flatness tolerances for "stock" material

[RoboBill]

Several people at my company always put flatness tolerance callouts on faces that are indicated as being "stock" dimensional thickness and then use them as a datum, typically Datum A. We're not talking sheet metal here either, we're talking stock anywhere from 1/4" plate to 1" bar stock. I always wonder if that's "legal". It seems that we're telling the machinist to use the stock material, but ensure that it meets some flatness tolerance. In some cases the flatness is specc'ed as 0.2mm for 1/4" thick aluminum plate over a distance of a meter! Other times they call out flatness of 0.2mm for a 3/4" stock thickness piece only a couple of inches long and call that out as Datum A. Is it right to call out a flatness tolerance on material listed as "stock".

Thanks,
Courtmann

 

[MintJulep]

This thread286-330445 may interest you.

 

[RoboBill]

Thanks for the redirect, MintJulep. This thread seems mostly concerned with tolerancing the dimension specifically rather than flatness. After further search I'm wondering if using the flatness tolerance with the free state modifier might be appropriate (at least for the larger 1/4" to 1/2" plate pieces). It still seems somewhat ridiculous to me to flatness tolerance finished pieces only a few inches long made from 3/4" stock with "stock" called out for the thickness.
Any other responses on the appropriateness of flatness and perpendicularity tolerancing finished pieces largely using stock dimensioned material would be appreciated.

 

[Belanger]

Well, Y14.5 says that Rule #1 doesn't apply to stock sizes, because it's understood that stock materials already have other commercial standards governing the form. So I would say that a flatness spec is not required for a stock size, but to add one is not wrong; it's just being more specific.

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

http://www.gdtseminars.com

 

[Belanger]

On second thought, Rule #1 refers to an automatic form control on a feature of size. You are asking about form (flatness) on a surface, so my previous post isn't all that relevant. But the conclusion is the same: flatness on a surface of a stock material is perfectly fine.

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

http://www.gdtseminars.com

 

[ctopher]

It's pretty rare I receive flat stock that is flat. Unless you order it with a specific flatness, expect to machine it to your flatness.

Chris
SolidWorks 11
ctopher's home
SolidWorks Legion

 

[RoboBill]

Well I guess my question really comes down to just what am I telling a machinist/fabricator when I apply a flatness tolerance (or straightness or perpendicularity tolerance, for that matter) to a surface that is also dimensioned as being "stock"? It seems like I should have one or the other but not both. Should the stock dimension be given as reference?
Thanks

 

[Belanger]

Stock size has a tolerance by default. Is the flatness that you wish to apply to the surface a smaller tolerance?

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

http://www.gdtseminars.com

 

[RoboBill]

That's just it, Belanger, I DON'T want to apply a flatness tolerance. I think it's contradictory to specify that something is both "Stock" thickness and to apply a flatness tolerance to it. It's that it seems to be standard practice at my company that makes me ask the question. As I said in my first message sometimes the flatness tolerance they call out is insane, like flat to 0.2mm on a 1/4" aluminum plate 800mm long which is going to be welded into an assembly. Then they'll call out that "flat" face as a datum. Similarly, they'll apply a 0.2mm flatness tolerance on the flat face of a piece only a few inches long with "stock" 3/4" thickness called out. My point is, are you asking your machinist to verify that every piece of 3/4" stock he uses is flat to that tolerance and also that it is within the thickness standards for whatever association makes the standards of form for that particular stock material? Otherwise how can you justify putting both pieces of information on the drawing?

On the drawings I'm doing/correcting I've started calling out flatness of 1mm in the free state for the longer 1/4" plate pieces and eliminating flatness tolerance callouts entirely where "stock" thickness callouts also exist.

Thanks,

 

[SDETERS]

Can you use three equally spaced out Datums pads on the surface? Then the control those three pads with a surface profile. Then let the rest of the surface have the stock flatness to it? Would these help your situation?