Cutting T-Slot Extrusion at 45° for Mitred Corners

A mitre is a saw cut taken at an angle across the profile instead of square across it. Two pieces cut at 45° meet at a right angle with their outside faces flush, so the corner reads as one continuous frame.

A mitre saw, or any saw with an adjustable fence, will do the job. What decides whether the corner closes is rarely the machine: it is the blade angle and whether the two cuts are mirror images.

Why mitre a corner

A square-cut butt joint shows the end of one extrusion against the side of the other. That end face is mostly slot and thin wall, so the joint reads as a seam.

A 45° mitre hides that end: the cut faces sit against each other, the outside faces line up, and the corner looks like a bend rather than a junction.

Be honest about the trade-off, though. A mitre is a cosmetic joint in this material, not a structural one. See when a bracket is the better choice.

What a 45° cut does to the profile

• The cut face is wider than the profile. A 45° cut across a square section makes the cut face √2 times the section, so a 20 mm section produces a cut face about 28.3 mm across.
• The two pieces must be mirror images. Cut both the same way and the faces lean the same direction and never meet.
• The slot disappears at the cut end. Mitring removes the slot, so there is nothing for a T-nut to slide into.
• The cut can lean on either axis. A mitre can be taken across the height axis or the width axis of the cross-section, so on a 20 × 40 or a 45 × 90 the two are not the same cut.

Setting up the saw

1. Square the blade to the table first. Cut a scrap piece at zero and check both faces against a square. An out-of-square saw passes its error to every mitre.
2. Then set 45°. Use the detent or the scale, but verify it the same way.
3. Cut a test corner. Two offcuts, cut as the two halves of a corner, held against a square. If it will not close, adjust the saw.
4. Support the length. Extrusion is stiff but it will twist, and a long piece sagging off the table cuts at the wrong angle.

Blade choice, speed and feed are machine-specific: follow your saw's manual for aluminum.

Clamping and supporting the cut

Aluminum extrusion is easy to clamp and just as easy to distort. The section is hollow with thin walls, so a clamp tightened across the middle of a face bows it.

Clamp close to the cut, on the side the blade pushes the work into, and bear on a solid face rather than across a slot. If the clamp must sit over a slot, back it with scrap.

Support the offcut as well. A piece left hanging drops as the cut finishes, pinches the blade and can snatch. Let the blade do the work.

Checking the angle off the saw

Do not judge a mitre with the work still in the saw. Take both pieces off, bring the cut faces together on a flat surface and press them against a square.

• A light gap in the middle: the angle is off. The faces touch at the outside edge and open up towards the inside.
• A gap at one edge only: the fence is out of square with the blade. Re-square it and cut fresh test pieces.
• The corner rocks: usually a burr, or one cut leaning on the wrong axis.

Then hold the joint up against a light. A joint that looks shut in your hand often shows a line of light through it.

Deburring before you assemble

A saw cut in aluminum leaves a burr: a raised lip along the edges the blade exited, and often a thin fin inside the slot. That burr holds two good faces apart, and it is the most common reason a mitre will not close.

Run a file or a deburring tool along the outside edges and inside the slot, on both pieces. A few light strokes; you are removing raised metal, not squaring the cut.

When a bracket is the better choice

Often, in this material, it is. A mitred end has no slot, so the joint cannot be bolted through it. If the corner carries load or gets leaned on, bracket it.

A bracket joint is stronger, faster, adjustable and can be taken apart, and it leaves the slot free for later fixings. The cost is visual.

The rule I use: mitre when the corner is seen and lightly loaded, bracket when the corner does real work, such as a bench or a machine base.

Slot dimensions by series

These are the metric-standard values the generator uses, in millimetres. At a mitred end you are sawing through an open channel and thin walls, not solid bar.

Mitres in the generator, and printed corners

A mitre angle can be set independently on each end of a profile, from −80° to +80°, and the cut can lean on either the height axis or the width axis of the cross-section. Mitres can be combined with drilled through-holes in the same model.

A printed corner gusset can sit inside a mitred joint, and the model shows whether it fits before you print. The printed part is the weak link.

Frequently asked questions

Set the mitre before you cut metal

Set the angle and direction on each end, choose the axis the cut leans on, and add the holes for the bracket that will hold the corner before you commit a length to the saw.

Open the aluminum extrusion generator →

More on this material: slot dimensions by series · connector selection · T-slot nut dimensions · designing and printing T-nuts · 2020 vs 2040.