Josh On Design

I've been posting little teasers of the CNC machine I'm working on.  It's time to reveal a bit more.

In my spare time I have been working on a tiny two axis CNC machine that will be powerful enough to move a pen around like a plotter. Future versions will be larger and handle more powerful tools, but this is a good start to work out the kinks.  

So here she is in all her glory:

She's a two axis CNC. The X axis is the lower level and the Y axis is the upper, set at 90 degrees. A future version will have a Z axis as well. The structure is made of aluminum extrusion from Open Beam. This version uses about 4 meters.  You can see the nice ABS brackets on the corners, also from OpenBeam.  

Each axis consists of a structure sitting on sliders created from roller skater bearings. For movement I'm using standard stepper motors from Spark Fun powered by EasyDriver boards.  The steppers convert rotation into linear motion through the long threadscrews. While you can spend a great deal on threadscrews, these are just standard steel all thread from the hardware store (~3 USD). Everything is bolted together with Open Beam screws, which are standard M3 6mm screws and matching nuts.

Electronics

Stepper motors have to be powered and controlled by a driver board. I'm using the open source EasyDriver from SparkFun.  There are more powerful motors and boards available, but these will suffice for version 1.  They are cheap, effective, and very simple to wire up as you can see in this photo:

Each stepper motor has it's own driver board.  The four wires go into the four motor pins of the board, labeled A1, A2, B1, and B2.  These aren't actually labeled on the motors themselves so I had to test and label them by hand.  Each board also has power and ground.  They receive more than 5V since the motors usually need more. In my case I've connected them directly to a 9V battery. This would never be enough power in production, but it's okay for testing and very portable.  I'll dive more into the EasyDriver's later, but for now just know that they can take almost any power you can throw at them and convert it safely to run your motors. 

Finally the drivers have ground, step, and direction pins connected to the Arduino, which you can see in this photo:

The Arduino code toggles the two step pins over and over, occasionally changing the direction.  That's all there is to the electronics.  Arduino and the EasyDriver make it very simple.

Now let's look at the mechanicals:

Hardware

The beams are aluminum extrusions from Open Beam. In addition to being open hardware (the schematics are available to make your own) it has a few nifty features.  Take a look at this end cut:

There is one channel per side. Each channel takes a metric M3 nut or screw.  Hex nuts are the perfect size to slide in easily and not rotate. Because these are standard sizes you can buy screws from other vendors, such as these black anodized 16mm screws I got from Amazon.

I've cut most of the extrusion by hand using a hacksaw and plastic miter (the kind used for cutting wood). Eventually I bought a cheap 6" power saw from Harbor Freight that uses cutoff disks. The final cuts are smoother than a hacksaw can produce and are done in a quarter the time. It's good enough for 60 USD but eventually I will probably upgrade to a proper miter saw with a metal cutting blade.

Brackets

The extrusions are put together using injection molded ABS plastic brackets, also from Open Beam.  I mostly use angle brackets with a few Ts thrown in.

The stepper motors are attached with Open Beam stepper brackets. These are designed to accept any NEMA 17 stepper motor and come with matching brackets for the other end.

But here is the genius part: Open Beam sells an adapter which accepts #608 roller skate bearings, which look like this:

These brackets make building a lead screw very easy. I chose a 5/16 inch all thread screw at my local department store (~3 USD). It fits inside of a roller skate bearing perfectly.  The blue stuff in the photo is the skate bearing.

Speaking of skate bearings, they are completely awesome. But that will have to wait until tomorrow.

Next Time

I'll go into the bearings and shaft coupler tomorrow, two of the most critical parts of a CNC machine. Oh, and one more question. What should I name this contraption?