How do you miter angle iron for a box corner?

Lay the three angles in place, overlapping at the corner, and scribe a 45° line on each flange from the inside corner where the pieces cross out to the edge. Mark the waste side, cut to the lines, refit and tack.

What angle do you cut angle iron at for a corner?

On equal-leg angle meeting at a square corner, each flange gets a 45° miter. On unequal-leg angle the line runs from the inside corner to the outside corner, so the angle depends on the two leg widths.

Why scribe angle iron in place instead of measuring?

Because the real corner is rarely exactly square. Marking off the pieces captures what's actually there, so small errors don't stack into a gap at the point where all three meet.

How big a gap is OK before welding an angle iron corner?

Aim for under 1/32 in (0.8 mm) on the dry fit. Bigger gaps take more filler and more heat, and more heat means more distortion.

JoineryMetal & Tube JoineryThree-way angle iron corner

How do you get a clean three-way corner where three angle irons meet?

Three angle irons meeting at one corner. A table frame, a crate edge, the top of a box. On paper each piece just needs a 45° on both flanges. In real life the box is a hair out, the angle isn't perfectly 90°, and a cut-list miter leaves a hole you end up filling with weld. Welderman's angle iron corner joint skips the cut list: lay the pieces in place, scribe every line off the steel, cut.

Cutting angle with a grinder: keep the guard on, use a cut-off wheel rated for the grinder's speed, wear a face shield over safety glasses, gloves and hearing protection, and clamp the work instead of holding it. Never side-load a cut-off wheel to grind to the line; switch to a grinding or flap disc. Welding the corner: a helmet at the right shade, covered skin, leather gloves and ventilation. Grind any galvanizing or paint off the weld area first, since zinc fumes cause metal fume fever.

⚡ Rule of thumb

For a three-way angle iron corner, lay all three pieces in their final positions with the ends overlapping, then scribe each miter line in place with a 45° square, from where the pieces cross out to the edge. Hash-mark the waste side of every line before anything moves, then cut to the lines.

Starting point

45° line on every flange, scribed in place · waste side hash-marked

Measured

From the inside corner where two pieces cross, out to the outside edge

Confidence

Creator technique

▶ Source video — Connecting three angle irons into a precise connection — Weldermanwatch on YouTube ↗

Applies to

Process: Welded fabrication — box frames, crate and cabinet edges, table corners
Materials: Equal-leg steel angle, on a sheet or box carcass
Setup: Combination square with a 45° head, soapstone or marker or scribe, clamps, angle grinder
Orientation: All three pieces clamped in their final positions

Why it matters

Each of the three pieces needs two miters, one on each flange, and all six have to land at one point. Cut them off a list and the small errors stack. A degree here, a whisker of overcut there, and the outside corner opens into a hole.

Scribing in place records the corner you actually have. If the box is a touch out, the lines follow it.

And a tight corner welds better. Less filler, less heat, less pull.

What good looks like

The three outside edges meet at one point
Every miter seam is tight and continuous
No flange laps over another at the corner
Dry-fit gaps under 1/32 in (0.8 mm)

How to apply it

Clamp the three angles in their final positions, ends running past the corner so they overlap.
On one face, set the square's 45° head where the two pieces cross and scribe a line from the inside corner out to the edge.
Do the same on every face of all three pieces, the vertical one included.
Hash-mark the waste side of each line before you unclamp anything.
Cut each piece to its lines, leaving the line on the waste side.
Refit, check that the three outside edges meet at one point, then tack.

Test it on your setup

Dry fit after cutting. Gaps should be under 1/32 in (0.8 mm) before you tack; more than that and you're filling with weld.
Sight the corner: the three outside edges should meet at a single point.
Tack one seam, recheck, then tack the next. Welding pulls, and a tack is easier to tweak than a full bead.

Exceptions & tradeoffs

This assumes equal-leg angle. On unequal-leg angle the line still runs from the inside corner to the outside corner, but it won't be 45°, so draw it with a straightedge between those two points instead of the square's 45° head.

If the carcass corner isn't square, scribing in place is still the right call. The lines just won't be true 45°s. Trust the parts, not the square.

A cut-off wheel takes its own thickness with it, typically 0.045–1/16 in (1.1–1.6 mm). Cut on the waste side and grind back to the line.

Building a run of identical frames? Scribe the first corner, then use those pieces as templates for the rest.

Common mistakes

Cutting all six miters to 45° off a cut listScribe in place so the real corner sets the lines
Skipping the waste marks, then cutting the wrong side of a line on a piece that's now shortHash the waste before anything moves
Scribing with the pieces looseClamp them; a piece that shifts makes every line on it wrong
Running the wheel straight down the middle of the lineCut to the waste side, then grind to the line

Frequently asked questions

How do you miter angle iron for a box corner?Lay the three angles in place, overlapping at the corner, and scribe a 45° line on each flange from the inside corner where the pieces cross out to the edge. Mark the waste side, cut to the lines, refit and tack.
What angle do you cut angle iron at for a corner?On equal-leg angle meeting at a square corner, each flange gets a 45° miter. On unequal-leg angle the line runs from the inside corner to the outside corner, so the angle depends on the two leg widths.
Why scribe angle iron in place instead of measuring?Because the real corner is rarely exactly square. Marking off the pieces captures what's actually there, so small errors don't stack into a gap at the point where all three meet.
How big a gap is OK before welding an angle iron corner?Aim for under 1/32 in (0.8 mm) on the dry fit. Bigger gaps take more filler and more heat, and more heat means more distortion.

WeldermanSource & visual reference

Scribing a three-way angle iron corner in place with a 45° square, hash-marking the waste, and the finished closed corner — Connecting three angle irons into a precise connection..!! ↗ at 0:04–0:24.

Summarized and independently edited by Start Here from Welderman's YouTube Short: sampled frames and the title, no caption text, no audio. Laying the pieces in place, scribing 45° lines with a combination square and hash-marking the waste are clearly on screen. The cut itself barely is (a grinder edge, then the finished corner), so the cutting step is inferred from the result. The gap target, the kerf numbers, the unequal-leg note and the tack-by-tack check are mine. Same creator as Scribe a corner brace; this is the three-piece version of the same idea.