As most know who have attempted to make new wood for their cars, there are areas where long finger joints are used for joining pieces to make curved pieces. Areas like the the bottom of the C pillar on four doors or the deck rails on coupes, roadsters, and cabriolets. In order to make these joints the size needed, either a special cutter needs to be fabricated or the joints cut by hand. I recently have the need to make these joints for a Canadian built 32 Olds convertible roadster I am completely re-wording. All commercially made finger joint cutters will only cut to a maximum depth of 5/8” so in order to get a cutter to cut to the depth of 1 1/4” (most common depth on our early Chevys) one has to be custom made. The total quoted price of a six row cutter on a mandrel was over $1,800! I decided to try and make a box joint cutter to make the joints as I didn’t have a way to make finger joint blades. While a box joint is not originally used, the integrity of the joint is what’s important and these joints will no be seen once the body metal is installed. A box joint is similar but instead of the tenons and grooves tapered like a finger joint, they are the same thickness from the back to front. When I looked for a mandrel I also found that one long enough for what I needed would also have to be custom made. I decided to attempt my own cuter by making my own mandrel and using small circular saw blades. I ordered 18, 4.5 x 1/16”, 5/8” bore circular saw blades. I turned a 3/4” drive end diameter mandrel out of 1” steel with a 5/8” shaft diameter on my lathe. Threaded the end 1/2”-20 and a recessed washer to apply pressure to the blade stack. With the mandrel done, three blades at a time were sandwiched together with the carbide teeth staggered to get a width of 3/16”. (The average width of the original finger joints) Then each stack was spaced at .238 apart with the recessed washer and nut put on. The cutter is intended to be used in a vertical mill and it ended up working perfectly. The cost was less than $150 in materials and about 6hours in my time. I’m able to easily make 1/2” deep passes in the ash with nice smooth cuts and the two pieces slip together with just light taps from a rubber mallet. The cutter has the ability to make joints 1 3/4” deep if deeper joints are needed.
I agree that in the overall scheme of things your straight cut finger joint will be as strong as the tapered finger joint once the glue has set. From my view the main advantage of the tapered finger are that it is easier to start the assembly.
Here is a 32 Oldsmobile DCR from CN that I’m rewooding and welding all new sheet metal where needed. Used the joint cutter in the deck rails and the lower belt rail. It produces great joints. Still a lot more work to go. I’m about twenty hours into the wood so far. Though it’s an Olds, many pieces are very similar with a few even being the same as a 31-32 chevy cabriolet.
