so a couple days ago i finished a 12" radius fork blade bending mandrel from the H. James plans. upon further research, it became clear that radius would only be suitable for the recently-more-popular Continental or Italian camber, in which the blades curve rather gradually from near the top to the dropout. I decided that in keeping with the style of the bike, I wanted a more old-school British camber, in which the blades arc tightly only near the dropout, with perhaps the top two thirds straight and the bottom third curved.
despite having just finished the 12" mandrel, i undertook the construction of a new mandrel, based on the designs of John Clay of Tallahassee. this one has a varying radius, starting at 7" near the end where the dropout will be secured and changing to 12" at the other end. following are the steps i undertook to make the rough cut:
• determine a center point at least roughly 8" from the left edge and 12" from the top edge of a piece of 3/4" thick cabinet/high-grade plywood
• use a carpenter's square to draw lines radiating at 90° angles from the centerpoint: one 7" long to the left and one 12" long to the top
• use a compass to draw arcs 8", 9", 10", and 11" from the centerpoint within the area described by the right angle lines
• use a protractor to mark 18° increments in the relevant quadrant
• use a ruler to sequentially connect the centerpoint, an 18° mark, and an arc, starting with the 8" arc and the 18° mark closest to horizontal, progressing towards the 11" arc and the 18° mark closest to vertical
• now sketch the curve, attempting to connect the intersections just marked. the curve will have a slight teardrop shape.
now rough cut with a jig saw (after clamping down the plywood, donning goggles, and practicing), file to a smooth curve (a 4-in-hand rasp is perfect), and file an even semi-circular trough along the center of the curve (triangular, round, half round, chainsaw, and knife files are useful). figure out a way to hold the dropout end of the blade in place while bending (still working on that myself).