【转帖】gdt for controlling excessive twis

yang686526

gd&t for controlling excessive "twist."
anybody know of a better way - other than straightness - to specify against excessive "twist" when welding to a long, narrow flat?
i know straightness will govern "bowing along the full length, but does it also govern "twist" along an axis tangent to the long axis?
chris in nc
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give us a little background on this weldment.  are you using assembly fixturing to control this?  what's your datum structure?
this piece - while simple - will have a high-visibility function on a beta-unit that will be going around to a number of [customer's] plants for "ooohs and ahhhs."
it's a 48-inch long by 2-inch wide x 1/2-inch flat bar onto which will be welded some l-angle pieces with plugs and threads. this is a very, very simple fabrication - which means lots of room for a screw-up during welding.
while drafting this up, it occured to me that i wasn't sure how to specify this piece not to have any perceiveable twist along its full length. straightness will cover bowing along its full length, but if you think about it, specifying straightness across the large piece doesn't do the job. the thing could "corscrew" and still incrementally fall within flatness or straightness.
if you specify flatness tight enough, there will be no perceptable "corkscrew".  the problem comes in producing it.
whotmewory,
   i agree with ewh about flatness.  a profile tolerance will let you control the face with respect to your mount points or some other datum surface.  it depends on how much control you want.
   just don't forget this is a weldment.  they cannot weld to within a profile of .001".
                        jhg
are you worried about twist or are you worried about the location of the brackets?
i think flatness will work, but if you want to get complicated set a datum at one edge, then use a positional tolerances on the welded-on brackets.  their position should control the twist in your part if welded correctly.
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if this is for an oooh and ahhh response, i would suggest that you not overly complicate the weldment. i would perhaps you could forget about gd and t and verbally describe what it is you are wanting. try to be generous with any tolerances.
instead of (or as well as) stating flatness across the piece, state it in the drawing for along the piece.  therefore straightness will take out bowing as you suggest and flatness will take out corkscrew as ewh suggests.
just a quick opinion
asm
naturally i also agree with madmango's datum set and comments about tolerances on what seems to be some sort of prototype / display model.  constructability is a major factor in things/designs like this.
regards
asm
ok. let's pretend this isn't a simple bracket and that - maybe i'm blowing this out of proportion - but given that this is surely something that comes up, how do we specify for preventing receiving or qa from accepting outsourced fabrications that are twisted or distorted but not called out opn any drawing?
is this an oversight in gd & t?
chris in nc
i still don't understand why you find that flatness is not an appropriate call out in this case.  it will control straightness as well as flatness of a surface (if it's not straight, how can it be flat?).
in answer to your last post, if it is not specified on the drawing (or contract), then receiving or qa has no basis for rejection.  you can tell them to reject any parts that don't meet your expectations, but the supplier still has the right to be paid for manufacturing something that meets the specifications they were given (contract or drawing or both).