On 12 May 2011, at 14:36, Mark Lerman wrote:
>
> My biggest problem with cnc of pcbs, especially with triangular bits
> is that the pcb material is not even thickness, and more importantly
> does not lie flat. This causes traces to be uneven, areas to be not
> cut at all, and some gouges. The better pcb routers "test" the board
> before cutting and compensate for these things as they cut.
>
>
>
I have one of those CNC machines (an EverPrecision EP2002H in fact - see
http://everprecision.com/e-epseries-3.html, the EP2002 and EP2006 have the same specs as far as I know), and it really can do 6 mil trace/space accurately (4 is pushing it. It can do that on a good day). Even with the machine running a grid-test before starting the engrave process, though, you can still have a (severely) warped board cause you problems.
The way the machine tests the depth is to connect a GND wire to the copper surface, then scan a (typically 1cm spacing) grid across the work-area and as the tip of the carve-tool touches the copper, a circuit is completed and the machine records the Z value of the tool at that point on the grid. Most of the time this works great. Sometimes I get copper-plated FR4 that is so warped that this is defeated - we're talking a few mm of deflection in the middle of the board, with the board bowing up, for example. Since the circuit is made at the instant of touch, the bow is never removed, but when you come to engrave, there's more downwards pressure, and the bow deflects and insufficient copper is removed by the tool.
I've always thought another aid would be a vacuum table underneath the PCB - if the PCB is actively being sucked down onto the platform it would remove the bow and make the depth-measurement far more accurate. My suspicion is that a vacuum table would in fact work just as well as the copper-sensing approach I have now, so if you put one of those on a standard CNC, you might end up with a pretty darn accurate surface, and hence a pretty darn accurate PCB.
One of these days, I'll get around to using the K2 CNC machine to mill out a vacuum table for the PCB plotter. One of the side-benefits of the electrical-contact approach is that the tools have their height-above-zero auto-detected, so adding a (thin) vacuum table ought to be feasible.
[warning: Rant mode on]
One thing: DO NOT BUY the EP200x machines - they're absolutely fantastic at what they do, but the service completely sucks. The current dealer won't upgrade my software (even for an additional cost) on the machine I have because I bought it from the previous dealer. Ever-Precision themselves (in China) don't seem to care either. The software I have is buggy (presumably because it's an older version now) and even reporting bugs to them in the hope of a limited fix didn't get me anywhere. I now have a $10k machine that can't be upgraded...
Example of one of the bugs in the software: if you have a pad that is a hexagon rather than a circle/square, the PCAM software will "rub out" the entire hexagon. Looking at the gerbers in gerbview shows the hexagon is fine, and inspecting the gerber file shows that the CAD software (Eagle) uses a different style of macro for hexagons... That style of macro doesn't seem to be supported by PCAM, and since I can't upgrade the software, all I can do is transform hex pads to circles (luckily Eagle can do this automatically. It sucks to be 75% of the way through a two-hour job when you realize you didn't do it this time, though; unluckily, Eagle uses hex pads for pretty much any pin-header so it's something I'm constantly having to remember to do).
[/rant]
There's a couple of other things to consider. You don't get plated-through holes, so your designs for home will probably be different to anything you send out to a PCB - to make sure vias are reachable, and soldering is mainly on the bottom-side of the PCB for example. This isn't any different to etching, but it's worth bearing in mind that this isn't a complete replacement for a PCB house.
On the plus-side, the machine does all the drilling of holes for you as well - and if you have holes that are too big for any drill you have, it can interpolate a router-bit in a circle to make that hole. It can also route out any shape of PCB, which is occasionally useful to me.
Bottom line: even with the crappy service, and even though I could never in conscience recommend the company, I still love my EP2002. Being able to reliably turn around a 2-sided PCB in an hour or so is, well, brilliant. I never managed to quite get the chemicals method down, so it was still a bit hit & miss with etching for me. The router is worth its weight in gold, it just irks me that the company's attitude spoils it.
Simon
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