I'll throw a nickles worth in.

Gee, I wonder why they call them Spar Slotters?

Spar Slotters -
Perma-Grit has re-introduced these very useful spar slotting tools in 3 convenient sizes.
These 6" long steel bars have the Tungsten Carbide course grit bonded to one side allowing neat, true slots to be cut into plywood, balsa or composite formers and ribs. Two sizes available, SP9C has been discontinued.
3mm or 1/8" #3c
6mm or 1/4" #6c

This guy knows a little bit about building models airplanes and he does the same thing...

http://www.airfieldmodels.com/inform...raft_wings.htm

A good read... http://www.auf.asn.au/scratchbuilder/beams.html