Place raw shaped soundboard on top of piano, make sure it covers the entire inner rim and belly. Trace on the bottom of board along the outer rim, mark where belly line is with some dots. Then flip board over, place on a work surface, connect the belly line dots (shown in picture on the right). Clamp board down on work bench.
Cut the soundboard in shape on the marked line with a jig saw. Move the board and clamps around when cutting so the jig saw is close to the bench and produce less vibration or not bounce up and down much.
Next step is to dry the board to the ideal moisture content. The board is at 5260.2g before drying. The square mark is where the squeeze clamp is under that weighing scale. Write down the date and weight. Put soundboard into kiln for conditioning/heating/drying.
Take some cut off pieces from soundboard, weigh and record data on the piece, then place in oven at 100°C to dry them completely. This will tell us how much the samples weigh at zero moisture content, thus help calculate how much the soundboard shall weigh at target moisture content, about 10-12%.
Cover inner rim completely with tape, ribs in place. Rub wax on tape and inside of outer rim so epoxy won't cling. The masonite soundboard pattern was made ahead of time. It was trimmed to fit in piano with a gap around the edge or with room for epoxy filling.
Use long pieces of wood and go bars to fasten pattern edge. The perimeter shall be solidly sitting on inner rim. If not, epoxy filling will leak under. Fill epoxy around the old pattern. The filling level needs to be at least flush to the pattern surface. make sure there is no missed spot. The purpose of this step is to make a perfect sized pattern or soundboard shape for this piano with ribs underneath. Let dry over night. Note, there is no clamps at belly. This is a mistake. Clamping at belly will spread the pattern solidly flat thus make it just fit for this piano. When dried, sand the top and bottom side of pattern to create a flat surface for router to ride on (not shown here).
According to calculation by the shop owner, the soundboard shall weigh 5035 grams after drying. When the weight is right, take board out of kiln, place and fasten under the pattern we just made and sanded. With a jig saw, trim the excessive material off, leaving about 1/8" over hang. This will make routing easier, which is the next step.
Set up router bits, bearing riding against edge of masonite pattern. Trim soundboard to shape.
All trimmed, belly side being left alone to be fitted later. Now we are ready to glue ribs on the soundboard.