# How to design a 3D printed bracket for 2020 aluminum extrusion Slot widths, T-nut sizes, bolt clearance, and gusset design for 3D printed 2020 extrusion brackets, plus when to print one and when to buy the metal part. --- You are building something on 2020 aluminum extrusion: a printer enclosure, a camera arm, a spool holder, a shelf for a control box. The metal corner brackets you have do not sit where you need them, or the angle is wrong, or you want to mount a sensor to a face that has no obvious way to grab it. So you go to design a printed bracket, and immediately hit the questions nobody puts on the product page: how wide is the slot, what bolt hole do I need, and will this thing actually hold? Printed extrusion brackets are one of the best uses of a 3D printer, as long as you match the design to the load. This post covers the numbers that matter, which loads to print for and which to buy metal for, and how to size the whole thing so it lines up with your slots on the first try instead of the third. ## What size is the slot on 2020 extrusion? On standard 20-series extrusion the profile is 20 mm by 20 mm and the slot opening is usually 6 mm wide, which is what the common M5 T-nuts are made for. But "usually" is doing work in that sentence, so measure before you commit. The outside dimension being 20 by 20 does not guarantee the slot is 6 mm. There are many variants of 20-series extrusion, and while the outer size is standardized, the slot width, the edge chamfers, and the center bore vary between profiles. Some slots are 6 mm, some are narrower. Drop a caliper jaw into your actual slot and read the opening before you design anything that has to slide a nut into it. If you have never measured a slot cleanly, our guide to [measuring with calipers for a 3D print](/blog/measure-with-calipers-for-3d-printing) walks through getting a real number instead of a guess. The reason this matters: your printed bracket does not touch the slot. A steel T-nut does. Your job is to get the bolt holes in the right place so the nut lines up. The slot width mostly tells you which nut and bolt you are working with. ## What bolt and T-nut do 2020 brackets use? M5 is the default for 20-series extrusion. M5 T-nuts (drop-in or the spring-loaded roll-in style) are what most printer and CNC frames ship with, and they suit a 6 mm slot. You will also see M4 and M3 T-nuts for lighter work or tighter profiles. Match the nut to your slot, then design your bracket's holes around that bolt. For the clearance hole through your printed bracket, you want the bolt to pass freely, not thread into the plastic. A medium M5 clearance hole is about 5.5 mm. Model it at 5.5 mm and let it print. Remember that FDM prints round holes slightly undersized because the nozzle over-extrudes on tight curves, so a hole you draw at 5.5 mm often comes off the bed closer to 5.3 mm, which is exactly the range where an M5 bolt starts to bind. That shrink, and how to plan around it, is the whole subject of our [3D printing tolerances guide](/blog/3d-printing-tolerances-parts-that-fit). | Bolt | Clearance hole (model at) | Counterbore for socket head | |---|---|---| | M3 | 3.4 mm | 6.0 mm diameter, 3.0 mm deep | | M4 | 4.5 mm | 7.0 mm diameter, 4.0 mm deep | | M5 | 5.5 mm | 8.5 mm diameter, 5.0 mm deep | If you use socket-head cap screws (the standard for extrusion), a counterbore lets the head sit flush or recessed instead of standing proud and fouling whatever mounts next to it. The same clearance-and-counterbore logic, including countersinks for flat-head screws, is covered in more depth in [bolt and screw holes in 3D prints](/blog/bolt-and-screw-holes-3d-printing). ## Should I use round holes or slots in the bracket? Use a slotted hole when you want to slide the bracket along the extrusion before you tighten, which is most of the time. A round hole pins the bracket to one exact position, so it only works if you already know the final spot. A slot (an elongated hole a few millimeters longer than the bolt clearance) gives you a range of adjustment and forgives a hole you placed a millimeter off. A practical rule: put a slot on the piece you want to adjust, and round holes on the piece you want fixed. For a bracket joining two extrusions at a corner, slotting one arm lets you square the frame up before locking it down. It is the printed-part equivalent of the adjustment you would otherwise get by loosening and nudging metal brackets. ## How thick and how strong does a printed bracket need to be? Thickness depends entirely on load, but here is a sane starting point for PLA or PETG under light-to-moderate loads: a 4 to 6 mm plate, printed with 4 or more perimeters and 30 to 40 percent infill. Thin flat brackets flex, and a bracket that flexes lets your frame rack out of square. The bigger lever than raw thickness is a gusset. A flat right-angle bracket loaded at the tip is basically a lever trying to peel your layers apart, and layer adhesion is the weakest direction in an FDM part. Add a triangular gusset (a web of plastic running diagonally across the inside of the corner) and the same bracket gets dramatically stiffer without much more plastic, because the gusset carries the load in compression instead of asking a thin plate to resist bending. If you want the full reasoning on why parts fail in the layer direction and how to orient around it, see [why 3D prints break along the layer lines](/blog/why-3d-prints-break-along-layer-lines). Print orientation matters as much as the gusset. Orient the bracket so the main load pushes across the layers, not so it tries to split them apart. For a corner bracket, that usually means printing it flat on the bed with the corner in the plane of the build plate, so the layers run through the joint rather than stacking across it. ## Should you print a bracket or buy the metal one? Honest answer: it depends on the load, and both have a place. | | 3D printed bracket | Metal corner bracket | |---|---|---| | Cost | Pennies of filament | A dollar or two each | | Custom geometry | Any shape, angle, or hole pattern you want | Fixed catalog shapes only | | Structural load | Light to moderate, gussets help a lot | High, this is what they are for | | Lead time | Print it tonight | Order and wait, or drive to the store | | Best for | Sensor, camera, cable, and panel mounts, odd angles, custom adapters | Load-bearing frame joints, anything safety-critical | Print the bracket when you need a shape the catalog does not sell: a mount at a weird angle, a plate that holds a specific board or camera, a cable guide, a spool arm, a gusset the metal ones do not include. Buy the metal bracket when the joint carries real structural load or failure would be dangerous. There is no shame in a hybrid frame that uses steel at the corners and printed parts for everything else. That is how most well-built extrusion projects actually look. ## How Meshra helps you get the holes lined up Here is where designing the bracket parametrically saves you the reprint that guessing costs. In [Meshra](/builder) you describe the bracket in plain language, and it writes real parametric CAD (CadQuery on the OpenCascade kernel) and builds an exact solid, not a triangle mesh you can only re-roll. You might ask for "a flat 2020 bracket 40 mm long with two 5.5 mm bolt holes 20 mm apart and a triangular gusset," and get a solid part with those holes actually placed. The payoff is the first test fit. Every dimension Meshra generates becomes a slider: plate length, thickness, hole diameter, and the spacing between holes. If your bolts do not quite line up with the T-nuts, you nudge the hole spacing a millimeter and the same code re-executes deterministically with the new number. The holes move, everything else stays put, there is no AI call, and it is instant and free on every plan. If an M5 bolt binds because the hole printed tight, drag the hole diameter up 0.2 mm and reprint. That beats reopening a mesh editor or starting the model over. Meshra also runs a printability check on the loaded part and will flag a wall thin enough to be a printing problem, for example "2 walls may be too thin to print reliably, estimate only," which is a useful nudge if you made a gusset or a plate too skinny. Be clear on what that is: a geometry-reliability check, not a structural load calculation, so it does not replace matching the bracket to the load as above. When the part is right, export STL for your slicer, STEP if you want to open it in another CAD tool later, or 3MF for a modern slicer. The related [wall bracket guide](/blog/design-a-wall-bracket-that-fits) covers the same measure-then-tune workflow for brackets that mount to a wall instead of extrusion. ## FAQ ### What is the slot width on 2020 extrusion? On standard 20-series profiles the slot opening is usually 6 mm, sized for M5 T-nuts. But slot width varies between extrusion variants even when the outside is 20 mm by 20 mm, so measure your actual slot with calipers before designing anything that has to accept a specific nut. ### What size hole for an M5 bolt in a printed bracket? Model a clearance hole at 5.5 mm so the M5 bolt passes freely. FDM prints holes slightly undersized, so if the bolt binds, open the modeled hole by 0.2 mm and reprint. Add a counterbore around 8.5 mm diameter and 5 mm deep if you want a socket-head cap screw to sit flush. ### Are 3D printed extrusion brackets strong enough? For light-to-moderate loads, yes, especially with a gusset, 4 or more perimeters, and print orientation that keeps load across the layers rather than peeling them. For load-bearing frame joints or anything safety-critical, use metal brackets. Mixing the two on one frame is normal and sensible. ### Do I need slotted holes or round holes? Use slotted holes on the bracket arm you want to slide and adjust before tightening, and round holes where the position is already fixed. Slots forgive a hole placed slightly off and let you square a frame before locking it down. ## Where to start If you have an extrusion project that needs a bracket the catalog does not sell, measure your slot and bolt first, then describe the bracket in the [Meshra builder](/builder) with those numbers called out. Print one, check how it seats against a T-nut, and tune the hole spacing and diameter with a slider from there. The [template gallery](/templates) has brackets and mounts with parameters already exposed if you want a starting point instead of a blank prompt, and you can build your first part free without an account, then [create an account](/sign-up) to save it and keep iterating. See [pricing](/pricing) for current generation allowances. A printed bracket that fits your frame on the first try is not luck. Measure the slot, size the holes for your bolt, add a gusset where the load wants to peel the layers, and when the first fit is a hair off, fix the one number that is wrong instead of starting over.