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Does Flatness or GD&T add to dimensional tolerance stacks? 2

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livloud

livloud

Mechanical
Jul 14, 2015
1
I'm creating an RSS tolerance stack of an assembly and was told I have to include flatness callouts of each surface in my stack, whether it's on the drawing or not. I have never added flatness tolerance to my RSS tolerance stacks before. Is dimensional tolerance inclusive of any GD&T modifiers?
 
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Per ASME:
Flatness comes into play in with the "To or Thru Rule".
[ul]
[li]If the stack goes to a surface, the form of the surface must be accounted for in the stack.[/li]
[li]If the stack goes thru a surface, the form of the surface is automatically included in the limits of the dimension.[/li]
[/ul]

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'.
 
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Are you operating in ISO or ASME world?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
 
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