huangyhg

datum planes
hello to everyone!
i apologize for the question....but i have never understood this issue
if i have a flatness surface (datum a) e the i use two holes for datum b and datum c, which mutually normal planes i must consider?
could you help me, please?
thanks in advance
luke
can you tell us the sequence in the fcf? that will be the determining factor.
perhaps a sketch of your problem.

usually the primary datum is a plane and most often is called datum a. if another hole is called datum b and a third hole is called datum c, here is how it probably is referenced in the feature control frame - a|b|c.
if that is the case, then datum a is a plane and datum b is the intersecting point of 2 perpendicular planes while datum c only orients the part. now we have 3 mutually perpendicular planes - one created from datum a and 2 more from datum b.
hope this helps.
dave d.
there is a figure showing exactly dingy2 example in asme y14.5m-1994. i don't ahve my copy but take a look. i think it's probably in section 3 but i could be wrong.
kenat,
thanks to everyone!
last week i started to study the asme y14.5.m-19 and, with the help of this forum, i have understood the main issue.
now, when i have a drawing with three datum planes it's clear the meaning but, when i have a different drawing it's not clear the meaning for me.....
can i have always three mutually perpendicular planes also when in the drawing i have, for instance, a flatness plane (datum plane a) and two holes (datum b and datum c)?
also three holes can "fix" my part in the space but which planes i must consider like "datum planes"?
in attachment a drawing of the situation.
luke
the datum is the plan or center line that you fixture to when inspecting the dim. it is ok to have commonly perp datums a long as there is a reason. however, in the case of your drawing, i'm inclined to suggest against it. if those holes are related, then they should be judged by the same process.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i referenced the wrong section, though i would have hoped you could have found it looking through 14.5. figures 4-8 & 4-9 and more importantly the relevant paragraphs (especially 4.4.3) explain what i understand your original question to be.
on your sketch though, the holes aren't perpendicular to your primary 'a' datum which changes things. in fact, as set up i don't believe your datum structure works. you'd have to look at it again based on function to try and determine what's really required.
kenat,
i think that i have just a little confusion because i have never understood what a datum plane rappresents. are they determinated by the function or by the inspection method?
which datum planes (secondary and thirtiary) i can use if i have a surface with only a little flatness face?
in addition.....(i don't have a big practical experience about it)....for every part i produce should i have a specific tool that simulate the datum planes re-called in the drawing part?
is also for this reason that is better to have three mutually perpendicular planes?(reduce cost linked to a lower number of tools)
thanks
luke
the primary datum plant on a noncylindrical part is its mounting surface when assembled. the secondary and tertiary datums are usually holes since they are more important to the assembly than sides. the secondary hole should be the first operation in the assembly. the part mounts on datum a and a bolt or shaft is next installed in a hole. that hole will be the secondary datum. all dimensions will come from that hole since it is the intersecting point of 2 perpendicular planes. the tertiary datum is probably another hole used in assembly and it only orients the part.
if you part does not have holes used for assembly, then the primary datum is the largest surface while the secondary datum will be the longest side and the tertiary datum will be the end of the part.
dave d.
dingy2,
a minor clarification, if i may, to your previous. two planes will intersect in a line and not a point.
however, the line (secondary datum) is the axis of the hole thru which a set of mutually perpendicular planes may be passed.
ringster:
right on - i stand corrected since it is an an "axis" per 4.4.2 page 55 rather than a point.
dave d.

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