Build Your Own CNC Foam Cutter

After printing out, taping together and hand cutting about 20 Flite Test planes, I decided there had to be a better way.

I've had an interest in building my own CNC machine for many years, but the cost was always a barrier to entry. That all changed after I acquired a 3D printer. I found this really cool machine on Thingiverse called the Mostly Printed CNC (MPCNC). The designer Ryan, has come up with a rigid design that uses inexpensive components like electrical conduit, roller skate bearings, and 3D printer electronics. He has a version for 3/4" conduit, 25mm conduit, and 1" stainless steel tubing.

Print the Parts

If you do not have access to a 3D printer, Ryan sells these parts for $195.

Follow the instructions on https://www.vicious1.com/assembly/, and you end up with something like this.

Add Some Tools

This machine can use many different tool heads like Dremels, routers, spindles, vinyl cutters, drag knives, extruders, lasers, etc... Search Thingiverse for MPCNC and you will find mounts for them. But the best thing for foam board is the needle cutter that was shared with us by djk4linux in his forum thread Cutting-foam-sheets-with-a-needle! This long running thread has seen many versions of the cutter. My current version looks like this:

Assemble the Needle Cutter

The printed parts are the frame/mount, the flywheel, and the needle mount. They can be found at https://www.thingiverse.com/thing:2429886. My version is set up for a 2212 brushless motor. The flywheel mounts like a prop saver.  

The "needle" is a length of 0.032" music wire. It is inserted through the tip of the holder into the hub. There is a 90 degree bend on the end in the hub and it is pulled tight into the groove in the hub. The holder is then pressed onto a 10 mm O.D. bearing from an RC car. This bearing is screwed onto the flywheel in the first off center hole. Optionally, you can bend a loop in the wire and wrap it directly around the bearing. This is the way we have been doing it, but every time I've had a needle break, it was at the bend just below the bearing. I have not broken a needle since I started using the plastic holder.

Once the bearing/needle holder is mounted, you have to balance the flywheel using screws and nuts in the provided holes around the outside of the flywheel. djk4linux has provided a more detailed balancing procedure in the thread.

I am powering my ESC from the same ATX power supply that powers the Arduino/Ramps 1.4 driving the machine. I have a servo tester connected to the ESC to control the motor speed.

Guide the Needle

I have used athletic ball inflation needles for guides. They work well, but wear out after a few hours of use. I am currently using .035" MIG welding tips. You can find them in the welding supply section of your orange or blue store. The 3/4" block of wood has a 5/64" hole drilled all the way through. On the bottom, the hole is enlarged to 5/32" deep enough to thread in the MIG tip. I added some aluminium cooling fins to this one, but they are not really necessary unless you are going to try for higher speed cutting. 

Once the guide is mounted, cut the wire so that when the flywheel is in the full up position, the wire flush with the bottom of the guide. Use a file or stone to dress the tip into a conical point. A chisel point works, but the kerf will be wider in one direction than the other.

Hold Down the Foam Board

I used a piece of blue insulation foam for the bed/waste board of my machine. After it was secured, I mounted my small router and with a 1/2" bit, milled the bed level with the machine. Then you can place the foam board, crown side down on the bed and secure it around the edges with T-pins. This method works OK, but is time consuming and sometimes score cuts can be through cuts.

After FliteFest 2016, I added my vacuum hold down. I simply milled out 1/4" wide, 1/2" deep channels every 2" across the bed and spray glued down a sheet of foam board with 1/4" holes in a 2" grid. The hole in the lower right goes all the way through the base and is sized for my small shop vacuum.

Get the Files

The process for converting PDF plans to GCODE is not simple, but basically, convert the PDF to a .dxf file (most of us are using Inkscape), import the .dxf into a CAM program (Eslcam works great and is inexpensive) and define your cuts, then save to a SD card to load into the MPCNC.  Jason Hitesman has fully documented the process in a YouTube video.

Cut Your Planes

Set the needle speed to get 10 strokes per mm of travel. A good speed to start with is 600 mm/min travel and 6000 rpm on the cutter. Here is an example of a cut at these settings

You can see that it is a clean square cut.

Cost

PLA for parts ~ $30

3D printer electronics $35

Stepper motors $55

Gt2 timing belt $9

Bearings $40

Hardware $45

Power supply from scrap PC

Motor and ESC ~ $25

Conduit $8

Wire from scrap bin (speaker wire)

Total: $257

Alternately, if you do not want to source your own parts, Ryan sells a parts bundle from $252 plus $195 for the printed parts.

Conclusion

For a little money and a large investment in time, you can build your own CNC foam cutter. It takes about a half an hour to prepare a set of plans for cutting and about 15 minutes per sheet to cut them out. There is less fear in flying for me knowing that I can just cut another one.