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Full text of "Modern Mechanical Engineering Vol-I"

i8o                          THE  MACHINE-SHOP
CHAPTER   II
The Tools
GROUP   I
Single-edged Gutting-tools.—These are used in the lathes, planers,
shapers, and slotters. They are so termed because each tool has but one
edge, which distinguishes the group from the reamers and milling-cutters,
which have several edges acting in quick succession. They are an obvious
survival from the period when tools were presented by hand.
Formerly these tools were ground solidly with the shank or bar on the
end of which they were forged. Later, they have been more frequently
made separately, as tool points, to be gripped in holders, of which there are
many scores of designs. The expense of a higher grade of steel can then
be incurred for the small tool point, and in many instances tool points can
be disposed and operated to much greater advantage than when they are
forged solidly on long shanks. These holders occur in several machines,
but principally in the newer lathes, in automatic turning machines, and
turret lathes. They reach their highest developments in the latter.
Tool Angles, Rake.—The term " single-edged" includes some
dozens of ends and edges shaped differently, some being true cutting-tools,
others scrapers only. The essential difference between cutting-tools and
scrapers is that the first has top rake, the second has none, that is, in the
first the top face of the tool makes an angle of something less than 90° with
the surface of the work, if plane, or with its tangent if circular; while in the
second the angle is 90°.
The " tool angle ", the angle of " clearance ", and the angle of " top
rake ", are shown in figs, i and 2. The tool angle is a measure of the ability
of the tool to resist the pressure of the cut, and it is therefore maintained as
large as possible. The clearance of 6° (figs, i and 2) need not vary much,
since this clearance is provided merely to prevent friction and heating between
the tool and the surface of the work. It may range between 3° and 7°,
though many tools that are hand ground have a larger amount, by virtue of
which they cut more freely, but at some sacrifice of endurance. The angle
of top rake is varied with the material to be tooled in order to give a good
cutting action, and to permit the chips or the shavings to come away freely.
The tool angle ranges from 50° to 85°, both being exceptional. Keen
angles would give an easy cut, but the edge would not be permanent. Two
standard angles have emerged, roughly 70° for the softer steels, and 80° for
the harder steels and cast iron.
Figs, i and 2 show standard Sellers' tools. Two sets only of angles are
adopted (fig. i), the " blunt tools " for cast iron and the harder grades of
steel, and (fig. 2) the " sharp tools " for wrought iron and the softer grades
of steel. Both are made as right- and left-handed straight tools, or as right-