LATHE
Lathes are
considered to be the oldest machine tools.
Designed in 1797 they are probably the most versatile machine tools
developed. Primarily the lathe was
designed to perform turning, facing and boring operations on a cylindrical work
piece. However, operations such as
drilling, reaming, tapping, knurling, grinding, milling, threading, tapering
are possible on the lathe when various cutting tools and attachments are used. Lathes can be classified according to their
drive mechanism (direct or indirect), feed mechanism (hand, power, or
automatic), or production capabilities (non, semi, and production) and lastly
manual or computer controlled.
Lathe structure
Regardless of
classification, all lathes have five basic parts.
-Headstock
It contains the
gears, pulleys, or a combination of both, which drives the work piece and the
feed units. The headstock contains also the motor, spindle speed selector,
feed-unit selector and feed-direction selector. It provides a means of support and rotation to the work piece by
attaching a work-holding device to its spindle. Headstocks have a hollow spindle to which work holding devices,
such as chucks and collets, are attached, and long bars can be fed through for
various turning operations.
-Bed
It provides
support for the other units of the lathe.
V-shaped ways are located on the top of the bed providing alignment of
the headstock, bed and tailstock. The
top portion of the bed has two ways, with various cross-sections, that are
hardened and machined accurately for wear resistance and dimensional accuracy
during use.
-Carriage
It slides along
the ways and consists of an assembly of the cross-slide, tool post, and apron.
The cutting tool is mounted on the tool post, usually with a compound rest that
swivels for tool positioning and adjustment.
The cross-slide moves in and out, thus controlling the radial position
of the cutting tool, as in facing operations.
The apron is equipped with mechanisms for both-manual and the
cross-slide, by means of the lead screw.
-Tailstock
It can slide along
the ways and can be clamped at any position, supporting the other end of the
work piece. It is equipped with a
center that may be fixed (dead center), or it may be free to rotate with the
work piece (live center). Drills and
reamers can be mounted on the tailstock quill to produce axial holes in the
work piece. A hand wheel allows for the
extension of the tailstock spindle.
-Feed rod and lead screw
The feed rod is
powered by a set of gears from the head stock.
It rotates during operation of the lathe and provides movement to the
carriage and the cross-slide by means of gears, a friction clutch, and a keyway
along the length of the rod. The lead
screw is used for cutting threads accurately.
Closing a split nut around the lead screw engages it with the carriage.
Work holding devices
They are important
in machine tools. In a lathe, one end
of the work piece is clamped to the spindle by a chuck, collets, face plate,
mandrel or between centers.
A chuck is usually
equipped with three four or six jaws.
Three and six-jaw chucks generally have a geared-scroll design that
makes the jaws self-centering and hence are used for round work-pieces, such as
bar stock, pipes, and tubing. Four-jaw
chucks (independent chucks) have jaws that can be moved and adjusted
independently of each other and thus can be used for square or rectangular, as
well as odd-shaped work pieces. They
are more ruggedly constructed than three-jaw chucks and hence are used for
heavy work pieces. The jaws in both
types of chuck can be reversed to permit clamping of the work pieces on either
outside surfaces or inside surfaces of hollow work pieces, such as pipes and
tubing. Chucks are available in various designs and sizes. Their selection depends on the type and
speed of operation, work piece size,
production and accuracy requirements and the jaw forces required. High spindle speeds can reduce jaw forces
significantly because of centrifugal forces.
Process of cutting
Cutting processes
such as turning on a lathe, drilling, milling or thread cutting remove material
from the surface of the work piece by producing chips. The major independent variables (those that
we can change directly) in the cutting process are:
- Tool material
and its condition
- Tool shape,
surface finish, and sharpness
- Work piece
material, condition, and temperature
- Cutting
conditions, such as speed and depth of cut
- Use of a cutting
fluid
- The
characteristics of the machine tool, such as its stiffness and damping.
It is important to
note that a chip has two surfaces: one that is in contact with the tool face
(rake face), and the other from the original surface of the work piece. The tool side of the chip surface is shiny,
or burnished, which is caused by rubbing of the chip as it climbs up the tool
face. The other surface of the chip
does not come into contact with any solid body. This surface has jagged, step
like appearance, which is caused by the shearing mechanism of chip
formation. The type of chips produced
significantly influences the surface finish produced and the overall cutting
operation.
Continuous chips
They are usually
formed at high cutting speeds and/or high rake angles. The deformation of the material takes place
along narrow shear zone, called the primary shear zone. Continuous chips may develop secondary shear
zone at the tool-chip interface, caused by friction. The secondary zone becomes deeper as tool-chip friction
increases. In continuous chips,
deformation may also take place along a wide primary shear zone with curved
boundaries. The lower boundary is below
the machined surface, which subjects the machined surface to distortion. This situation occurs particularly in
machining soft metals at low speeds and low rake angles. It can produce poor surface finish and
introduce residual stresses, which may be detrimental to the properties of the
machined part. Although they generally
produce good surface finish, continuous chips are always desirable,
particularly in automated machine tools.
Built-up edge
(BUE) chips may form at the tip of the tool during cutting. This edge consists of layers of material
from the work piece that are gradually deposited on the tool. As it becomes larger, the BUE becomes
unstable and eventually break up. Part
of the BUE material is carried away by the tool side of the chip; the rest is
deposited randomly on the work piece surface.
The process of BUE formation and destruction is repeated continuously
during the cutting operation. BUE is one
of the factors that effects surface finish in cutting and changes the geometry
of cutting. As cutting speeds increase,
the size of the BUE decreases, or it doesn't form at all. The tendency for BUE to form is reduced by
decreasing the depth of cut, increasing the rake angle, and using a sharp tool
and an effective cutting fluid.
Although BUE is generally undesirable, a thin, stable BUE is usually
regarded as desirable because it protects the tool's surface.
Serrated
(segmented or no homogeneous) chips are semi continuous chips, with zones of
low and high shear strain. Metals with low thermal conductivity and strength
that decreases sharply with temperature, such as titanium, exhibit this
behavior. The chips have a saw tooth
appearance.
Discontinuous chips
Discontinuous
chips consist of segments that may be firmly or loosely attached to each
other. Discontinuous chips usually form
under the following conditions:
-Brittle work
piece materials, because they do not have the capacity to undergo the high
shear strains developed in cutting;
-Materials that
contain hard inclusions and impurities;
-Very low or very
high cutting speeds;
-Large depths of
cut and low rake angles;
-Low stiffness of
the machine tool;
-Lack of an
effective cutting fluid;
Because of the
discontinuous nature of chip formation, forces continually vary during
cutting. The stiffness of the
cutting-tool holder and the machine tool is important in cutting with
discontinuous chip as well as serrated-chip formation. This affects the surface finish and
dimensional accuracy of the machined part and may damage or cause excessive
wear of the cutting tool.
Cutting forces and power
Knowledge of the
forces and power involved in cutting operations is important for the following
reasons:
-Power requirements have to be
determined so that a motor of suitable capacity can be installed in the machine
tool.
-Data on forces are necessary
for the proper design of machine tools for cutting operations that avoid
excessive distortion of the machine elements and maintain desired tolerances
for the machined part.
-Whether the work piece can
withstand the cutting forces without excessive distortion has to be determined
in advance.
The forces acting
on the tool in orthogonal cutting are shown in fig. The cutting force Fc, acts in the direction of the cutting speed
V and supplies the energy required for cutting. The thrust force, Ft, acts in the direction normal to the cutting
velocity, that is, perpendicular to the work piece. These two forces produce the resultant force, R. The resultant force can be resolved into two
components on the tool face: a friction force F, along the tool-chip interface,
and normal force N, perpendicular to it. The resultant force is balanced by an
equal and opposite force along the shear plane and is resolved into a shear
force, Fc and normal force Fn.
The coefficient of
friction in metal cutting generally ranges from about 0.5 to 2.0, thus
indicating that the chip encounters considerable frictional resistance while
climbing up the face of the tool.
Although the magnitude of forces in actual
cutting operations is generally on the order of a few hundred Newton’s, the
local stresses in the cutting zone and the pressures on the tool are very high
because the contact areas are very small.
The majority of
turning operations involve simple single-point cutting tools with the fallowing
turning parameters;
-Rake angles
This is important
in controlling the direction of chip flow and to the strength of tool tip. Positive angles produce a small included
angle of the tool tip, which, depending on the toughness of the fool material,
may cause premature tool failure. Side
rake angle is more important than back rake angle.
-Relieve angles
They control
interference and rubbing at the tool-work piece interface. If the relieve angle is too large, the tool
may chip off; if too small, flank wear may be excessive. Cutting edge angles affect chip formation,
tool strength, and cutting forces to various degrees. Nose radius affects surface finish and tool-tip strength.
Cutting tools
All cutting tools
must have certain characteristics in order to produce good quality and
economical parts. These characteristics
are:
-hardness, particularly at elevated temperatures, so
that the hardness and strength of the tool are maintained at the temperatures
encountered in cutting operations.
-toughness, so that impact forces on the tool
in interrupted cutting operations, such as milling or turning a splined shaft,
do not chip or fracture the tool.
-wear resistance, so that an acceptable tool
life is obtained before the tool is re-sharpened or replaced.
-chemical stability or inertness with respect
to the work piece material, so that any adverse reactions contributing to tool
wear are avoided.
Tool materials are
usually divided into the following general categories:
-Carbon and medium-alloy steels.
They are
applicable for low-speed cutting operations.
Although inexpensive and easily sharpened and shaped, these steels do
not have sufficient hot hardness and wear resistance for cutting at high
speeds, where temperature rises significantly.
They have been used widely for drills, taps, broaches, and reamers.
-high-speed steels (HSS)
They are the most
highly alloyed of the tool steels.
These steels can be hardened to various depths, have good wear
resistance, and are relatively inexpensive.
Because of their high toughness and resistance to fracture, high-speed
steels are suitable for high positive-rake angle tools and for machine tools
with low stiffness that are subject to vibration and chatter.
There are two
basic types of high-speed steels;
Molybdenum
(M series), containing up to 10% molybdenum, with chromium, tungsten,
vanadium, and cobalt as alloying elements, and
Tungsten (T series), containing 12%-18%
tungsten, with chromium, vanadium, and
cobalt as alloying elements. The M
series generally has higher abrasion resistance that the T series, undergoes
less distortion during heat treating, and is less expensive.
-Cast-cobalt alloys
Commonly known as
stellite tools, these alloys have the following ranges of composition: 38%-53%
Co, 30%-33% Cr, 10%-20% Wo. Because of
their high hardness, typically 58-64 HRC, they have good wear resistance and
maintain their hardness at elevated temperatures. They are not as tough as high-speed steels and are sensitive to
impact forces. These tools are now used
only for special applications that involve deep, continuous roughing operations
at relatively high feeds and speeds as much as twice the rates possible with
high-speed steels.
-Carbides
Because of their
high hardness over a wide range of temperatures, high elastic modulus and
thermal conductivity, and low thermal expansion, carbides are among the most
important tool and die materials.
There are two basic groups of carbides:
-Tungsten carbide (WC)-used
for cutting nonferrous abrasive materials and cast irons. It is a composite material, consisting of
tungsten-carbide particles bonded together in a cobalt matrix. These tools are
manufactured by powder-metallurgy techniques, in which WC powders are crushed
together with cobalt in a ball mill, with the cobalt coating the WC
particles. The amount of cobalt
significantly affects the properties of carbide tools. As the cobalt content increases, the
strength, hardness, and wear resistance of WC decrease, while its toughness
increases, because of the higher toughness of cobalt.
-Titanium carbide (TiC). It has higher wear resistance than tungsten-carbide
but is not as tough. With a
nickel-molybdenum alloy as the matrix, is suitable for cutting hard materials,
mainly steels, and cast irons, and for cutting at speeds higher than those for
tungsten carbide.
-Inserts
The more
traditional cutting tools are made of carbon steels and high-speed steels. These tools are formed in one piece and
ground to various shapes. After the
cutting edge wears, the tool has to be removed from its holder and
reground. The need for more effective
method has led to the development of inserts, which are individual cutting
tools with a number of cutting points.
They are usually clamped on the tool shank with various locking
mechanisms, or they may be brazed to the tool shank. Clamping is the preferred method because each insert has a number
of cutting edges, and after one edge is worn, it is indexed (rotated in its
holder) for another cutting edge. Carbide inserts are available in variety of
shapes, such as square, triangle, diamond, and round, with or without
chip-breaker features for chip-flow control.
The strength of the cutting edge of an insert depends on its shape.
-Coated tools
With their unique
properties such as high strength, coated tools can be used at high cutting
speeds, reducing the time required for machining operations and cost.
Coating materials
commonly used are titanium nitrides, titanium carbide, and ceramics.
-Ceramics
These tool
materials consist primarily of fine grained, high-purity aluminum oxide. They are cold pressed under high pressure,
sintered at high temperature. Ceramic
tools have very high abrasion resistance and hot hardness. Because they are more chemically stable
than high-speed steels and carbides, they have fewer tendencies to form a
built-up edge. A good surface finish is
obtained with ceramic tools in cutting cast irons and steels. Ceramics lack
toughness, resulting in premature tool failure by chipping or catastrophic
failure.
-Cubic boron nitride (CBN)
It is the hardest
material next to diamond. CBN is made
by bonding a 0.5-1 mm layer of polycrystalline cubic boron nitride to a carbide
substrate by sintering under pressure.
While the carbide provides shock resistance, the CBN layer provides very
high wear resistance and cutting edge strength. Cubic boron nitride tools are also made in small sizes without a
substrate. Because CBN tools are
brittle, stiffness of the machine tool is important.
-Silicon-nitride base tools (SiN)
SiN base tool
materials consist of silicon nitride with various additions of aluminum oxide,
yttrium oxide, and titanium carbide.
These tools have high toughness, hot hardness, and good thermal-shock
resistance. Because of chemical
affinity, SiN-base tools are not suitable for machining steels.
-Diamond
It is the hardest
substance of all known materials. It
has low friction, high wear resistance, and ability to maintain a sharp cutting
edge. It is used when good surface
finish and dimensional accuracy are required, particularly with soft nonferrous
alloys and abrasive nonmetallic materials.
Single-crystal diamonds of various carats are used for special
applications, such as machining copper-front surface mirrors. Single-crystal diamond tools have been
largely replaced by polycrystalline diamond tools (compacts), which are also
used as wire-drawing dies for fine wire. These materials consist of very small
synthetic crystals, fused by high pressure, high temperature process to a
thickness of about 0.5-1 mm and bonded to a carbide substrate, similar to CBN
tools. The random orientation of the
diamond crystals prevents the propagation of cracks, which improves its
toughness. Diamonds tools can be used
satisfactorily at almost any speed, but are suitable mostly for light, uninterrupted
finishing cuts. Because of its strong
chemical affinity, diamond is not recommended for machining plain-carbon steels
and titanium, nickel, and cobalt-base alloys.
Drilling operations
Cutting processes
that produce internal circular profiles are boring and drilling. Reaming, tapping and die threading are
processes for finishing work pieces.
Chip formation in all these processes is essentially the same. Its removal can be a significant problem
especially in drilling and tapping and can lead to tool breakage.
Drilling is one of
the most common machining processes.
Drills usually have a high length-to-diameter ratio and are capable of
producing deep holes. They are most
commonly made of high-speed steels (M1, M7, and M10) and can be coated with
titanium nitride for increased wear resistance. Carbide-tipped or solid-carbide drills are available for hard,
high temperature metals and abrasive materials.
The most common drill is the standard-point twist
drill. The main features of the drill
point are a point angle, lip-relieve angle, chisel-edge angle, and helix
angle. The geometry of the drill tip is
such that the normal rake angle and velocity of the cutting edge vary with the
distance from the center of the drill.
Generally, two spiral grooves (flutes) run the length of the drill, and
the chips produced are guided upward through these grooves. The grooves also serve as passage ways to
enable the cutting fluid to reach the cutting edges.
Reaming is an
operation used to make an existing hole dimensionally more accurate than can be
obtained by drilling alone and to improve its surface finish. The most accurate holes are produced by the
following sequence of operations:
-centering
-drilling
-boring
-reaming
A reamer is a
multiple-cutting edge tool with straight or helically fluted edges, which
removes very little material. The
shanks may be straight or tapered, as in drills. The basic types of reamers are hand and machine. They are usually made of high-speed steels
(M1, M2, M7), solid carbides (C-2), or have carbide cutting edges. Reaming speeds are generally lower than
drilling for the same material, but feeds are higher.
Internal threads
in work pieces can be produces by tapping. A tap is basically a threading tool
with multiple cutting teeth. Taps are generally available with three or four
flutes. Three-fluted taps are stronger
because of the larger amount of material available in the flute. Tapered taps are designed to reduce the
torque required for tapping through holes.
Bottoming taps are for tapping blind holes to their full depth. Chip removal can be a significant problem
during tapping because of the small clearances involved. If chips are not removed properly, the
resulting excessive torque can break the tap.
Tapping may be done by hand or in drilling machines, lathes, or
automatic screw machines, using tapping head to hold the taps. Special tapping machines are also available
with features for multiple tapping operations.