specifying "flatness" of rod or tubing bent into a U-shape

[RoboBill]

Hi all,

I'm scratching my head over how to specify something that seems like it should be simple to do. We've got some 1/2' Dia. rods that are bent into U-shaped pieces where the "horizontal" section of the U is appreciably longer than the vertical legs.The two ends of the U should be co-planar and each be perpendicular to the rod face. The bends should be 90 degrees. My question what datums should I use? I want to be able to specify that when placed on a flat surface there is no more than a certain size gap between the piece and the table anywhere along its length. In other words I want the centerline of the rod to be planar, but I can't figure out how to set up datums to do this. Can I use datums target lines for the (theoretical) line of contact of the piece on a flat table and call that one datum?

Also, Ive seen other references stating that the standard in the tubing industry is to dimension bend radii to the centerline of the tube, not the inside. Is the same true for solid rods?

Thanks much,

 

[MikeHalloran]

The tube industry dimensions to the tube centerline. ... but usually measures to the OD or outside tangents, and doesn't get all worked up about it, because the tolerances are usually not all that tight.

... until they buy a measuring arm. Then they go insane. ;-)


I'm not sure that GD&T buys you anything on this part, as in being somehow superior to a note to just lay it on a table and go over it with a height gage.



Mike Halloran
Pembroke Pines, FL, USA

 

[RoboBill]

Mike, thanks much for the reply. Unfortunately (maybe) my company's standard is that ALL drawings have GD&T applied, so it's not a question of whether it gains me anything. I was just trying to find the easiest/best way to specify that the two bends are in the same plane. Thanks again.

Bill

 

[CheckerHater]

Well, according to GD&T, simple envelope dimension will indirectly control your “flatness”.
If you are following ASME, envelope requirement is implied; in ISO you can specify it with (E) symbol.
Having dimension with tolerance actually means having “GD&T” on your drawing.

 

[btrueblood]

Agree with CheckerHater. Assuming somewhere on the drawing you invoke the relevant GD&T standard, i.e. in a drawing note or the tolerance block or somewhere.

 

[RoboBill]

Thanks very much for the input. Since I'm really new to this I'm not sure exactly how this is indicated on a drawing. Are you saying that the envelope principle means that I don't have to call out some range of acceptable flatness as long as the part "appears" to be flat as drawn? Since there's no such thing as "perfectly" flat, what tolerance applies? The title block tolerance for angular dimensions?

Thanks again.
Bill