GD&T Parallelism Question, 9 Engineers Stumped 2

[severeundersteer3000]

[–]svereundersteer 1 point 4 minutes ago

So I want to control the consistency of the thickness of this large metal plate. I don't actually care about the dimension of the thickness, or the parallelism of the part. 9 of us engineers have no ideas and we even hired a GD&T expert to consult us on it and he was stumped.

Its failing this parallelism GD&T often in inspection, but I usually end up with a consistent thickness this way. My current GD&T just doesn't communicate what I want it to.

Any ideas?

 

[greenimi]

What standard are you using?
What is the tolerance for the 16mm dimension?
Can you even identify which side is datum feature A and which one is the controlled feature?

 

[greenimi]

Re:
“So I want to control the consistency of the thickness of this large metal plate. I don't actually care about the dimension of the thickness, or the parallelism of the part”

Assuming Y14.5 and 16mm tolerance (total tolerance) is bigger than 0.05mm (.002 inch) I would use composite profile (instead of parallelism) . Upper segment xxx to A (xxx bigger than 0.05mm) and lower segment 0.05 to A. That’s means I don’t care where the surface is (I don’t care within xxxx) but wherever it is should be oriented within 0.05mm to A (so the thickness consistency is controlled)
The rub comes in when you have to identify which surface is which.
Probably the design wants that the part to be inspected from both surfaces and rejected if from one surface comes not good.

 

[drawoh]

severeundersteer3000,

What is "consistency of thickness"?

How flat is your plate going to be? On large, thin plates, I usually have a note next to the datum saying something like "DATUM[ ]A APPLIES WHEN THE FACE IS CLAMPED TO A FLAT REFERENCE SURFACE". In ASME Y14.5-2009, they talk about Restrained Condition. See section[ ]4.20, and Figure[ ]4.42. In ASME Y14.5M-1994, look for Free State Variation, Section[ ]6.8, and Figures[ ]6.53 and[ ]6.54.

--
JHG

 

[severeundersteer3000]

This thing is bolted down with Qty 20, M12 Bolts to a theoretically perfectly flat machine frame. The only thing i care about is that the thickness of the plate at any possible point you could measure it's thickness at is + or - .005mm from any other point. so the plate's thinnest part and the plate thickest part can be no more different in thickness than .010mm at the MOST. So yeah. Pretty accurate. This thing is Blanchard ground and is VERY expensive to make. I don't want to accidentally make them hold it even MORE accurately than I need it.

So the current Parallelism Tolerance isn't really what i want? At least I don't think so?

 

[severeundersteer3000]

I don't really care if the part is flat at all, heck this thing can look like a rainbow, and it will flatten out when I bolt it down. Its the thickness at each individual cross section of the part that I care about.

 

[drawoh]

severeundersteer3000,

My suggestion above is bang-on. The plate will be forced flat when it is used. You need to force it flat when you inspect it. What you want is parallelism with respect to the forced-flat primary datum.

You seem to be using ANSI Y14.5M-1982. I have not checked to see how it handles flexible components.

--
JHG

 

[ctopher]

You will need to call out flatness, otherwise the machinist 'can' cut the holes while the parts isn't flat.

Chris, CSWA
SolidWorks '15
ctophers home
SolidWorks Legion

 

[pylfrm]

Parallelism of the right-hand surface to datum A in the restrained condition sounds like it might do the trick.

This only controls the one surface though. Depending on the restraint procedure you specify, there will likely be the possibility of a gap between datum feature A and the datum feature simulator.

To truly control local thickness variation you would need to limit the gap. This can be done with parallelism or profile of a surface to datum A, again in the restrained condition, applied to datum feature A itself. Note that if you use profile of a surface, the tolerance value must be twice the maximum gap.

Assuming you use parallelism for both surfaces, the sum of the two tolerances is the maximum thickness variation allowed.

- pylfrm

 

[gabimo]

I would just call the maximum thickness you can live with (16mm ± 0.005mm) and then state: PERFECT FORM AT MMC NOT REQUIRED.
I think that's all you need.
No flatness, no parallelism.

 

[pylfrm]

Gabimot,

That would work, but holding a tight tolerance to a specific dimension may increase cost.

I don't actually care about the dimension of the thickness

Perhaps a better note would be something along the lines of "VARIATION OF LOCAL THICKNESS NOT TO EXCEED 0.005 mm" if you wanted to avoid flatness, parallelism, profile, etc.

- pylfrm

 

[greenimi]

Did we found a common ground on this thread? I do not think so

 

[aysuio]

pylfrm,

In gabimot's idea, what it does is to control the actual local size at each cross-section. Thus it has avoided MMC perfect form control and meet Ob's requirement. Of course size tolerance does nothing with parallelism.

I don't quite get you for "VARIATION OF LOCAL THICKNESS NOT TO EXCEED 0.010 mm"(I suppose you mean 0.010). I thought it does the same thing as gabimot's idea, and would get the same cost. Would you mind explaining to me a little bit about your idea? Thx

@pylfrm



Just add-on
PERFECT FORM AT MMC NOT REQUIRED-----Y14.5-1994
Independent symbol-----Y14.5-2009
May be the same as 1994?(Appendix D doesn't state the change)-----Y14.5-1982

 

[pmarc]

If I am reading OP's post correctly, main concern is to hold thickness variation in a free state within 0.01 mm without tightening thickness "overall" tolerance down to the 0.01.

If that is true, I do not think there is a standardized symbolic tool in ASME/ANSI GD&T that would clearly and unambiguously express the design intent. A text note defining what exactly 'thickness variation' is will have to be used in conjuction with directly toleranced dimension for the thickness. Additionally, the directly toleranced thickness dimension will have to be defined in a free state, and depending on which ASME/ANSI standard governs the drawing, the free state requirement will be expressed differently:
- for ANSI Y14.5M-1982 - FREE STATE note will have to follow the thickness dimension;
- for ASME Y14.5M-1994 and Y14.5-2009 - free state modifier (F in a circle) will have to follow the thickness dimension.

Regardless of the version of the Y14.5 standard, a note PERFECT FORM AT MMC NOT REQUIRED or Independency modifier will not have to be used to override Rule #1 because that rule by default does not apply for parts/features subject to free state variation in the unrestrained condition.

A text note defining what the 'thickness variation' is could be something like:
"FREE STATE VARIATION OF LOCAL (TWO-POINT) THICKNESS MEASUREMENTS NOT TO EXCEED 0.01 mm".

PS1. I assume a restrained note is going to be specified on the drawing according to which all other part's geometric characteristics will be verified, otherwise the free state notation on the thickness dimension will make no sense.

PS2. In ISO GPS/GD&T language there is a standardized modifier that would pretty nicely grasp the design intent without a need to use a descriptive note. It is SR modifier enclosed within an oval that follows a numerical value defining the maximum allowable difference (range) between max and min values of local (two-point) size measurements. For more details see ISO 14405-1:2010.

 

[aysuio]

Pmarc，

Thx! No matter whether OP knows how to deal with his problem, as a newbie, I just learned a valuable application from your post and found my ignorance in many sections of GD&T.

 

[severeundersteer3000]

Thanks for all the responses: This was a solid discussion and i learned alot.
Here is what we went with: + or - .005" for the thickness (our title block tolerance) and a note that says "VARIATION OF THICKNESS NOT TO EXCEED .0002" the people Blanchard grinding this plate were cool with that, as was our part inspector guy.

 

[greenimi]

Thank you pmarc.
Happy New Year!

 

[pylfrm]

aysuio,

Looking back, perhaps it wasn't clear that I intended the "VARIATION OF LOCAL THICKNESS NOT TO EXCEED 0.005 mm" note to supplement a looser thickness tolerance such as 16 mm +/- 0.127 mm.

I used 0.005 mm based on one of OP's earlier posts. I know 0.010 mm was mentioned as well. I basically just picked one at random.


pmarc,

Your comments on free-state variation prompted me to reread the relevant sections in ASME Y14.5-2009. I didn't find the explanation very clear, especially for when the exception to the envelope principle applies. Where do you draw the line between a 1 inch cube of steel and a sheet of .001 inch thick plastic film? Or is the distinction actually something else entirely?


- pylfrm

 

[greenimi]

What is the free state?
free state: the condition of a part free of applied forces.

Gravity?

 

[Belanger]

What is the free state?
free state: the condition of a part free of applied forces.

Gravity?

In its definition of free state (paragraph 5.5), the Y14.5 standard refers to that variation as "principally due to weight and flexibility of the part..."
So yes to your question, greenimi (and I guess pylfrm too), since weight comes from gravity.

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