Homebrew PCBs

Hi Charlie,
> Yes, there are two worries that I mainly have about using a 3D router:
>
> burring and minimum trace width/clearance.
>    

With the right equipment you can get quite fine traces. I have seen 
boards made this way with two tracks fitting between 0.1" pitch pads. To 
achieve that sort of resolution you need a very accurate spindle as even 
0.005" out of center can make a big difference. If your spindle is not 
perfectly concentric the cut width will increase and you often get 
problems with burring. If you get burring, stop cutting and rotate the 
cutter in it's collet by 45 degrees and try again. Repeat until you find 
the 'sweet spot'.  A worn cutter will of course also cause burring.

> I'm pretty sure surface mount would be "out"...but maybe not.
>    

Fine pitch such as QFP packages is out but other surface mount is fine.

> On the other hand, once it's setup and debugged, it's repeatable, fast

Not as fast as you would think. Routing is slow but you can just leave 
the machine to get on with it. It is also generally quite noisy due to 
the high spindle speeds (15,000RPM to 100,000RPM, depending on how deep 
your pockets are).

> , and the automated hole drilling is not to be sneezed at; I find the hole drilling part by far the biggest hassle and would love to have it done automatically....I suppose
>    

Yes, drilling is far faster and more accurate than hand drilling. Before 
I went over to routing boards I used to photo etch then CNC drill. 
Aligning the board was always a PITA.

> However, if the hole diameters were standardized to one size (in some circuits this is doable, even if it isn't optimal), one bit could do it all: routing and drilling...all in one multi-stepped set of passes. That would be pretty cool. It would have to be a carbide or even diamond bit, at a very high speed, maybe even with water cooling, but I'm sure it's achievable.
>
> And, now that I think about it more, actually larger holes would, of course, be achievable: just program it in to to mill the hole out larger.

Don't go there. Thin routers break VERY easily. In my experience 
anything below 1.5mm (1/16") is asking for trouble. Cut the tracks with 
a V cutter like these 
<http://cgi.ebay.co.uk/5-x-45-Carbide-PCB-Engraving-Bits-CNC-Router-Tool-T14-/250522253434> 
then drill with normal carbide drills. You can get plastic collars that 
fit on drill shanks to set the depth. Once you have the machine and 
collars set up you can change cutters quite quickly because they always 
fit in the collet at a fixed depth.

You don't need any form of coolant. It just makes a mess. Use decent 
vacuum extraction to keep the abrasive dust out of your machine and out 
of your lungs. As far as hold-down is concerned, vacuum works quite well 
if the vacuum is strong enough. A vacuum cleaner is definitely NOT 
strong enough. When drilling and routing parts out you need to cut all 
the way through plus a bit so you need a backing sheet of some sort. For 
vacuum this sheet needs to be porous. 6mm (1/4") MDF works and is pretty 
cheap. I just use double sided adhesive tape to hold the boards down. It 
is quick and easy. You need to experiment with the tape. White 'carpet' 
tape sticks well but is a pain to remove and tends to clog cutters. 
Transparent tapes generally work quite well but can be difficult to find.

For double sided boards, drill a couple of locating holes in your blank 
board and use them to locate the board on pegs on your table. These 
precisely locate the board when you flip it over.

One stumbling block is the software to generate tool paths. It can be 
quite expensive. The free offerings are generally rather quirky. I quite 
like Visolate <http://sourceforge.net/projects/visolate/> though it 
takes some trial and error to get it working correctly. Instead of 
slavishly following the track outlines it calculates the minimum cut 
length to achieve the desired connectivity. The resultant tracks look 
odd but it works well. Cutting time and cutter wear is considerably less 
than most other methods. There are some pics here 
<http://www.mit.edu/~vona/Visolate/Visolate-info.html>. Visolate won't 
do the drilling.

I use SheetCam <www.sheetcam.com> for drilling. Actually what I do is 
generate the tracks in Visolate them import the tracks and drill files 
into SheetCam. I can then lay them out as I want them before cutting.

Les