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CNC

Last update: 31/07/2015

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.

The whole pcb milling process, even double side, is a simple matter but it requires a series of small shrewdnesses that, if not considered, can lead to a bad realization of the printed circuit creating short circuits or malfuncionings (not wished micro-tracks or errors on the pcb itself).

Usually this process is done, by the cnc machine, at relatively low speed, a simple cnc machine even home made can be considered perfect for the job to be done.

The 'secret' to obtain a virtual zero backlash resides on the movements of the axis. Without considering ball bearing screws, usually too expensive (and in many cases useless) the solutions consists inusing normal tpn screws (few cents of euros per meter) realizing all the nuts in plastic materials like delrin or teflon. Using such nuts the speed of the cnc machine will be a little reduced but the backlash will be very closed to zero.

Surface co-planarity: It is really important. On your cnc machine table look for a place with an high co-planarity. During the milling process even a small difference of the co-planarity of the surface will cause a different thickness of the track. If tracks are really small of very close each other the tracks can be even destroyed.

frese pcb

The router bits: An high quality router bit, double cutter, 0.4-0.5 mm, costs a lot but it is not indispensable. Using a small drill we can sharp broken drill bits. and cutting the head of the tip. Such a kind of router bits will do their job very well at zero cost. The trick consists in a double pass of the pcb (with the first pass the pcb will be only scratched, while with the second pass the pcb surface will be cut). In the picture a pcb realized in this way using old drill bits.


At the end of the work, a quick sanding of the surface will clear completely the surface itself.

 

pcb erroreerrore pcb

A tester and a magnifying glass: are necessary to check the tracks before the soldering process. Sometimes mistakes are so small that cannot be easily seen. Using a cutter all mistakes of the milling process can be corrected.

In the picture are shown a couple of classical mistakes obtained during the milling process.  The first picture shows a small uncut piece of copper inside the track, while the second picture shows a missing part of the track caused by the software (wrong dimension of the cutting tool in pcb-gcode.ulp).

 

In any case this mistakes must be corrected and cut by hand.

 

Double side pcb: Someone thinks this is difficult to obtain it but it is not true:

  • create normally the pcb with Eagle Cad and the tracks path with pcb/gcode.ulp to create all necessary files for the milling and drilling process;
  • position the pcb and move to the x0y0 location: make an hole. Move again to a fixed position (x100y0) and make another hole. Mill and drill the top of the pcb.
  • turn the pcb to mill the bottom side. Position the head exactly on x100y0 hole we have drilled before and set this position to x0y0;
  • move to x-100y0 (the old 0.0 position) and correct the pcb position. Move the machine head a couple of times to x0 and x-100 just to be sure to center the holes we have done before. Set firmly the pcb and start the bottom side milling process. The job is done.


More difficult to describe than to do. Here an example of double side pcb realized (by no cost router bits).

 

pcb ok

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