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Thursday, August 01, 2013

Jigs And Joints

This past week I have been making a jig to assist cutting what I call a "modified scarf joint" for the caprail trim. Perhaps there is a better name for this as I have seen it used often on caprails? Basically the joined ends of the caprail planks are cut such that they interlock together.

I used CAD to design a jig that slides onto the trim lumber and is secured with screws using index holes, cut with CNC. The index holes also match the final screws that will hold the caprail trim in place. Practicing on some scrap lumber, it took three different versions of the jig to dial in the tolerances to ensure a snug fit.
Jig attached to a practice piece of wood.
The jig is attached to the lumber to mark the rough cut, where waste material is removed with a jigsaw. Then a router with a trim bearing bit is run along the edge to cut the lumber flush with the jig. Do the same for the matching end (rotated 180 degrees).
Two ends ready to be joined. Note the biscuit on the top piece.
Next, the pieces are joined and clamped for a test fit. The center of the seam is marked for the biscuit joiner. Take the biscuit joiner and cut a slot at the marked point, on both pieces. Insert a biscuit, join and clamp the pieces for a test the fit. The biscuit ensures the top edges of the joint will always be flush with respect to the other piece and that either piece will not "lift". As this is plastic wood, I am using plastic biscuits (plus a little polyurethane adhesive).
Test pieces joined.
With this technique, I expect the ends of the caprail trim will match just nicely and everything should look symmetric.

Next is to do this on the real PlasTeak caprail trim.

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