Correct Application of Reference Dimension?

[ModulusCT]

Good morning, gentlemen (hah, gentlemen ;-) )...

Simple question. I told my boss yesterday that I didn't believe the R.030 dimension on my drawing should be a reference dim. My reasoning was that the radius could really be any value and therefore it was wrong to infer that because the wall thickness is .010 and the internal radii is stated as being .020 that the resultant outside radii must be .030.

However, now I'm having second thoughts. I can see it both ways... I'm wondering though, if some kind of additional note or GD&T callout (profile?) is necessary to indicate that the wall thickness stated on the drawing is constant throughout the part. Any ideas?

I've attached a jpg of the view in question... It's a domed lid for an enclosure. It is pressed or otherwise formed from a single piece of uniform thickness.

My bosses reasoning for wanting the reference dimension was because he said that we don't care what the radius is... We want them to shoot for .020, but we don't want anyone to inspect to that value and if it's higher or lower, we're OK with that... gah.

For the record, I think the correct way to handle this is to either mark the dimension simply as "R" to indicate radius but leave the size value out of the situation entirely, or mark it as R.030±.030 which basically states that we'll accept any value from zero all the way up to twice the stated nominal size.

Anyway, thanks for the input! I'm sorry to show up here at these forums and flood the place with questions... lol Well, that's what it's for, right?



I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[ewh]

I'm with your boss on this one.
"R" by itself is often interpreted as "FULL R", which is meaningless here.
"R.030±.030" is still an inspectable dimension.

Technically, the glass is always full.

 

[ModulusCT]

I'm with your boss on this one.

OK, cool... So it IS understood then, that the thickness of .010 is constant throughout the part?

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[dgallup]

Reference dimensions are there only for information. You could measure it just to see where you are but there is no pass fail criteria. This is an excellent place for a reference dimension. Note that you have another radius on the other side of the R.002+.003/-.002 that is not called out at all. You can add a reference dimension there or not, it makes no difference.

Since this part appears to be a stamping with a coining operation to make the thinner flange, you can not assume that the material will be uniformly .010+/-.001. There will likely be some local thinning in places. It is up to you to determine the function of the part and what you are willing to accept.

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[KENAT]

Not necessarily the way you're showing it. You may be able to clarify with an 'all around' or 'wall thickness' note though on the .010 dimension. I don't think 'all around' really works because you relieve the bottom face so 'wall thickness' might be better.

As is you aren't explicitly controlling the material thickness anywhere except at the top.

I assume some other missing information is on other views - though will bring to your attention that the .226 dimension has no centering dimension/tolerance on your drawing as presented.

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[ModulusCT]

Kenat, can you tell me what the correct wording would be for that wall thickness note? Would it just be .010±.001 WALL THICKNESS?

You couldn't have known this, but all dimensions are controlled by the title block tolerance note, u.o.s. - I guess the .236 dim should be controlled with another dim from the outer most surface to the left extension line for the .236 dim, otherwise, you're right, it could shift all over the place just as long as it's .236±.005. That would be bad.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[KENAT]

I'm not gonna claim it's super robust or explicitly in the standard etc. but yeah I was thinking ".010±.001 WALL THICKNESS" would be the minimum to clarify your need.

As to your .236 dimension, you could also use position to locate it if appropriate. In this case you'd need an appropriate datum structure to tie it back to.

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[ModulusCT]

Please look at the attached image.

If I use centerlines the way I have in these drawing views, does that locate the .226 dimension, to title block tolerances, to the center of the part?

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[KENAT]

No, just showing center lines doesn't define how far off nominally centered the features can be.

Assuming you're still to ASME Y14.5M-1994 take a look at section 2.7.3.

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[ModulusCT]

ok, this is what I decided on... Pretty simple I guess.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[KENAT]

So long as your tolerance allows it that works.

(Might want to check on the double duty arrows though, that may be a no-no)

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[ModulusCT]

Double duty arrows? I don't know what you mean... Are you referring to having the .015 dimension and the .026 dim overlapping?

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!