huangyhg

datums on a weldment
hi all-
when drafting a weldment with several parts on several sheets, how does one organize the datums?
in other words, say you have a square block with a hole in it, and a cylindrical peg to be fitted into the hole and fillet welded around the perimeter. when detailing the welded part, let's say datum a is the bottom of the square block, and datum b is the axis of the cylindrical peg. when detailing the individual piece parts, should the datums be the same? or when detailing the peg, should the axis of the cylinder be "reset" to a, instead of being b.
i hope i'm being clear, as it's a little difficult to explain. this may very well be covered in y14.5, but i don't have it.
thanks in advance.
v
check out our whitepaper library.
apply the datums in the method in which the part assembles in the next operation. your weldment drawing will have different datums than the individual parts that are welded together.
the weldment primary datum is the mounting surface in the next operation. usually a hole is the secondary if the part actually has a hole. the tertiary is either a hole or other feature used to orient the part.
now the parts that go into the weldment will have a different primary, secondary and tertiary. how does the part sit on the weldment assembly. that will be the primary datum. most often the sides of the individual part may butt up against another feature on the assembly and that would be the secondary.
hope you get my drift here.
dave d.
so, basically what you're saying is that, 2 separate datums on the same drawingfff"> (irregardless of what part they belong to) should never have the same letter, correct?
this is what i thought, but i wanted to make sure.
v
i have not seen where this is directly specified, but not repeating datum letters is good practice and helps to eliminate confusion. unfortunately, to follow this practice, i often have to go to double digit datums.
believe it if you need it or leave it if you dare.fff"> - robert hunter
vc66,
this is more of an information point than a suggestion, but here goes.
when your weldment arrives on your loading dock, you do not know how they assembled it, and strictly speaking, you do not care. your inspector will have to deal with a single piece of metal, which requires three datums, as per asme y14.5m-1994.
your drawing should show the final, assembled part, with the critical features located to the three primary datums. when you show separate details of the to-be welded parts, you are merely being helpful. i would leave it to the welder to figure out how to achieve the final, as-assembled tolerances. there may be several ways to do this, and your welder hopefully understands the problem better than you do.
jhg
i would suggest keeping the same datum "name" referencing the same data in all the drawings! but re
you are drawing each piece in the drawing before assembly? why? portray only the end item. to draw each part and worry about how they fit together is irrelevant as long as you get your end item. with all due respect, you are doing more work than necessary and stifling the creativity and ingenuity of the mfgr. you are also driving up your costs because the takeoff estimator will see an eight sheet e-size drawing with many monodetails instead of a two-sheet e-size drawing showing your end-item. although this effect may be purely psychological, the bottom line is you will end up paying more!
tunalover
tunalover-
1. i completely agree. it is not my choice to do it this way.
2. i've had a note put on the drawing basically saying that the components are just for reference, and the dimensions on the weldment are the only driving dimensions.
v
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