If you look up "table saw box joint jig" (or "... finger joint jig") on the web, you'll find instructions for two versions. One is a complicated apparatus with moving parts that can cut the fingers of a joint at nearly any width. The other is a simple two-piece jig that gets clamped to a miter gauge (or crosscut sled), that must be remade and matched to the saw blade or dado stack for each width of joint finger.
I've made a few of the second, simple type. One for the 1/8in. fingers matching the kerf of my rip blade. One for the 3/8in. fingers I wanted on my dresser's drawers. One for the just-over-1/4in. fingers I wanted to make on a small kitchen project this week. But just as I was about to use that last one, my dado blade exited the chat.
I wanted to finish that project sooner than I could get a replacement dado blade, but I also wanted to keep the finger size I had designed. I couldn't just use the 1/4in. jig, though, because the slots cut by my 1/8in. rip blade wouldn't fit over the 1/4in. pin. After some thinking, and with the aid of an example to debug, I figured out how to make it happen with just the 1/8in. jig that matches my rip blade! Now I'm going to write down the process and share it, so that I don't forget how to do it before next time.
If you would find a video explainer easier to follow, or otherwise useful as supplemental material, I've posted one on Youtube.
Box Joints, Any Size, With Just a Rip Blade
To use this method, you need a finger joint jig matched to your rip blade, including the spacer piece used to set the jig's position. I recommend 3x3 Custom's video on how to make one quickly. That video should also familiarize you with the basic use of the jig.
You will also need two pieces of material that are each the thickness of the fingers you want in the joint. If you want a square finger (width the same as thickness), these two pieces can be the workpieces themselves (or offcuts from them).
The usual setup for the jig puts the spacer between the pin sticking out of the jig, and the saw blade. Instead of doing that, use your two finger-width pieces. (Fig. 1, top) But don't cut yet! After you've set the jig with those references, clamp a scrap to your miter gauge or crosscut sled, against the side of the jig, on the same side of the blade as the pin. Now unclamp the jig, and insert the usual jig spacer (the one that is the width of your saw blade) between the jig and the scrap. (Fig. 1, bottom) Re-clamp the jig, remove the scrap and spacer, and now you're ready to cut.
Begin making these first cuts exactly was you would for a thin tooth joint. (Fig. 2) Start with one of the workpieces against the pin, then put that slot over the pin, then move the next slot over the pin, etc. When you get to the other edge of the board, if your saw lines up to take a cut less than the full width of the blade, do not make the cut. (Fig. 3) You'll have a slightly thicker tooth on this end, instead of a slightly thinner tooth on the mating workpiece. Making the thinner tooth on the other piece correctly requires an additional jig setup that I haven't had time to test.
Make all the cuts in the second piece, exactly as you would for a thin tooth joint. Get the first cut lined up by flipping the first workpiece around and placing the first slot over the pin. Butt the second workpiece up against it (Fig. 4), clamp it in place, remove the first workpiece, and make the cut. Then put that open-ended slot over the pin, make the second cut, and continue the process through the end of the board. (Fig. 5)
When you've made all the cuts in both boards, they will each have a wide-tooth, narrow-gap pattern. If you hold the outer edges together, you'll see that one side of each gap aligns with one side of a gap on the other piece. (Fig. 6)
Unclamp the jig and reposition it with just one of your finger-width pieces between the pin and the blade. (Fig. 7) Now you'll run each piece through the jig again, but with two major changes.
When running the workpiece through that you started with in the first setup, you'll keep its orientation the same, but move it in the opposite direction. For example, if you had the top edge pointed toward the right, and moved it left to right, this time you'll keep the top edge pointed right, but move it from right to left. Place the pin in the second to last gap you cut. (Fig. 9) (If you place it in the last gap you cut, it should be the case that either the place is past the end of the piece, or it would cut a partial gap that would require the additional jig setup I haven't tested. (Fig. 8)) Make the cut, then move to the next slot (remember - in the opposite direction as before), repeating until you make a cut with the edge of the workpiece resting against the pin.
When running the second workpiece through, you will also move it in the opposite direction. But you also need to flip it over so that the opposite side is facing you. (Fig. 10) Place the gap over the pin, make the cut, and move to the next. But do not make a cut with the edge of the board resting against the pin. To finish the final gap in the second workpiece, remove all material between the last slot and the edge of the board. (Fig. 11)
If your tooth size is less than twice the width of your saw blade, you're now done. (Fig. 12) If your tooth size is wider, you'll need to clean out the waste inside the gap (see video @ 10:24 for an example). Holding the workpieces edge-to-edge should help to illustrate which bit is the waste, and which is the tooth. You'll need to remove the jig, so the pin doesn't get in the way, but I recommend using a backer board to avoid tear out.
I've used this method on two projects now, and both have been completely acceptable joints. Loose enough to fit together without forcing, but not so loose as to make the joint weak, or even to rattle. It's more accurate than I would have gotten trying to adjust my fence by ruler, and quicker than it would have been to align the saw to scribe lines on each board. I'll replace my dado blade eventually, but for now this method will do the job.
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