PG20 40×40 mm Eight Slot Aluminium Strut Profile 20 mm Pitch T-Slot Extrusion for Dense Fixtures

Two systems, one envelope

The 40 mm square is the most useful size in aluminium framing because it is large enough to be structural and small enough to handle. Both the 40 series and the 20 series use it, and from the outside the two sections look almost the same. The difference is entirely in the groove and the pitch, and it decides what the member can be used for.

A 40 series groove is 10.2 mm wide with an 8 mm mouth and takes M8 hardware. That is a real bolt: it carries load, and one slot per face on a 30 mm pitch is enough because a structural joint only needs one or two fixings. A 20 series groove is 6.2 mm with a 6 mm mouth and takes M6 hardware. It carries less, but it can be repeated — two per face on a 20 mm pitch, which is what turns the member into a mounting surface rather than a beam.

More slots is not automatically better

Doubling the number of grooves costs material, and the material comes out of the web. On this section the web is 2 mm where the 40 series section has 4 mm, which is why the second moment of area is roughly 45 % lower even though the outside dimensions are identical. A frame built from the eight-slot section is a lighter, softer frame than the same frame built from the four-slot section, and it will deflect more over the same span.

That is a fair trade only when the frame’s job is to hold things in position rather than to carry them. Jigs, fixtures, inspection frames, sensor mounting plates, test stands and adjustable shelving all fall into that category: they need many fixing points at fine intervals and they do not need to resist large bending moments. Put the same section under a machine base or a load-bearing beam and it will be the wrong member.

What a 2 mm web means for machining

The thin web is the practical constraint on this section, and it is worth understanding before the drawing is released. Two millimetres of aluminium behind a 6.2 mm groove is enough to hold a nut captive and to resist the pull of a tightened M6 bolt, but it is not enough to take a thread or to survive an over-torqued fastener.

• No direct tapping: a thread cut into a 2 mm web has roughly two threads of engagement, which will strip long before the bolt reaches working load. Every connection goes through a T-slot nut or a machined block.
• Torque limit: specify a torque for M6 fixings, typically 8 to 10 N·m, and hold to it. The groove lips deform before the bolt fails, and a deformed groove stops a nut sliding along the member.
• No self-tapping screws: they split the web rather than cut it. Where a light cover has to be fixed directly, use a slot nut rather than a screw into the extrusion.
• Machining is still fine: cut, drill, counterbore, notch and mitre operations are all normal on this section. It is only the tapped hole in the web that has to be avoided.

Specifying it so the frame assembles first time

1. Confirm the system before the section State PG20 or PG40 on the order, not just 40 x 40. The two are not interchangeable and a mixed delivery cannot be assembled without adaptors.
2. Give the fixing schedule Because the point of this section is fixing positions, the drawing should show where they are. We can pre-drill and counterbore to that schedule so the assembler is not measuring on site.
3. State the torque limit on the drawing A 2 mm web relies on the assembler respecting a torque figure. Put it on the drawing rather than in a note that never reaches the bench.
4. Decide the finish with the frame in mind If the frame is visible or is used near food or clean areas, anodise the whole order in one batch to avoid a colour step between deliveries.

Frequently asked questions