【转帖】flatness requirement and callout questions

yang686526

flatness requirement and callout questions
i designed a plate in which some areas inside are hollow.  the plate is 31.0" x 22.5".  it is made of two .100" thick aluminum alloy 6061 plates vacuum brazed together; the overall thickness i specified is .200+/-.005".  
the flatness i specified on both surfaces is as follows:
unrestrained condition
.025" global
.002" per 1.00 x 1.00 local
restrained condition
.010 global
my question is: does the flatness requirement make sense?
hwl
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is the flatness called out on each piece or after they are brazed together?
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
your +/-.005 size tolerance is at the limit of mil (stock)tolerances for the 2 sandwiched sheets, allowing no room for machining to hold the specified flatness, thus distortion due to braze heating and braze material thickness must be accommodated within your parameters. a welding expert could answer this better as to amount of possible distortion, but flatness is normally considered to be within the size tolerance and perfect form at mmc. it appears your not thinking about asme y14.5 form limits, but that is what i am referring to.
chris,
my drawing only shows the brazed assembly and have the flatness callout on both sides.  i don't have separate drawings for the individual piece.
hwl
i did specify the flatness within the size tolerance .010", but it is under restrained condition.  did i do it right?
did you start with mil stock finish or ground plate stock?  unless the plate stock had some preprocess you might have trouble holding that local flatness callout after brazing.
best regards,
heckler
sr. mechanical engineer
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heckler,
   by the sound of it, he wants it to be hollow, sealed and flat.  if he specifies the flatness after all fabrication, then the fabricator will have to figure something out.  yet another problem is that i do not think there is a .100" mill size.  the nearest one is .102" (10 gauge).
   if the fabricator works with two pieces of .125" sheet, they can meet thickness specifications and flatness by milling off both faces.  
   we should specify what we want, not how it is made.
leung2005,
   when i do crazy stuff like this, i take a close look at all my tolerances.  if you back off your thickness tolerance a little, it might make it easier to hit your flatness.  at least, it shows the fabricator what your priorities are.  
   you are specifying the flatness for both unrestrained and restrained condition.  i do not know what you are doing with this, but it seems to me it functions in one state or the other.  the tolerances only matter in that state.  read section 6.8 in asme y14.5m-1994.  this very topic is discussed.
   otherwise, your flatness specification makes sense to me.
                          jhg
the manufacturer will start with two thick plates, machine to make pockets on each plate and braze them together, then machine the outside surfaces to make the final product .200+/-.005" thick.
this plate will be attached to an electronic assembly using screws, ~70 of them.  the complete assembly will be slided into a slot.  under the worst condition, the air gap between the plate to the wall of slot is only .025". i have been thinking the flatness requirement under the non-restrained condition is not really important and necessary.  what i care is the flatness under restrained condition and the plate won't hit the wall. right?
hwl
leung2005,
   it appears to me that your requirement is to maintain an air gap of .025".  is there a flatness requirement in addition to this?
   if you specify an overall thickness tolerance of +/-.005" and you clamp your part to something flat, you have a flatness tolerance of .01".  you should not specify flatness unless you have a requirement different from .01".
   this could go either way.  a flatness specification of .001" says you are not absolutely concerned about the thickness, you need it flat.  the fabricator can make another machining pass if necessary.  
  a flatness of .05" in the free state says that you need the thickness, and that you can tolerate waviness.  this may be what you require.  a thickness of .200" is not very rigid.  if your part is attached with ~70 screws to something flat to .005" and rigid, it will flatten out to within .015".

                     jhg