Aluminium Angle Profile 40x40x5 mm 6061 T6 Extruded Structural Angle

Where a 40 x 40 x 5 angle earns its extra weight

The step from a 4 mm wall to a 5 mm wall is a 25 % increase in thickness and about 22 % in weight, but the second moment of area rises by roughly 70 %, because stiffness in bending scales with the cube of the thickness. On a bracket that spans a change of plane, that is the difference between a part that deflects visibly under load and one that stays put, and it is the reason this size exists alongside the lighter 40 x 40 x 4.

It is bought for four jobs in practice, and on all four the angle is loaded across the heel rather than along either leg.

Bolting it to a 40 series profile with T-slot nuts

The standard connection between this angle and a 40 series extrusion is an M8 T-slot nut in the profile slot and an M8 bolt through the angle. The nut is captive in the slot and can be slid to any position, so the joint is adjustable after assembly, and no thread is cut into the aluminium angle at all. That last point is the whole reason the connection is made this way rather than with a tapped hole in the angle. The nuts are standard T-slot fastening elements for the 40 series and for the wider strut profile range, so no special part is needed.

The hole in the angle is a plain 9 mm through-hole for an M8 bolt. The hole has to be in the leg that lies against the profile face, and its position along the leg decides how the angle sits relative to the slot. The slot mouth is 8 mm wide on an 8 mm slot, so the bolt needs about 6 mm of clearance from the profile face to the centre of the T-nut body; with a 5 mm angle leg and a washer, that fixes the bolt length rather than leaving it to choice.

A bolt that is too long bottoms in the slot before the joint is tight, which feels like a stripped thread but is not; a bolt that is too short engages only the first few threads of the T-nut and pulls out under load. Measure from the underside of the bolt head to the bottom of the slot and take the bolt length from that, not from the thickness of the angle alone. Where two angle legs are bolted back to back through the same nut, add their thicknesses before choosing the length.

Why not just tap a thread into the angle

It is tempting, because a tapped hole removes the nut and looks cleaner. It also removes the strength of the joint. Thread strength depends on how many full threads engage, and the accepted minimum for a steel screw in aluminium is about one times the bolt diameter, with one and a half to two times the diameter being where the bolt itself becomes the weak part. A 5 mm wall does not reach that on any thread that matters.

The arithmetic is short. An M6 coarse thread has a 1.0 mm pitch, so a 5 mm leg gives about five turns, of which roughly four are full threads after the chamfer — about 0.67 times the diameter, well under the one-times rule. An M5 at 0.8 mm pitch gives about six turns and about five full threads, which is close to one times the diameter but only for a small bolt. So the only thread that is defensible in a 5 mm wall is an M5, and an M5 carries a fraction of the load of the M8 T-nut connection it replaced.

Round holes or slotted holes

A round hole locates the joint and a slotted hole lets it move. The choice is decided by whether the assembly has to be aligned on site or only bolted together. Both are produced on CNC machining centres from the hole pattern in your drawing.

• Round holes are the default where the frame is jig-built and the position is fixed by the cut lengths. Drill 9 mm for M8, deburr both faces, and keep the hole at least one diameter from any edge so the leg does not tear out.
• Slotted holes are used where a panel, a rail or a foot has to be aligned after the frame is up. A 9 x 11 mm slot gives about 2 mm of movement in one direction, which is normally enough to take out the tolerance stack of a large frame.
• Slots across the leg are better than along it where the movement needed is perpendicular to the profile face, because the bolt then stays in the same slot position and the T-nut does not have to move.
• Oversize holes are not a substitute for a slot. A 12 mm hole for an M8 bolt gives 4 mm of movement but also 4 mm of slack in every direction, so the joint can shift under vibration even when it is tight.

6061-T6 or 6063-T5

This section is supplied in 6061-T6 as standard, because a structural angle is bought for its strength: 6061-T6 yields at roughly 240 MPa against roughly 110 to 150 MPa for 6063-T5, and it resists bolt bearing and local dishing far better. Where the angle is a load path, or where it is drilled and used as a cleat, 6061-T6 is the correct alloy. 6063-T5 is offered where the part is visible and has to anodise cleanly, but on a 40 x 40 x 5 structural member the appearance argument is rarely the deciding one.

Frequently asked questions