【转帖】weldment gdt guidance

yang686526

weldment gd&t guidance
we are in the process of starting up gd&t implementation at my company. i am curious as to how detailed we should be as we begin using gd&t on large weldments. there are lots of small blocks welded onto plates as well as mating surfaces with bolt holes, welded on fabricated brackets with peviously machined details. what level of gd&t would you who have experience reccommend? should we use gd&t on the drawings of simple blocks details? some are no more than a rectangular block, some are a block with one or two tapped holes. should we only use it on critical mating surfaces? should we do all mating details? are there some guidlines?
i arrived here last year, and am trying to bring the company into the twentieth century. we have lots of poor drawings and every assembler has a grinder.   
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all parts should have gd&t based on the design and how the parts fit together. you need to be as detailed as possible to make sure of proper fit.
the guideline to follow is asme y14.5m-1994.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
ctopher and john,

i believe the application of gd and t, (y14.5) on weldment
drawings should be approached with caution.  if interchangeability is an issue, then the application is more valid.  otherwise it would seem of limited value on a 'typical' weldment drawing.
'weldment drawings' are limited with gd&t, the parts themselves are not. the weldment usually rely on associated tooling, which have their own drawings with gd&t applied.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
chris and ringster,
thanks for your input. unfortunately, at this time the company does not own a copy of asme y-14.5.
i should be able to get the new standard soon, so i am waiting for that. in the mean time, i've been using a very good textbook by james meadows to bring myself up to date, in preparation for teaching the design and drafting staff here. meadows does not, however address weldments in this book. should weld-on details be located using positional tolerance, or is it permissible to use toleranced dimensions? when i have two opposing planes, one of which is a datum and the other a face of a weld-on block, is that a feature of size? what does asme y-14.5 have to say on the subject?   
there is no difference between a welded part than a machined part. after welding it becomes one part.
johnj52,
a weldment is an inseparable assembly. the specification of the assembly typically details the type of (weld, bond, press, over-mold, rivet, etc.), its location, its boundaries "if critical", and its properties "integrity, performance criteria, etc."  the specification also typically includes some obvious specifications for measurement of alignment and location of one detail to another. individual details are typically fully dimensioned and toleranced on their separate specifications and are assumed to retain that dimensional integrity throughout the assembly process.
there is an asme specification for inseparable assemblies but i cannot re  
asme y14.24-1999 gives the description of inseperable assembly drawings at section 4.2 & gives an example fig 9.  the example is a welded assembly and includes reference to 14.5m-1994, however it doesn't have any gd&t on it.
the standard says "an inseperable assembly drawing fully defineds the end product as assembled" as such any required dimensions & their associated tolerances are needed.  per 14.5 the preffered way of doing this for many situations is to use some kind of gd&t.
so, use gd&t on your weldments where it is appropriate.
kenat,
thanks everyone,
your input has been most helpful. it looks like we will settle on a hybrid approach, using gd&t for most functional features, but allowing some linear postional dimensions, if they do not affect fit or function.for now we will concentrate on modifying existing drawings in areas where we have experienced assembly fit-up problems, using gd&t to clarify fabrication and machining, inspection expectations.
we can also address new drawings as they occur.
thanks again,
john j