CNC controllers

Having just looked at the physical CNC machine, we’ll next take a look at the unit that supplies all of the power and signals to the motors, switches, etc.

The photos below show the front and back of the control unit for my little CNC machine. Larger CNC machines have controllers inside them, as well as large custom control panels to control every aspect of the machine while it’s running. Smaller systems like this one often use a PC and a control box connected between the computer and the CNC machine. This is a fairly typical setup for DIY and even small commercial CNC machines.




Front and back of a typical small CNC controller

You may be surprised to learn that our old friend the parallel port is still alive and well in small CNC machine setups. No doubt this is due to several factors, not the least of which is the fact that it’s got an adequate number of control signals, has been commonly available on computers especially during the era when much of the PC controlled CNC software was evolving, and it’s also well coupled to the computers architecture in terms of being very responsive to commands issued by the computers CPU. Simple, reliable, effective, and still available on many desktop computer systems and/or motherboards you can still buy today. As time goes on, these will continue to disappear, but you can still get a PC with a parallel port if you look around a bit.

On this controller, and many like it, the parallel port signals are connected to the control unit by a simple cable. On the circuit board inside the control unit, these signals are connected to chips that directly control the motors. These have been developed over time to suit the needs of CNC machines, CNC software and general stepper motor control via PC.

A slightly simplified model of how the parallel port controls these stepper motors is as follows. The CNC software determines that one or more of the stepper motors needs to move a step in the desired direction in order to make the tool follow the toolpath, so it sends a short pulse to one of the parallel port pins. For each stepper motor controlled by the CNC software, a dedicated pin on the parallel port is used. There is also pin dedicated to each motor that determines which direction the motor should move. Every single step that each stepper motor makes in the parallel port type of interface has an associated pulsed signal. It may take thousands of steps just to move the motors an inch of travel. It’s a good thing computers are pretty fast! We’ll see how this is set up in the CNC control software in the next article.

Extra credit for those who notice that I replaced the spindle speed control with a toggle switch. I burned out a component on the variable speed control and replaced it with a simple on/off switch. Perhaps a story for another day :)

We’ll have a look at some CNC controller software next.

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