Spherical Tolerance Zone

[CheckerHater]

I recently run into question that I thought could be interesting for the rest of this forum: what spherical tolerance zone is good for?

ASME Y14.5-2009 Fig. 7-35 shows obvious application for locating spherical feature of size.

Now, if I want to locate point in space, there is nothing wrong about that point being contained within spherical tolerance zone.
One possible use is shown on the enclosed picture; all opinions are welcome.

 

[KENAT]

I could imagine situations with balls in grooves/kinematic mounts and that kind of thing where it may make sense.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

Is point a feature of size so that Position can be used?

 

[CheckerHater]

It’s extension of principle.
If we can locate to zero tolerance, why not use zero size?
Also, what if point is “derived” from something that has size?
Actually this is the whole point of my post: collect different opinions on how far can we extend our principles.

 

[pmarc]

Assuming the point is derived from something that has size (like a spherical feature), I do not see anything on your print that would fall under "extension of principles" approach. Everything seems to be in line with the standard. Unless I am missing something?

As for applying this concept to a physical single point = a feature without size, I could possibly buy it for Position at RFS, but for sure not in case of Position at MMC or LMC.

 

[CheckerHater]

Unfortunately the approach varies both in time and space.

Say, in 1994 flatness at MMC was a big no-no. In 2009 flatness that is derived from feature of size may be specified at RFS, MMC, and LMC.
And of course in ASME angle is not a size, but in ISO it is a size and cone is “feature of size” as well.

So we are in the grey area, not universally understood and not universally accepted all the time and in all the places.
That brings another question: should we “extend our principles” to close the gap?

 

[pmarc]

You will not provoke me again, CH.

 

[CheckerHater]

I just want to be convinced.

 

[pmarc]

If your own words in the thread about runout application for partial surfaces do not convince you, I am pretty sure nothing will.

 

[Belanger]

Keep this in mind about extending principles... some things can't be extended due to a logical impossibility (such as talking about MMC on something that has no size, as pmarc mentioned -- i.e., a point). But in the case of flatness at MMC, in the 1994 system you may have been able to argue that it was logically possible, if you attempted to apply the flatness to a feature of size.
Unfortunately, at the time the standard told us to use straightness for form control on any feature of size -- even a planar block, which really didn't make sense since the concept of straightness is one-dimensional. But that was the rule at the time.
CheckerHater, you really like to keep us on our toes, huh?

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

http://www.gdtseminars.com

 

[CheckerHater]

No, it’s just this thread:

http://www.eng-tips.com/viewthread.cfm?qid=328006

Really wild interpretation of simple position made me think that we are far away from understanding each other, and everybody extending principles their own way is not an answer.

 

[fsincox]

ISO allows position on points, In the older revisions, I know that.
Frank

 

[fsincox]

I am with Ken, I have spherical applications I use all of the time.
Frank

 

[fsincox]

pmarc,
I do not see a point as a feature of size.

 

[pmarc]

I do not see it too, but if the position is applied at RFS basis the size would not really matter. Don't you think?
Agree, per Y14.5 position shall be applied to features of size only, however I do not see a "logical impossibility" against applying it to a point (at RFS of course).

PS.: Nice to see you back.

 

[Belanger]

Welcome back, Frank! I recall you had a thread (or contributed to one) several months ago that discussed a spherical tolerance zone for a point hovering above a plane. That was all about using a spherhical diameter symbol if the control was only constrained in one direction. Ring any bells?

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

http://www.gdtseminars.com

 

[fsincox]

Thanks guys, you are too kind.
I have been looking for those:
thread1103-314840
thread1103-322523
pmarc,
Does ISO still allow position of a point?

 

[pmarc]

Frank,
The newest edition of ISO 1101:2012 as well as the older one (issued in 2004) show an example entitled: "Position tolerance of a point", however the point is derived from typical spherical feature of size. It is not a physical portion of the part. Does this mean that position applied to a physical point is illegal? I am not able to tell.

 

[axym]

CH,

I generally don't have a problem with extension of principles.

The thing that I always watch out for is whether or not everything is clearly defined on a real part. In your cone example, the tolerance zone appears to be reasonably well defined. It's a spherical zone at a certain distance from Datum A and centered on Datum B. Datum feature B is a bit non-standard - presumably you would want a cylindrical simulator for B even though the feature itself is conical.

I would have more concerns about the definition of the toleranced point itself, however. The standard does not address how to establish the apex of an as-produced conical feature. On a real part with form error, there are many ways one could use to find an apex and they would all potentially give different results. What did you envision for the method of establishing the apex point?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

If we can “derive” virtual center of a sphere, we should be able to “derive” virtual sharp of a cone, shouldn’t we?