LP and GX for cylindrical feature ISO GPS

[Kedu]

Folks:
Do you think LP and GX combo is valid for the same cylindrical feature? Is it allowed based on ISO GPS set of standards?
What would be your opinion?

Thank you for any input in this matter.

 

[Belanger]

It depends on what you mean by "combo." (For those who aren't familiar with this stuff, it comes from ISO 14405-1.) For a cylindrical feature:

LP means "two-point size"
GX means "maximum inscribed association criterion"

Since those could result in different measurements, then I wouldn't typically use the on the same feature. But I don't think it's illegal to do both; perhaps you have a different diameter number given for LP and GX. To me it just seems like an awkward way to control the part's roundness.

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

 

[CheckerHater]

On the internal feature LP will produce "maximum" local measurement, GX will produce "minimum" inscribed size.
Together they will produce "envelope requirement".

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

 

[greenimi]

LP and GX probably is acceptable for a hole, but might not be for a pin.
Does any ISO standards specifically forbidden this practice for an external cylinder?

Never seen LP and GX for an outside feature. Maybe its legality could be questioned at best.

 

[CheckerHater]

Do words "inscribed" and "circumscribed" have a meaning?

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

 

[greenimi]

Do words "inscribed" and "circumscribed" have a meaning?

Yes, they do.

But why cannot be INSIDE of the material in the same way actual minimum material envelope (unrelated or related) is defined in ASME?
Thinking out loud......

Again, do you see a specific prohibition in ISO of such approach? (I am learning ISO, along with the OP, probably)
Thank you for chime in.

 

[CheckerHater]

There is no prohibition. Just limited use (if any).


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

 

[greenimi]

LP and GX probably is acceptable for a hole, but might not be for a pin.
Does any ISO standards specifically forbidden this practice for an external cylinder?

Never seen LP and GX for an outside feature. Maybe its legality could be questioned at best.

CheckerHater (Mechanical) 2 Aug 19 14:42

There is no prohibition. Just limited use (if any).

CH,

If has limited use, then how would you show on an ISO drawing that the design intent is to control the minimum distance between two components or a minimum wall thickness?
Let's say a hole that should not be too close to the edge of the part? Isn't it the same -conceptually- as it is in ASME? (size of the hole and its applicable tolerance AND position modified at LMC).

Also, on the same token, if the feature that must be controlled (for structure integrity/ minimum wall thickness) is a surface then , probably, profile it is the best geometrical control to be used? Am I correct? And modify the profile at LMC (or at MMC for that matter) is still a no-no in the ISO book (as it is in the ASME world).

 

[mkcski]

Given this thread, and many in the recent past, I am always curious about the differences between Y14.5 and the ISO GPS "system". Buying the collection of GPS standards is almost prohibitive. We have a few at my place of work. They are really "dry" compared to the Y14 series of standards and hard to interpret. Could anyone recommend a book or collection of publications that is comprehensive with the numerous GPS standards and does a better job of explaining the content than the standards themselves?

Certified Sr. GD&T Professional

 

[greenimi]

mkcski,

I would love to find an ISO book/ publication within which the latest ISO concept is explained in "down to the earth" terms.
The best I could get my hands on is Henrik Nielsen's handbook ("The ISO Geometrical Product Specification"), but that book is covering 2012 version of 1101. As I said I like to find one which is covering 2017 version of 1101 (for example).
I guess does not worth ($$$) to write ISO books as they become very quickly outdated as ISO stuff is changing very often.


CH,
Attached you will find a snap picture from ISO Pocket Guide (Alex Krulikowski) (written based on ISO 1101:2012). It is shown a drawing -rectangular piece dimensioned for its width 12.8-13.2-- and its maximum inscribed size shown as 12.7.
And yes, the size it is 12.8-13.2 (not a mistake) an outside dimension.
So, looks like maximum inscribed size could be used on a pin/ tab.
Am I missing something or seeing (conceptually) something wrong in ISO?

I self admitted I do not have knowledge in this system and that is why I came here and ask questions......
Having different level of experience and different perspectives is always helpful.

 

[mkcski]

greenimi

Thanks much. I have Alex Krulikowski's book from 2013 - very good - but somewhat abbreviated - I think. I will look for Henrik Nielsen's handbook on the net/amazon. Please post if you find/see something newer in the future.

Certified Sr. GD&T Professional

 

[SeasonLee]

Per the 2017 versiob of ISO 14405-1, its legal to use both modifiers(LP and GX) on the same feature, pls look at the bottom of Table 3 below, and it means Envelope requirement.

Season

 

[greenimi]

Season and all, (edit "and all"--as the question is intended not only for Season, but for anyone who might know the answer--)

Thank you for the table from the ISO standard.
I see that LP and GX is legal to be used on the same feature so that solve the OP question……… probably.

My follow-up question has been if could be used on an external surface (shaft, pin, tab) since GX stands for maximum inscribed association criteria.

Would that be acceptable? Or GX it’s a dedicated modifier for internal features only?
(maybe the same question could be asked if GN “minimum circumscribed association criteria” could be used for the holes or it’s a dedicated modifier for external features only)

I cannot find a definitive answer to my own question(s).

 

[SeasonLee]

Yes, you are right greenimi, GX is for an internal feature while GN is for an external feature, pls see the snapshot below.

Season

 

[greenimi]

The question still remains if GX could be used for external features and/or GN could be used for internal ones? In a way to mimic the unrelated actual minimum materal envelope (per ASME verbiage which probably is incorrect in ISO).
I cannot find a definitive answer (read prohibition) of such practice. [pre][/pre]