CNC Build – Part 2: The Mechanicals
May 8, 2011 Leave a comment
The idea behind a CNC is that the unit moves the router around on its own. This being the case, I needed to design a way of incorporating automated movement into my mill. Turning from woodworking to electronics, I began planning how I would move the various parts of my mill electronically.
Relying on every other CNC mill that I’ve seen, plus all of the robotic theory at hand, the obvious choice for my actuation is stepper motors. Stepper motors, like regular electric motors, can spin continuously and can turn in either direction. Unlike a conventional motor, however, a stepper motor moves in precises increments called “steps”. If you add power to a regular motor, it will spin until power is turned off. In this fashion, it is difficult to specify how many degrees the shaft has turned. With a stepper, however, it will only turn one step. The motors I purchased have step of 1.8 degrees. This means that I can control the rotation of the shaft by 1/200th of a turn. Regardless of voltage or amperage supplied to the motor, it will only turn 1.8 degrees at a time.
Using a stepper motor, I can control rotation, but I need to convert that to distance across an x, y, and z axis. There are several options I could have used to convert rotational movement to linear movement. The two common ones in the CNC universe are belts and lead screws. Using a belt, the object that must move linearly is attached to a point on the belt. As the motor turns the belt, the object moves accordingly. This works nicely, but I chose the other route: lead screw. Attached to my stepper motor is a 3/8″ diameter threaded rod. The rod passes through the object via a t-nut. As the lead screw turns, the t-nut threads up and down the rod, moving what the nut is attached to accordingly.
In my CNC, I will have three lead screws; one per axis. Each lead screw has its own stepper motor. As I had mentioned, the stepper motor turns in 1.8 degree steps which means 200 steps per rotation. The threaded rod had 16 threads per inch (a nut would have to turn 16 full rotations to move one inch). This equates to 3200 (200 x 16) steps per inch, meaning that each axis will have a resolution of 1/3200″ (minus any slop). I feel pretty good about those numbers.