Rule #1 Question - Implied Flatness

[sooner1031]

Quick scenario, followed by a question:

A vendor fabricates a large circular panel. It is comprised of two thin aluminum face sheets bonded to aluminum honeycomb core in between. Holes were drilled in this panel, located with positional tolerances. The thickness dimension on the panel is 0.80", +-.1". No explicit flatness is defined. The fabricated part is warped (by a half inch in some places!).

Given that a drawing note indicates to interpret the drawing per ASME Y14.5M-2009;

Does Rule #1 apply to the thickness dimension of this panel (i.e. implied flatness of .2" if the part is built to the nominal dimension)?​

Or, does the "stock item" argument hold any water here, thereby nullifying Rule #1? The face sheets were called out as .020" 2024-T3C aluminum, and the data sheet for the aluminum core only mentions a thickness tolerance of +-.006".
​

I realize that there *should* have been an explicit flatness callout on the drawing to avoid confusion, but unfortunately that's water under the bridge at this point.

 

[BillPSU]

Thickness and flatness are two totally different dimensions. I've had this argument on many occasions. You can buy a sheet of steel and see out of flatness conditions of 3/4" or greater be on the sheet and still meet the material this thickness. You may stand a chance by calling the problem a workmanship problem however the vendor will then point out put the tolerance on the print. Try to use the part as is or add a flattening operation or order new parts which meet your requirements.
Bill

 

[KENAT]

If the drawing has the .80"+-.1" clearly stated on it for the assembled panel then my understanding would be that yes "Rule #1 <does> apply to the thickness dimension of this panel (i.e. implied flatness of .2" if the part is built to the nominal dimension)?"

This assembly dimension/tolerance would override the stock dims of the piece parts.

If you don't need tighter than .020 flatness then you don't actually need to add separate flatness tol. However, because places frequently overlook the application of rule 1 to part thickness it can be a good idea to add it as a preventative measure.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[3DDave]

If the finished part is given a finished thickness dimension then that controls the flatness/straightness. You should not need a flatness control unless +/-.1 was larger than required.

The 'stock' tolerance is to avoid substituting the title block or note dimension tolerance and control for an existing one which may be better or worse.

 

[Belanger]

Agree with the others. But just to clarify, the implied flatness tolerance would NOT be .2" if the part is built to the nominal dimension. The nominal dimension is .80, which only affords .1 warpage. (Only a part at the least material condition would get the full .2 for flatness purposes.)

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

 

[sooner1031]

Thanks for all the responses. Much appreciated!