ISO Envelope Principle vs OTE/ITE

[[Ri...]]

GD&T gurus,

What are the differences between using the 2-Point Diameter with the Envelope Principle and evaluating the diameter using the OTE/ITE? This is in regards to a single circle only.

I'm seeing some weird results - huge deltas between the OTE/ITE with the roundness values being 1/3 of the delta. In addition, when I tried the 2-Point Diameter with the Envelope Principle turned on, the Min value was greater than the LSQ, so I got even more confused.

Lol.

[[Br...]]

The 2 point is the actual local size. In other words, its like measuring it with a pair of calipers. It takes 2 points directly opposed from one another. It should actually give you 2 results(Maximum and Minimum) if I'm not mistaken. OTE/ITE use an algorithm where a perfect circle/cylinder expands or contracts about the data until it makes full contact. So as you can imagine, there may be irregularies in the surface that will illicit different results depending on which method you choose. Regarding the LSQ thing, I would think the two results you get from the 2 point calculation would be on either side of the LSQ measurement since LSQ is more of an average.

[[Br...]]

It looks like depending on if you have an OD or an ID, when the envelope is selected, the two results you get are the actual mating envelope, and then the 2pt for the opposite size. So if its an ID, the MIN will be the envelope, and the Max will be the 2pt. It would be reversed for an external feature.

[[Ri...]]

Hi Brett,

Thank you for the information. This is good stuff.

The customer is confusing me as this is what is on the drawing:

"DIAMETRAL SIZE SPECIFICATIONS SHALL BE INTERPRETED PER RULE #1 (ENVELOPE PRINCIPLE) AND ALL POINTS ON THE DIAMETER SHALL FALL WITHIN A MAXIMUM INSCRIBED CIRCLE AND
A MINIMUM CIRCUMSCRIBED CIRCLE THAT ARE DEFINED BY THE LIMITS OF SIZE AND ARE CONCENTRIC."

The drawing is ASME, but they are spouting ISO terminology. Which one do you want?? Lol.

[[SH...]]

Please sign in to view this quote.

I think rule number one is enough, rest of them is superfluous..

[[Da...]]

Please sign in to view this quote.

It is not. Rule 1 only requires that the entire surface fits within the outer boundary of size, and that the smallest cross section is larger than the lower limit of size. It does not require that the max inscribed and min circumscribed circles are concentric.

Basically, this note converts all the diameters into profile callouts with no datums, and no simultaneous requirement.

[[Br...]]

Please sign in to view this quote.

How is it even possible that a minimum circumscribed circle and a maximum inscribed circle are perfectly concentric on the same feature? I think they just mean they don't like the 2pt/actual local size requirement from ASME. As David touched on, they should just use profile tolerances on all their diameters then.

[[Br...]]

Also, ASME does actually refer to Rule #1 as the envelope principle.

Capture.JPG

[[Da...]]

Please sign in to view this quote.

Smells like something using Chebyshev algorithm. How to do it, no idea.

[[Jo...]]

I haven't experimented, but my assumption was 2 point is any local dimension, while envelope is a simultaneous requirement of the same center/axis.

In the case of a cylinder, I use profile of a surface bilateral zones to double check envelope condition. Unconstrained or freeform tells me if it "fits" the envelope.

Derived axis and location gets confusing for me as well.

[[An...]]

_

[[Da...]]

Please sign in to view this quote.

The MCC and the MIC are never concentric. If you have an inscribed and a circumscribed circle with both being concentric, they’re Chebycheff circles and then the minimum is not the max inscribed and the maximum is not the min circumscribed anymore. The text on the drawing is BS.

[[An...]]

_

[[Br...]]

Please sign in to view this quote.

Really, the thing that is really messed up about this note is that all the points on the diameter will always fall within the MaxInscribed, and MinCircumscribed no matter what because those are literally the inner and outer envelopes that contact the extreme points. So all the rest of the points would be between those envelopes.

The engeneer is confused about terms. He/she thinks MaxInscribed, and MinCircumscribed are synonymous with Inner and outer boundary. Otherwise the statement "...THAT ARE DEFINED BY THE LIMITS OF SIZE AND ARE CONCENTRIC." makes no sense. Both because they cant be concentric, and they have nothing to do with the limits of size. They are not "defined by the limits of size" they are defined by the actual data.

[[Aa...]]

Please sign in to view this quote.

You should change the title of the thread. The Envelope Principle is called Rule #1 by ASME, and not (to my knowledge) by ISO.

[[Aa...]]

Please sign in to view this quote.

What is the ISO terminology here?

[[Aa...]]

ASME Y14.5.1-1994 says a cylinder passes the size criteria if the whole surface falls between half-space boundaries generated by pulling spheres sized at the limits of size along spines. It does not say that those two spines have to be identical. Rule #1 additionally says that the Spine of the MMC boundary must be perfectly straight.

The language your customer used saying the circles have to be concentric seems like a 2-D way of saying the spines have to be identical. That means that all points along the length of the cylinder must pass a roundness requirement of the difference between the size limits.

[[Ri...]]

Please sign in to view this quote.

That was a misreading on my end. I just saw Envelope Principle, and thought of ISO where you have to specify if you want anything other than LSQ.

My original question is still valid where I was trying to understand the difference I was seeing between using the Envelope evaluation inside of Calypso, and MIC/MCC.

[[Da...]]

Please sign in to view this quote.

There’s no difference. Depending on whether the element you evaluate is an ID or an OD, the envelope condition evaluates an MIC (ID) or an MCC (OD). Additionally, the local size (two-point diameter) is checked, the max local size with an ID, and a min local size with an OD.

That’s all there is to it.

The only thing you should keep in mind is that it doesn’t make much sense for a circle. You should always use a cylinder for evaluation.

[[Ri...]]

Please sign in to view this quote.

There obviously is a difference. Like I said, I had a circle that I evaluated the ITE, OTE, as well as the Envelope, and none of the Envelope results matched the ITE/OTE - what was even stranger was the fact that the LSQ results were smaller than the ITE results which made no sense.

I agree on your second comment, but the customer is dictating the measurements. It's a pretty long cylinder, so they are wanting to see MIC/MCC on 40 cross sections along the OD.

[[Da...]]

That is strange and shouldn’t be the case.

[[Me...]]

Please sign in to view this quote.

There will be a difference. This has been addressed in the ASME Y14.5.1-R2019 where the "Actual Local Size" has been defined for both opposed points & circular elements using MIC / MCC.

[[Ri...]]

Please sign in to view this quote.

Can you expand on what exactly you mean. Are we sure Calypso would be using this?

[[Me...]]

2.3.3.2 Evaluation of Actual Local Size (Opposed
Points).
If actual local size is to be evaluated by the
opposed points method, an actual local size exists:

(a) Definition. The actual local size for a particular
evaluation line is the Euclidean distance between the
opposed points where the evaluation line intersects
the actual feature. If there are more than two intersecting
points, no size exists for this evaluation line.

(b) Conformance. If the distance between the two
opposed points satisfies the size limits, this actual local
size conforms to the tolerance.

(c) Actual Value. The actual value for an individual
actual local size is the Euclidean distance between the
two opposed points where the evaluation line intersects
that actual feature.

NOTE: Conformance to the actual local size requirements is
neither a necessary nor a sufficient condition for conformance
to the size requirement based on swept spheres.

2.3.3.3 Evaluation of Actual Local Size (Circular
Elements). If the actual local size is to be evaluated by
the circular elements method, two actual local sizes
exist for every point on the local size spine in the
cross section perpendicular to the local size spine at
that point (cylindrical features of size), or every plane
passing through the center point (spherical features of
size).
– Where Rule #1 does not apply, both “maximum material”
and “least material” local sizes are of interest. If Rule
#1 applies, only the “least material” local size is of interest.
– Cross sections are formed by using a cutting plane for
cylindrical and spherical features of size.
– Circular element actual local size is not defined for
parallel planes features of size.

(a) Definition. The actual least material local size in a
particular cross section is the diameter of the minimum
circumscribed circle (internal features) or the maximum
inscribed circle (external features) corresponding to the
actual feature in that cross section.

(b) Conformance. If the diameter of the defined circle(s)
satisfies the limits of size, this actual local size conforms to
the tolerance.

(c) Actual Value. The actual value for an actual local size
(s) is the diameter of the defined circle(s).

NOTE: Conformance to the actual local size requirements is only
intended to be an estimate of conformance to a size specification
by the swept-sphere interpretation.