Background
CNC routers are versatile, programmable machines that use a high-velocity cutting bit to cut complex patterns out of a piece of stock. Usually, CNC routers are used to cut 2D patterns out of large pieces of flat stock compared to a CNC mill, which tends to work with a bar or block stock. Unlike many other 3-axis CNC machines, routers tend to have fixed stock and move the head in all axes. CNC routers tend to be ~$400-$2000 depending on size and machine precision.
Concept
I wanted to build a machine that was rigid and precise, but also inexpensive and easily repeatable by others in the open-source community. This influenced many of my design decisions at both high and low levels. The bulk of the frame is made out of 2020 aluminum extrusion. Aluminum extrusion is a fantastic tool for building motion systems as it is strong, relatively lightweight, maintains excellent straightness, and has very versatile mounting options. Additionally, it is relatively inexpensive and is easy to acquire, making it an optimal primary material for an accessible build like this one. Another thing I wanted to try was to use high-quality steel linear rails for the motion axes. Linear rails offer low friction and top-notch rigidity and are excellent for precise motion systems like this one. For this project, my custom components needed to be versatile as well as rigid. To achieve this, I employed a technique I’ve been looking to implement for a while: interlaying aluminum plates within 3D printed parts and connection points to maximize rigidity while maintaining the ability to employ many mounting points and complex geometries of 3D prints. For motion, I employed 16mm ball screws and lead screws. Ball screws offer precise, low-friction motion control for high-load systems. Belts would be an acceptable and cheaper alternative for X and Y motion, but belt stretching would result in a flexibility in motion which I wanted to avoid.
I used a 500W spindle for the first iteration of the machine. The spindle is definitely the weakest point of this machine (ball screws+linear rails are used for heavier-duty machines than this). This was intentional, in case I ever wanted to upgrade this machine with a more powerful spindle. I over-engineered the gantry now because that would be much, much harder to change later compared to the spindle head.
To run a job on a cnc router, you need to first program the toolpaths of the specific job you want to run, including tool type and size, speed, feed, stepover, depth of cut, etc. Once that has been done, a universal Gcode sender needs to interpret these toolpaths and turn it into Gcode: a script that a CNC controller can understand and interpret into motor movements. I am using linuxCNC to accomplish this. Once the Gcode is sent to the computer on the router (I chose to use an Arduino UNO and a CNC breakout board), the computer uses the Gcode and the pre-programmed firmware (GRBL) to turn the Gcode commands into specific commands for each motor.
Fabrication
The entirety of this CNC router was fabricated at the Flowers Invention Studio using a variety of tools. All of the 3D printed parts were printed at the Hub and EcoLab printers. All Aluminum plates were cut on the OMAX waterjets, chamfered, and sand-blasted in the Metal Room. The extrusion pieces were cut on the Wood Room miter saw and then drilled, counterbored, and tapped in the Metal Room.
Assembly
One of the biggest challenges with assembling a CNC gantry is ensuring squareness; that is, that each axis of motion is orthogonal to the other two. To ensure a relatively square machine (no more than 0.01 in over its range of motion), all mating surfaces between structure and motion were constrained by machined surfaces or by the locations of precisely located holes. To allow the machine to be trammed (calibrate the axes), several oversized mounting holes were used so that the motion-defining rails could be adjusted and re-measured.
Conclusions
I am very happy with how this machine came together with no major snags or required redesigns. It will take time for the machine to be tuned for optimal settings to allow it to make clean cuts in woods and metals, but the bones of this machine are strong and capable, and should act as an engine for creating many cool projects in the future!
Parts List
Funded and supported by the Invention Studio at Georgia Tech, a student organization
