eShapeOko Part 1

I was partway through writing a draft for a post about PCB milling when I realised I hadn't yet written anything about the machine I've been using, so I think it's about time for a post about the eShapeOko CNC machine I built.

I ordered the mechanical kit only, so no motors or electronics were included. To kill time while the much awaited parcel made its way to me, I ordered a pair of EasyDriver stepper controllers and some stepper motors to experiment with. The EasyDriver boards aren't particularly cheap but very easy to use, just supply power, ground, connections to the motor and signals for Step and Direction.

I soldered some pin headers onto the boards to break out all of the pins I needed. With them plugged into a breadboard I could easily experiment with driving them via a Minimus USB microcontroller board. Thanks to the simplicity of the stepper controllers I had it driving the motors nicely within an hour, though unfortunately I shortly after discovered several ways to fry the Allegro A3967 chip that the EasyDriver is based on.

EasyDriver stepper boards

The first mistake was forgetting to power off the boards while unplugging a motor. The controller keeps the coils in the stepper motor energised continuously in order to hold the motor steady. Suddenly removing the load tends to fry the controller. When I make a permanent controller board for the machine, I plan to use locking connectors to avoid absent mindedly unplugging the motors in this way.

The second mistake was playing around with circuit on the breadboard while it was powered up. I accidentally connected the Step input to my 12v supply instead of the regulated 5v output that the EasyDriver provides.

Either way, I now had two unusable EasyDriver boards. Fortunately the A3967 chips are available for about 1/4 of the price of the EasyDriver board, so I was able to order some replacements. Despite being surface mount chips they were relatively easy to replace, with a fine tipped soldering iron. The hardest part being removing the old chips as the solder wicks under the pins. Some brute force was required, but not too much as it is easy to peel up the tracks.

With the stepper controllers proven to work (once refurbished) I ordered an additional board to control the third axis.

The next step was to get something in place to act as a controller. I briefly considered writing my own but decided it would add a lot of work and even more unknowns. After looking around online I decided on GRBL. GRBL is a CNC controller firmware designed to run on an Arduino, and thanks to a conveniently timed workshop at Nottingham Hackspace I had an Arduino board I'd built lying around. GRBL takes instructions in G-Code — which is a fairly standard language for controlling CNC machines — and outputs step and direction pulses.

GRBL is designed to read G-Code through the Arduino's serial port, so a USB to serial converter was required (and was also required to program the Arduino). Fortunately, the trusty Minimus board came to the rescue, along with some instructions provided by Reading Hackspace. With a Minimus acting as a USB to serial adapter, I had the electronics side of the machine in place (albeit cobbled together).

Cobbled together CNC controller

I'll cover the mechanical side of it in another post.

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