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Full text of "Modern Mechanical Engineering Vol-I"

182

THE   MACHINE-SHOP

£ND  ELEV.

Fig. 3.—Double-edge Roughing Tool

and left-handed bent tools.   The angles are stated on the drawings.    The
difference lies in the side top rake.

Side Top Rake.—When considering top rake it is necessary to bear
in mind the direction in which a tool is fed in relation to the work. If it is
traversed laterally, as in turning, then a straightforward nose with front rake
only is not the best possible, because the tool angle is not in the line of travel

A                                 and the lateral strain on the

j^                                 tool is increased.    But if

rake is provided in the
direction of travel—" side
top rake "—the tool can be
fed more easily, will cut
more freely, and the chips
will be deflected away from
the tool support. This
explains why the majority
of roughing-tools have side
top rake, and why, when a
straightforward tool is em-
ployed, it is generally set at an angle for traverse cuts, and why so many
tools with top rake are bent at the points right and left to correspond
with the direction of their traverse. The blunt tool in fig. i has 14°
of side top rake, and the sharp tool (fig. 2) has 22°.

Plan Outlines, Roughing and Finishing.—The curvature of the
nose of a cutting-tool is important. Figs, i and 2
show " round-nose " tools. These gouge-like tools
remove material with the maximum of effect. The
amount of convexity varies considerably, and generally
those with the longer radii are used for the heavier
duties. These are termed roughing-tools, notwith-
standing that they are often retained for finishing.
The distinction between tools for roughing and
finishing is not observed to the same extent as of old.
The spring-tool, so long a favourite with turners, is
obsolete. Tools with double edges (fig. 3), such as
are commonly used in lathes, rough with the leading
edge, while the small following radius leaves a smooth
surface.

Knife Tools.—The knife or shaving-tools are
employed extensively in turret lathes. They rough
and finish. They cut normally to the knife edge
(fig. 4), and remove broad shavings with fine feeds,
leaving a finished surface on the work. They are made straightforward,
left-handed, and cranked. A clearance of 6° and a side top rake of 12° is
suitable. Allied to the knife tools are the narrow parting-tools, used for
severing pieces of work. These are made straightforward, left-handed

SECTION A 9

A -

PLAN

"B

Fig. 4.—A Knife Tool