M or L is required for a datum created by a pattern, then I might buy it; even that isn't an absolute requirement.

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

http://www.gdtseminars.com

 

[DanStro]

If its an older book to the pre-1994 standard could they mean that it had to have M,L, or S? If that's the case then I guess technically it's true because the S is implied if no modifier is shown.

If it's a newer book than it's wrong.

Dan

Han primo incensus

 

[SeasonLee]

The statement quoted from Alex’s Advanced Concepts of GD&T Textbook, page 14-10, please ref to the attached for details.

SeasonLee

 

[Belanger]

Wow, that is poorly worded!

The book is trying to say that since the datum feature isn't a surface, then it must be simulated correctly -- not just setting the part onto a surface plate or gage table. So it must be one of three things: MMC, LMC, or RFS (or in today's parlance MMB, LMB, or RMB).

That paragraph does allude to Rule #2 ("assume RFS"), so I guess all is OK. But that one sentence isn't right, no matter how you slice it, because of the parentheses.

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

http://www.gdtseminars.com

 

[SeasonLee]

John: Sorry for the misleading, I should quote exactly from the book.

From that figure, you can see the MMC on the referenced datum A, I am still interesting to learn “why” and “when” MMC is needed on the referenced datum.

SEasonLee

 

[Belanger]

No apologies needed. That sentence is flat-out wrong. It would be correct if it were:

"When a cylinder...is used as a datum feature, any feature control frames that reference the datum feature must include the material condition (MMC, LMC, or RFS) under which the datum feature simulator is to be constructed."


At any rate, one of those three is needed because a FOS can, of course, change in size from part to part. So the designer needs to tell the inspector if he wants a fixed or variable hold upon that datum feature. RFS means that the inspector must hole the datum feature firmly, regardless of its actual size (and any GD&T). But the M or L modifiers on the datum reference would imply that a constant-size gage is adequate, even if the part is loose. (I'm simplifying it by explaining it in terms of physical gaging, because that's not practical with L. But the general idea should make sense.)

When to use? In its function, if the datum feature is a clearance fit, and a little looseness or play is acceptable, then the M modifier should follow that datum letter when referenced. If the function dictates that the datum feature will be firmly held, like a press-fit dowel/hole, then the RFS condition is the way to go.

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

http://www.gdtseminars.com

 

[axym]

I completely agree with J-P on this one. That one sentence in the book is incorrect, and could easily lead the reader to an incorrect conclusion if they did not look into what Rule #2 states. The idea is that a datum feature of size must be referenced at a certain material boundary condition - either explicitly with (M) or (L), or implicitly with implied RMB.

As far as why MMB or any other material boundary modifier would be referenced on a particular datum feature, fit and function of the datum feature would primarily drive this. If the datum feature interfaces with a mating feature with a clearance fit (e.g. fixed size mating feature or fixture element), then an MMB reference usually makes sense. If the datum feature interfaces with a mating feature with a self-centering fit (e.g. press fit, threads, adjustable or tapered fixture element), then an RMB reference usually makes sense. If the datum feature will have material removed and must align the workpiece with a cutter path, then an LMB reference usually makes sense. It all comes down to how the workpiece will be constrained to a datum reference frame (coordinate system) using contact with the datum feature surface. If the datum feature is not functional and was chosen arbitrarily, then I would usually go with RFS as it will be the simplest and most repeatable (no issues with datum feature shift).

There may be other reasons to reference a datum feature at a certain material condition, that go beyond this simple analysis. These usually relate to tolerance stackup considerations and limiting tolerance accumulation in assemblies. For example, a datum feature that mates with a clearance fit might still be referenced RFS (or possibly even LMC) to limit tolerance accumulation in the assembly.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[SeasonLee]

Can I say MMC modifier on the referenced datum will create a repeatable datum axis since it has a constant size gage?

SeasonLee