One each rib, mark two places on the ends to install 1/4" diameter soundboard locator pins (shown in the mid picture). Drill with a 1/4" bit on the mark as deep as the locator pin goes.
Insert locators and return ribs to piano. On the side of ribs, mark where locator pins are.
Insert soundboard, press down with go bars. Press ribs upwards at the mark to make indentation on the soundboard. Take the board out and check that all marks are visible, see the right picture.
Remove locator pins from ribs, glue in 1/4" dowels, then trim short with a saw. The amount of protrusion depends on how deep of a hole to drill on the soundboard. At the marks on soundboard, drill with 1/4" drill bits for the locating dowels on ribs to go in. The soundboard is only so thick, i.e. less than 9mm. Be careful not to drill through.
We use air press to glue on ribs. The detailed gluing process is not shown here because of tight time frame. The ribs are dry fitted on soundboard with air pressure. If fitted nicely with no gap, the pressure is relieved, and ribs can be officially glued on soundboard with air pressure applied. The cauls between taper/feather and air hose are pre-shape cut off pieces, each feather has its own. We use Titebond 50 for the gluing job. The clamping time is usually over night although Titebond says 30 minutes minimum.
When glue dries, the ribs can be shaped and sanded. The lime colored planer rides on rib molding to shape the wood. The belt sender refines the tapers further more. We use to sand the rib edges by hand to round them. A router with a round over bit like this will do the job more evenly and faster. After all the machining, sand with 220 grid sandpaper before sealing the wood with sanding sealer. It is better to return soundboard to hot box or kiln to let dry before applying sanding sealer. I once didn't go through the drying process and applied sealers. The board grew way bigger in high humidity and the sealer sealed all moisture inside the wood. The board ended up having to be trimmed before gluing in piano.
After the board is dried in kiln over night, the gluing edge (area to be glued on piano inner rim) can be taped and sanding sealer applied to seal the wood. Be sure to clean the spray gun before and after spraying.