Aluminum Extrusion Connectors: How to Choose

There is no self-joining extrusion. Every T-slot frame is held together by something that bridges the joint: a bracket bolted into the slot with T-nuts, a plug in the centre bore, or a plate spanning two ends.

This page covers the seven connectors the generator can produce, the hardware each needs, and where printed parts stop being sensible. Dimensions are millimetres, from the metric-standard values the tool uses.

Wall thickness, corner radius and slot tolerance differ between manufacturers, so measure your own profile before printing a part that clips into it. The size chart lists slot dimensions by series.

The seven connector types

Connector	What it is	When to use it
Corner bracket (L)	A flat L-shaped bracket bolted onto two profiles with T-nuts; a gusset version adds stiffness.	90° joints. It sits flat on the profile, so it is the most versatile option.
Outside corner (3D)	Wraps an outside corner: two load-bearing faces and a triangular gusset.	Outside corners, where the joint should be covered, not just held.
T-nut / spring nut	Slides into the slot and locks any connector in place; printed ones can add a spring arm.	Essentially every bolted joint.
Inline joiner (hidden)	A plug that fits the centre bore and aligns two profiles end to end.	End-to-end joints, or when the joint must be concealed.
90° joiner (hidden)	An elbow plug joining two centre bores at a right angle.	Right-angle joints where no bracket should show.
Splice plate	A flat plate bridging two profiles end to end and spreading the load; longer versions add reinforcement.	Extending a run, or a stronger joint than a hidden joiner gives.
Extrusion profile	The frame member itself, at any length, with 45° mitre cuts and drilled holes.	Cutting the members that everything else connects.

T-nuts: the hardware behind every bolted joint

A bracket on its own achieves nothing. Something must sit in the slot and give the screw something to pull against: the T-nut, whose flange sits in the undercut behind the slot lips.

Undercut per side is (inner channel width - slot opening) / 2. The 20 series gives (8.0 - 6.0) / 2 = 1.0 mm, the 30 and 40 series (11.0 - 8.0) / 2 = 1.5 mm, the 45 series (14.0 - 10.0) / 2 = 2.0 mm. The flange must pass through the opening yet be wider than it to lock, and the neck must clear the throat: 1.5 mm on the 20 series, 2.0 mm on the 30 and 40 series, 2.5 mm on the 45 series.

The 30 and 40 series share one slot, so a single T-nut fits 3030, 3060, 4040 and 4080; the 20 series needs its own for a 6.0 mm opening, the 45 series for a 10.0 mm opening. See which nut fits which slot, or how to print a T-nut. A spring arm holds the nut while you start the screw.

Hidden joiners and the centre bore

The centre bore is the round hole down the middle of the profile, and it is what a hidden joiner plugs into: ⌀4.2 on the 20 series, ⌀6.8 on the 30 and 40 series, ⌀8.5 on the 45 series.

An inline joiner enters the bore of both profiles and lines them up end to end; a 90° joiner is the same idea bent into an elbow. Both leave the joint invisible, which suits display frames and machine enclosures.

The limit: the load path runs through the material around a hole, and the joint cannot be adjusted once the frame is closed. Where a joint carries tension or shock, use a splice plate or an outside bracket.

Printed connectors vs metal ones

Metal brackets are the baseline: the supplier publishes load data, they barely protrude, and they do not creep. The limit is the catalogue — standard angles and sizes, only the shapes somebody chose to cast.

Printed connectors win on geometry: any angle, any extra feature such as a cable clip or sensor mount, costs minutes of CAD instead of an order.

They are weaker. Layer direction decides which way a bracket is strong: an L-bracket loaded across its layer lines splits along them, so orient the part to load the layers in plane. No load ratings are quoted on purpose — the answer depends on material, layer height and infill, and a borrowed number is worse than none.

Rule of thumb: printed brackets for light frames such as 3D printers and camera rigs; metal for anything carrying a person, hanging overhead or holding load for years.

Match the connector to your slot series

If you are printing a connector, this table decides whether it fits.

Series	Common models	Slot opening	Undercut per side	Centre bore
20 series	2020 · 2040 · 2060	6.0	1.0	⌀4.2
30 series	3030 · 3060	8.0	1.5	⌀6.8
40 series	4040 · 4080	8.0	1.5	⌀6.8
45 series	4545 · 4590	10.0	2.0	⌀8.5

A 2040 takes the same 6.0 mm slot as a 2020, and a 4080 the same 8.0 mm slot as a 4040, so rectangular profiles use the hardware of their series. For angled ends, see 45° mitre cuts.

Frequently asked questions

Which connector should I use for a 90° joint?

Start with the corner bracket (L), bolted onto both profiles with T-nuts and sitting flat against the extrusion. Use the outside corner (3D) for joints on the outside of the frame, or a hidden 90° joiner when nothing may show.

Do the 30 series and 40 series use the same T-nut?

Yes. Both share one slot profile: an 8.0 mm opening, an 11.0 mm channel, 1.5 mm of undercut per side and the same ⌀6.8 centre bore. The 20 series needs its own nut for a 6.0 mm opening, the 45 series a larger one for 10.0 mm.

Can I 3D print load-bearing connectors?

For light frames, yes: printed brackets, T-nuts and joiners hold 3D printer frames, camera rigs and small enclosures. For anything carrying a person, hanging overhead, or seeing sustained load and vibration, use metal connectors and the supplier load data.

How do I join two profiles end to end?

Push an inline joiner into the centre bore of both profiles, which keeps the joint hidden and aligned, or bolt a splice plate across the two ends, which spreads the load. The bore is ⌀4.2 on the 20 series, ⌀6.8 on the 30 and 40 series, ⌀8.5 on the 45 series.

What if my joint is not a right angle?

Use a bracket that matches the angle, or print a custom part, which is where printed connectors beat metal: you are not limited to the angles somebody chose to cast. The generator can also add 45° mitre cuts so angled ends meet flush.

Build the connector for your profile

Pick your series, check the slot dimensions against your profile with calipers, then export an STL for corner brackets, T-nuts, hidden joiners, splice plates or the profile itself.

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