How do you check square tubing for square?

Put a framing square in the corner with its edges bearing on the flat walls of both tubes, above the rolled corners. Raising the square on small standoffs keeps it from sliding under the radius. On a full frame, equal diagonals are a second check.

Why does my framing square slide under square tube?

Square tube has rounded corners, and a thin framing square lying on the table sits down in that curve. Lift the square on tabs a little thicker than the corner radius so its edge lands on the flat side.

How thick should the standoffs be?

A little more than the outside corner radius of the tube you use most. About 1/8 to 1/4 in (3 to 6 mm) covers a lot of common light-wall tube. Too tall on small tube and the square reads off the top corner instead.

Can I weld to a framing square?

On a steel square, yes: small tacks, cool between them, then re-check it. Grind off any plating first. On an aluminum square, screw or epoxy the tabs instead.

MeasuringLayout & Site WorkRaised framing square

Why won't a framing square sit flat against square tube or pipe?

You lay two lengths of square tube on the table, push a framing square into the corner, and it reads perfect. It's lying. The square has slid under the rolled corner of the tube and is checking a curve. Adam Fleisch welded small pieces of flat bar under the tongue and blade, so the framing square rides up onto the flat face of the square tubing where the check means something.

Welding hazard. Arc light burns eyes and skin: wear a welding helmet at the right shade, gloves and long sleeves, and screen anyone nearby. Weld with ventilation and keep flammables clear of spatter. Never weld through zinc plating or galvanizing; grind it off first, because the fumes cause metal fume fever. The square stays hot long after the arc stops, so let it cool before you handle or check it.

⚡ Rule of thumb

Raise the framing square off the table on short flat-bar standoffs, a little thicker than the tube's corner radius, so the tongue and blade bear on the flat wall of the tube above the rolled corner. Flat on the table, the thin square slides under the radius and checks the curve, not the side.

Starting point

Flat-bar tabs about 1/8–1/4 in (3–6 mm) thick under the tongue and blade, near the heel and out along each leg

Measured

Square's edge bearing on the flat tube wall above the corner radius

Confidence

Creator technique

▶ Source video — Flat bar on the tongue and blade of a framing square — Adam Fleischwatch on Facebook ↗

Applies to

Process: Metal fabrication fit-up and layout
Materials: Square and rectangular tube, round pipe
Setup: Steel framing square, short pieces of flat bar, welder (MIG in the clip)
Orientation: Square lying on the welding table, standoffs down
Not for: Wood or flat stock, where you want the square flush on the table

Why it matters

Square tube isn't square at the corners. The edges are rolled, with an outside radius that grows with the wall thickness. A framing square is thin, so lying on the table its edge sits right down in that curve.

So it wedges under the radius, or rides up it, or rocks. It's touching something, just not the flat side of the tube. And the flat side is the thing you're trying to put at 90°.

Lift the square and the problem goes away.

On standoffs, the edge sits above the radius and lands on the straight wall of the tube. Round pipe has the same problem in a bigger way: the higher the edge sits toward the middle of the pipe, the firmer it bears.

What good looks like

The square's edges touch the flat tube walls above the radius, along their full length
The square slides on its standoffs without rocking
It still checks true after the tabs go on

How to apply it

Check the square is true before you touch it (flip test below).
Cut a few short pieces of flat bar, a bit thicker than the corner radius of the tube you use most.
On a steel square, grind any coating off the tab spots, then tack the tabs to the underside of the tongue and blade, near the heel and out along each leg. Short tacks, and let it cool between them.
Re-check the square once it's cold.
Lay your two tube members at the corner, set the square on its standoffs, and push it into the corner. The edges should bear on the flat faces.
Same move on round pipe.

Test it on your setup

Flip test: hold the blade on a straight edge (a factory edge of sheet goods works), draw a line along the tongue, flip the square over and draw again from the same point. One line means square. Two lines that spread apart means it's out by half the gap.
Run the flip test before and after the tabs go on.
Then check it on tube: set it into a corner you know is square, like a fixture block or a machined part, and look for light along both edges.

Exceptions & tradeoffs

Welding on a square can pull it out of true. If you don't want to risk it, screw or epoxy the tabs on instead. That's also the way to go on an aluminum square, which you can't weld steel flat bar to.

Standoffs make the square worse for wood and flat stock, where you want it flush on the table. If you do both kinds of work, keep a second square or make a dedicated fab square.

Standoff height has to suit your usual tube. Too thin and the square still slides under the radius. Too tall on small tube and it reads off the top corner instead.

Mind you, this fixes where the square touches. It doesn't fix a bowed tube, or mill scale and spatter on the face.

Common mistakes

Trusting a square that's only touching the tube's rounded cornerMake sure the edge bears on the flat face along its length
Laying heavy beads on the square to make the tabs 'really stick'Short tacks, cool between, re-check with the flip test
Reaching for the MIG gun on an aluminum squareScrew or epoxy the tabs; steel won't weld to aluminum
One tab size for every tube from 3/4 in to 4 inSize the standoffs for the tube you use most

Frequently asked questions

How do you check square tubing for square?Put a framing square in the corner with its edges bearing on the flat walls of both tubes, above the rolled corners. Raising the square on small standoffs keeps it from sliding under the radius. On a full frame, equal diagonals are a second check.
Why does my framing square slide under square tube?Square tube has rounded corners, and a thin framing square lying on the table sits down in that curve. Lift the square on tabs a little thicker than the corner radius so its edge lands on the flat side.
How thick should the standoffs be?A little more than the outside corner radius of the tube you use most. About 1/8 to 1/4 in (3 to 6 mm) covers a lot of common light-wall tube. Too tall on small tube and the square reads off the top corner instead.
Can I weld to a framing square?On a steel square, yes: small tacks, cool between them, then re-check it. Grind off any plating first. On an aluminum square, screw or epoxy the tabs instead.

Adam FleischSource & visual reference

Flat-bar standoffs welded under a framing square so it registers on square tube and pipe above the rolled corner — I welded small pieces of flat bar to the tongue and blade of my framing square... ↗ at 0:04–0:54.

Summarized and independently edited by Start Here from Adam Fleisch's Facebook clip, read off the frames and his on-screen captions with no audio. The flat-bar standoffs and the rolled-edge problem are clearly his. He doesn't state the flat-bar size or how many tabs he used; at least two are visible, so the 1/8–1/4 in thickness and the placement are my starting points. The flip test, the aluminum-square warning, the coating and heat cautions and the round-pipe explanation are mine.