Machinery Repairman

igidity of the tool and work and where you can
maintain high machining speeds. Negative rake tools
require more horsepower. Under these conditions,
negative rake tools are stronger and produce
satisfactory results.
You may use negative rake inserts on both sides,
doubling the number of cutting edges per insert. This
is possible because end and side relief are provided by
the angle of the toolholder rather than by the shape of
the insert.
Use positive rake inserts where rigidity of the tool
and work is reduced and where high cutting speeds
are not possible; for example, on a flexible shaft of
small diameter. Positive rake tools cut with less force
so deflection of the work and toolholder should be
reduced. High cutting speeds (sfpm) are often not
possible on small diameters because of limitations in
spindle speeds.
INSERT SIZES.—Select the smallest insert that
can sustain the required depth of cut and feed rate.
The depth of cut should always be as great as
possible. The rule of thumb is to select an insert with
cutting edges 1 1/2 times the length of cutting edge
engagement. The feed for roughing mild steel should
be approximately 1/10 the depth of cut.
TOOLHOLDER AND BORING BAR
STYLES. —Tool style pertains to the configuration of
toolholders and boring bars used to hold a carbide
Figure 5-16.—Toolholders for carbide inserts.
5-12
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insert. To determine style, you need to know
something about the particular machine tool you are
using and the operations you will do on it. Figures
5-16 and 5-17 show some of the styles available for
toolholders and boring bars.
TOOLHOLDER AND CARBIDE INSERT
IDENTIFICATION. —The carbide and toolholder
manufacturers and the American Standards
Association (ASA) have adopted a system to identify
toolholders and inserted carbides. The system is used
to call out the toolholder geometry and to identify
inserts. You’ll find these charts in manufacturer’s
catalogs; copy them and keep them in your toolbox.
We have given you an overview on carbides;
certainly not everything you need to know. You also
must work with personnel who know how to use them
and then learn by using them yourself. Also, a
number of carbide manufacturers offer schools to help
you understand carbides and their uses.
CERAMIC
Other than diamond tools, ceramic cutting tools
are the hardest and most heat resistant available to the
machinist. A ceramic cutting tool can machine metals
that are too hard for carbide tools, and they can
sustain cutting temperatures up to 2,000°F.
Therefore, you can use ceramic tools at cutting speeds
two to four times greater than cemented carbide tools.