【转帖】recommend a gd

yang686526

recommend a gd&t
so we have this part fabricated from 7075-t6 aluminum.  it a 4"x6" flat plate.  because of the all the machining the part becomes warped.  so on the detail drawing i want to specify a gd&t.  i've read through numerous books and it seems that a straightness tolerance is the one to useo rpossibly a flatness.   
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flatness
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

yes flatness, not straightness to cover the whole surface.
so can you guys give me more of an explaination as to why flatness and not straightness?   
straightness is 2-dimensional, like a beam over a certain length.  flatness is 3-dimensional, like the surface of a plate.
straightness will only control "flatness" in one dimension.  you can have an extruded wave that will meet a straightness requirement.
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

gbor and ewh just explained it well.
read pages 157-161 of y14.5m-1994 to see the differences---single individual elements for straightness vs. all elements (whole surface)for flatness.
you can also call out flatness on certain areas and not the whole part if needed, to make machining easier.
chris
solidworks/pdmworks 08 3.1
autocad 06/08
londonderry,
the flatness or straightness call-out depends on the amount or warpage you are can allow and the material thickness tolerance. re  
jv1,
i disagree,
rule #1 does not apply to flatness since it is a surface form control... (at least it does not yet with asme y14.5m-1994 but possibly with asme y14.5m-2xxx if flatness (m) is permitted to apply to a fos "thickness").
straightness although can be applied to a planar fos in the current asme standard by specifing it under the size callout. you are correct that it is a constant value if no (m) is specified in the tolerance but it is a variable value if the (m) is specified.
in your examples rule # 1 does apply to size so a 0.25 +/- 0.01 thickness would be limited to 0.26 "perfect form at mmc" and flatness constrained to its limit on the specified surface(s).
straightness (m) is not constrained by rule #1 therefore  if the straightness was applied to the size callout and thickness 0.25 +/- 0.01 and straightness tolerance was 0.03(m) then the overall effctive thickness would be 0.25 +0.01 +0.03 which is 0.29.
with straightness applied to the size without (m) the overall effctive thickness would be limited to +0.03 above the actual maximum thickness.
paul
  
  
flatness is a surface condition and does apply in this situation according to asme y14.5m-94. rule 1 is not applicable to a surface but only to a feature of size.
i would suggest, like the others, that you apply a flatness tolerance on the surface.
dave d.