You’ve seen paint-grade moldings at the home center — the stuff made up of those short pieces joined end-to-end with finger joints. Have you ever thought how great it would be to “stretch" a board like that? Did you know you can cut that joint on your router table?
The finger joint is a positive-negative interlock, in which tapered projections (the fingers) on one piece fit into tapered grooves in the other. It expands the glue area threefold, but more importantly, it exposes long grain surfaces for gluing. That’s why it works for end-to-end joinery.
It works great for miter joints as well as for joining plywood panels to each other and to edge banding. The bit that cuts the joint is not cheap, and when you see it, you understand why. The Sommerfeld bit comprises a stack of four-wing finger cutters and a thicker shoulder cutter, along with a ball bearing guide on a 1/2"-shank arbor. The full assembly is impressive, though at just over 1-1⁄2" in diameter, it can be run at full speed in a 1-1⁄2 HP router.
Out of the box, the bit has five finger cutters stacked atop the bearing and capped with the shoulder cutter. You must reorder the stack according to the stock thickness. For example, you use only two finger cutters and the shoulder cutter to mill stock that’s 5/8" to 13/16" thick.
To set up, you remove the spindle nut and the cutters. (It’s easiest to do with the arbor chucked in a router.) Leave the bearing at the bottom. Add the appropriate number of finger cutters for the given stock thickness and then the shoulder cutter. The shoulder cutter always tops the “working” stack. The remaining finger cutters go on top of that, where they’ll be “out of play.”
You rout one workpiece face-up, the other face-down. When the bit height is correct, the two pieces should slide together with their faces perfectly flush.