The go-bar method is a trial for now. Before, we used machine screws to fasten bridge cap to root. But to use machine screws, many holes are dilled on the cap, which weakens the bond. The go-bar method saves time on dry-fitting, also creates a stronger glue joint.
This piano does not require a new soundboard. The bridge cap was removed with a router; bridge pin holes were plugged; and surface smoothed with a power plane. Bridge cap was made from hard maple, milled to match original bridge height plus 3mm additional thickness to make room to set down-bearing later on.
First, drill 3 locator holes at each end and center on bridge root. We use #6 bridge pins, 0.076" in size, for locating bridge cap. The drill bit used here is also 0.076". The hole shall be just deep enough to hold a pin. It will be trouble if a pin is lost/pressed all the way in the hole. Hammer locator pins in the holes till bottomed up, snip the pin top, leaving a 1/8" or 3mm tip sticking out.
Place bridge cap on root. Fasten one go bar above each locator pin. Pay attention to the go bar pressure. When tightening go bar number 2, number 1 may get loose and fall down. Trace the root shape now. the special tracing pencil is made of a pencil tip glued on a stick.
Remove go bars, check that the locator pin made marks on the cap. The pencil line for the root is very rough, and parts of line missing. It is ok. It will be refined later. Saw outside the pencil line, leaving about 3-5mm room for adjustment. Sawing too closely may cause trouble. Tweaking a little at a time is better here. On a drill press, drill through the marked locator hole on cap with the drill bit for locator pins.
Erase pencil marks on cap, drive locator pins through the locator holes. The pins will go into the holes first drilled on bridge root. Trace root pattern again. This time the line will be more clear. Saw outside the lines leaving about 1/16" or 1-2mm room just incase the cap moves slightly during gluing process.
To prep glue surface, mark pencil lines all over the top, sand with 80 grid sandpaper till all marks are gone. This is to make sure the surface is flat. Bend a razor blade, scrape till a smiling face is formed, deep in the middle and high on two edges. This makes room for glue to stay.
The root surface is prepped the same way. Scrape one 12-20" section at a time. Keep checking with a straight edge for that smiling face. The left picture shows the opposite of a good glue surface. The center needs to be scraped down. Keep bending razor blade back and forth when it gets dull. A sharp blade shaves more wood off, you can feel it. When bending doesn't make the other side of blade sharper, change blade.
Before gluing, trim the two ends of bridge cap as flush to the root as possible for easy machining later on. The trim can be done either by hand or on belt sender. Also make sure the locator pins go into their holes on root each time dry fitting.
Apply wood glue on root, Titebond 50 is used here. Spread glue evenly with a foam brush. Place cap on root, locator pins go into their holes. Fasten the first go bar in the middle, followed by bars at two ends, then bars in between. The soundboard is crowned upwards, fasten bridge cap firstly on one end may cause the other end to tilt very high. A good squeeze out shows good glue joint. Clean excessive glue with rag soaked in hot water.
We don't have enough go bars for both bass and treble bridges. The bass bridge was glued first thing in the morning (left picture). Tidebond 50 calls for 30 minutes clamping time. We gave it 2 hours just to be safe. Then treble bridge cap was glued. The next day, The go bars can be taken off, and bridge cap routed flush to the root.