huangyhg

flatness/parallelism question
hi all-
i have what might be a simple question that i can't wrap my head around. i'm working to y14.5-1994.
if i have a square block of metal, and i state that i want one surface to be datum a and be flat within, say, .005". the i have the opposite surface that i want to put a parallel callout on. can the parallelism be tighter than the flatness?
my immediate inclination is to say no, but i'm not positive. please let me know what is correct, and the reasoning behind it.
thanks for the help.

v
check out our whitepaper library.
does this help?
perfect, ctopher!
thanks for the help.
v
ctopher
the web site you suggest just doesn't seem right. it states that flatness is created from a 3 point set up (yes i agree) but then it states that this is the mid plane which is not correct. it goes on to state that a 0.25 flatness is 0.125 on either side of the 3 point set up which is not correct.
flatness is a unilateral tolerance in which the surface must not exceed using a 3 point set up to create a plane. it can also be performed on a cmm with a scan mode using a best fit condition.
vc66
i cannot find anything in the standard that states that the flatness tolerance must be less than the parallelism but it does make sense.
once we have a flatness requirement that is met, we then have a datum that is assumed perfect. from a 3 point set up on the datum surface we can check the opposite side for parallelism.
theoretically, we should be able to place the datum surface on a flat plane simulating assembly to check the parallelism but if the surface was convex at all, we could not check the parallelism. the part would rock.
dave d.
this or similar was posted a while back, i think i posted some information from 14.5 at that time. i can't find it now but take a look, i think it will answer your question.
if i recall in the previous question the datum a surface didn't even have a flatness call out. the other side had a parallellism to datum a. looking at 14.5 6.6.3 7 figure 6-30 this was a perfectly legit callout.
plane a is derived from the corresponding surface but is not the same as it. the parrallellism is: "a tolerance zone defined by two parallel planes parallel to a datum plane or axis, within which the surface or center plane of the considered feature must lie".
so i would say that yes the parallelism to datum a can be tighter than the flatness of datum a. if this in practice achieves what you are trying to get is another matter.
kenat, probably the least qualified checker you'll ever meet...
sorry, not "flatness of datum a" - flatness of the surface corresponding to datum a
kenat, probably the least qualified checker you'll ever meet...
i agree with kenat. the parallelism tolerance can be smaller than the flatness tolerance on the datum feature.
the danger is that increased flatness error on the datum feature leads to increased potential for convex shape and rocking. if the simulator (surface plate) rocks relative to the convex datum feature, the parallelism tolerance zone rocks right along with it.
evan janeshewski
axymetrix quality engineering inc.
this is what i figured. it's not a design that i've created, but an old design that i'm updating.
thanks for all the replies.
v
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