Friday, October 7, 2016

Broken bits - CNC isolation milling and V-cutters

I buy inexpensive V-cutters for milling printed circuit boards (and recently switched to 20°, sharper ones). Often, the very end of the cutter breaks on the first contact with the copper layer (it is just so delicate). Usually I have either to compensate for the missing Z depth manually, or restart the job... else it will not carve enough and will not go through the copper layer.

From now on I think I will break the tip of the V-shaped mill bit myself before using one! Why?

V-cutter for milling PCB: pristine, slightly damaged and obviously broken. But the last one is my choice!

Check the two pictures below, that were made with the second and third cutter shown above.

Leaving it to chance is no good, the second one did a very poor work, as the edge was damaged "randomly" during the first millimeters.

It is not the first time I observe this, but I get better results when the very end of the tip is slightly broken. So here is the trick: it works well when the break has a very slight angle, so that the "incoming" corner hits the copper first when the bit is rotated in the usual spindle direction.

The best way to achieve it is to break the tip willingly. It also makes it more robust, as it often breaks randomly on the first millimeters anyway... which in turns ruins any proper calibration in Z.

I reduced the carving depth slightly in pcbgcode, since the width is made wider this way without having to plunge as far as before into the board. The tip is left small enough so I easily route a trace between two 0.1" pads.

Unexpected broken edge: not good at all!
Almost unusable result because the schrapnell may shortcut traces.
For this reason I sanded it with a 1000 grade paper, because using
a steel wool as I do usually would probably rip the traces off the board :(
Broken tips are quite OK, especially when they are slightly angled,
so that the lowest part hits the copper first when it rotates.
I did no post-process this board (i.e. result as is).


A recent and better job, shown in transparency.
The thinner voids in the bottom left corner are composed of a single pass.