Surface Profile with respect to datum

[AMontembeault]

Good Morning

There is some confusion in our design department regarding the interpretation of using surface profile w.r.t. a datum as it may (or may not) control size/orientation/location.

Numerous reference materials we've pulled up have all said some flavor of "Surface must be at true profile within X and located with respect to the datum reference frame established by datum features A,B,C". In our case, we are trying to control the inside radius (as a basic dimension) of a cylindrical sheet metal part within a surface profile about an axis datum A (not defined from this cylinder, but from another feature), the interpretation being that in this case, surface profile is essentially doing the same as the accumulation of a typical size tolerance range and positional tolerance (which would be applied to the diameter instead of radius, of course) combined - it's really just defining the virtual condition this surface may have with respect to the datum. However, some are convinced that this is an ugly/incorrect interpretation of what surface profile can be used for in this case. Thoughts?

 

[CheckerHater]

I am not sure if I visualize your case correctly, but I am sure profile is never specified at MMC / LMC, so no "virtual condition" is present.

Other then that, yes, profile may control size, orientation, or location depending on how it is applied.

 

[pmarc]

There is nothing wrong (wrong = in conflict with the standard) in applying profile wrt datum(s) to a closed cylindrical contour defined by basic radius or diameter. And it is indeed a combined control of size and position (as a matter of fact it limits form and orientation of toleranced feature too).

However, in many cases single profile tolerance is not a best choice from functional point of view. Where the requirement is to have loose size (and form) tolerance and tight location tolerance, or the opposite - tight size tolerance and loose location tolerance, single profile tolerance will not do it, thus the idea of controlling size of the feature through directly toleranced dimension and location of the feature through positional tolerance at RFS/MMC/LMC.

It is also worth to remember that other ways to split size and form of the feature from its location exist.
1. Composite profile tolerance, with upper segment containing reference to datum feature(s) - location and orientation control, and lower segment containing no datum feature references - size and form control.
2. Position at MMC + profile combination as shown in fig. 6-19 (Y14.5M-1994) or in fig. 8-24 (Y14.5-2009).

As for CH's caveat about "profile is never specified at MMC / LMC, so no "virtual condition" is present", it is of course true (we are talking about tolerance value), but I believe this small misunderstanding is caused by not too fortunate use of the term "virtual condition" by AMontembeault. In Y14.5 world we think about the Virtual Condition, if a geometric tolerance used to control form, orientation or location of a feature of size is applied at MMC or LMC basis. Since profile tolerance value can't be applied at MMC/LMC, we should rather think in terms of Outer Boundary and Inner Boundary. Granted, the OB and IB do not exist in reality, so are virtual by nature, but if we want to speak the same language and avoid confusion like this, the proper terms to use are Inner Boundary and Outer Boundary.

 

[CheckerHater]

@pmarc:

The small misunderstanding was caused by the following wording:
" the interpretation being that in this case, surface profile is essentially doing the same as the accumulation of a typical size tolerance range AND positional tolerance COMBINED "
This description clearly describes "virtual condition", so, driven by the best of intentions, I felt like I have to warn OP that VC will not exist in such case.

In fact, the closest thing to Profile tolerance applied to basic diameter will be zero positional tolerance applied at MMC - we will know where material can and cannot be present, but there will be no shifting.
Which brings me to another question: when dscribing similarity between Position and Profile, do you think the term "Resultant condition" as in "worst-case boundary" can be applied to Profile?

 

[pmarc]

CH,
To me the term "Resultant Condition" goes hand in hand with the "Virtual Condition". RC is one boundary, VC is the other. If there is no VC (like here), there is no RC.

Besides, look at the definition of the RC in the standard:
1.3.51 Resultant Condition
resultant condition: the single worst-case boundary generated by the collective effects of a feature of the size’s specified MMC or LMC, the geometric tolerance for that material condition, the size tolerance, and the additional geometric tolerance derived from the feature’s departure from its specified material condition. See Figs. 2-12, 2-13, 2-15, and 2-16."

1. Profile tolerance does not specify tolerance for MMC or LMC of the feature. It just defines a bandwidth within which the actual surface of the feature must fall.
2. There is no additional tolerance derived from the feature's departure from its specified material condition.
In other words, rather nothing in that definition matches to what profile tolerance actually defines, therefore I would stay from using the term "Resultant Condition" to describe one of the boundaries generated by the profile callout.

 

[CheckerHater]

Thank you