various styles of tools required in machinery, such as
allows cutting speeds approximately 60 percent
turning, facing, threading, and grooving are available
greater than for high-speed steel tools. However, cast
with different grades of carbide tips already brazed
ahoy tools are not as tough as the high-speed steel
onto steel shanks. You can also get small carbide
tools and they cannot bear the same cutting stresses,
blanks and have them brazed onto shanks.
such as interrupted cuts. Clearances ground on cast
alloy cutting tools are less than those ground on
When you use cutting tools with brazed-on
high-speed steel tools because of the lower degree of
carbide tips, chip control may be provided by either
toughness. Tools made from this metal are generally
feeds and speeds or by chip breaker grooves ground
known as Stellite, Rexalloy, and Tantung.
into the top of the carbide tip. The best way to grind a
chip breaker is to use a chip breaker grinder with a
CEMENTED CARBIDE
diamond impregnated wheel. The depth of the chip
breakers averages about 1/32 inch, while the width
A carbide, generally, is a chemical compound of
varies with the feed rate, depth of cut and material
carbon and metal. The term commonly refers to
being cut. Grind the chip breaker narrow at first and
cemented carbides, the cutting tools made of tungsten
widen it if the chip does not curl and break quickly
carbide, titanium carbide, or tantalum carbide, and
enough. You may also use these same types of chip
cobalt in various combinations. A typical composition
breakers on high-speed steel cutters.
of cemented carbide is 85 to 95 percent carbides of
tungsten and the remainder a cobalt binder for the
Mechanically Held Tip (Insert Type)
tungsten carbide powder.
Cemented carbides are made by compressing
Mechanically held carbide inserts are available in
various metal powders and sintering (heating to weld
several different shapes--round, square, triangular,
particles together without melting them) the
diamond threading, and grooving--and in different
briquettes. Cobalt powder is used as a binder for the
thicknesses, sizes, and nose radii. In the following
carbide powder.
paragraphs, we'll discuss the most important criteria
you'll need to select an insert.
Carbides have greater hardness at both high and
low temperatures than high-speed or cast alloys. At
OPERATING CONDITIONS.--You must use
temperatures of 1,400F and higher, carbides maintain
three variables to establish metal removal rate: speed,
the hardness required for efficient machining. This
feed, and depth of cut. Cutting speed has the greatest
makes possible machining speeds of approximately
effect on tool life. A 50 percent increase in cutting
400 fpm in steels. The addition of tantalum increases
speed will decrease tool life by 80 percent. A 50
the red hardness of a tool material. Cemented
percent increase in feed will decrease tool life by 60
carbides are extremely hard tool materials (above
percent. The cutting edge engagement or depth of cut
Rc90), have a high compressive strength, and resist
is limited by the size and thickness of the carbide
wear and rupture.
insert and the hardness of the material being
machined. Hard materials require decreased feed,
Cemented carbides are the most widely used tool
speed, and depth of cut. The depth of cut is limited by
material in the machining industry. They are
the strength and thickness of the carbide insert, the
particularly useful for cutting tough alloy steels that
rigidity of the machine and setup, the horsepower of
quickly break down high-speed tool steels. Various
the machine, and the amount of material to be
carbide grades and insert shapes are available and you
removed.
should make the correct selection to machine a
particular material. We'll now briefly discuss
Edge wear and cratering are the most frequent
brazed-on tip carbides. Since mechanically-held tip
tool breakdowns and they occur when friction and
(insert type) carbides are more widely used, we will
abrasion break down the tool relief surface. They are
discuss them in more depth later in the chapter.
also caused by the tearing away of minute carbide
particles from the built-up edge. The cutting edge is
Brazed-on Tip
usually chipped or broken in this case. Lack of
rigidity, too much feed, or too slow a speed causes
The brazed-on carbide-tip cutting tool was the
chipped or broken inserts.
first carbide cutting tool developed and made
Thermal shock is caused by sudden heating and
available to the metal cutting industry. A brazed-on
cooling that causes a tool to crack, then break. This
tip can be easily ground to machine such jobs. The
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