A compound, or parabolic curve is a curve in which the radius is constantly changing along the length of the curve. A circular curve has a constant radius that does not change. Preparing plywood for any curve is done by a process called kerfing, where grooves are cut into the plywood allowing it to bend. The spacing of these kerf lines determines the radius around which the plywood will bend. Since the radius of a compound curve changes continuously the kerf lines will have constantly increasing or decreasing spacing.
- Skill level:
Things you need
- Circular saw
- Chalk line
- Screws or glue
- Tape measure
- Work bench or table
- Carpentry pencil
Determine the length of plywood needed by laying out the chalk line along the length of the curve. Then, pull the string straight and measure. Cut the plywood to this length.
Set the depth of the circular saw blade to the desired kerf depth. If using 3/4 inch material, set the depth to 5/8 inch. When using 1/2 inch material or thinner, subtract 1/16 inch to determine kerf depth.
Mark a square line using the chalk box at about the halfway point on the plywood. Cut the first kerf line along this line.
Clamp the plywood to a workbench. The kerf line should be positioned a distance equal to the starting radius of the compound curve from the edge of the table.
Gently lift the side that is not clamped until the kerf line closes, then measure the height from the edge of the table to the surface of the plywood. This is the distance from the edge of the wood to the first kerf line. Mark this kerf line from one edge of the plywood.
Repeat steps 4 and 5 using the ending radius of the compound curve as the distance from the edge of the table. Then, mark this line from the opposite end of the plywood sheet.
The next step involves a bit of trial and error to get correct. Start from the end with the shortest spacing. Add 1/8 inch to each space and mark each kerf line. If the last line before the end with the larger spacing is equal to the large spacing, you've correctly laid out the kerf lines. If this last space is too small, add 1/16 to each progressive spacing until the lines are laid out correctly.
Once the lines are properly laid out, carefully cut each kerf line. Once this is completed, test fit the plywood to the curve. If the kerf lines were laid out properly, the fit should be snug.
Attach the plywood to the curve using screws for heavier applications like curved roofing or skateboard ramps. For lighter applications like furniture, clamps and wood glue are recommended.
Tips and warnings
- When laying out the kerf lines, make very light marks to begin with as it is likely that it won't be correct on the first try.
- This is an advanced carpentry technique. Make certain to be comfortable with the tools and procedures involved to ensure safety and the best end result.
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