【转帖】straightnesflatness requiremen

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straightnes/flatness requirement.
we have a part, which is the shape of a large washer.  its a steel disc with a sintered material (9 in. od x 6.5 in. id x .575±.015 thickness).  the disc is then cut into multiple sections of equal size.  the manufacturer is having difficultly maintaining the default straightness of ±.015 (rule #1 of gd&t).  we don't care how warped the disc becomes, as long as the straightness/flatness within a 4 in. area is maintained.
any suggestions on an appropriate gd&t callout for the drawing to maintain what is required.
thanks,
rich     
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if it's the surface you are concerned with, you should use flattness, and define the area.  since you are concerned with only a portion of that pie, look in asme y14.5m-1994, section 6.4.2.1.1
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i would create something that looks like a pcb. one piece of mtl that has dotted lines for ref with the flatness callout note indicating each section.
or, does it have to be cut from a 'washer' shape? why not have the dwg show the final part and let the vendor cut it to the shape from whatever stock they please?
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
runz,
  do you mean they are chopped up into pie slices?
                          jhg
i'm with ctopher on this one.  send the vendor (or shop floor) a print of the actually part you want in the end.  put the flatness callout on that drawing & don't worry about what they need to do to get there.
gk
i appreciate all the information, but i am still unsure if any of this solves my problem.  the manufacurer makes the washer shaped component, then we cut the parts in house to the desire size (pie shaped).  the problem is that the manufacturer can not make them to print (default straightness of ±.015).  it is actually not the surface itself, but just the warping of the part.
i previously looked at asme y14.5m-1994, section 6.4.2.1.1 as madmango mentioned, but i am not sure that solves the problem.  per the specification, does the part not have to be straight within ±.015.  we can allow as much as they need (warp), so long as they can control the straightness or flatness to within ±.015 in every 4 in. length.  the only way to override the default straightness is to place a straightness on the fos or add a note "perfect form at mmc not required".  i am no gd&t expert, but if appears that section 6.4.2.1.1 of the spec, would not allow for addition straightness or flatness deviation, which the manufacturer requires..
i need to allow the plate (washer shaped) to deviate like fig. 6-5 if section 6.4.1.1.4.  if i add a straightness callout of ".015 / 4.00" to the thickness dimension, would that do what i am trying to accomplish?
thanks,
rich
you could call out flatness per unit area.  you could also specify zones to which flatness applies.
thetick,
does the entire part still not have to maintain a straightness of ±.015 by default?  flatness callouts in zones sounds good, but if the part is straight within ±.015, that would be permissible per our requirements.  but like i mentioned, this is not possible by the manufacturer.
the parts seems to warp from one end to the other.  so long as the change is not drastic within a 4 in. length, the warping is of no concern.
if a centerplane or surface has a straightness callout, is that straightness implied in all directions?
sorry for all the questions and comments, but i am just trying to understand.
thanks,
i think i would call out only one.
parallelism (top surface to btm plane datum a), flatness, or straightness (point to surface in one front view with a tol, and point to same surface in right view with a tol...2 direction tol).
a call out of straightness and flatness may be too hard or confusing for a sheetmetal part.
just a thought.
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
here is my last attempt to describe the problem.  the parts in question are similar to a brake lining (steel ring with a sintered material added).  the total thickness has to be held to ±.0075.  we don't care if the parts come in looking like a bowl, so long as the change in straightness or flatness does not deviate more then .015 over a 4 inch length.  we receive these sintered rings and then segment them for other uses.  the maximum size is 4 inches, hence the 4 in. length in the straightness callout.  we also need to maintain the .015 straightness for proper application of the segments, but not .015 over the entire length of the unsegmented part.  
so in short, we need a large tolerance window over the length of the parts, but a smaller one within any 4 inch segment (while maintaining the ±.0075 thickness tolerance).
i am not sure if i am conveying this info in an easlier understandable manner.
thanks for all the comments... its really appreciated.
rich
runz,
   what actually is the problem here?  is the manufacturer unable to meet your flatness specifications, or is he unable to understand your drawings?
   as per asme y14.5m-1994, you can specify straightness as an absolute value, or per length.  see section 6.4.1.1.4.  in your case, you would specify .015/4.
   flatness may be specified per area -- .015/4x4. see section 6.4.2.1.1.
                        jhg