How I Engineer Functional 3D Prints (Sim Rig Quick Release!)

I’ve been designing and modelling parts for possibly up to a decade at this point, although I’ve massively ramped up my abilities and the number of things I’ve created recently as I’ve found more reasons to jump into a CAD program and make something. I’ve designed cases for the tools I sell worldwide – the open source testing tools – I’ve made custom 4040 extrusion mounts for my Logitech X56 HOTAS to mount to my DIY simulator rig, and most importantly for today, I designed a custom quick release mount for 4040 extrusion, but it also acts as a right angle bracket for the 80 by 40 extrusion that makes up the main arm of my sim rig. This is a structural part, and a quick release mount all rolled into one, and I figured with how proud I am of this design that it’s worth showing off, but more so it’s worth explaining how I made it so hopefully you’ll be able to pick up at least a rough idea of how you can get started making stuff yourself. 

First thing’s first, we need to assess the problem we’re trying to solve. For me that was how to best attach my flight sim hardware to my sim rig. I had already designed a sort of quick release mount that bolts to the cross bar and then you need to drill alignment holes in the extrusion to secure the flight stick or handbrake to the rig, but that is clunky, a pain to secure, not very secure, and way too high. It also has limited adjustability and is a right pain to drill the holes in the extrusion for. So I need something better. The problem is just moving the mounts to the underside means the flight hardware and my legs start to occupy the same space making it very uncomfortable. I realised the only way to fix this was to replace the cast aluminium right angle brackets with custom 3D printed ones.

The design process for me almost always starts with a vision of what I want in my head, followed by a pencil and paper sketch. My desk is littered with little pages filled with designs and layouts. This is all the sketching I needed to do for this part, but it’s basically a way to work out what you’ll want the thing to look like, and this is the stage that often irons out most of the kinks in the design. Initially I drew the extrusion holder part, but while drawing that I was thinking about how it would attach to the double wide extrusion, and how secure it’d be given it’s printed plastic and not cast metal, which is why I added the tabs. Thinking about the load paths is useful for structural components like this. My wheel is mounted to this cross bar, so the main forces this mount will see are twisting loads front-to-back, and a bit of twisting left to right too. With that in mind, adding the tabs adds four more attachment points with leverage, on top of the four-per-side points internally.

Once you’ve got the rough idea it’s time to start modelling in your CAD program of choice. For me that’s Fusion 360 – that isn’t an endorsement, in fact much like Adobe I kinda hate that it is the program I learned to use first – but there are plenty others for you to choose from. I’ve heard FreeCAD has improved its UI recently so I do intend to give that a go. For this model I am re-using a project that I’ve designed a whole bunch of parts in for the sim rig already which is also handy because one trick I’ve picked up recently is a process called ‘Parametric Modelling’. Basically you create a list of parameters – the extrusion height, what you want the inside tolerance to be, how big you want the holes for bolts, all that sort of stuff, and when you change these, everything auto updates without having to go through and change sketches. It’s really useful! It saves a lot of time and hassle later when you realise you need to change, say, the tolerance by 0.1 of a millimetre, where you only need to change one number and everything auto-updates, rather than having to manually go and change 100 different sketches.

The way at very least Fusion works is basically a collection of 2D sketches that you extrude features from. While there are a thousand ways you could design this part, and I’m sure I’m not doing it the ideal way, what I tend to do is pick a face to start with, in this case the extrusion shape and tab makes the most sense to me, then design it using the parameters I mentioned. I’ll often have to use some maths to work out where things go – for example the tabs that index the extrusion need to be in the middle of the interior space, and for my 4040 extrusion they are 10mm wide slots. So they need to be half the extrusion width from the bottom corner up, but then you need to add the half the inside tolerance as the inside corner is offset by half that amount, then you need to subtract half the slot width so the starting point for the rectangle is low enough. But remember to add back in half the tolerance so the tab has a bit of clearance to actually fit inside the slot. That’s a bit of figuring to get right, but you get the hang of it. Then you can extrude your features. In my case because I wanted a gap for a T slot nut to fit snugly in I extruded the slots the full width, then cut half plus half the width of the slot nut away, then added back half minus half the nut width to end up with the right shape. 

The holes for the bolts that make this a right angle bracket were pretty tricky. I knew that I needed four bolt holes on the back and top faces, but I also knew I wanted a hole for the hex tool to be able to reach the bolts, rather than trying to awkwardly tighten them from inside the extrusion, so my sketches for those include three hole diameters, one for the bolt itself at 9mm (M8 bolt with a bit of clearance), 7mm for the wrench, and 18mm for a washer. I then extruded, cut, extruded, cut, extruded repeatedly until I got the pattern I was looking for. I used my digital calipers to measure the bolts, washers and extrusion for all of this by the way. These are basically a must-have if you are making anything, especially if you want the thing you’re making to work with existing stuff. These are like £20 and are really handy. Highly recommend getting yourself some calipers. Anyway, with the holes made all that’s left to do is chamfer and fillet everything, and that’s basically my part done. An important feature though is the bolt-head-knob that makes this whole quick-release thing usable. I initially designed a round scalloped knob that you slide the M8 bolt through and acts as a built-in spanner, but it wasn’t overly comfortable to use so I changed it to a straight bar which works so much better. 

I’ve somewhat skipped over a few key parts here so let me rewind a little as I think this is pretty important. When it comes to structural things like this it is absolutely important to consider the strength of the material you’ll be, in this case, printing with. Plastic, especially FDM 3D printed plastic isn’t exactly known for being incredibly strong, so considering things like wall thickness is really important here. My previous prints have been 6mm thick, but that was only holding the accessories, not the main bar, so I doubled it for these brackets to 12mm. If you were sending this off to be metal 3D printed you absolutely could make this a lot smaller. If you knew you’d be printing in PETG instead of PLA again you could likely slim it down, and resin is a whole different ball game. Part of this comes from experience printing and using these materials, although if you want to be professional you can do stress concentration simulations and actual engineering, but for DIY stuff like this I’m fine with just making it thicker and spending 50p extra per part in filament to know it’ll be solid. 

Once you have your model, the next thing to do is make it real by printing it. There are a myriad of services that will do this for you at fairly reasonable rates, or if you have your own printer you can get cracking. I’ve been using both my Elegoo Mars 5 Ultra and Bambu Lab P2S a lot, although for a print of this size generally FDM printing is your best bet. I’m using my white PLA Matte for this, in part because I think it looks good on the silver extrusion, and in part because I’ve got a full roll in the AMS I need to use up to clear space for a roll of PETG. When FDM printing you absolutely need to care about the print orientation. This is honestly the most crucial thing to consider, because 3D prints are uniquely weak in shear across the layer lines. If I printed this part as it sits on the rig, it would be weak in the main load axis. It would want to split apart if you pushed on the accessory, and that’s no good. I printed it on its end so you basically have a solid hoop around the accessory, and a solid plate for the tabs that wrap around the vertical extrusion. This isn’t perfect either as these tabs are prone to shearing off with enough force, but with the rest of the mounting points included I’m not so worried. For the loads that this will see, this orientation is more than strong enough. It is of course worth also thinking about wall loops and infill amount. The stronger you want the part, the more wall loops and infill you should include. I think I went with something like 6 wall loops and 50 percent infill for this thing, which is a lot and means it ends up being around 250 grams of filament, or a quarter of a spool, but I’d rather it be solid and last than save 50 grams of filament. 

While the part itself is pretty simple, I think it’s illustrative of my design process. It’s basically a second iteration of my sim rig quick release solution, but integrating a corner bracket into it too for much more comfortable placement of accessories. I have been designing much more complex models – I’m in the process of designing my own sim racing shifter which has been a pain and a half, and this version one is basically complete and yet is a fail that I need to start over with, so I guess be prepared to re-print the same thing over and over as you tweak your designs. And I also want to make clear that designing stuff like this is a skill that takes time to develop. I started making incredibly basic stuff and couldn’t fathom designing even relatively basic parts like this in a single day – yeah I came up with this idea on Sunday morning and by Sunday evening I had this first one printed and in my hands, and by Monday night I had the second, and I’d redesigned the knob – this is something I’ve just been working at for years. Hell, I only started using the parameters this year, everything before that including all the cases for the tools were done manually. I can’t tell you how many times I ended up just starting from scratch with the case builds because the steps I took made it impossible to change something like the final height by a millimetre. A lot of the improvements I’ve made to my modelling ability is in reducing the number of actions taken, such that making changes doesn’t break the whole thing. The only ‘natural’ part of this for me is in imagining the parts in the first place, as I have a photoreal imagination which helps a lot. But even then my ability to come up with designs and figure out how a part might work is something I’ve absolutely developed over the years, so don’t get disheartened if you’re struggling right now. I know how dejected I feel when it feels like I’m just incapable of doing something. It’s a skill, you can learn it. You got this.