Check Out a Newer and Cheaper Electric Discharge Cutter by Rack Robotics

The problem with traditional CNC machines

Traditional machining typically revolves around three or four types of tool heads, all of which are mounted upon multi-axis systems. Subtractive manufacturing is normally performed by removing material from stock with either a fast-spinning mill, a laser, water jet, or a jet of hot plasma. Meanwhile, additive manufacturing is achieved through the use of melted plastics, such as fused filament deposition, or via a resin/small particles that are bonded together by a laser in a series of layers. Scaling these up, however, requires far larger components and additional force, thereby increasing costs.

What is electric discharge machining?

Electric discharge machining is in the subtractive manufacturing family, but rather than moving a jet of light/water or a spinning mill trough stock material, it uses many small sparks to cut. This allows for the tool head to remain extremely light, with the only additional weight coming from a larger power supply off to the side. Currently, prices for EDM machines range from $3,000 all the way to $30,000, making them outside the budgets of most hobbyists. But the startup Rack Robotics has announced their own desktop version, called the Powercore, that only costs a few hundred dollars while maintaining the ability to cut through sheets of solid metal. Best of all, it comes as an open source kit that simply adds onto an existing CNC machine.

The base device

No matter the number of axes, all CNC machines rely on a series of steps that tell the tool head where to go, how quickly, along with other information about the entire system. 3D printers normally have three linear axes for X/Y/Z motion, and this is why Rack Robotics decided to go with existing printers as their base device. Before machining begins, the user jogs the end to the corner of the stock, sets the home position, and begins cutting with a GCODE file.

Delivering power for cutting

Because EDM machines use sparks to cut, they also require ample amounts of power, and this needs to be delivered by a suitable power supply. The Powercore PSU's life started out on a breadboard, with a buck-boost converter providing the correct voltage, an Arduino Nano outputting the PWM signal, and a current sensor for detecting short circuits. When the team moved on from the breadboard design to a PCB, they swapped out the Nano for a Raspberry Pi Pico due to its larger number of pins, faster processor, and ease of programming.

Designing the PCB was a challenge in and of itself since no one on the Rack Robotics team knew how to create them. This meant the first iteration has messily routed traces and was lacking the ground plane pour, thus causing the power MOSFETs to burn out. However, one week of further revisions resulted in a far more effective design. Finally, the team realized that the seven expensive power resistors could be replaced with a single PCB containing a space-filling Hilbert curve that had the same electrical characteristics.

Assembly and safety features

Assembly started by 3D printing each duct and some of the housing, where the components were the attached to each other with screws. A large heatsink was added on top of the power resistor PCB to adequately cool it while a ducted fan blows air over top. Lastly, the controller PCB was installed near the front along with two plugs for connecting the cutting end.

In order to ensure safety while in use, a continuous 24V must be supplied to one of the front connectors. Additionally, there is another port which outputs a voltage that is proportional to the amount of current being used, and it can assist in overcurrent protections. The last connector is an alert signal to indicate when an error is occurring.

Next steps

The Powercore from Rack Robotics is recently ended it funding period on Kickstarter, but there are plans to open source all design files once the campaign has finished. For more information, you can watch the team's demonstration video here on YouTube.