![]() Because of the limited clamping pressure in the middle, I want the epoxy a touch on the runny side so that any excess can flow out. I mixed up some west system epoxy and used just a touch of cab-o-sil to slightly thicken it. I then set the small piece back on top with the joint lined up with the bottom piece (like it was when I finished routing). I put plastic and mylar tape over everything that I don’t want the epoxy to stick to including the bottom support piece. The gap below the clamping caul is intentional to put pressure on the middle of the epoxy joint Time for epoxyĮverything looks good, so I dismantle the caul and clamps. As I drew the clamps tight on the other side, I could see the pressure disperse towards the middle. The other side sat about 5/8″ in the air. After assembling the caul, I set it in place and test clamped one side. Using a track saw, I took 5/16″ off either end tapering to nothing towards the middle leaving a flat spot about 10″ wide in the middle. Before assembling the caul, I screwed the two uprights together. I made the caul slightly belly shaped which will help put clamping pressure on the middle of the joint. Since the joint is 10″ long, I made the clamping caul 14″ wide, which would give me a little pressure beyond the feather edges of the joint. Next was a test clamp using the clamping caul. One side was sitting slightly high, so I flipped the piece back over and sanded on both mating surfaces in this area. I then flipped the small piece over and into place to check for dry fit. The feather edge wasn’t perfect, so cutting it back gives you a nice straight line. I laid out each piece and cut off the feather edge until the plywood was maybe 1/32″ thick. Once that was complete, using a track saw I cut 2′-2″ off from the top piece (measuring from the short end of the angle). I sanded off all of the router marks and made sure the lines/plies were straightish. With the routing complete, I switched to a 6″ sander with a hard backing pad. I then set the sled over each 1/2″ section and made the final passes. I adjusted the height of the router bit to just touch the angle. Now the sled was sitting flat on the angle. I unclamped the small piece of plywood on the top that the sled was riding on. These kept the sled in plane until the end. I left two small areas less than 1/2″ wide near the middle. I repeated this process coming in from both ends until I met in the middle. I’d blow off the dust, move another 11/16, and cut again. I’d move down about 11/16 and make another cut. Using a 3/4″ bit, it was a lot of passes. I made a few test cuts and dialed in the bit height. It was in the shape of a “T”, using the bottom support plywood as an indexing face. With the pieces aligned, I made a quick router sled. Offsetting the plywood for scarfing Router sled With this setup, I’m cutting both scarf joints at the same time. This will help keep my router guide flat. On the top sheet, I also measure back 10″ and clamp on another piece of 18mm ply in place. On the second sheet up, I mark back 10″ and align the top sheet to those marks. The bottom sheet is for support only both for cutting the scarf and for the glue up. In the pics below, there are three sheets of plywood. Using 18mm marine grade baltic birch plywood, I measured back 10″. This is towards the longer, stronger side of the scarf joint ranges. For a 1:7 scarf joint on 1″ thick material, you’d measure back 7″ and that would be the angle of the scarf. The “1” in the ratio stands for the thickness. Scarf joints can range in ratios from 1:7 all the way up to 1:20. A scarf joint is basically an angled lap joint. In the boat building world, structural repairs are made using a scarf joint. The 5×12 sheet will be used in a structural application, so I needed a strong joint. The largest they could get in marine grade was 5×10. Really! They asked me if I could create a 5×12 sheet of plywood. I had a client ask me if I had a board stretcher. JScarf Joining plywood to make a 5×12 Sheet ![]()
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