This simple wooden comb guides the router bit as it cuts the fingers of your box joint. It is easy to make, but it must be precise, so you should resign yourself to the fact that you're going to make several "almost-but-not-quite-perfect" combs before you get one exactly right.
This is my stack of imperfect combs. I had a comb that was perfect, but my router bit gave out, and when I bought a new one--you guessed it--it was slightly different and I had to start all over.
The best way to cut the fingers of the comb is to use either a simple box joint jig or an adjustable box joint jig. Making a simple jig will take about fifteen minutes, but tweaking any jig to make perfectly sized fingers could consume hours.
You want the fingers to start about two and-a-half inches from the edge of the jig, which is difficult to accomplish. The easiest way I've found is to use a miter gauge to make a slight notch at the place where you'd like the fingers to begin.
Make sure your miter gauge has an extension so that it supports the work right next to the blade. If yours did not come with an extension, you can easily make your own out of scrap wood. Cut it just deep enough so that it will sit on the spacer block of your jig.
Place the notch you just made on the spacer block of your jig and cut a full-sized notch in your wood. (The exact length of the fingers is not important, but I've found that an inch-and-a-half is about right.)
Now flip the wood right-to-left and place the notch you've just cut on the spacer block and complete the cut.
Flip the wood back the way it was in the beginning and cut the rest of your notches, leaving about 2 1/2 inches at the end.
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