How do you join square tubing at a corner without a separate cap?

Let the rail make the cap. Cut across the rail's side walls one post width from its end, free the bottom wall of that end section, and tap the end down over the post top. The rail's own top wall closes the post end. Then weld the seams.

Do the rail and post have to be the same size tube?

The clip uses the same size for both. The end section's side walls spring open by one wall thickness each side to lap the post, which is easy on light-gauge tube. Whatever the sizes, cut the end section to the post's width.

Is a folded rail cap strong enough without welding?

No. The folded end is a locator and an end cap. It holds the rail in place while you fit up, but the corner needs welding or a mechanical fastener before it carries load.

What's the difference between this and a notch-and-fold corner?

A notch-and-fold corner bends one piece of tube into an L. The rail cap joins two pieces, a rail and a post, by folding the rail's end down over the post. Both come from the same Craft Hacks Studio clip.

JoineryMetal & Tube JoinerySlit-and-fold rail cap

How do you cap a square-tube post with the rail so it sits square before you weld?

Butt a square-cut rail onto a square post and you've got two problems. There's an open tube end staring at you, and the rail skates around on the post top while you try to tack it. This square tube rail to post joint fixes both with the rail's own steel. Cut the end, fold it over, and the rail end becomes the post cap.

Cutting tube with an angle grinder: keep the guard on, use a thin cut-off wheel rated for the grinder's speed, and wear a face shield over safety glasses, gloves and hearing protection. Clamp the rail to cut it; never hold the work in your hand. The cut edges and the freed flap are razor sharp, so deburr and wear gloves when you fold them. Hammering a springy flap can throw the tube or a chip, so clamp the post and keep eye protection on. Galvanized tube gives off zinc fumes when welded: grind the coating off the weld area and ventilate.

⚡ Rule of thumb

For a rail-to-post corner in the same size square tube, cut across the rail's side walls one post width from its end, leaving the top wall whole. Free and fold the bottom wall of that end section out of the way, then tap the end down over the post so its top wall caps it.

Starting point

End section = 1 post width long · top wall left uncut

Measured

From the rail end back to the cut across its side walls

Confidence

Creator technique

▶ Source video — Don't Weld Until You See This — Craft Hacks Studiowatch on YouTube ↗

Applies to

Process: Metal fabrication — rail-on-post corners for frames, gates, railings, table bases
Materials: Light-gauge square tube, rail and post the same size
Setup: Angle grinder with a thin cut-off wheel or a hacksaw, hammer, square, welder or fasteners
Orientation: Post vertical, rail running across its top
Not for: Heavy-wall tube, or a joint that has to carry load before it's welded

Why it matters

A square-cut rail sitting on a post leaves an open end you have to cap later, and a joint with nothing holding it in place while you tack. One bump and the corner's out.

Folding the rail's own end over the post changes both. The end section's side walls lap the outside of the post, so the rail can't slide off sideways, and its top wall sits on the post end as the cap. No separate cap plate. No magnet square fighting you.

It's a locator, though. Not a joint.

What good looks like

The rail's top wall runs unbroken over the post top
The end section's side walls lap the post tightly on both faces
The rail end sits flush with the outside face of the post
A square in the inside corner shows no daylight

How to apply it

Mark a line across the rail one post width back from its end. With same-size tube, that's one tube width.
Cut across both side walls on that line, stopping at the top wall. The top wall is the hinge you keep.
Free the bottom wall of the end section by cutting along its two corners, and bend it out of the way. Trim it off if it fouls the post. What's left is an open-bottomed cap: the top wall plus two side walls.
Set the rail across the post top with the cap over the post.
Back the post up, then tap the cap down with a hammer until its top wall sits on the post end and its side walls lap the post.
Check the inside corner with a square, then tack and weld the seams (or fasten it).

Test it on your setup

Do it on two offcuts first. Tap the cap down and check the inside corner with a square.
Cap stops short of seating: the cut across the side walls is too shallow, or the bottom wall is still in the way.
Side walls won't open far enough to lap the post: the tube is too stiff for this. Cut two pieces and weld a conventional corner.

Exceptions & tradeoffs

Same-size tube means the cap's side walls have to spring open by one wall thickness each side to lap the post. That's easy on light gauge and a fight on anything heavy. Expect a small step, one wall thick, where the cap laps the post.

Mind you, this is fit-up. The folded end locates the rail and closes the end; the welds hold the corner. Don't load it until it's welded or fastened.

Rail and post in different sizes: the end section is still cut to the post's width, so measure it off the post, not the rail.

The same clip ends on a third trick, a threaded rod through the post with a cross bolt holding the rail. It isn't clear enough on screen to write up.

Common mistakes

Cutting the end section shorter than the post is wide, so the cap stops short of the outside faceMake it one full post width
Running the cut through the top wall tooStop every cut at the top wall; it's the hinge and the cap
Hammering the cap with the post unsupportedBack the post so the blows fold steel instead of denting the tube
Treating the folded cap as the finished jointTack, check square, then weld or fasten it

Frequently asked questions

How do you join square tubing at a corner without a separate cap?Let the rail make the cap. Cut across the rail's side walls one post width from its end, free the bottom wall of that end section, and tap the end down over the post top. The rail's own top wall closes the post end. Then weld the seams.
Do the rail and post have to be the same size tube?The clip uses the same size for both. The end section's side walls spring open by one wall thickness each side to lap the post, which is easy on light-gauge tube. Whatever the sizes, cut the end section to the post's width.
Is a folded rail cap strong enough without welding?No. The folded end is a locator and an end cap. It holds the rail in place while you fit up, but the corner needs welding or a mechanical fastener before it carries load.
What's the difference between this and a notch-and-fold corner?A notch-and-fold corner bends one piece of tube into an L. The rail cap joins two pieces, a rail and a post, by folding the rail's end down over the post. Both come from the same Craft Hacks Studio clip.

Craft Hacks StudioSource & visual reference

Cutting a rail end free on three sides and hammering it down over a same-size post so the rail caps the post — Don't Weld Until You See This #metalworking #welding #tips ↗ at 0:16–0:22.

Summarized and independently edited by Start Here from Craft Hacks Studio's YouTube Short: frames and the video description, no audio, no narration. The rail end cut free, set over the post and hammered down until the rail caps it, then checked with a square, is on screen. The cut pattern goes by in a couple of seconds, so exactly which edges he slits, and whether he folds the bottom wall away or trims it off, isn't certain; the step order here is mine and gets you the same result. The one-post-width length is my reading of the frames, not a number he gives. Welding it, the test, the wall-thickness step and the mistakes are mine. Same clip as the notch-and-fold tube corner, which bends one piece instead of joining two.