【转帖】rue position tolerance to a face

yang686526

true position tolerance to a face
hello folks
hope i'm ok posting same question in multiple forums, i'm new to this one and was recommended to post here rather than my original choice.
ok, my question-
we have shiny new designer in the company, who is producing drawings with faces toleranced by true position back to other datums/planes. i'm perfectly familiar with true position of holes, but i've never come across this style of tolerancing a face. i don't understand how a face can be toleranced in this way, am i thick, or is my new designer nuts?
i hope that makes sense.
here is a sketch (if i've managed to load it correctly) where there are 2 faces toleranced as true position. one is the end face of a valve feature, where there is a threaded cap assembled up to the face, and the other is a milled face on the right hand side to accommodate the two ports drilled through. both are toleranced back to a fictional plane. does that make sense in this context?
cheers
positional tolerances are only on features of size such as holes and pins and not surfaces. this drawing in not drawn according to asme y14.5m-94 and is wrong.
the designer should use the geometrical symbol profile of a surface on faces rather than positional.
good luck!
dave d.
the position tolerances on the surfaces are incorrect. a profile tolerance needs to be used to locate those surfaces.
david strole
engineering systems administrator
gdtp s-0132
thanks guys, that's what i thought, but i'm just a humble machine jockey, i just like to get my facts straight before arguing with the grads.
appreciate the advice.
i belieive you might have other problems too.  i don't believe the std provides for datum identifiers to be attached to cl.
i could be wrong, but i don't believe it's acceptable to use a positional tolerance on any feature that cannot be captured at least 180° around, i.e. half circle or slot.
v
ringman:
you are correct here. datums are developed through features that create the c/l rather than the c/l itself. apparently, the iso standard allows a centre line to become a datum though.
dave d.
iso 1101-1983(e) uses position in that fashion.
is the document stated to be asme y14.5m-1994 complient?

"is the document stated to be asme y14.5m-1994 complient?"
if you mean the drawing i'm working to, then no. we design/develop and manufacture our own products complete, so this is a drawing from my design friend 20yds further down the office. i will educate him on his drawing style, but i just wanted some expert opinion on this specific issue, as it was a new one to my dodgy eyes. as you've already noticed on that small section i showed, there are many other issues too. the fact that he has 14 datums specified on this product, being one of them. features being toleranced back to 3 or sometimes 4 other features, needlessly, being another. nightmare for the poor guy who has to try and create cmm prog to measure it. (we are still in the dark ages there, no model import function for us, all manual cmm programming).
i think you've all answered my question admirably, thank you again for your time and experience.
cheers
if the drawing isn't required to be y14.5 compliant, and lacking any company standard addressing this issue, then i think you just out of luck.  if there is no standard, how can you accuse him of not following it?
i completely agree with the others though that it is not drawn per y14.5.
believe it if you need it or leave it if you dare.fff"> - robert hunter

"iso 1101-1983(e) uses position in that fashion.
is the document stated to be asme y14.5m-1994 complient?"

errr, maybe it might have helped if i'd thought to mention (doh!), but i'm in the uk, so we will be working to bs or iso. i gather that makes a significant difference.
oops!