GD&T Parallelism Question, 9 Engineers Stumped
GD&T Parallelism Question, 9 Engineers Stumped
(OP)
[–]svereundersteer[S] 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?
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?





RE: GD&T Parallelism Question, 9 Engineers Stumped
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?
RE: GD&T Parallelism Question, 9 Engineers Stumped
“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.
RE: GD&T Parallelism Question, 9 Engineers Stumped
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.
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JHG
RE: GD&T Parallelism Question, 9 Engineers Stumped
So the current Parallelism Tolerance isn't really what i want? At least I don't think so?
RE: GD&T Parallelism Question, 9 Engineers Stumped
RE: GD&T Parallelism Question, 9 Engineers Stumped
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.
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JHG
RE: GD&T Parallelism Question, 9 Engineers Stumped
Chris, CSWA
SolidWorks '15
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RE: GD&T Parallelism Question, 9 Engineers Stumped
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
RE: GD&T Parallelism Question, 9 Engineers Stumped
I think that's all you need.
No flatness, no parallelism.
RE: GD&T Parallelism Question, 9 Engineers Stumped
That would work, but holding a tight tolerance to a specific dimension may increase cost.
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
RE: GD&T Parallelism Question, 9 Engineers Stumped
RE: GD&T Parallelism Question, 9 Engineers Stumped
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
RE: GD&T Parallelism Question, 9 Engineers Stumped
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.
RE: GD&T Parallelism Question, 9 Engineers Stumped
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.
RE: GD&T Parallelism Question, 9 Engineers Stumped
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.
RE: GD&T Parallelism Question, 9 Engineers Stumped
Happy New Year!
RE: GD&T Parallelism Question, 9 Engineers Stumped
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
RE: GD&T Parallelism Question, 9 Engineers Stumped
free state: the condition of a part free of applied forces.
Gravity?
RE: GD&T Parallelism Question, 9 Engineers Stumped
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
RE: GD&T Parallelism Question, 9 Engineers Stumped
RE: GD&T Parallelism Question, 9 Engineers Stumped
You have to be careful with terminology.
Straightness is applied to surface element or derived median line.
In case of median line straightness symbol is combined with FOS dimension, but median line implies cylindrical body.
For the body that has "thickness" straightness symbol is applied to surface which may be controlled by FOS dimension.
Since your question was slightly off-topic you could benefit from starting separate thread.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: GD&T Parallelism Question, 9 Engineers Stumped
RE: GD&T Parallelism Question, 9 Engineers Stumped
Well I don't see anything wrong with the parallel tolerance but if the part is failing inspection after grinding, it suggests to me is that there might be residual stress in the component which is released post grinding.
Is there any stress relief called for on the component prior to final grinding?
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein