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CNC

Last update: 31/07/2015

Macchina CNC

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.

But how is made a CNC machine? sometimes only the personal fantasy is the limit. Numerical controlled apparates are everywere, automatic vending machines, factories assembly lines, robots, lathes etc. In our case we'll use a cnc machine mainly for two/three reasons: to realize prototypes of pcb and to realize components of aluminium or plastic/wood (yes: the real passion is small planes/boats modeler).


The structure of a cnc machine is quite simple: a particular software drive, through the parallel (or usb) port of the computer an interface. The interface transform the code of the pc in impulses that are sent to some, usully three, stepping motors that, connected to screws, will move an electrical appliance, usually a drill/milling machine to replicate exactly the shape we have realized on the pc.Astonishing is the accuracy: a normal stepping motor is driven at least at 400 (later on we'll see why) steps/turn. If the motor is connected to a 2mm screw the precision will be 2/400 of mm, it means 0.005 mm. Such theoretical accuracy, that has something of incredible, can be really obtained even using common and not expensive components.

Just to give some idea of the differences look at the following pictures:

vite   vite tpn  ricircolo sfere  

There are three different screws normally used in a cnc machine:


The first is a normal long iron screw: the price is 1/2 euro per meter (pitch 0.75-2mm), the second is a TPN screw - 20-25 euro per meter (pitch 3-4mm), the third one is a ball screw, the perfection, but the price is quite high 150-250 euro per meter including the nuts (3-10mm pitch). It is really obvious that a ball screw represents the best choice in terms of speed and positioning but are we ready to spend 500-700 euro only to get the three screws necessary to our machine if we can obtain the same precision using a normal iron screw and only 5-6 euro? In our opinion the game doesn't worth the candle even because, normally, all milling processes are done at very low speed simply to save the life of the end mills and the quality of the manufacts.

alluminioWhat about the body of the machine? The body of the machine can be built by a plethora of materials. (iron, aluminium, wood, plastics) Many people prefer aluminium profiles. They are easy to cut and join together, enough cheap and using them a cnc machine can be created in a very short time. We prefer normal, rectangular, tubes of iron. The mass is important and in my opinion aluminium is too light and in any case the iron is less affected by vibrations (to reduce almost to zero the vibrations it can be filled with polyurethane foam). Iron is very cheap but it is quite hard to work as per the fact that joins are made by a gas  or electric welding torch not always available.

In any case the personal creativity plays a big role into the project of a cnc machine.

 

motoreAnother critical component of a cnc machine is the motor. The motor should not be too small and should not be over dimensioned. Sometimes an excess of power is counterproductive (also for the power supply current needed). Remind to yourself that it is better the stuck of the machine than an end mill broken.
Naturally the power supply must provide enough power to move the axis without problems, with linearity and speed sufficient for our needs.
Nowadays 99% of the motors on the market are hybrid motors; it means that we can use them in a bipolar or unipolar configuration using, of course, a bipolar of unipolar electronics (see the section).
The price of these motors it is not excessive: remember that a good motor last for a life.

 

guida lineare

Slides:

To support the movement we've used linear ball slides. Probably are more expensive than other linear axis but using such a guides many small positioning mistakes  (distances and linearity) can be corrected in a snap. Are really resistant and can support a very high weight on every rail.  The market is plenty of other solutions but the use of ball guides and rails can be considered the best choice.

Normally, as we said before, we use a cnc machine to realize our prototypes of printed circuit boards using a machine create with the listed simple (and poor) material. The picture shows the quality of a pcb realized with such a machinepcb. This is just an example (a double optoisolated solid state relais) but the picture underlines the quality and the high precision of a manufact created by a milling procedure. Really impressive.

The electronics of a CNC machine is a simple but often confusing matter. In the picture (down) is shown the schema of a full system of four axes. Sometimes the breakout board is not present and the parallel port connected directly to the controllers. The main advantage of using single stepper controllers enable the user to connect different motors even in a mixed configuration (unipolar or bipolar steppers) tuning the CNC machine performances to the job to be done.

schema cnc

A common and simple solution is the use of a board where the optoisolator (if any) and the controller/drivers are present in the same board, all-in-one. This solution allows generally a quick connection and a simple wiring schema but force the use of three or four identical (or very similar) stepper motors.
schema cnc

Generally the first solution is limited to professional cnc systems while the second regards hobbystic or semi-pro environments.

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