Cutting and Machining of Metals
metal shaping processes are forging, rolling, extrusion, etc.,
that do not involve the removal of metal by means of cutting
tools. Many important shaping processes are based on cutting
and similar operations. The tools used are made of special
steels (tool steels), hard metals (Cemented carbide alloys),
oxide ceramic materials, and diamonds.
principles of the various methods are briefly outlined, without
detailed descriptions of the machines used for performing
the shaping operations. For each of these methods a whole
range of tools has been developed, each type of tool being
employed for a particular purpose. In chiseling (Fig.1), the
cutting edge of the tool is driven into the surface of the
workpiece by the action of blows. To ensure even and regular
removal of the chips it is essential to hold the chisel correctly
and take care that it does not slip on the metal surface or
dig too deeply into it. Chiseling is used chiefly for cutting
off and for the removal of edges, burrs, fins, etc.
Machining operations like planing, shaping and slotting (Fig.2)
are comparable to chiseling, characterized by the removal
of the chips in one direction, the tool being moved to and
fro or up and down in relation to the workpiece. In sawing
(Fig.3) the removal of metal is effected by a series of saw
teeth. With power-driven band saws and circular saws, cutting
can be performed in continuous operation.
shape, spacing and number of teeth vary greatly for different
saws. Large diameter circular saws may have interchangeable
teeth or interchangeable segments comprising a number of teeth.
Sawing is used mainly for cutting off and for cutting plate
material of not too great a thickness. Thick pieces of metal
can, while still hot from the furnace, be cut with hot sawing
machines or with cutting discs. The latter achieve very high
cutting rates, cutting being effected by melting of the metal
due to frictional heating. Band-type cutting devices are based
on the same principle of heat generated at the cutting surface.
is another important basic process (Fig.4). By using suitably
shaped files it is possible to cut metal to any desired shape.
The actual cutting is performed by the teeth of the file.
Roughing out the shape is first done by means of coarse files,
followed by finishing with finer files. Files are available
in great variety of shapes, sizes and grades. They are classified
and named according to sectional shape (e.g., half-round,
square, triangular, round), length, and the relative fineness
of cut of the teeth. With regard to fineness, the following
classes of file are distinguished: bastard, second-cut, smooth,
dead smooth. If there is only one series of parallel teeth,
the file is known as a single-cut file. If the first series
is intersected by a second and finer series, so as to form
diamond shaped teeth, the file can also be called as double-cut.
broach (Fig.5) is a tapered tool provided with a series of
cutting teeth which are lower at one end of the tool than
they are at the other. Broaching is mainly employed for machining
out holes or other internal surfaces, but can also be used
for external surfaces and for burnishing already-formed holes.
The cut starts with the smaller teeth, which enter the hole,
and finishes with the larger teeth, which bring the hole to
the finished size. Fig.5 shows an internal broach. Its cross-sectional
shape may be round, rectangular, etc., depending on the desired
shape of the hole. The broaching operation is performed by
a machine that pulls or pushes the broach through the workpiece.
(Fig.6) is one of the most important machining processes.
It is the process of reducing the diameter of material held
in a lathe. The workpiece is attached to a driven spindle
and, while rotating, is brought into contact with a cutting
tool. The position of the tool in relation to the axis of
rotation can be varied so as to cut the workpiece to the desired
shape. In longitudinal turning, the tool is moved parallel
to the axis of the rotation, so that cylindrical shapes are
screw thread can be cut by a tool forming a spiral groove.
In transverse turning, also known as facing, and in forming,
the tool is moved at right angles to the axis. Workpieces
of any desired tapered or other axially symmetrical shape
can be produced by suitable combinations of longitudinal and
transverse tool movements.
tools are available in a wide range of shapes and types. The
cheapest are made of high carbon steel, hardened and tempered.
Alloys known as high speed steels are used for tools that
can be operated at much higher cutting speeds. In tipped tools
the cutting tip is made of a special hard material. E.g.,
a cemented carbide, particularly tungsten carbide.
(Fig.7) is a rotary cutting operation for producing holes.
The tool most widely used for the purpose is the twist drill,
provided with helical cutting edges, which rotates and is
fed forward into the material under pressure. The combination
of rotary and feed motion cuts away chips of the material,
which are removed from the hole. For drilling a hole in a
solid workpiece, it is necessary first to make an indentation
for the center of the drill to revolve in. A tool called center
punch is used for the purpose. It is advisable first to use
a smaller drill and then follow up with a drill of larger
(Fig.8) is a process related to drilling and is employed to
form a cylindrical hole of large diameter at the end of an
existing hole. E.g., to receive the head of a screw or bolt.
If the enlarged hole is formed with tapered sides, the process
is called countersinking.
(Fig.9) is another important machining process, in which the
workpiece is shaped by means of a rotating cutter provided
with a number of teeth. Usually the work is fed against the
teeth, the work-feed direction (in relation to the cutter)
being longitudinal, transverse or vertical. Milling machines
are very versatile and can be used for a great variety of
work, including screw-thread cutting. In circular milling
the cutter and the workpiece are both rotated; in straight
milling the cutter rotates while the workpiece performs a
straight feed motion
(Fig.10) is the operation in which an abrasive wheel or disc
is used to remove metal. It is employed as a finishing treatment
to give parts already machined the necessary precision by
the removal of excess material. It is also employed as a machining
process in its own right – e.g., for roughly forged
or cast parts or for the shaping of hard materials. Centerless
grinding is used for small cylindrical parts and is performed
between two grinding wheels. Grinding wheels are made from
artificial abrasives, usually of the aluminum oxide or the
silicon carbide type, embedded in suitable bonding agents.
Wheels are available in a vast number of different combinations
of abrasive, grain size, type of bond, hardness of bond, and
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