Hey, forget those old-fashioned methods! I use a 3.5 KW excimer laser which I CA'd to a precision, high-speed XY flatbed plotter. I feed in the plans in a binary format derived from Autodesk's DXF format. The 20 megabyte on-board buffer in the plotter holds the entire file with room to spare. (I usually keep several menus form local Chinese takeout kitchens in there as well). I then place the balsa sheet on the plotter bed which has been enhanced by replacing the bed material with that from an air-hockey table with the air pump inlet and outlet swapped. Then I turn on what is now the vacuum pump. This holds the Balsa sheet flat and tight against the bed. I slip on my safety goggles and run up the laser onto a water-cooled graphite target until it comes up to full power, then I fire up the plotter. In about 2.5 seconds an entire piece of 1/16" x 3" x 36" wood is reduced to a highly accurate collection of ribs, gussets and other assorted parts. The entire setup only cost me about $175,000.00 to slap together! Since I've already built 3 planes this way, I figure it only cost me just over $58,000.00 each. (Seems reasonable). One last note: if you try this, don't forget to vent the vacuum pump outside or the smell of burning balsa might be overwhelming or even set off your smoke detectors.
I also highly recommend that you keep your cat out of the room during the entire operation.