【转帖】controlling bow of part in one direction

yang686526

controlling bow of part in one direction
i have a plastic injection molded part that is .120" with +/-.004 and flatness call out on the bottom surface of .010". to me this doesn't make sense. anyway, what i'm trying to do is to control the thickness and the direction of the bowing. i would rather bow in one direction so that it will be flush to the mating surface when assembled. the length is 6 inches and width is say .25". what would be the best way to dimension this and how to measure it. i know that i can't use the part that bows .010" from theoretical straight line at mmc (.124) at assembly.
thanks
hi kinsrow
any chance of a drawing or sketch?
regards
desertfox
  having a tolerance of +/-.004 with a flatness of .010 on one surface does not make sense, as you stated, because the size tolerance has an indirect flatness control of up to .008 depending on the actual size of the feature. this is consistent with general rule #1. if the actual thickness winds up being .124 then can have no error in flatness. if it winds up being .116 then you can have .008 of error. the .010 flatness is not a refinement of tolerance and thus is illegal.
  you may be able to use profile of a surface on the surface that sits flush against whatever it assembles to and unilaterally dispose the tolerance zone to the outside. this will create up to a convex surface when the tolerance zone is used.
  anything i've said really depends on whether i've understood the original question. i'm envisioning a rectangular piece that is 6" long, .25" wide and .124" thick and there may be a counterbored hole on each end. the direction of the bow decides whether it comes out during assembly.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
it depends on whether you are using ansi y14.5 or not.  if you are then the flatness must be a refinement of the overall size tolerance & typically less than have the size tolerance.  if you are using iso 8015 then there is no envelope principal and the dimension tolerance is just a 2 point measurement.  it can look like a banana for all iso 8015 cares & you must use gd&t to control form.
in any case you can use a modifier like "not concave" if it is only permissible do deform in one direction.
"ansi y14.5" or the current asme y14.5m-1994 version.  the op is i believe in the states so i'd guess asme but it's a good point.  
kinsrowfff"> - what standards are you working to as this is a case where it may make a difference.
how about specifiying it in the restrained position and giving the 'clamping' info on the drawing?  see section 6.8 of the current asme standard & fig 6-54.
kenat,
the reason the flatness vs. thickness does not make sense is because you're not putting enough thought into it.  thickness is independent of flatness.  imagine a  cylindrical tube with the same wall thickness and tolerance.  thickness could be perfectly uniform, in, spec, and the plate still be warped.
insterad of flatness, try a bilateral profile tolerance.
it is per asme y14.5m-1994 version. i can't check it when it is clamp because that is exactly what i'm trying to control. if the bow is more than, say, .020" i have to modfity assembly fixture which is a no-no.
i might try profile tolerance like tick and powerhound and see if we have the tools and set up to measure it.
powerhound,
you understood the part i discribed except i've got many lighting holes between and 2 controlled mounting holes ( one rivet and the other is clearance for screw).
thanks  
tick,
  when interpreting a print to asme y14.5m-1994, thickness tolerance and flatness tolerance are related via general rule #1. saying that the thickness can be uniform but the part can be warped is not correct. if the thickness is at mmc then the part cannot be warped at all nor can there be any flatness error. general rule #1, in a nutshell, states perfect form at mmc. straightness and flatness are form controls and thus must be perfect if the feature is produced at mmc.
  you started out describing a cylindrical tube then finished off talking about a plate so i'm not sure i really understood what you were trying to say. maybe i'm addressing a non-issue.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
well, i talked to several people including the senior qc and they all said what tick said. the part can be +/- .004 and can have an additional flatness tolerance of .010". they said that the flatness is reference to it self. this still doesn't make sense to me, but if this is wrong, there are tons of wrong drawings that are released for production. where exactly can i find this info.
thanks

when i used to design military parts, straightness was the most important. flatness sometimes, but because of variable tolerances across the surface, they were usually a tighter or looser fit and not acceptable by the customer.
does this work for you kinsrow?
"where exactly can i find this info."
have you tryed looking in the dimensioning & tolerancing standard, asme y14.5m-1994?
specifically 6.4.2. deals with flatness.
6.4.2.1 states:
quote:
where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance.
i don't know what standard your qc guys are working to but the above sounds pretty definitive to me, unless i'm missing something.
  
kenat,