Smithy Midas 1220 XL - Manual (Page 38)

Chapter 8
Metalcutting Theory
A machine tool is no more efficient than its cutting edge. Because lathe operations require
continuous regrinding and resharpening of the machine's cutting tools, operators should
know some metalcutting theory.
All cutting with a sharp edge, whether with the thin blade of a knife or the almost square
edge of a closely supported carbide tool, is basically a wedging-apart action. The first
essential of any wedging tool is a penetrating edge. The narrower the blade, the less
force is needed to wedge it through the material. Therefore, when cutting comparatively
soft materials with a cutting tool made from a much harder, stronger substance, the blade
can be very thin and sharpened to a long, thin edge.
As the material hardness (or resistance to separation) increases, the strength of the
cutting edge must also increase. A knife whose edge is too thin dulls quickly, even when
cutting comparatively soft materials. This explain why, in Figure 8.1 the knife edge breaks
off almost upon contact with the metal while the more obtuse cutting edge of a cold
chisel stands up to continuous pounding.
Figure 8.1 The knife edge breaks off almost upon contact with the metal
because its cutting edge is not as strong as that of a cold chisel.
The primary requirements of the cutting edge of any metalcutting tool are that it be (A)
strong and (B) closely supported. This understandable when we realize how much
downward pressure is exerted against the cutting edge. Pressure against cutting tools as
great as 250,000 per square inch (psi) have been measured on large metalcutting lathes.
While the workpiece revolves, a strong, rigidly held cutting edge is forced under its
surface. As it presses down against the cutting edge, small chips or a continuous ribbon
of metal wedges away (Figure 8.2). Only in soft, ductile materials is this wedging action
continuous.
8-1
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