For a current project, I need to match boards together along a curved line that will result in a tight glue joint. Basically I need to cut two boards that will fit perfectly into convex and concave curves on an existing board. Unfortunately the curves are not the same. I tried using a router with bushings to cut an offset template and then repeat the process with the same bushing riding against the offset template to match the original curve, but the results were marginal. I have attached a simple drawing to hopefully make this easier to understand. Obviously, I can cut and sand a sample and trial fit it until I am satisfied with the results (and then use this as a template to flush rout the final pieces), but since I have encountered a similar probem before, I am wondering if there are any foolproof techniques. Any ideas? Thanks.
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Yeah, one template can't work exactly. You're cutting one curve on the outside of the router bit, and the other on the inside of the bit. If you use only one template, you can improve the situation by using the smallest possible diameter bit and the smallest possible bushing. The trouble is that a small bit can't cut very deep. A way around that is to use the small bit to establish the curves, and use a pattern bit to cut away the rest of the board's thickness.
There was an article in FWW, I think in the late 80's or early 90's about a guy using a router to cut joints very similar to yours. I don't remember all the details, but he was using a template router and I believe two templates. The part I don't remember is how he made the two templates. As far as I could tell from the pictures in the magazine, the joints fit perfectly.
Another possibility, if you don't have many linear feet of joint to make, would be to cut the new pieces to the approximate shape and then scrape and fit until it's tight enough. The trick with that is how do you mark the high spots that need to be scraped (sanded, planed) off.
Use your existing board as a template. Cut an MDF/ply pattern from it using a router and top bearing pattern cutting bit running the bearing anti clockwise along your existing board. Next set up a base that will hold all the boards that need to be joined. Rough cut the new boards to the approximate shape, a bit on the generous size. Then fix your existing board accurately atop your newly cut pattern, and space the new boards that need to match just a bo'hair less than the diameter of your top bearing pattern cutting bit (as used earlier) and fix them in place with cramps, double sided tape, etc.. Now follow your existing board with the pattern cutting bit to cut an edge on the new board to match the profile of your existing board. Facing towards the edge of your new board that you are routing, you go left to right. After routing, the new edge will match the existing edge at glue up. Slainte.
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