GD&T for controlling excessive "twist."
GD&T for controlling excessive "twist."
(OP)
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
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





RE: GD&T for controlling excessive "twist."
RE: GD&T for controlling excessive "twist."
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.
RE: GD&T for controlling excessive "twist."
RE: GD&T for controlling excessive "twist."
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
RE: GD&T for controlling excessive "twist."
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.
"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: GD&T for controlling excessive "twist."
RE: GD&T for controlling excessive "twist."
Just a quick opinion
ASM
RE: GD&T for controlling excessive "twist."
Regards
ASM
RE: GD&T for controlling excessive "twist."
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
RE: GD&T for controlling excessive "twist."
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).
RE: GD&T for controlling excessive "twist."
I suggest using a flatness callout to control camber, and additionally limiting twist by means of a limit on the position of one corner relative to a line datum at the opposite end and a point datum at the adjacent corner.
Maybe I can make that more clear with a different example: Twist in automotive connecting rods is specified with extended tolerance zones implying the presence of 6" long gage bars pressed into the big and small ends, and twist is specified as a maximum deviation from planarity of one end of the smaller gage bar, relative to a plane defined by both ends of the larger gage bar and the opposite end of the small gage bar. At least that's how they did it in olden days, before GD&T left the toolroom.
Mike Halloran
Pembroke Pines, FL, USA
RE: GD&T for controlling excessive "twist."
I'm new to this site, so I don't know the experience / training level of any of the respondents. If it helps the discussion, I am ASME Y14.5M-1994 GDTP-S certified, and I've had considerable training from a number of renowned instructors. I have used the composite flatness callout on similar applications many times; it works when the tolerances and unit area are appropriate.
Also, ewh is quite correct about the supplier's right to be paid if you don't provide them with a spec. However, you might put out a general notice to your vendors that covers such items, and cc your receivers. Much better idea to put it on the drawings though, as some parts may need tighter / looser tolerances than the norm.
RE: GD&T for controlling excessive "twist."
"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: GD&T for controlling excessive "twist."
RE: GD&T for controlling excessive "twist."
Checking for flatness is tough on the shop floor though.
Do not use profile since this relates to datums while flatness is to itself.
Hope this helps
DD