See this discussion-link below- and speaciily Evan J answer (in bold) :
Hi All,
Interesting debate.
One problem I always had with the description of size in Y14.5 is the emphasis on the "size also controls form" idea. The first sentence in the Limits of Size section states "Unless otherwise specified, the limits of size of a feature prescribe the extent within which variations of geometric form, as well as size, are allowed". This would naturally lead one to believe that size controls form. But the "size controls form" idea is not a rule, and does not bring in specific form controls. For the width example in the OP, the size tolerance does not impose a flatness tolerance of 0.6 on each surface. The form control from a size tolerance is indirect, and is a consequence of two specific geometric requirements:
1. The feature must conform to the Rule #1 boundary (a boundary of perfect form and MMC size)
2. The feature's actual local sizes (opposed point distances) must all be within the size limits.
When these two requirements are applied to regular features of size whose points are all opposed, then the "indirect form control" concept works. If we look at the possible as-produced surfaces that meet both requirements, we see that the form error of the surface cannot be larger than the size tolerance.
But if the requirements are applied to features whose points are not all opposed, then things change. In other words, if the feature has points that do not have a corresponding opposed point. This may be by design, as in the parts with partially opposed widths that have been discussed (or cylinders with a blind hole on one side). This may also be a symptom of the as-produced part, where a cylinder with a nominally perpendicular end face is produced with a tilted end face. In either case, we get some non-opposed points. What do we do with those?
I don't think that the problem is deciding whether or not Rule #1 applies. The problem is with the definition of actual local size. In Y14.5-2009, actual local size is defined as "the measured value of any individual distance at any cross section of a feature of size". There are several problems with this definition (this discussion could fill another thread), but the main one for this thread is the idea of a "distance". This can't deal with non-opposed points. In the places where the points are not opposed, we can't define the actual local size. What is a distance when there is only one point? So the unopposed sections of the surface can pretty much do anything within the Rule #1 boundary, and we don't get the indirect control of form. To appreciate what type of control the actual local sizes apply, imagine measuring the size of the feature using a micrometer with pointed anvils. All you can measure are sets of opposed points. If the surface has areas that are not opposed, you can't measure those areas. It's as simple as that.
So what do we do with those areas that are not opposed? Strictly speaking, the form in those areas is not controlled. In most cases, this does not cause a practical concern. If the unopposed areas are relatively small as in CH's part with the blind hole, we probably assume that the surface is continuous enough that there won't be significant local deviations in the non-opposed areas. If the unopposed areas are relatively large as in CH's part with the rectangular cavity, we might not assume this. Where do we draw the line? There are no rules - this is where "practical considerations" come in. As CH alluded to, it comes down to risk management. It is up to the designer to decide whether to go with just the size tolerance or to add additional controls (if they feel that the size tolerance would allow undesirable outcomes that have a significant chance of occurring).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
