Odd All-Around Profile Application

[Nescius]

Putting this question into words is awkward at best, so please do see the attachment. What is the interpretation of an all-around profile tolerance that controls a part outline where one of the controlled faces is partially represented by a hidden line in the drawing view in which the profile tolerance is applied?

 

[CheckerHater]

Would that be too much to ask, what datums A, B, and C are?

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

 

[Nescius]

Not at all, CheckerHater. How about specifying A and B, and get rid of C in the FCF. I believe this would be legitimate. See attached, where A is a surface and B is a hole normal to A.

 

[CheckerHater]

From degree of freedom point of view, you still need clocking datum to stop your part from spinning around |B|.

Now, when it comes to "invisible" profile, at least part of the feature(s) is visible on the view where profile is specified.

According to the the standard, GD&T controls apply to entire feature "unless otherwise specified".

For me it's enough to extend application of profile into "invisible" area, even if it wasn't what creators of "all around" originally intended.

But wait 'til the real experts kick in

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

 

[Nescius]

I appreciate your input. If anybody does feel that the hidden portion of the surface is controlled, what happens when this hidden portion isn't coplanar? How far do we extend this principle? See the attached example where some of the flat plane in question is hidden...and then a curve, tangent to this face.

 

[CheckerHater]

I got your idea.

The way I see it, the curved part of the protrusion becomes different feature, so profile will stop where the flat stops.

There is also one thing I believe in: if you feel that your drawing may be ambiguous, it is your job to clarify possible misinterpretation, for example, using "profile from A to B", etc.

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

 

[mkcski]

There is nothing wrong with adding a note to clarify which surfaces are included in profile control.

 

[Nescius]

I agree, mkcski, but I'm not knowledgeable enough to know if I'd be overstepping by making such a clarification. A clarifying note can only guide interpretation within certain bounds. In this case, I have no idea where those bounds may be.

CH, it sure does "feel" right on some level to say that the hidden portion of the flat surface is controlled, and that anything else is not controlled. I'm tripped up when I try to reconcile the situation where the half-hidden face is not planar at all. What if this half-hidden face is irregularly wavy, for example? See attached.

 

[mkcski]

I understand. I could also suggest labeling each surface that is part of the profile and state under the FCF "surfaces labeled X-X"

 

[drawoh]

Nescius,

Are you preparing this drawing, or reading it? If this is your drawing, you can make it more clear.

--
JHG

 

[djhurayt]

May I ask a question from my non-expert perspective? Nescius, you have added some rounds to your example and the waves in the last one, and (from what I can tell) implying that this changes the interpretation. I think you are saying because it creates a profile that is not made of straight lines.

But my understanding is, straight or curvy profile has nothing to do with it. As long as the profile is dimensioned, you can put a fence on each side of it. For that mater a lengthwise cross section of a potato could have a profile callout proved every little dip or bump in the outline can be dimensioned in some manner.

In your last examples the face of the lower left iso view that is facing the lower left corner of the page (minus the hole) makes up the profile you gtol is controlling. In effect this would be a top and bottom horizontal line, a vertical line on the left, two short vertical lines on the right, and the bottom horizontal, right side vertical and top wavy portion (with 1.5 humps) is all you are controlling. The hidden portion is not controlled regardless of its shape.

Now saying that, if you gtol was on the top view all it would control is the face of the upper left iso view that is facing the lower right corner of the page (one horizontal line, two vertical lines and a 5 hump wavy line.

Is my understanding way off base???

 

[Nescius]

drawoh, I am creating the drawing. As you suggest, I have decided to avoid the issue altogether. That is unless somebody can give a truly definitive answer.

I only wish the geometry of my part were as simple as the examples I created. I am concerned about creating discontinuities or steps in the profile in question; aesthetics are a driving factor here. My part has radiused transitions between the "friendly" part of the profile and and the problematic part of the profile, as shown in my attached example. I must control any mismatch or step at points X and Y, the tangent points of the radii.

The profiles in question will be milled in house and there with be no problems to speak of no matter what I do, simply because the CNC magic machines are so great. The issue is vexing me because these parts may be metal injection molded in the future. Describing some good control over these aspects will then be mandatory, or else we'll blow $20K on tooling to produce crummy looking parts. Young engineer Nescius might then find himself in a "meeting".

Anyway, I'm working through it and greatly appreciate all the input!

 

[Nescius]

djhurayt, you say:

"The hidden portion is not controlled regardless of its shape."

That very well may be the case. This is the heart of the issue: Is any control applied to the hidden portion?

The whole curved vs. straight business was speculation between me and CheckerHater about how the rule could be applied. My wavy example was meant to challenge our experimental notion that the hidden portion of a planar face was subject to control simply because the visible portion of the same face was controlled.

If the profile controls only the visible portion, that would be OK in my eyes. It would also mean that applying a profile tolerance in such a case would probably rarely be the best option.

 

[pylfrm]

I agree there is often much clarity to be gained with the between symbol instead of the all around symbol, even without hidden lines. I think your examples could have been similarly ambiguous if the profile all around had been applied in a view from the opposite direction.

Another possible alternative is model based definition. It can allow you to unambiguously specify complicated tolerance boundaries with relative ease, but it's certainly not appropriate for all circumstances. As far as I know, it's mainly early-adopter territory at the moment.

- pylfrm

 

[CheckerHater]

Nescius:

Your "corrugated" face is NOT a feature, but rather a collection of features. Those not visible are not controlled.
Unfortunately the ASME definition of "feature" is so sloppy, it can be stretched in several directions.

I'm not knowledgeable enough to know if I'd be overstepping by making such a clarification. A clarifying note can only guide interpretation within certain bounds.

You are the boss of your drawing. You cannot "overstep" anything. There is an uncomfortable thought that you have to take responsibility for the results, but it comes with the profession you chose.

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

 

[Nescius]

I believe I'll put that in the title block, "Nescius is the boss of this drawing".

Seriously though, I understand and appreciate what you're saying. I am in a position where great importance is assigned to my opinions, most all of them, including small drawing details. I am a fine engineer, but the great confidence placed in me isn't necessarily all deserved; it's partially because nobody is in a position (knowledge-wise) to challenge me on many issues, GD&T being one. Like many lone engineers at small companies, I must be my own opponent in these debates.

 

[CheckerHater]

Nescius:
Think of it this way: You find perfect, genius, elegant way to specify the requirement, that only you and couple people on this forum can understand and appreciate.
There is a good chance your supplier may be not satisfied and ask for further explanations.
You mention molding the parts in the future. If you are familiar with ASME Y14.8-2009, it has 2 pages of sample notes one places on the drawings to take care of things, appearance being among the others.
Very often when you feel your symbology is not enough you just create verbal note: this place to be smooth, free of partng lines, ejector pin marks, etc. ("there be dragons"?)
And remember: we never stop learning and our drawings are growing up with us
Good luck!

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

 

[mkcski]

I am in a similar situation at my "small" place of work. I am NOT in the Design Eng'r Dept; I am in Mfg. There is no GDT guru in Design, so they come to me for guidance and critique. This is OK with me as I "love" the subject - I'd better after 35 years of "study". I teach and consult too.

Anyway, I am so glad to discoverer this Forum. The Posts confirm my understanding of the subject, which I consider a philosophy, as it is a thinking tool more than anything. Applying the controls makes for robust designs that give maximum flexibility to Mfg (read.. lower cost).

Thanks to all who participate in this Forum.

 

[drawoh]

Nescius,

The all-around profile is one of my favourite tolerancing tools. I would still apply profiles to each of your vertical faces.

--
JHG

 

[djhurayt]

Nescius,
(in response to your 10 Mar 16 22:28 reply to me)
... on the other hand if you had put this gtol on a top view (based on the current one being the bottom view) then the tolerance would apply to the basic rectangle plus all (section overhanging plus section on top of rectangular piece) of the wavy piece.

I think, ???