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"unit basis" for profile of a line
2

"unit basis" for profile of a line

"unit basis" for profile of a line

(OP)
Hello All,

I am very beginner at GD&T. Have an urgent question, please help me guys :)

The question is: Is unit basis applicable for profile of a line?

RE: "unit basis" for profile of a line

Urgent question: ISO or ASME?

RE: "unit basis" for profile of a line

It would be an extension of principle so my take is that it is okay per ASME. You may require a note. I can't speak per ISO as I don't know a lot about it. There might be a case where it is not appropriate though so maybe you can post a picture that we can all look at and argue about. wink

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: "unit basis" for profile of a line


If I understand correctly, the whole idea of “per unit basis” application is to reduce abrupt variations within larger tolerance zone.

This is what ASME Y14.5-2009 says about “abrupt variations”:

Section 5 Tolerances of Form:

Para. 5.4.1.3: “Straightness may be applied on a unit basis as a means of limiting an abrupt surface variation within a relatively short length of the feature.”

Para. 5.4.2.2: “Flatness may be applied on a unit basis as a means of limiting an abrupt surface variation within a relatively small area of the feature.”

Section 8 Tolerances of Profile:

Para. 8.3.1: “Since the surface may lie anywhere within the profile boundary, the actual part contour could have abrupt surface variations. If this is undesirable, the drawing must indicate the design requirements, such as rate of change and / or blend requirements.

In my opinion this means, Straightness - YES, Flatness – YES, Profile – NO (but you can add the note).
Am I the only one seeing the difference?

RE: "unit basis" for profile of a line

I think applying profile on a unit basis is a valid way to indicate a restricted rate of change.

So, of course I have a tec-ease tip to refer to. It cites 8.3.2.1:

Link

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: "unit basis" for profile of a line

The tip says exactly this: “This revision of the Standard states that profile of a surface may be applied on a unit basis similar to straightness or flatness. [8.3.2.1]”

Para. 8.3.2.1 explains how to indicate non-uniform tolerance zone on the drawing. It has nothing to do with per-unit basis. Neither this nor any other paragraph “states that profile of a surface may be applied on a unit basis similar to straightness or flatness”. Such paragraph simply doesn’t exist.

In absence of better word I would call the tip “wishful thinking”

RE: "unit basis" for profile of a line

I do not believe that is valid. Profile is deviation from a basic dimension. Your reduced tolerance zone of .1 bylateral would have to be 0.05 on either side of the basic 20mm. It can not be anywhere within the global 0.6 bylateral tolerance zone. This violates the entire concept of profile.

I think you have to use either straightness or flatness per unit size if you want to refine the profile to control abrupt variation of form.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: "unit basis" for profile of a line

Dgallup,
You beat me to it.
I was preparing another quote from Para. 8.3.1: “The boundaries of the tolerance zone follow the geometric shape of the true profile”.
You cannot have smaller “per unit” tolerance zone floating within larger tolerance zone; it will violate the definition of profile itself.

RE: "unit basis" for profile of a line

Guys,
Aren't you about forgetting composite profile tolerance concept applied to a single feature as shown in figs. 8-19 and 8-20, but especially 8-19? The lower segment defines a tolerance zone that is not equally disposed around true profile. It can float within larger tolerance zone.

In the same way profile tolerance can be refined by on a unit basis. There is nothing wrong with it in my opinion.

If we are talking about profile of a line tolerance, the only problem I see is how to properly define a unit length if the toleranced contour nominally is not a straight line. The smaller length would probably have to be defined as length of arc in the feature control frame (instead of circular area as in Tec-Ease tip), or - to avoid confusion - with an explanatory note.

RE: "unit basis" for profile of a line

Pmarc,
I have nothing against 8-19.
In fact, I would be happy if 8-19 solved OP’s problem.
But on 8-19 it is the whole tolerance zone derived from the entire profile that is allowed to float. You still see no difference between “the whole” and “within a relatively short length”?
And when it comes to explanatory note – I am all for it. I am probably the last one believing that “must indicate the design requirements” is actually stronger statement than “may be applied”

RE: "unit basis" for profile of a line

By the way, where exactly in 8-19 "The lower segment defines a tolerance zone that is not equally disposed around true profile"?

RE: "unit basis" for profile of a line

I should be more specific. The tolerance zone in lower segment IS equally disposed around true profile of the hole, but the true profile itself is not oriented/located wrt datums B and C, so it is not centered relative to larger tolerance zone.

RE: "unit basis" for profile of a line

Look at attached picture - I know it is crude, but I think it shows how the per unit area profile concept works.
As it was hard to clearly present everything, I made some simplifications:
1. Case #2 and Case #3 - smaller tolerance zones are shown parallel to datum plane A, while in reality it does not have happen.
2. Case #3 - Dia. 10 areas are shown in front view as lines.
3. Case #3 - in order not to obscure the picture, 10 mm long lines are shown as if the per unit portions were checked "adjacent" to each other, not overlapped.

I think that if we change profile of surface symbol to profile of line and delete diameter symbol from "dia. 10", it will be a case for <<"unit basis" for profile of a line>>.

http://files.engineering.com/getfile.aspx?folder=f...

RE: "unit basis" for profile of a line

A lot has happened since I've been away from my desk.

CH, Considering that the cited paragraph in the tip doesn't specifically address the issue, I think there is one of three possible things going on:

1. The cited paragraph is a typo. The NOTE at the end of paragraph 8.3.2.2 sure sounds like what we're talking about.
2. The idea was that since the paragraph alludes to individual segments of a profile, it's not a stretch to imagine such segments as unit based.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: "unit basis" for profile of a line

Oops, I guess that was only two things.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: "unit basis" for profile of a line

Pmarc,

I agree, you can use per unit basis, where profile can be safely replaced by flatness or straightness.
Can you do the same thing with Fig. 8-7 or 8-25?

Powerhound,

Both 8.3.2.1 and 8.3.2.2 are dealing with non-uniform tolerance zone. Nothing to do with per-unit definition. The only 2 things that happen are:

Tec-ease made false statement. Standard doesn’t say that profile of a surface may be applied on a unit basis similar to straightness or flatness.
Tec-ease used bogus reference to back that statement.

It is possible that SOME paragraph in DRAFT VERSION of Y14.5 was dealing with per-unit profile but was removed from final edition. Tec-ease based tip on draft version and didn’t fact-check later. This is why I called it “wishful thinking” – they expected it to be in final release, but it didn’t happen. Get over it.

RE: "unit basis" for profile of a line

Like I mentioned, if the nominal contour is straight/flat I see no major problem with the concept. If it is curved, then we start walking on thin ice.

As for Tec-Ease referring to 8.3.2.1, I guees it was discussed on the forum some time ago. Why is it there? I do not know. But instead of speculating I would simply ask Don personally - the easiest and probably the fastest way to solve the dillemma.

RE: "unit basis" for profile of a line

Pmarc,

We finally start agreeing on something.

There was a reason for ASME Y14.5 committee to use the careful wording they used, so there is a reason for us, mortals to be careful. smile

RE: "unit basis" for profile of a line

So what is the note at the end of 8.3.2.2 referring to?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: "unit basis" for profile of a line

Easy now, CheckerHater. From the published Standard at the end of 8.3.2.2: NOTE: A profile per unit length, similar to that shown in Fig. 5-4 for the control of straightness, may be used to control abrupt transitions that occur when profile tolerances are specified on adjoining segments of a feature.
Not clear how you can say that profile on a unit basis is disallowed. It is not "wishful thinking" as you stated. The section reference is a typo that I will correct immediately (if you never do anything, you never do anything wrong). Yes, it talks about using the concept to control abrupt transitions but it does not disallow using it elsewhere. In fact, I have a customer who makes knee implants who has used it very effectively. Their scanning software is capable of inspecting the profile of a surface on a unit basis tolerance. Don't want a snappy knee joint.

RE: "unit basis" for profile of a line

GDTcoach,
I am glad to see some clarification.
The note still part of Para. 8.3.2
It only applies to “abrupt transitions” that occur within non-uniform tolerance zone (You quoted it in your own post), not to abrupt changes in profile itself.
It does not universally apply to the entire Paragraph 8.
But let’s go back to your tip.
When I “imagine a flexible coin that could conform to the basic contour to visualize the tolerance zone” I see that coin sweeping exactly the same area as good old vanilla profile tolerance zone.
This is why I see idea of per-unit requirement meaningless when applied to profile.
Per-unit requirement works well with datumless controls where feature is related to itself, where it probably belongs anyway.

RE: "unit basis" for profile of a line

CH,
If it matters to you, I fully agree with Don on this.

Your argumentation that the note from 8.3.2.2 applies only to 8.3.2.2 is not convincing. Read para. 8.3.1. It says that: "Since the surface may lie anywhere within profile boundary, the actual part contour could have abrupt surface variations. If this is undesirable, the drawing must indicate the design requirements, such as rate of change and/or blend requirements." Don's tip is exactly the indication of required rate of change.

I also do not agree with the statement: "Per-unit requirement works well with datumless controls where feature is related to itself...". Case #3 from my sketch is exactly showing per-unit requirement related to datum plane A - notice that each smaller tolerance zone is parallel to A. This is what isn't required by profile without datum references, thus my remark: "1. Case #2 and Case #3 - smaller tolerance zones are shown parallel to datum plane A, while in reality it does not have [to] happen."

RE: "unit basis" for profile of a line

Well, CheckerHater, as you said, and I agree, "Per-unit requirement works well with datumless controls" which is exactly what I show in the Tip. Since the unit basis control does not have a datum feature reference, it is free to float (in location and orientation) within the larger profile tolerance that is controlled relative to a datum. Your comment that "it only applies to abrupt transitions" is like saying that tangent plane only applied to orientation tolerances because that is where it was shown in the 1994 revision. That sort of reasoning made us add the note to the end of the current tangent plane section. For the readers of this thread, often it is necessary to use extensions of principle to clearly define design intent. You will not always find your exact application in the Standard. Ask yourself if what you have said on a drawing will have one clear meaning to anyone knowledgeable of the Y14.5 standard. Often people ask "Where does it say I can do that?" A better question might be "Where does it say I cannot do that?"
As a member of the Y14.5 committee and Section Sponsor of Profile, I am reluctant to participate in these discussions because some folks just like to take shots at the committee and our efforts.

RE: "unit basis" for profile of a line

Hi pmarc, we were posting at the same time. You bring up a good point regarding having a datum feature reference to control orientation.

RE: "unit basis" for profile of a line

Pmarc,

If Note was meant to apply universally to all profile cases, it would not be added to Para. 8.3.2, but to Para. 8.3.1 right after it said “the drawing must indicate the design requirements”.

I m not happy with the way Y14.5 handled it. I said it and I can repeat that it looks like afterthought, last minute adjustment, but face it:
When explaining how to deal with abrupt variations in profile standard DOESN’T SAY “use per-unit principle”. It says what Para.8.3.1 says.

I am still not convinced.

We had this discussion before. If it’s not in the standard, it has to be explained on the face of the drawing.

RE: "unit basis" for profile of a line

And I am not going to further convince you - I do not want to end up searching forum history for how many times we both agreed that "extension of principles" is acceptable or not depending on circumstances. That is too tiring for me.

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