Is a internal hexagon a regular feature of size?

[EliteEDM]

Is a internal hexagon a regular feature of size? If not, please explain why. Thanks in advance, Carl.

 

[Belanger]

Two ways to answer that -- the hexagon in it's entirety is an "irregular feature of size," not a regular FOS. However, it can be said that each pair of opposing flats is a "regular feature of size."
Since your question just says "internal hexagon" as if you mean the entire thing, then I'd go with my first answer.

This all comes from paragraph 1.3.32 of the ASME Y14.5-2009 standard. In order for something to be a regular FOS, it must be a "cylindrical or spherical surface, a circular element, [or] a set of two opposed parallel elements or opposed parallel surfaces..."

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

 

[EliteEDM]

My problem is just what you have confirmed, there are two ways to answer that. It seems to me that a standard would not leave that gaping hole. My book has it as an irregular fos, but the dimension is 3x the dimension across the flats. I thought the dim across the flats would be two parallel planes, all sides fully opposed, but I guess it is not cylindrical.

Is a full radius end slot is a regular feature of size; it has a parallel width dimension and a parallel line dimension at the ends. If this is a regular feature of size, I have a hard time figuring why a hex would not be regular, unless it was called out with a profile tolerance rather than the direct dimension across the flats.

Any additional information to help me figure out regular vs irregular fos would be helpful (I know the obvious, non-FOS and multi feature surfaces are not reg. fos).

Thanks in advance.

 

[EliteEDM]

Belanger,
I see what you mean with regards to the hex in its entirety. If I came across a test question that has an internal hex or octagon and I was instructed to fully dimension and define the feature, do I assume it is irregular and use geometric tolerances for the profile or should I dimension across the flats directly and approched is as 3 sets of parallel planes, or dimension the flats with a basic dim and use profile, or ..... ? I have a copy of the standard and reference it often, but some things still seem to vague. Thank you again.

 

[Tarator]

Hexagon (internal or external doesn't matter) is NOT a regular FOS... See the attached image. Does the orange line belong to Surface A or B? It would have different opposing element depending on which surface it belongs.

 

[racookpe1978]

21 Sep 13 20:08
Hexagon (internal or external doesn't matter) is NOT a regular FOS... See the attached image. Does the orange line belong to Surface A or B? It would have different opposing element depending on which surface it belongs.

But I would argue that the sketch is NOT defined by anything "inside" a true circle - a circle defined by a diameter that is. That sketch has two irregular sides of six possible sides, and the entire figure isn't symmetrical within a hexagon at all.

Given the original problem, I would NOT argue about the definition of the hexagon as a figure, but rather argue strongly about how to ensure the gadget is always machined to the needed size to the needed accuracy. So, if it were to be operated by a 3/4 inch wrench, the "flats" dimension (not the length of each flat) is critical and MUST be defined with tolerances.

If, on the other hand, the 'socket" hole is important (as if the final gadget were to operated by a hex head wrench), the both the hex cross-section and "flat-to-flat" and "length of flats" and "angular accuracy of the flats" ARE ALL important and need to be defined. If any of these values are off by more than the tolerance, the hex key will not fit in the opening, the hex key flats will not turn all sides of the operator evenly, or the hex key will bind against the opening edges. In both of these example cases, and you will easily find others, the "diameter that the hex fits inside of" is strictly a nominal number and if used in those dimensions at all, is used only as a reference place to start the "important" measurements.

 

[Belanger]

A hexagon (internal or external) is not a regular FOS. Sorry if I confused things by my expanded answer

For your other question... A hexagon is a feature and I would use a profile tolerance to control it. And even with toleranced dims across the flats the hexagon is still a feature, and more specifically a FOS.

A radius of <180º is not a regular feature of size, and it's not an irregular feature of size. It's simply not a FOS of any kind, because it lacks the idea of opposing elements, which is intrinsic to the definition of a FOS.

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

 

[pmarc]

Just to expand a bit on J-P's most recent comment, the idea of opposing elements is indeed intrinsic to the definition of a FOS, but in case of irregular FOS "opposing" should not always be understood in a way it is understood in case of regular FOS. A hexagon contains "classic" opposing elements, but for example features shown at figs. 4-33 through 4-35 in Y14.5-2009 do not have such, yet they are called irregular FOS.

Having said that, in the broadest sense for regular and irregular FOS "opposing" means that a FOS may contain or be contained by an actual mating envelope and size/form of the envelope is definable. If a feature can't contain or be contained by clearly definable actual mating envelope, like for example in case of a radius of <180º, it is not a FOS.