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Most tablesaw jigs for cutting finger joints (also called box joints) mount to the miter gauge and must be tweaked every time they are reattached. I took a different approach, making a simple sled to cut one size of fingers. The sled ensures repeatable results without tweaking, and it rides in the tablesaw’s miter slots, so it doesn’t depend on the miter gauge.
To make a sled for 1/2-in. fingers, start with a Baltic-birch plywood base. Add hard-maple runners, sized to fit the miter slots, then add front and rear fences, also made of hard maple. After assembling the sled and cutting a 1/2-in. dado slot through it, use the same dado setup to cut a notch 1/2 in. wide by 1/4 in. deep in the lower edge of the rear fence face. Don’t bolt this fence face to the rear (fixed) fence just yet.
Glue a snug-fitting key in the notch and, using a scrap of the key stock as a spacer, position the fence face so the key is 1/2 in. from the slot in the base (this dimension will change if you use different size fingers). Clamp the fence face at this position, then install small recessed carriage bolts from the front of the fence face, enlarging the holes through the fixed fence slightly. Lock the fence with washers and hex nuts and cut a trial joint. If you need to adjust the setup, loosen the hex nuts slightly, tap the fence face left or right as needed, and retighten the nuts.
To use the jig, mount the appropriate dado blades, set the depth of cut to accommodate the stock thickness, and go. You can add a sacrificial backer board (1/4-in. hardboard) to reduce tearout when cutting fingers that are shorter than the slot in the fence. The 1/2-in. fingers work well for most of my needs, but if you need other sizes, make a separate jig for each size.
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