Measuring straightness vs flatness

I'm very comfortable measuring flatness with a surface plate and the part resting on 3 points all exactly the same height, sweeping under the part with an indicator, or using feeler gages depending on part configuration and accuracy required. I have a customer asking me for ideas to measure straightness of a piece of rectangular tubing (NDA prevents sharing a drawing). I have explained that straightness is a 2 dimensional measurement between two points, or along the edge of a cylinder, but the quality engineer (who I don't personally know and am not dealing with) has repeatedly stated through a third party, yes he wants a straightness measurement because that is the symbol on the print ( from nuclear facility, so it wont get changed). I asked him how he wanted to measure it and he replied back, he's looking for ideas from me, which leads me to believe he has no idea how to do it. Am i thinking about this correctly? Can you measure straightness of a plane? If you clamped a straight edge to a surface plate and ran a surface gage with an indicator along the straightedge (like a fence) and held the part paralell to the straight edge, would you be measuring straightness ? We make all kinds of gages to measure diameters, lenghts, runout etc. but never been asked for a gage to measure straightness.

Guess a plate measure might be as set on two jo blocks and sweep but the weight of the part would make it slack and so error the test. Even set on the plate and checked to the side the setting would change a long rectangular tube..
Can he give the limits required...

Well thought out question, at least on your end. It does sound like what you describe inevitably is a surface, as presumably this tube has both lenght and height. So if you have a straightedge used as described and the reference indicator shows a deviation within specification I'd thing that would "pass" the part.

But as you're dealing with a nuclear plant part, I'd ultimately put the onus on the customer's quality assurance guys to give you a defining method, this shouldn't be your call.

Just like checking any single axis when evaluating a surface plate.

Set the scope up, and run the target down the part.

Supporting the part is another issue.

How straight does the tubing need to be? Cold finished bars are normally spec'd at 1/8 inch in 5 ft. To measure the straightness bars are supported at 2 points (by off set rollers) and rotated while indicated at 3 to 5 points along the bar, with no point more than 5 ft apart. Sag only comes into play with very small diameter product.

Straightness applies in one direction, as indicated on the print. It can be applied to any geometry that can have a straightness to it.

You can check it like you would flatness, but you'll just want to move it in a straight line.

The direction is dependent upon how it's pointed out in the drawing view, iirc. I'd have to open up my copy of Y14.5 to be sure. It's not a control I see very often.

It got me curious, so here's the def from -2009:

5.4.1 Straightness
Straightness is a condition where an element of a surface,
or derived median line, is a straight line. A straightness
tolerance specifies a tolerance zone within which
the considered element of a surface or derived median
line must lie. A straightness tolerance is applied in the
view where the elements to be controlled are represented
by a straight line.

Click to expand...

Pic attached of relevant figure 5-6 which shows it being applied in two different directions on a plane, and how it's interpreted.

Hope this helps.

ETA: Shit, I can never get this stuff to attach full size on the first try