Lego to Fusion: Designing a Missile Launcher

This story, like many others, starts with Lego.

We all started with Lego at some point. I don’t care who you are, I don’t care what your gender is — we’ve all had our hands on Lego. It’s one of the high points of developmental life in the civilized West; access to Lego is unrestricted, which you discover for the first time in a personal way once you step on one of those little bricks in the middle of the night. At that point you may actually wish access was a little more restricted.

Maybe even a two week waiting period.

The rest of the time, you absolutely adore the ability to make anything — as long as you can come up with a way to put it together from roughly 4.5 mm studs. People can do some amazing things with Lego pieces which are on the market.

Consider the pieces that people build for Mobile Frame Zero, a tabletop wargame of giant robot combat — where everything is made of Lego.

(There’s also Intercept Orbit with fleet combat in deep space for those inclined.)

And if you wanted to play games with Lego that weren’t necessarily giant robot combat or space fleets, there’s good ol’ straight-up Brikwars.

Effectively, what I’m saying here is that everyone loves Lego, everyone’s played with Lego, and for the foreseeable future (and probably into the era in which we have actual giant robots to clash against one another) will all continue to enjoy Lego.

Which brings up an interesting fact coupled with the recent developments in 3D printing, making whatever custom object you want evermore able to be designed and implemented in a matter of days.

What if you wanted your own custom Lego block?

Yes, yes, I know — Lego currently makes a truly disgusting number of little fiddly blocks of various sorts, uses, and purposes. Odds are good that whatever you want in terms of constructible architecture is already available from Lego themselves. In fact, if you want to play around with the entire catalog of Lego components, they make it easy.

Lego Digital Designer is effectively a 3D CAD system, produced by Lego, which lets you assemble any Lego pieces in the collection, however you like, and then get a parts list of what it takes to build that wonder that you’ve put together. Sadly the service that allowed you to take your model and have it delivered in its own Lego branded box has been out of play for five years or so, but you can still get the parts list, and you can still create your own assembly instructions from that model.

If you want to step outside the bounds of the “official,” LDraw still exists and has an even larger collection of “virtual pieces,” some of which have been designed by the community. For the most part, none of those have really seen the light of day in a physical form, but the idea is there.

There are companies that are making “Lego compatible” bricks and accessories for minifigs, one of the biggest being Brick Warriors which makes helmets, clip-on armor, and other accessories for figures both fantastic and incredibly modern (including a surprisingly dense collection of weapons and gear for World War II). They manage to produce quality pieces at a level which earns them invitations to major trade shows and conventions. Brick Warriors are extremely clear in their discussion of their work that their pieces are injection molded from models created in CAD software, printed in heat resistant polymers, and then the injection mold made from those initial prints.

Therein lies the rub for home designers interested in producing Lego compatible blocks. Lego themselves are made from high density ABS plastic injection molded it to ridiculous tolerances. The precision necessary to make Lego in mass quantities is fairly staggering. We’ve all had some of those knockoff blocks which never seemed to go together with the Lego that we already had; it never quite linked, it never had that satisfying click into place — it just was never good enough.

But times have changed.

Traditional 3D printing (if we can be said to have such a thing quite yet) is FDM, which means it’s based on a hot head full of melted plastic squeezing it out like toothpaste and having it cool so that more can be deposited on top as the head moves around. Surprisingly, that’s really cheap — comparatively — and well within the reach of the home fabricator at this point. But it’s not the first kind of additive manufacturing that we developed.

That would be SLA. SLA printers work by having a pool of resin which gets shot with a laser, turning it from liquid to solid along a surface, whether that surface be the print bed which lifts slowly out of the pool or part of the print which is itself attached to the print bed. Because the resin is far more expensive to maintain than just rolls of plastic loops, and because the mechanisms for accurately aiming lasers are inherently more expensive than the mechanisms for moving a hot plastic squeeze tube around, SLA printers have been more expensive.

But they’re getting cheaper.

For the sake of this discussion, the important thing is that they become cheap enough that small businesses in your area probably already have one, and very likely are willing to produce pieces for other people. Oh yes, and there about an order of magnitude more accurate in build tolerances than FDM printers.

Some people have been getting acceptable tolerances for Lego compatible bricks from hobbyist FDM printers, which is awesome. But you can get absolutely acceptable tolerances for Lego compatible bricks from an SLA printer.

Which brings us — at long last — back around to my project:

Could I design a Lego compatible brick for a singular, specific purpose, which would be “reasonable” to purchase?

Let’s take a look at the process.

You’ll notice I start with what looks like a Lego base already in place, which probably seems like cheating.

That’s because it is cheating. Sort of.

See, here’s the thing. I like to work in parametric CAD systems, which means that all of the volumes and shapes are geometrically described by volume. With a parametric model you can have arbitrary accuracy in measurements, shapes, and designs. It appeals to the engineer in me.

If you go searching for “Lego CAD models,” you come up with a lot of STL files — and STL is great for printing on your desktop FDM printer but it’s lousy for doing actual engineering off of, because it’s made up of thousands of tiny triangles which are an approximation of an underlying geometric shape but typically vary in detail because of the limitations of the mesh description.

I’ll show you a close up of this base model converted to STL as an example.

The edge thickness analysis mode for models on Shapeways is surprisingly useful for this — and points out another issue which we’ll talk about shortly.

Notice how rough and approximated that mesh is compared to the actual design. Surfaces which should be flat are only “mostly” flat, and particularly when it comes to one curve shading into another, a triangular mesh doesn’t necessarily describe the geometry as perfectly and repeatedly as one would like. This analysis has the roughness exaggerated slightly, but not by much when looking at pieces this small.

You’ll also note that the thin edges of that front inset are marked as well below the threshold of thin wall that Shapeways is comfortable doing — in the base plastic. This is another side effect of manufactury, even on an extremely high end, industrial plastic printer; there are limitations in resolution based purely on the material you because of inherent inaccuracies in working with it.

Short version: I went looking for actual Lego pieces to work off of and realized what I needed was not STL files of the shape of one but an actual CAD-level procedural/volumetric file.

Lucky for me, other people are just as geeky and have already come up with procedural generation of Lego compatible bricks, often using a free CAD solution called OpenSCAD, which takes a short program and generates geometry based on that code. It’s extremely good at creating geometry which is mathematically regular.

If nothing else, Legos are mathematically regular.

I did a lot of digging around looking for a really good parametric brick generator and started with a lovely piece that I found off Thingiverse. It’s not exactly where I ended up, but if you’re interested in doing your own pieces it’s a fine place to start.

It could never be over that easily. The output from OpenSCAD wasn’t actually in a format that Fusion 360 could use, so I had to go and take the CSG file from OpenSCAD into FreeCAD, another free CAD software, in order to output an STEP file — and that, dear friends, is an excellent volumetric/CAD description file which should work in your favorite parametric CAD modeler, no matter what it is.

In this case, I started with a simple 1 x 2 base with a flat top, one third of a block high, and with a little bit of a lip around the bottom so that you can pry it loose when you’re done. I considered excluding the lip, but I’ve had enough issues with real Lego to know what happens when you put something like this at an edge and want to get it loose later.

Getting that into Fusion was the first step.

Then the missile launcher support needed to be put together. There are a a lot of ways that you can do stuff like that, but I decided on a nice, fat neck with a little bit of a slope at the top in order to give the thing a little of that “elevated” feel. All I did was create a side-on sketch to define the slope of the top and how far up it would go, then I created a circle on the top of the block and extruded up to a plane which I based on that first sketch. Easy, and then you have a nice plane to build and ground the rest of the missile box off.

Things are going to look a little strange when you build sketches which are not parallel to one of the axial planes. Make sure that your camera is in orthographic mode and you should have much less trouble, but you will find yourself spinning the camera around just so you can get a better idea of where your sketch entities are, so resign yourself to that now.

You can do amazing things with just and extruded box and fillets/chamfers. You can also do amazing things with rectangular patterns in your sketches, but if you’re me you’ll end up spending half of the time designing the whole piece trying to get a nice, straightforward, repeating pattern of circles which fill the front plane of the inset. What I ended up doing in the sketch itself was getting half of the circles and places I felt worked out then mirroring across a vertical line on sketch to the other side. It felt kind of hack-y, and I’m sure there’s a far better way to do that, but no amount of playing with the rectangular pattern really made me happy.

I thought about using a series of spheres like I did on my previous piece inspired by the Astroneer terrain deformation gun, but while rectangular patterns do work with bodies and features, it just felt like I wasn’t getting the right tool for the right job. Ultimately I ended up simply extruding the circles into the body and putting a fillet on their bottoms to make them almost spherical.

The targeting array on top was really kind of an afterthought. I looked at things and really had the sensation that they were just “too naked.” There’s nothing going on in the targeting array which is really worth talking about from perspective except for the fact that it’s built off of a rectangle sketched on to the top plane of the missile box body, which means that underneath it at the front and along the curved side there is open space. I considered my options on that and ended up simply extruding the whole thing downwards into the body as a whole but not so far as to enter the gaps in the missile tubes, because trying to “extrude to face” wasn’t working as there are multiple faces underneath it. That’s a serious limitation that I don’t run into in Onshape because it has “extrude to body.”

Once I had the general shape of the array put together, I merged it with the missile box body, and at that point I could treat the area at which it joined as just another edge where I put a bit of a fillet. Completely unnecessary and probably less than the resolution of any printer that this thing will ever see, but it made me feel good and that’s what’s important.

Once the array was together, the piece was done. Then it struck me that I had built something with a handedness. If you build something that is intended for other people to design with and it has a particular left-hand orientation, it makes sense that you should also build a right-hand model. Parametric modeling makes that easy; you just mirror across a plane and you have what you need.

The rear tubes are just extruded cylinders again with fillets. Nothing fancy, nothing exciting, just the circles from the front projected on the rear face, extruded, and filleted. Tedious only because each of the inner circles had to be manually clicked on.

And that’s it for the piece in terms of design.

So now let’s talk about the last important thing about pieces like this, and that’s “how much would it cost me to have one?”

That’s a big deal if you actually want to be able to use these missile pods in your MFZ giant robot or just on that cool Mad Max post apocalyptic Lego dune buggy you’ve been working on in the garage. I know you have one. I’m not judging.

You can have one of these sweet little honeys in PLA plastic from Shapeways for about $3. (Without shipping.) The detail level will be terribly low, it’ll be kind of rough, but I suspect that the Lego interaction would be “good enough.” At that price you could have ten or so to create an entire army if you didn’t mind doing a lot of cleanup on the pieces.

If you really want to get a quality piece for using with your Legos, you’re going to want to use their Frosted Ultra Detail or Frosted Extreme Detail resin offerings, and the cheaper of the two will run you about $9.50 each (without shipping). Frankly, that’s a little rich for my blood unless I absolutely had a purpose in mind for a custom piece. For some people that will be an absolute steal.

Because I am a completist at heart, I had to check with the local offerings for resin printers were, and there is one stop shopping for that in my area and probably in yours: 3D Hubs.

I popped the model over into 3DH and checked my local suppliers who have a Form 2 SLA resin printer, and there’s a guy here in town who is willing to do the print for just south of $6, with no shipping because I’ll meet him at the local Starbucks to get the piece. That could be within my margin of “willing to spend that much on a single custom piece” if I had a very specific project in mind — say I was putting together an MFZ army to take to conventions. It very well might be worth my time and money to drop for several.

This project exists because I was curious. Lego is fantastic. It is one of the best and easiest ways to get both kids and adults into thinking about mechanical engineering. But sometimes, just sometimes, you want “the right part” and that part doesn’t exist. I wanted to know if it was reasonable and possible to design an accurate piece which served a singular purpose and which could be purchased and used for a reasonable amount of money.

I think the answer is yes.

Let me know what the price point would have to be for you to be interested in buying custom Lego pieces, or even if it’s a thing you imagine would be worth doing. I’d really like to know.