Custom Lego® Parts (Howto?)

Note: If you thought this was a neat story, you'll probably be more interested in my new series, which shoehorns a 400Mhz strongarm into a 5x11 3 tall lego block, along with bluetooth, i2c networking, and a Linux kernel ;) More details here: Lego Hacking: The GumBrix® Part 1.

I was working on my robot balancer earlier today, and realized that I was completely bored with the project, as well as a little disappointed with the direction it was taking. Hours and hours of debugging assembly optimizations for a PID loop are not my idea of fun (in fact, that is one of the things I do for a job), so I suddenly just put it to an end.

I have had a Lego Robotics Invention System (1.5) for a long time now, and had a lot of fun building a variety of robots with it, but grew bored when I got the feeling that I had outgrown it's capabilities. While it is true that 3 channels of input and 3 output drivers are not enough for everybody, I underestimated (as I always do) the complexity of building up the robot from scratch. I realize now that a better approach is a compromise.

That is why I decided to build my own lego parts. Drawings, NC code, and 3d models follow in the extended...

Of course, not everybody has access to their own CNC machine (thanks Andre!), but for those who do, here is a lesson in building up custom lego parts. This is not necessarily a design just for those of us lucky enough to own a CNC machine. A manual milling machine, or even hand tools and a great deal of patience will do the job. This post IS however directed at somebody who owns some sort of CNC milling machine, has access to some plastics and the tools appropriate to cut them, and somebody who is not a rank beginner at building lego robots.

Measurements of the lego parts are easy to come by on the net, and I was able to easily build up a model of the part I wanted to build (a foot for a walker) in a couple of hours. My part was not available in any kit, so I was forced to build it, but really it was an experiment as much as a necessity.

The first thing to do is get your hands on some plastic. The parts in this tutorial were cut from ³/16" polycarbonate. This is by FAR the best material I can find for this job. Polycarbonate sheet is strong, light, hard, and relatively easy to machine. Barring this, acrylic and nylon are good choices. Delrin is good for parts that do not need to stick together, but a brick made from delrin will rarely stay attached to regular lego; it is just to slippery. Polycarbonate sheet is available at a variety of online vendors, but I got mine from the scrap bin at Industrial Plastics and Paints in Langley. They are just a 2 minute drive from my house, and they always have good stuff in the junk bin. Highly recommended.

Next up, get yourself a ¹/₈" endmill (flat). My program used this for ALL cuts, including the holes for the axles. Plunges are all really shallow, but I still recommend a lot of coolant to keep chips breaking nicely, and clearing the tool (you do always use coolant don't you)?

If you do not have a milling machine, the axle holes can be made with a ³/₁₆" drill, and are REALLY close.

Next up, load the G CODE program into the NC program of your choice. This one is designed to work equally well in either the amazing Mach 2 Mill, or the inexpensive/free TurboCNC. I have used both, and believe that this design will mill better on Mach2, as TurboCNC has a little bit of trouble with small diameter circles, but this may just have been because I didn't calibrate correctly. Your mileage may vary.

Start up your CNC and place the tip of the endmill at the bottom left corner of a piece of 4"x4" polycarbonate stock, with the tip of the endmill touching the plastic. Zero your machine and then move the Z to 2" above the stock. You can now run the program, making sure to use a lot of coolant (it will keep the plastic shiny and smooth).

How shiny and smooth?
The image doesn't do it justice. Not quite glassy smooth, but better than 800 grit sandpaper.

If this program doesn't work for you, I have also attached an IGES file at the bottom of this post. I have also included a DXF file if you are 3D challenged!

* Lego is a registered trademark of Lego corporation. Please don't sue me, I am poor.