From: BILL KELLY, EASTERN REGION.
Taken from “A Manual of Carpentry And Joinery” J. W. Riley. First printed 1905.
Curved surfaces are often “cut out of the solid” and dressed to the required curvature. It is occasionally necessary, however, to bend a board to obtain a curved surface. A method often adopted is to make saw-cuts (kerfs) in the face, which is to be concave, at such distance apart that in order to close the kerfs the board must be bent to the required curvature.
A ready appliance for obtaining the exact distances apart of the saw kerfs is illustrated in Fig 1. It consists of a lath of exactly the same thickness as the board to be bent. About the middle of the length of this lath, a saw cut is made. The lath is then bent until the kerf closes, and the angle through which it has been turned from the straight line is obtained. An arc of a circle is then struck with A as centre and a radius equal to the radius of the curve required.
The chord BB of the arc is the distance apart of the kerfs to be cut in the board to be bent. Similarly, with a curve of radius AC, CC gives the distance apart of the saw kerfs and again, DD would be the distance apart of the kerfs required for a curvature of radius AD.
The following precautions must be carefully attended to:
(1). The lath must be exactly the same thickness as the board to be bent.
(2). All the saw-kerfs must be made with the same saw, i.e. the saw used for cutting
(3). The depth of all the kerfs must be equal, and deep enough to allow bending
without breaking the fibres on the convex side of the board.