Aluminium 2020 Extrusion 20 x 20 mm for 3D Printer Frames

01Is 20 x 20 stiff enough to build a 3D printer from?

For a static frame, usually yes. For a moving gantry, usually no. That single distinction explains most of the disagreement you will find online, because two people arguing about 2020 are often describing two different jobs. A 400 mm cube frame made of 20 x 20 mm four slot extrusion at roughly 0.62 kg/m is rigid enough to hold a bed and carry a gantry, provided the joints are braced. The same section used as a 400 mm unsupported gantry beam will deflect visibly under its own carriage and will ring for a second after every direction change.

So the useful question is not “is 2020 strong enough” but which part of the machine is the compliant one. On a typical desktop printer the answer is almost never the frame. It is the belt, the toolhead mount, or an unsupported span. Spending money on a heavier frame before identifying the compliant part is the most common way to build a printer that is heavy and still rings.

02Where does the deflection in a printer frame actually come from?

Four sources, in descending order of how often they are the real limit. The frame section is fourth on this list, not first.

Note the third row. Deflection falls with the cube of span and only linearly with stiffness. Going from a 400 mm span to a 200 mm span is an eightfold improvement for the cost of one bracket. Going from 20 x 20 to 20 x 40 is also an eightfold improvement in bending, but it costs twice the material and adds mass to whatever has to move. If the span can be shortened, shorten it first.

03How much stiffer is 20 x 40 or 40 x 40, really?

For a beam in bending, stiffness goes with the cube of the depth in the direction of bending and linearly with the width. Taking a solid rectangular section as a reference, the scaling is easy to see. Real extruded sections are hollow, so the absolute numbers are lower — but the ratios between sizes hold closely, and the ratios are what the decision actually depends on.

That is why gantry beams on production machines are tall and narrow rather than square. If you are choosing between a 40 x 40 and a 20 x 80 for a gantry, the 20 x 80 is roughly twice as stiff per kilogram in the vertical direction and about the same weight — but it is only stiff in that one direction, so the cross members have to carry the rest.

04What does ringing have to do with section choice?

Ringing is the machine oscillating after a sudden change in direction. Its frequency is set by stiffness divided by moving mass, and its visibility on the part depends on how many oscillations happen before the motion settles. A stiffer frame raises the frequency, which shortens the period, which means the oscillation damps out within a shorter distance of travel. That is the whole mechanism: higher frequency equals shorter ringing marks, not weaker ones.

It also means that adding mass to a moving part makes ringing worse even if it makes the part stiffer, because frequency depends on the ratio. A 20 x 40 gantry beam is eight times stiffer than a 20 x 20 but twice the mass, so the frequency rises by about a factor of two — a real gain, but nothing like the eightfold gain in static deflection. On a moving axis, weight is not free.

05Can a linear rail be bolted straight into the 6.2 mm slot?

Mechanically yes, but not with the rail screw. The 20 series slot is served by M5 and M6 slot nuts, and a typical MGN12 rail is fixed with M3 screws. So the rail does not bolt into the slot directly — you need either a 20 series slot nut with an M3 thread, or a machined adapter plate that picks up the slot with M5 and presents an M3 pattern for the rail.

The adapter plate is the better answer on a machine that has to hold tolerance, for one reason: the slot is not a datum. An extruded slot has a positional tolerance relative to the outside faces, and the outside faces are what the rest of the frame is built from. Bolting a rail into the slot means the rail follows the slot, not the frame. Machining a flat on the profile and mounting the rail to that flat puts the rail on a surface you control.

• Slot nuts are the fast route and are fine for a hobby machine or a jig.
• For anything with a stated straightness, machine a reference face on the profile and mount the rail to it.
• Keep the rail over the web of the profile, not over the slot opening, where the wall is thinnest.
• Use a slot cover strip after assembly — an open slot next to a rail collects chips and they end up under the carriage.

06How tight do the joints have to be?

Tighter than most people expect, and the reason is slip rather than strength. A bolted aluminium joint with a plain washer carries load by friction between the two faces. If the preload is low, the joint does not fail — it slides a few hundredths of a millimetre under a reversing load, and the machine slowly loses square. On a printer that shows up as a frame that was trammed on Monday and is out by Friday.

Three things fix it, in order of cost: a corner brace on every joint that sees a reversing load, a thread locking compound so preload does not decay, and re-torquing after the first thermal cycle — the first time the machine runs warm the joint beds in and the preload drops.

07Questions from people building a desktop machine

Related sections and categories

Browse 20 Series Aluminium Profiles, Strut Profiles, the 20 x 20 mm four slot section, or see the 20 x 40 mm six slot beam for the full section data.