somewhere in my metalworking past I remember being told that rapid transitions from one plain to another in metal, particularly in forgings, can often give rise to breakages.
Milling or turning the back of the crank to remove forging " flash" or similar waste metal and thereby producing a sharp corner is similar to the effect created by sawing a piece of wood across its grain.. thereby severing that grain and jeopardising the strength of the timber If the spare timber were to be sawn off along the grain thereby creating a step in the timber, there could be a shearing effect or splitting in the case of the timber,
When patterns are being prepared for castings all sharp corners are radiused in order to avoid the possibilty of stress raisers in the castings and also to help distribute any stresses.
A similar principle applies when bronze-welding ( fillet brazing) a frame. It is possible to join tubes by blowing the filler material around the mitre of the joint thereby leaving a fairly sharp juncture. However the radiused bead produced by a weld reduces the stress at such a juncture by distributing it more evenly.
In recent years improvements in forging techniques have vitually eliminated the need any form of cosmetic turning or milling. Some long years ago - perhaps as many as 12, I was handed a pair of wonderful looking cranks by a salesman on the TA stand at the Paris Show. They were prototypes of the Alysee and the Zephyr. I ordered several pairs but had to wait several more years before they arrived because TA , who apparently no longer do their own forging, could not find anyone capable at that time of forging to the shape, standard, quality and finish required..
Norris Lockley.. stroking a fillet weld..