Drilling Eagle Printed Circuit Boards

I use Cadsoft’s Eagle for schematics & circuit board layouts, then build the boards in my basement laboratory using Pulsar’s laser toner transfer and ferric-chloride etching. My Sherline CNC milling machine pokes the holes in the board, which means they actually wind up in the right places. I don’t mill the outline into any fancy shapes, generally using a tin snip and maybe a little filing; glass-fiber dust is a nuisance.

My Eagle ulp routine (in the Useful Stuff page) extracts the holes from the circuit board layout, sorts by drill size, then visits each hole in nearest-neighbor order. It starts by touching each hole with a center drill, which probably isn’t necessary, but it makes me feel good and provides a last-minute check that everything is lined up properly.

Figuring the tool path is obviously the traveling-salesman problem in disguise, but a strict nearest-neighbor order is close enough for boards of this size. You could probably optimize it by brute-force exhaustion and that would be appropriate for production use, but I rarely make more than one version of each board.

Eagle’s standard part libraries use a weird set of hole diameters, which my routine rounds off to the nearest mil. I don’t have a vast array of drills, so I don’t pay much attention to the differences between, say, 0.024, 0.025, and 0.027 inch drills. Tool changes are strictly manual and I don’t have to change the drill if I don’t want to!

Got a bunch of teeny carbide drills as resharps from DrillBitCity a long time ago.

I double-stick-tape the board (center and corners) to a flycut sacrificial plate, which makes it flat enough for these purposes.The pic below shows a 60-mil board held down with masking tape; it’s the same layout as in the screen shot above.

Tool changes use a 2-inch block (plus a sheet of paper) as a height reference. You can tweak the ulp file for your setup.

My board layouts have a giant via at each corner, with the lower-left corner at (0,0). Drilling doesn’t require any fussy alignment, because I etch the board after drilling: the holes serve as bright lights to line up the pads & vias. I’ll have more to say about this elsewhere.

Speeds and feeds are on the sissy side; I crank the 10k rpm head up to a dangerous chattering whine and feed the drills at 5 inches/min (call it 125 mm/min). Both of those are far too slow, but work OK.

Run a shop vac to suck up the dust as you drill! I doubt that a typical shopvac filter removes the fines, but it’s better than letting all the dust settle on the mill and in my lungs.

The clamps are these, mounted on studs screwed into the tooling plate.

Incidentally, the Sherline mill’s throat depth and Y-axis travel limits the board to about 4 inches along the Y axis; yes, with the spacer block installed. That’s just about exactly the maximum size the low-end version of Eagle can produce, so it’s a nice match.

There are other ways of doing PCBs. I haven’t tried trace-isolation milling, but PCB-Gcode looks like the ticket if you want to generate a breathtaking amount of glass-fiber dust. My quick check shows that it inserts semicolon-delimited comments into the tool-change commands, which EMC 2.2.8 promptly chokes upon, but that’s probably a quick configuration tweak and will change with EMC 2.3 anyway.

If you’ve got the scratch, there are commercial solutions: Chris Daniel (who was also at the Cabin Fever EMC booth) uses a T-Tech gantry router at work.

Memo to Self: Expect a call from a patent lawyer either telling me that I’m infringing somebody’s Nearest Neighbor Algorithm claims or asking me for my design notebooks to establish me as the Prior Artist.