datum target -- help?
datum target -- help?
(OP)
ASME Y14.5 did not tell how to dimension the datum targets. i tried to tell people it should be dimensioned relative to each other. but they continue to dimension it from part features. would anyone know where to find such guideline except "datum targets are ... dimensioned relative to each other" - drawing requirements manual for departments of defense and commerce, 3rd edition.
thanks.
thanks.





RE: datum target -- help?
Y14.5-1994 is vague on this question.
Believe it if you need it or leave it if you dare. - Robert Hunter
RE: datum target -- help?
Some designers reuse secondary or tertiary datum targets in subsequent processed operations thinking that the registry is equivalent and disregarding the surface's form effects in their stacks but that mistake can bite you if those actual errors are significant.
Bottom line... Define the targets amongst themselves...tolerance the processed features from them...and then "as functionally reflective" create "tolerance" other functional datum features.
Paul
RE: datum target -- help?
Were you saying that he is right and "they" are wrong? If so then I would agree that the datum targets should be relative to each other and not based on the location of any features that the part may have.
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RE: datum target -- help?
Simple example: A rectangular shaped casting.
Datum Targets A1, A2, A3 are all on Datum –A- (shown in a view perpendicular to –A-),
they and are dimensioned from –B- & -C- (shown in views perpendicular to –B- & -C-).
NOTE: All Datum Target dimensions will be BASIC (boxed dimensions with no tolerance).
Datum Targets B4 & B5 are on –B- and dimensioned from –A- & -C- as noted above.
Datum Target C6 is on –C- and dimensioned from –A- & -B- as noted above.
Complex example: A casting with 3D contour and no flat planes.
Actually the same process, except Datum Target Points will also have a dimension from the Datum they simulate.
Datum Targets A1, A2, A3 are all at different distances from Datum –A-, the dimensions will be shown in views perpendicular to –A-, -B- and –C-.
The same process is used for B4, B5 & C6.
The Datum Target Points are then used by manufacturing & inspection for the production of the product.
RE: datum target -- help?
this is the part "they" assigned all datum targets. the problems here ARE 1) they missed Z dimension for TP2A AND TP1C. 2) TP1A, TP2A, TP3A and TP1C are going to "define" datum plane A. (see attachment)
1) yes. the Z can be calcualted from the norminal dimensions. but it should not.
2) a physical part "sits" on the fixture will never touch those four point as the same time. only three.
comments?
RE: datum target -- help?
If you do indeed have a planar surface for datum A, why use datum points to redefine it as shown?
Believe it if you need it or leave it if you dare. - Robert Hunter
RE: datum target -- help?
thanks.
RE: datum target -- help?
RE: datum target -- help?
I could be easily wrong though (and have been) as I haven't used stepped datums such as this before.
It seems to be a very awkward datum setup.
Believe it if you need it or leave it if you dare. - Robert Hunter
RE: datum target -- help?
Small changes:
1. All Datum Points are dim'ed with BASIC box (no tolerance).
2. Added other dim's for complete definition of part.
Datum changes: (There are a ton of different ways, and the product desired end result drives the requirements, but, try to keep it simple, and, more importantly, easy to produce & inspect.)
1. Put A1 & A2 on same plane. Put A3 on center of shaft.
3 pt's will always align.
2. Put C1 @ 90 degrees to -A- on center of shaft.
Reason: Datum Target points hold a product in relation to the Datum Reference Frame. Try to make the points as strong in the degree/vector of the datum as possible, I.E.; the example shows A2, A3 & C1 at less than 45 degree's to each other on two different surfaces. The odds are that those three points will not all make contact because we are already resting on A1 (4 points strong in the -A- vector). C1 is not strong in the -C- vector, so it will not control -C- location as strong as we would like.
RE: datum target -- help?
as you said there are tons of different ways, it is up to "them" to define the datum targets. if the cylinderical part is more important, "they" can assign datum A to it. (e.g. ASME Y14.5 Fig 4.34)
in my view, "their" datum targets are not just improper, but WRONG. to persuade "them" to make change, i like to have supporting material to my view. or tell me that i am totally wrong.
thanks.