1/4 inch thick. An SD150R100B grinding wheel is
normally recommended.
Chip breaker grinders have a coolant system that
either floods or slowly drips coolant onto the tool
being ground. The main objective is to prevent the
grinding wheel from loading up or glazing over from
the grinding operation.
CUTTING TOOL MATERIALS
The materials used to make machine cutting tools
must be hard enough to cut other metals, be wear
resistant, have impact strength to resist fracture, and
keep their hardness and cutting edge at high
temperatures. Several different materials are used for
cutting tools, and each one has properties different
from the others. Selection of a cutting tool material
depends upon the metal to be cut and the conditions
under which it will be cut.
HIGH-SPEED STEEL
High-speed steel is a widely used cutting tool
material. High-speed steel tools can maintain their
hardness and abrasion resistance under the high
temperatures and pressures generated during the
general cutting process. Although the hardness of a
high-speed tool (Rc 60-70) is not much greater than
Figure 5-11.--Chip breaker grinder.
that of carbon-steel tools, high-speed steel begins to
lose its hardness at a tempering temperature of 1,000
to 1,100F. Machine shops generally use two types of
CHIPBREAKER GRINDER
high-speed tools: tungsten high-speed steel and
A chip breaker grinder (fig. 5-11) is a specialized
molybdenum high-speed steel. These designations
grinding machine. It grinds grooves or indentations
show the major alloying element in each of the two
on the top surface of carbide tools to control the
types. Both types resist abrasive wear, remain hard at
direction and length of the chips produced in cutting
high temperatures, and keep a similar degree of
metal. Later in this chapter, we'll describe the types
hardness. The molybdenum high-speed steel is
of chip breakers that are commonly ground on carbide
tougher than the tungsten and is more effective in
tools.
machinery operations where interrupted cuts are
made. In an interrupted cut, such as cutting
The chip breaker grinder has a vise you can adjust
out-of-round or slotted material, the cutter contacts
to four different angles to hold the tool to be ground.
the material many times in a short period of time.
These angles are the side cutting edge, back rake, side
This "hammering" effect dulls or breaks cutters that
rake, and chip breaker, and we'll explain them later in
are not tough enough to withstand the shock effect.
this chapter. The vise is mounted so you can move it
back and forth under the grinding wheel. Both the
CAST ALLOYS
cross feed used to position the tool under the grinding
wheel and the vertical feed used to control the depth
Cast alloy tool steel usually contains varying
of the chip breaker are graduated in increments of
amounts of cobalt, chrome, tungsten, and molybdenum.
0.001 inch.
Tools made from these steels are generally more
The chip breaker grinder uses a diamond wheel.
efficient than tools made from high-speed steel,
retaining their hardness up to an operating tempera-
It is usually a type 1 straight wheel but differs from
ture of approximately l,400F. This characteristic
other type 1 wheels because it is normally less than
5-8
