Straightness Tolerance concave vs convex?

[JLang17]

I have a metal door, just a flat piece with formed edges for rigidity. The prototype had an issue with concave bowing. Concave bowing is not acceptable, but x-amount of convex bowing is ok...I'm not quite sure how to call this out on the drawing. I immediately think of a straightness tolerance, but that would allow both concave and convex bowing within the limitations. Is there a way to use straightness to allow bowing one way and not the other? Or can anyone suggest an alternative way of clarifying this on a drawing?

Thanks!

 

[CheckerHater]

Providing you are in ASME world, you can use Unilateral Profile. That will permit variation only in one direction.

 

[pmarc]

I am not aware of any standard method in ASME's world of GD&T that would help you with this. The nature of bow is not what form or any profile tolerance aim to control. You can go with straightness tolerance (but in that case I would recommend applying two straightness tolerances in two orthogonal directions, as shown in fig. 5-6 in Y14.5-2009 or fig. 6-6 in Y14.5M-1994). You can apply flatness tolerance (as shown in fig. 5-7 in Y14.5-2009 or fig. 6-7 in Y14.5M-1994). You can even try to apply flatness per unit area tolerance (as shown in para. 5.4.2.2 in Y14.5-2009 or 6.4.2.1.1 in Y14.5M-1994) in order to limit abrupt surface variation within relatively small areas. But none of the methods contains direct message that convexity is allowed, while concavity is not. Unilateral profile tolerance won't be able to do this too.

Having said that, my advice would be to associate an additional text note with the chosen form feature control frame explaining design intent as clear as possible.

 

[CheckerHater]

Pmarc,
As far as I can tell, OP was about “bowing”, not about reversals.
Not only bowing can be controlled by unilateral profile, but you can also apply “new” 2009 concept of “non-uniform” profile and simply draw the shape you are trying to control.

 

[pmarc]

CH,
1. I was not thinking about reversals.
2. Even in your graphic (assuming I am reading it correctly), concavity will be possible. The surface will indeed have to lie above the true profile, but it still can contain concave areas inside this tolerance zone. But I agree that this option could be used - it at least assures that the surface won't fall below the true profile, so won't be concave in a classic way.

 

[CheckerHater]

The surface will indeed have to lie above the true profile, but it still can contain concave areas inside this tolerance zone.

This is exactly what they call "reversal"
From my experience with sheet metal having your material bulging in one direction (even with some bumps) is an achievement by itself.
I am glad you agree that using "non-uniform" at least gives good idea of what one is trying to achieve.

 

[JLang17]

Thank you for the replies!

 

[Tenkan]

How ironic, I’ve been lurking on this thread out of general interest (I used to work in precision sheetmetal…) and today I had an engineer come to me with this exact callout problem only on a machined surface. The process is proprietary, but the result was a convex profile that otherwise met the flatness spec. In our case, we decided we can have a concave surface tolerance but with no reversals in the concave profile….

So my question about applying the Y14.5-2009 ‘non-uniform’ unilateral profile callout is can I attach a note to or under the FCF to say “CONCAVE REVERSALS NOT PERMITTED” (or similar… suggestions?)?


PS: I hope my question compliments this thread and not creating a thread drift. If so I will start a new thread but it seems like its very in line with the OP.


lightweight, cheap, strong... pick 2

 

[pmarc]

Koda94,
I think adding a note like that may not suffice.

There was a discussion about controlled radius not so long ago where some interesting comments about flats and reversals were raised, although the thread wasn't too long.

http://eng-tips.com/viewthread.cfm?qid=347084

The bottom line is, the note has to be very precise as to what the CONCAVE REVERSALS really mean to you.

 

[Tenkan]

Thanks Pmarc, It does look like no matter how its called out a detailed note will be required. I'm not certain if a controlled radius could work, ideally we want the surface perfectly flat but anything proud makes it tippy which we are trying to eliminate (tall part...). I was hoping for a minute that somewhere in this new 09 callout was the simple solution...

lightweight, cheap, strong... pick 2

 

[pmarc]

I wasn't implying that controlled radius concept would work in your particular case. I was just trying to say that in case of flats and reversals control (no matter whether it is nominally flat surface or an arc or something else) on has to be quite precise in specifying what is really required (and acceptable) and what is not.

 

[dgallup]

Back to the OP. We have some very flat lapped surfaces that we add the note NOT CONCAVE to the flatness call out. Works for us.

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