For example, after the first variable (A) we quote,
"The proportion is as I in the case of semi-hardened steel or chilled iron to 100 in the case of a very soft, low-carbon steel." The meaning of this quotation is that soft steel can be cut 100 times as fast as the hard steel or chilled iron. The ratios which are given, then, after each of these elements, indicate the wide range of judgment which practically every machinist has been called upon to exercise in the past in determining the best speed at which to run the machine and the best feed to use.
(A) The quality of the metal which is to be cut; i.e., its hardness or other qualities which affect the cutting speed. The proportion is as 1 in the case of semi-hardened steel or chilled iron to 100 in the case of very soft, low-carbon steel.
(B) The chemical composition of the steel from which the tool is made, and the heat treatment of the tool. The proportion is as 1 in tools made from tempered carbon steel to 7 in the best high-speed tools.
(C) The thickness of the shaving, or, the thickness of the spiral strip or band of metal which is to be removed by the tool. The proportion is as 1 with thickness of shaving 3/16 of an inch to 3 1/2 with thickness of shaving 1/64 of an inch.
(D) The shape or contour of the cutting edge of the tool. The proportion is as 1 in a thread tool to 6 in a broad-nosed cutting tool.
(E) Whether a copious stream of water or other cooling medium is used on the tool. The proportion is as 1 for tool running dry to 1.41 for tool cooled by a copious stream of water.
(F) The depth of the cut. The proportion is as 1 with 1/2 inch depth of cut to 1.36 with 1/8 inch depth of cut.
(G) The duration of the cut, i.e., the time which a tool must last under pressure of the shaving without being reground. The proportion is as 1 when tool is to be ground every 1 1/2 hours to 1.20 when tool is to be ground every 20 minutes.
(H) The lip and clearance angles of the tool. The proportion is as 1 with lip angle of 68 degrees to 1.023 with lip angle of 61 degrees.