Smithy MIDAS MI-1220 LTD, Operator'S Manual, Page: 28 / 133

Chapter 7
Metal Theory
Tool sharpness
Instead of being the all-important factor in determining tool performance, keenness of
the cutting edge is just one of many factors. On rough or heavy cuts, it is far less
important than strength, because a false cutting edge or crust usually builds up on the
tool edge, and though the edge dulls, its angle often increases the cutting tool's
efficiency by increasing its wedging action. Cutter shape is usually more important than
edges, which generally are rough-ground and usually must be honed for fine finishing
cuts or work in soft, ductile materials like brass or aluminum.
Lack of clearance, which lets a tool drag on the work below the cutting edge, is a brake
on the lathe, greatly reducing pressure on the cutting point and interfering with tool
performance more than edge dullness. At the same time, excessive clearance weakens a
tool because of insufficient support to the cutting edge. Such an edge will break off if you
use the tool on hard materials.
Clearance requirements change with almost every operation, but there are certain
standards for all aspects of the cutting tool. You must not only provide clearance from the
cutting edge; there must also be end and side clearance. To help the chip pass with
minimum resistance across the top of the tool, it should often have top rake as well. You
determine the shapes and rakes to which you'll grind your tools by the tool holder you
use. TheCB-1220 XL LTD have a four-sided turret toolpost that accommodates four
high-speed-steel (HSS), carbide-tipped, or indexable carbide turning tools.
Heat
The energy expended at the lathe's cutting point converts largely into heat, and because
the energy expended is great, the heat is intense. Before today's HSS, carbide, and
ceramic tools, this heat created a serious machining problem. Machining could be done
only under a steady flow of coolant, which kept the tool from heating to its annealing
point, softening, and breaking down.
With HSS, you can usually cut dry unless a small lathe is running at extremely high speeds
on continuous, heavy-duty production work. HSS tools are self-hardening even when red
hot. They do not dissipate the heat, however, or in any way prevent the workpiece from
heating up. Because steel expands when heated, it is a good idea, especially when
working on long shafts, to check the tightness of the lathe centers frequently and make
sure workpiece expansion does not cause centers to bind.
7-1
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