Basic Dimension, positional feature with MMC. Legal? 3

[kimocal]

I am looking over some drawings and I saw some basic dimensions, each with a positional tolerance that has a MMC modifier (see attached). Is this 'legal'? I've searched eng-tips and and the ASME Y14.5M-1994 (specifically section 2.8 APPLICABILITY OF RFS, MMC, AND LMC) and can't find an answer.

Assuming the MMC modifiers were accidentally placed in both positional tolerance features, do the profile and circularity call outs affect the virtual size condition? I.E. do they add a bonus or expand/contract the virtual condition? I understand that the MMC on the Datum allows it it to shit, not add a bonus. Do these call outs have the same affect?

 

[ringster]

I DONT BELIEVE THEY sh__.

 

[ringster]

Kimocal,

I believe the mmc is legit callout for the datum. However, a couple things might merit reconsideration on your drawing. I dont believe it proper to use profile to modify the positional tolerance. Why not just apply the profile tol on both surfaces. Again beneficial to see how it assembles.

 

[kimocal]

I'm not creating this tolerance, I'm just trying to interpret it.

 

[kimocal]

Also the question pertains to the MMC on the Positional tolerance of .010, not the datums.

 

[dingy2]

kimocal:

The size of the features are shown as a basic dimension rather than a nominal size with tolerances. As such, the MMC size cannot be calculate and the positional tolerances at MMC just doesn't make sense. The features need to have a size tolerance.

I would suggest going to engineering and enquire about this situation.


Dave D.

http://www.qmsi.ca

 

[kimocal]

Thanks Dingy. This is an old drawing done by someone no longer here. I'm engineering so there's no one else to ask here, lol. Hence the reach out to the internet. I'm pretty sure you can't have a MMC modifier as a basic dimension has no max or min. I think its just an error =P

 

[KENAT]

Starting with the outside SR dimensioned on the left, looking at para. 6.5.5.1 & figure 6-19 of ASME Y14.5M-1994, it is legal to combine position and profile tolerances to control the boundary of a non cylindrical feature, it also talks about MMC. However, they don't explicitly state boundary in your drawing and I'm not quite sure this was their intent. Also I'm not smart enough to immediately explain any significance but in the example in fig 6-19, the profile & position control are separate FCF's.

The internal SR doesn't have any size tolerance does it? It has location and form control but no actual size control.

So the drawing is both incomplete and confusing. I realize you didn't create it but I'd go back and look at function and re-draw it. Perhaps using a single surface profile or even composite profile if appropriate.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[DesignBiz]

My logic for a profile callout in reference to the legallity of adding a material condition would be as follows.

Material condition (MMC / LMC) can add a bonus tolerance in direct relationship to a sized dimension.

The nature of the “profile” callout being defined by basic dimensions, which do not have size, leads me to believe that a material condition is illegal for the profile tolerance.

There would be no size variation to add as a bonus to the profile tolerance zone. In addition, the nominal profile is defined by basic dimensions and the tolerance zone itself is defined as a fixed boundary defined by the profile tolerance which is disposed around the shape equal laterally by default. Unilateral or bilateral boundaries can be indicated on the drawing if required.

The datums referenced can have a material condition added for allowance of datum shift. For instance a cylindrical hole referenced as a datum in the profile call could have a material condtion. The hole's representative pin in a gage would be made at the hole's virtual size, allowing the part to shift around in the profile boundaries by the amount of deviation the actual hole size departs from the material condition. That hole referenced as datum in the profile callout, would allow that particular DOF constraint some shifting.

For a refinement of location using a "positional callout" combined with a profile callout in regard to a non cylindrical shape, the "boundary" concept would need to be noted under the FCF as KENAT mentions can be found in 6.5.5.1 & figures pg 6-19 of ASME Y14.5M-1994.

My conclusion would be that the tolerance in a profile callout can not have a material condition and a reference
datum could have a material condition.

DesignBiz

"Quality is in the details"

 

[ringster]

Kimocal,

You made the statement that the MMC
modifiers might have been 'accidentally
misplaced'. Have you considered that
they may have been 'erroneously
misplaced' and created non verifiable
drawing? Just a thought.

 

[MechNorth]

Very unpleasant drawing from a GD&T perspective.

First, you can only use positional tolerances with features of size; they require directly opposed points (i.e. a MINIMUM of 180-deg of surface) and a limit on the size. Basic dimensions are "theoretically perfect", so no size limits.

Next, you barely have 180-deg of designed surface on the two hemi-spherical surfaces, and there's no guarantee that they would achieve 180-deg in fabrication; not a good place to use +/- size tolerances, so you'd be better off with the basic size and a surface profile wrt the datums (sizes & locates all in one) for both of these features.

Third, you CAN use a positional control at MMC to control the boundary of an irregular feature whose size control is established by a surface profile. The problem here is that the surface profile is also establishing location (since it has a DRF), so you have a conflict between the two controls for location.

Also, your 1.90-dia needs a size tolerance if you're going to modify the datum reference at MMC.

Surface profiles cannot be applied to a size dimension, whether basic or toleranced. Profile is a SURFACE control whereas when you put most controls on a FOS, you are controlling the center plane or axis.

There is no control on the surface indicated as datum feature A, and no relationship of datum feature B back to datum A. This relationship of datum feature B back to A isn't of much use if the thickness of the material is minimal, and is commonly dropped without issue.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com