parallelism versus flatness
 

parallelism versus flatness
i have read the gdt book which outlines examples on parallelism and flatness tolerancing and how one orientation and the other is form type of tolerance. can somebody help me interpret the attached jpeg example? i came across this example where parallelism was called out on the datum surface itself. my thought was - can this be converted into a flatness spec without hurting how the drawing will be interpreted?
thanks.
it is wrong! instead of converting it find out what is functionally necessary to constrain the geometry of the part and specify the necessary controls and dimensional tolerances for that purpose.
paul
if you want flatness, call out flatness.
reference "asme y14.5".
chris
solidworks 08, catia v5
"can this be converted into a flatness spec without hurting how the drawing will be interpreted"
no.
as above, it's just plain wrong/meaningless.
kenat,
maheshh
the parallelsim callout should be on the top feature, it can't be on the same side with the datum.
seasonlee
the example is wrong and one cannot replace the parallelism tolerance with flatness referencing datum a.

dave d.
on the upside, it would be very easy for a machinist to hold datum a parallel with itself.
seasonlee,
absolutly. the fcf can not call the // sym that featur in relation to datum a, as that surface is the datum. the symbol can be used on the top, but if the purpos was to define the profile, then the profile-of-a-surface shall be utalized.
v/r
nathan
cad technician/iso director
nathan
profile datum rule : a profile control should not be applied to the surfaces it references as datum feature.
this will avoid profile specifications that are confusing since you don't know how to interpret the profile tolerance zones that with datum.
the datum symbol on this case should always on top side no matter it's a parallelism or profile callout here.
seasonlee

"profile datum rule : a profile control should not be applied to the surfaces it references as datum feature.
this will avoid profile specifications that are confusing since you don't know how to interpret the profile tolerance zones that with datum."
seasonlee,
that is a good rule of thumb, but it can be done. see y14.5 fig.6-21. they call a poas for both "a" & "b", then they use thos two surfaces to call a poas on the middle surface plane. in this case, you are stating datum "a" in it's simulated manor. nevertheless, i would agree, that it's still a wise rule to follow, unless perchance the situation calls for it directly. even then, thos situations few and far between.
v/r
nathan
cad technician/iso director
seasonlee,
i almost forgot, he would need to remove the referance to datum "a" tho. as it is still datum plane a.

v/r
nathan
cad technician/iso director