How To: Scarf Joints; Part 1
The "scarf joint" is used to join materials of similar thickness in order to retain the original thickness. It is a way to join sheets of plywood to create additional length or width, as well as increasing the length of dimensional lumber. For the wooden drift boat, plywood side panels and bottom panels must be scarfed for the required length. The scarf is really only overlapping bevelled edges of the the plywood. The integrity of the joint is dependent on the fit of the bevels and the quality of the glue used to adhere them.
For plywood joints. a 12:1 scarf is used. This means; "the length of the scarf is 12 times the thickness of the material." for a 1/4" thick plywood panel, this would result in a 3" scarf.
Eye and Ear protection, tape measure, pencil, straight edge, clamps hand plane, belt sander, flat surface.
1. Determine which faces that you want to use as mating faces. ("A" faces or "B" faces)
2. Place the two mating surfaces together- I like to put the "good" faces together so the "B" faces are exposed.
3. Scribe a line 3" from the end of each sheet as shown. Place the edge of the top sheet on the line of the lower sheet.
4. Clamp the sheets to your work surface. The idea is that the "scarf" on each sheet will be made as the same time.
5. Using a hand plane, sanding block, power planer or belt sander, feather the edges and use the plies of the material as a guide. You want a smooth, continuous bevel from the top sheet to the bottom sheet. I prefer to use a belt sander and a 40-60 grit belt.
6. Test the fit of the scarf by flipping the top sheet over and matching the bevels. Adjust as necessary.
Gluing up next...