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RSS Calculation including Flatness in Sheet metal Chassis

RSS Calculation including Flatness in Sheet metal Chassis

(OP)
I'm copying from a closed thread that ALMOST answers my question perfectly...

"weavedreamer (Automotive)14 Jul 15 13:10
Per ASME:
Flatness comes into play in with the "To or Thru Rule".
1) If the stack goes to a surface, the form of the surface must be accounted for in the stack.
2) If the stack goes thru a surface, the form of the surface is automatically included in the limits of the dimension.
Think of two flat washers held together. If you can fit a feeler gage between them, you are checking surface to surface. Flatness would control the amount of gap.

If you set a micrometer to the outsides of the two washers, the flatness would already be included in the dimensional limits of the two thickness'."

I have a sheet-metal chassis that is pretty simple. End caps are roughly 11" deep by 24" tall. Front and Rear Brackets are all to be welded to End Caps. All of the parts are 11 gauge material (.119" thick). We are having a weld fixture built so all the parts are set in place and tacked together, then welded at key interior locations to finish the structure. I envision the weld fixture as my "Micrometer" as described in the above "To or Thru Rule". The flatness of the End caps influences the fit between the two limits of the weld fixture, so I'm interpreting my stack up approach as a "2) Thru" scenario. The question is what is literally meant by "the form of the surface is automatically included in the limits of the dimension".. I'm using an RSS method of tolerance analysis. My stack up is:
a) End Cap Material Thickness + Tolerance
b) End Cap Flatness
c) Length + Tolerance of Front (or Rear) Bracket
b) End Cap Flatness
a) End Cap Material Thickness + Tolerance

My nominal overall length calculation is Material thickness (2x) + Length of Front or Rear Bracket
My RSS calculated tolerance for the assembly is where things get sticky. Without flatness, I just RSS Tolerances from a) + c) + a)... does it make sense to include the flatness b) 2x?

Thanks

RE: RSS Calculation including Flatness in Sheet metal Chassis

The cited explanation was for flatness as applied to a feature.
The 2009 standard allows for application of flatness to a feature of size.

In a feature of size application, flatness could affect form. The flatness tolerance(s) would be added to the limit dimension(s).

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