Rib Depth of Guitars with Spherically Domed Plates

A Survey of Some Published Methods for Shaping the Ribs of Flattop Guitars, with Some Elaboration and Some Additional Information

by R.M. Mottola

Originally published in American Lutherie #84, 2005 and Big Red Book of American Lutherie Volume Seven, 2015



Modern flattop guitars generally do not have flat tops (or backs) at all. Instead, the plates of these instruments are generally forced into spherically domed shapes. The doming is accomplished using arched braces glued to the plate while it is pressed into a dished assembly board. Once the glue hardens, the plate permanently assumes the domed shape. The radius of the dome is generally quite large, typically ranging from 12´–30´, so the doming effect is slight. Still, this doming means the edges of the plate are not constrained to a single horizontal plane, and this means that the ribs of an instrument using this style of plate must vary in depth in order to uniformly contact the plate edges. For those unfamiliar with this style of construction, the issue can be imagined by considering a round, spherically domed plate sitting on a table. The edge all the way around the plate will contact the table top. If the round plate is now cut into the shape of a guitar plate and that plate is set on the table, the edge will not contact the table top all the way around, as shown in Fig. 1. While points at the neck and tail block positions will touch the table, the rest of the edge will vary in distance above the table top. Again, the ribs of a guitar with such a plate would have to vary in depth if they were to continuously contact the plate edge, without gaps.