A CNC machine, in simple words, is a numerically controlled machine basically used to realize, very precisely, shapes. The concept of the process is really simple: we realize a shape, using a cad, on our pc then, using the same pc and particular software, we can drive the cnc machine to realize phisically the shape we have drawn. Nowadays many, virtually all, prototypes of items we are using every day are realized by such a process (honestly even 3d printers are now widely used to create 3d shapes): cars, boats, models, phones, printed circuits and so on.
The parallel port of our computers is the most common device to control a CNC machine. Using this port the drivers of the motors of the cnc machine can be controlled very quickly and inexpensively because:
The tool-cutter connected to a cnc machine must be chosen according to the machining to be performed and to the materials to be milled. As already explained in another article of this site, if the machine is used for the production of printed circuits boards, engravings of plastics or aluminum, light jobs, a 50W Proxxon or a Dremel or any other drill/milling small router can be considered valid.
One of the most common and serious problem in a cnc machine is the step loss. In very poor words it means that a pulse coming from a parallel port cannot reach the motor (or it is not recognized by the motor driver - or it is not executed by the stepper motor).
The cooperation among Sanyo and Toshiba with some chinese distributors, recently appeared on the market some integrated circuits dedicated to the motion control world. According to the datasheets they seem to be really interesting. Unfortunately we can only talk about them because still now we were not been able to test them.
We cannot understimate the accuracy of a cnc machine when used to realize pcbs. If a machine is well done, without any backlash, the accuracy of the milling process can be really astonishing.
Difficult to suggest the right cutting tools for a cnc machine. To realize our pcb we use a 220V grinder made by Proxxon 240e. It is characterized by a regulated speed and by a very low noise but the real quality of this instrument is the rigid metal collar that allows, like the milling cutting tools of greater power, a vibration-free mounting.
It is very popular, nowadays, realize pcb prototypes using the milling procedure. There are many reasons: it is a fast (no uv lamps), it is clean (no acids, no photoresist), it is cheap.
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