Diametrical Position on a Square or Rectangular hole 1

[tharding247365]

Would it be acceptable to use a diametrical position call-out on a square feature? Trying to use a circular tolerance zone to control this, but I read somewhere you can only use the diametrical call-out on a circular feature? Can anybody explain this in detail?

 

[Belanger]

The pertinent paragraph in the Y14.5 standard would be 7.4.5. But to give a good answer, should I assume that you want to also include the MMC modifier? Or are you thinking RFS?

 

[powerhound]

I would think that you could do this but I would certainly question why.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[tharding247365]

RFS is how it's currently called out.

I believe the intent, is to usually include this modifier for the "more efficient" circular tolerance zone instead of the square tolerance zone.

It just caught me off guard, because a circle, for every point on a circle, the opposite side of the circle will theoretically intersect at the midpoint, and that's where you calculate the position tolerance

On a square or rectangular hole, this creates just a line of centerpoints. Does each center point have a theoretical tolerance for it's centerpoint?

Or do you get the centerpoint by taking the length of the height and base, and dividing it in 2?

Maybe I'm thinking of this completely wrong.

Link

 

[powerhound]

I don't think there is a clear concise answer to your question. I also don't think there is direct support for your exact situation in the standard since, at least on the surface, it doesn't seem to make sense. If this is a light switch and there is a rectangular component that fits into that rectangular opening, I can't see any sense whatsoever to be made out of this callout. I can also see where an MMC modifier would benefit this yet there is none.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[tharding247365]

Any words of wisdom? This part is sheet metal. I almost want to suggest to just put a general note "All parts shall conform to TP .5 ABC unless otherwise specified" and be done with it and leave out the diametrical modifier.

Not only does this make more sense in my head, this is laser cut sheet metal, and if everything is done in one setup, wouldn't that be the best way to tolerance this anyway?

Almost every feature is called out either TP .5 or 1.5 calling back to the same datums.

 

[monty011]

Would it be best to control the position of the rectangular feature to a center plane datum? This method would be without the diameter symbol in your FCF. The center plane of your feature would be controlled in one direction to be between two planes of width of your specified tolerance. Probably need to see a drawing (or simplified sketch) to get a better picture of what you have.

 

[chez311]

My instinct says that locating a rectangular/square feature in an identical fashion as a standard round/circular feature and producing the same circular tolerance zone is NOT possible. I may well be wrong, but I don't know of any way to achieve that exact result.

That being said, I believe the closest result you might be able to achieve in attempting to tolerance a square/rectangular feature in a similar fashion would be to utilize either a composite profile tolerance (RFS only), multiple single segment profile tolerance (RFS only), or a combined profile/position tolerance (allows use of MMC/LMC controls on position). Its not exactly what you're looking for but I think it achieves a similar result in controlling the location of the entire feature instead of each wall or pairs of opposing walls individually.

 

[Belanger]

My first reply asking about RFS vs. MMC was because I was thinking ahead to some sort of "boundary" application. That might be possible for the OP question, but only if the MMC modifier is used (which you said it isn't).

Profile would certainly be a good solution, but that requires that the size and form of the square also be controlled to the same accuracy as the location. So it might be too much?

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

 

[chez311]

Belanger,

It certainly isn't an equivalent to position control, as I noted, but its an option. Also your statement about the form controlled to the same accuracy as the location is only true for a pure profile tolerance. I was referencing composite and multiple single segment which would actually hold a tighter tolerance on the form and a looser tolerance of the location ie: a tight profile tolerance of the FRTZF/lower segment could float within a looser tolerance of the PLTZF/upper segment.

Also my last suggestion was to combine profile and tolerance - see section 8.8 of the Y14.5 standard to see an example. It references MMC controls used with position, however I don't see any reason why it couldn't be used with RFS (please - if someone knows otherwise feel free to correct me).

Edit: changed "pure position tolerance" to "pure profile tolerance"

 

[Belanger]

chez311... I disagree with your statement that form being "controlled to the same accuracy as the location is only true for a pure position tolerance." A position tolerance never controls form.

But to your point, I do like the idea of a combined control, because position and profile would work together controlling their respective qualities to different accuracies, as needed. However... the hitch here is that the OP mentioned RFS, not MMC. Paragraph 8.8 refers to using position as a "theoretical boundary," and that means MMC. So that's where I find it difficult to give a good answer to the goal.


(edited to correct RFS in the last sentence to MMC)

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

 

[chez311]

Belanger - Oops, that definitely was not what I meant to say! I apologize, I meant "pure profile tolerance" as I was comparing a standard profile tolerance to composite profile and multiple single segment profile tolerances in my statement. (I have edited the original post to reflect that) My original statement still stands, albeit with the edit previously noted.

After re-reading that section I think you may be correct, it may only be possible with the use of the MMC control. The OP would either be forced to change to an MMC callout or utilize a different method.

 

[pmarc]

To answer the original question, if the intent of the position callout at RFS is to control location of two center planes derived from two pairs of opposed walls of a square feature within a cylindrical tolerance zone, then that kind of callout generally does not make sense.

The only case I can think of where it could theoretically hold water is when size of the position tolerance zone would be equal to or greater than the LMC size of the square hole. Only then it would be possible to fit two center planes (of finite length and height) within a cylinder.

With that said, if a cylindrical position tolerance zone is trully functional requirement, the thing/object that could be controlled is a line derived from an intersection of two center planes. Of course to do that, some kind of additional note would have to be designed to clarify what kind of thing the positional callout is intented to control and how that thing must be constructed.

And even if that is done properly, the open point will still be a mutual orientation of the two center planes, because the positional callout itself would not be able to prevent the square from being manufactured as parallelogram, for example.

 

[CheckerHater]

I always had the feeling that cylindrical tolerance zone automatically implies Axis, not the Plane.
Also, combining position with profile (as was suggested before) only works with Boundary concept which also doesn't do well with Axis.
All together, using diametrical tolerance with anything but a round hole seems meaningless to me, although I haven't seen it explicitly forbiden (maybe I didn't look too hard )

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

CH, how about this... Using a diameter symbol on position for a hexagon (a non-round feature!), knowing that a hexagon forms an "irregular FOS"? I haven't seen it either, but one could argue that a circle scribed around a hexagon could then be positioned within a diameter.

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

 

[CheckerHater]

IF hexagon is free to spin around AND circle is INSCRIBED inside of hexagon AND Position Specified at MMC, then it may have some meaning, but even then I don't think everybody will agree.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future