"unit basis" for profile of a line 2

[concerta]

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?

 

[CheckerHater]

Urgent question: ISO or ASME?

 

[powerhound]

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.

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

 

[CheckerHater]

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?

 

[powerhound]

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

 

[CheckerHater]

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”

 

[dgallup]

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.

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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.

 

[CheckerHater]

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.

 

[pmarc]

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.

 

[CheckerHater]

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”

 

[CheckerHater]

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

 

[pmarc]

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.

 

[pmarc]

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/getfil...7-aa8e-4a51ceefc0a7&file=profile_per_unit.JPG

 

[powerhound]

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

 

[powerhound]

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

 

[CheckerHater]

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.

 

[pmarc]

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.

 

[CheckerHater]

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.

 

[powerhound]

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

 

[CheckerHater]

Referring to TRANSITION from one profile requirement to another profile requirement, not to profile itself.