Choosing 2020, 3030 or 4040 aluminium extrusion: slot width, T-nuts, corner brackets and stiffness

2020, 3030 and 4040 sizes and slot widths

T-slot extrusions are 6063-T5 aluminium profiles with a T-slot on every face. The number is the section: 2020 is 20 × 20 mm, 3030 is 30 × 30 mm, 4040 is 40 × 40 mm; 2040 and 2080 are 20 mm wide, 40 / 80 mm tall multi-slot profiles. The key buying spec is slot width, which decides which T-nuts and accessories fit.

Based on MISUMI HFS5 / HFS6 / HFS8 and common European-style profiles. Second moment and mass vary with each supplier's section — use the catalogue values for real calculations.

TIPSame slot width doesn't guarantee compatible parts: 3030 has an 8 mm slot; 4040 is usually 10 mm (e.g. MISUMI HFS8) though 8 mm-slot series exist, and the lip (≈ 2.2 vs 4.3 mm) and inner cavity differ. Check series and slot width before buying T-nuts and brackets.

T-nuts, corner brackets and joining methods

Types of T-nut

• Slide-in T-nuts: cheapest and strongest, but must be slid in from the profile end before assembly.
• Spring-loaded drop-in nuts: press in through the slot opening and stay put — ideal for adding parts later.
• Hammer-head (roll-in) nuts: drop in and turn 90° to lock — fast, but usually lower load than slide-in nuts.

Common joints

• L corner brackets: the default. 2020 uses 20 × 20 or 20 × 28 mm die-cast brackets with M5 × 8 or M5 × 10 screws and T-nuts — easy to adjust and remove.
• End tapping: tap one profile's centre bore (M5 for 2020), drill an access hole in the other and drive a socket screw through — clean and stiff, but needs machining.
• Internal connectors and joining plates: hidden in the slot or on the face, for tidy frames or multi-way joints.

Screw length ≈ bracket thickness + slot lip + nut thickness. With a 3 mm bracket on 2020, M5 × 8 is just right and M5 × 10 also works; too long and the screw bottoms out and pushes the nut away instead of clamping.

TIPThe classic mistake is finishing a frame and finding a slide-in nut missing. Pre-load one or two spares per profile, or use drop-in nuts on faces where accessories may be added.

Stiffness: second moment of area and deflection

Bending resistance depends on the second moment of area I. For a solid square I = S⁴ / 12 — proportional to the fourth power of the side. Hollow extrusions follow the same trend: doubling the side increases stiffness by 13× or more while mass grows only about 3×.

Simply supported, mid-span load δ = F · L³ / (48 · E · I)　　Cantilever, end load δ = F · L³ / (3 · E · I)　　E ≈ 69 GPa (6063-T5)

Deflection grows with the cube of length: with the same 100 N, a 2020 beam goes from 0.54 mm at 500 mm to 4.3 mm at 1000 mm (8×). Long gantries and cross-beams need a bigger section first — 2040 or 4040.

• 2040 is directional: with the 40 mm side in the load direction it is more than 3× stiffer than lying flat.
• Aluminium's modulus is about 1/3 of steel's, so the same section deflects 3× more; precision machines use larger profiles to compensate.
• Open T-slots make extrusions weak in torsion, and joints and brackets are usually the weak link — diagonal braces or gusset plates help most.

Cut-length tolerance and frame dimensions

• Standard saw cuts hold about ±0.5 mm; precision cutting or end milling reaches ±0.1–0.2 mm, depending on the supplier.
• Square frames depend more on matched pairs and end squareness: cut parallel members in one batch — equal length matters more than absolute tolerance.
• Profiles also have straightness and twist tolerances (see EN 12020-2); mill a datum face for precision rail mounting.
• Deburr after cutting and clear chips from the centre bore before tapping; anodised profiles show bare metal at the cut.

End-tapped frame: cross-member length = outer frame size − 2 × profile width　(e.g. 400 mm frame in 2020 → 360 mm)

With L brackets the profiles can run to the outer frame size; with end tapping the through member takes the full length and the tapped member loses two profile widths. A quick sketch with every length, tap and drill position avoids most ordering mistakes.

TIPThe aluminium extrusion generator downloads 2020, 2040, 3030 and 4040 STEP models of any length for assembly drawings and clearance checks; brackets and motor mounts can then be 3D-printed or CNC-machined.

Which one: typical applications

V-slot extrusion is a common variant with V-shaped slot lips so V-wheels can roll directly on the profile. It looks like 2020 T-slot but accessories don't always interchange. Choose V-slot when carriages ride on the profile; for plain frames T-slot has a wider accessory range.