Major Operations Performed By Machine Tools
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Major Operations Performed By Machine Tools
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| Major Operations Performed By Machine Tools |
A machine
tool is an electrically powered tool that is used to remove material,
usually metal, at a controlled rate to achieve a desired shape or finish.
A machine tool typically holds the workpiece and a cutting tool, and
moves either the workpiece, tool, or both to provide a means of machining the
material to the desired shape. Machining, another term for metal-cutting, is
performed by shaving away the metal in small pieces called chips. An average
machining operation can reduce the original workpiece weight by approximately
50%. The modern machine tool is a precision piece of equipment designed to cut
metal and produce thousands of parts to an accuracy of millionths of an inch,
which is approximately equal to 1/300 of the thickness of a human hair. Machine
tools range from very small bench mounted devices to large complex machines
weighing hundreds of tons. The major operations performed by machine tools are
milling, turning, boring, planing, shaping, drilling, power sawing, and
grinding.
1)Milling machines
Milling
machines comprise one of the largest categories of machine tools with many
different varieties and configurations available. A milling machine is
considered essential equipment in any machine shop because of its wide variety
of machining operations and its high metal removal rates. The workpiece,
mounted on a movable machine table, is fed against one or more multiple-tooth
rotating tools called milling cutters, or mills. The workpiece is usually held
in vises, special holding fixtures, or clamped directly to the machine table
and fed at right angles to the axis of the milling cutter to produce flat,
recessed, or contoured surfaces.
Classifications
Milling
machines can generally be classified according to the orientation of the
spindle, either vertical or horizontal. Vertical milling machines can also have
what is called multi axis capability where the vertical axis can tilt and swivel
to enable the machining of closed angles and contoured surfaces. Vertical
milling machines are extremely versatile and can machine horizontal surfaces,
vertical surfaces, angular surfaces, shoulders, grooves, fillets, keyways,
T-slots, dovetails, and precision holes.
Horizontal
milling machines are available in plain and universal types. Plain milling
machines have tables that are fixed at right angles to the knee. Universal
milling machines have a table that can be pivoted in a horizontal plane. This
allows the machine table to be swiveled to different angles for milling helical
grooves.
The
universal milling machine is widely used by maintenance machinists and
toolmakers because of its versatility. Computer numerically controlled (CNC)
mills or machining centers are available in vertical and horizontal
configurations and come with automatic tool changers that can store many different
tools in carousels. The major components of a typical milling machine include
the following: base, column, knee, elevating screw, saddle, machine table, ram,
head, and spindle. The base is the heavy foundation member of the machine that
can also be used as a reservoir for coolant or cutting lubricant often used in
machining operations. The base is a massive casting that helps to absorb and
dampen vibration from the machining process. The column, which is either cast
with the base or keyed and bolted on, supports the functioning members of the
machine. Horizontal ways on top of the column support the ram and head while
vertical ways on the column front face support the knee, saddle, and machine
table. The knee moves along the vertical ways of the column and is the basic
work-supporting member. The knee is equipped with ways on top to allow
horizontal movement of the saddle to and from the column face. The elevating
screw provides additional support for the knee and allows the knee to be raised
and lowered. The saddle mounts on the ways of the knee and has horizontal ways
at right angles to the knee ways to support the machine table.
The
machine table moves longitudinally on the ways of the saddle and supports the
workpiece. Combined movements of the knee, saddle, and machine table allow for
precise positioning and feeding of the workpiece left and right, in and out,
and up and down. This is called 3-axis movement (X = left and right movement, Y
= in and out movement, and Z = up and down movement). A rotary table can be
added to a 3-axis mill to give it 4-axis capabilities (typically rotation is
about the longitudinal or X-axis), while 5-axis mills are able to tilt and
swivel about the vertical axis. The ram is mounted on the horizontal ways at
the top of the column. It supports the head and provides horizontal movement
and positioning of the head at varying distances from the column face. The head
includes the motor, stepped pulley, belt drive (or in the case of heavier duty
mills, the gear drive), and the spindle. The head assembly provides for
rotation of the spindle and spindle feeding along the vertical axis using a
quill. The spindle contains the toolholding mount and drives the cutter.
2)Turning centers or lathes
Lathes
are considered to be one of the oldest machine tools in existence. Lathes were
typically foot-powered until water and steam power were harnessed. One of the
first machines driven by Scottish inventor and engineer James Watt’s
(1736–1819) steam engine was a lathe that is how it came to be known
as an engine lathe. The lathe operates by holding the workpiece in a rotating
holder, usually a chuck or collet, and then a single-point cutting tool is fed
into the workpiece. If the tool is fed along the axis of rotation of the
workpiece, it is considered to be a turning operation and any desired
cylindrical contour can be made. If the cylindrical contour is produced on the
inside of the workpiece, the operation is called boring.
In
addition to turning and boring, the lathe is also used for threading, tapping,
facing, tapering, drilling, reaming, polishing, and knurling. Some typical
parts a lathe may produce are pins, bolts, screws, shafts, discs, pulleys, and
gear blanks. Different attachments allow a lathe to perform milling, grinding,
and broaching operations. With the right combination of attachments, it is said
that the lathe is the only machine tool capable of reproducing itself. The size
of a lathe is given in terms of the maximum swing and length of bed. The swing
refers to the maximum diameter of work that can be rotated in the lathe. The
length of the lathe bed refers to the maximum length of the lathe ways, not the
maximum distance between centers of the chuck and tailstock. Many different
varieties of lathes are available ranging from the small precision lathe used
for making watch parts to the extremely large lathes used in producing mill
rolls and rocket casings.
Lathes
can generally be classified in one of the following five basic groups: engine
lathes, speed lathes, turret lathes, vertical lathes, and automatics. The engine
lathe, sometimes referred to as a geared-head lathe, is the most commonly found
lathe model. Speed lathes are used where the workpiece is polished or formed
(e.g., spinning) rather than cut. Turret lathes have a turret tool changer that
rotates to permit a number of different tools to be used in a certain sequence.
Vertical lathes have a vertical axis of work-piece rotation rather than
horizontal. Automatic lathes consist of high production turning machines such
as screw machines and single or multiple spindle chucking and bar fed machines.
All
of the five basic lathe groups can also be found in a computer numerically
controlled version, sometimes called a turning center. The main components of a
typical engine lathe include the following: bed, headstock, feedbox, tailstock,
and carriage. The bed is the base of the lathe that supports the other
components. The precision ways are the part of the bed on which the carriage
travels. The bed is a massive casting in order to absorb and dampen vibration
from the machining process. The headstock is mounted rigidly on the bed and
houses all the gearing and mechanism for the spindle drive and power takeoff
source for the feed-box. Controls for selecting and changing spindle speeds are
also part of the headstock. The feedbox, which may be an integral part of the
headstock or a separate unit, drives both the feed rod and the lead screw for
the feed rate or thread lead required.
A
direct mechanical connection with the spindle drive is required to provide the
proper relationship for feeding or threading operations. The lead screw is a
precision part and is usually only used for threading operations to avoid
unnecessary wear. Most engine lathes incorporate a feed rod that is used to
drive the carriage for operations other than threading. The headstock spindle
supports a faceplate, chuck, or collet, which in turn holds and drives the
workpiece. There are four types of standard spindles, all identified by the
type of nose: threaded nose, camlock, taper nose key drive, or flanged nose.
The threaded nose spindle is usually only found on smaller and less expensive
lathes. The camlock type allows faster changing of faceplates or chucks. The
taper nose key drive type provides greater support to the workpiece while the
flanged spindle nose permits mounting of special chucks or power operated
equipment and can be found on turret lathes and automatics. The tail-stock is
mounted on the bedways and may be positioned and clamped to support work for
turning. It may also use a tool mounted in place of the tailstock center so
that boring, drilling, or reaming can be done. The tailstock must be perfectly
aligned with the head-stock spindle in order to produce good parts. The
carriage is the tool platform of the machine. It supports and feeds the cutting
tool over the work. The carriage consists of the cross slide, which bridges the
ways to support the compound and tool post, or toolholder, and the apron. The
lead screw and the feed rod pass through the apron and transmit feeding power
to the carriage. The main controls for positioning and feeding the tool are
also located on the apron.
3)Boring machines
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| Boring machines |
Boring
machines are similar in construction to milling machines except they are
generally more massive and built lower to the floor, use different tooling, and
feed differently along the axis of the spindle. Boring machines are typically
located in very clean, climate controlled environments and are massive for
extra rigidity and vibration damping to ensure close tolerance hole sizes and
locations, one example being automobile engine piston bores.
Jig
boring machines are primarily intended for tool room use and are used to
produce precision dies, jigs, and gages, which are used to ensure the accuracy
and interchangeability of high volume production parts. There are three common
designs of jig boring machines in use, the open-sided or C-frame,
adjustable-rail, and fixed-bridge construction. Variations of the jig boring
machine include jig grinders, which are used to realign holes after hardening,
and the horizontal jig boring and milling machine, which is utilized for
general production operations.
The
base of the jig boring machine supports a saddle that moves in and out from the
operator to the column. A table moves right or left on the saddle to complement
the saddle movement. A massive column supports the spindle housing, which
adjusts to the work location by moving up and down the column ways. The spindle
moves inside a quill that is supported by the housing or spindlehead.
The
quill also moves up and down inside the housing to give a telescoping mechanism
which adds rigidity to the spindle. The spindle, quill, and housing are
manufactured under very careful and exacting conditions to eliminate any lost
motion. The housing is usually made of Invar cast iron to minimize errors due
to thermal expansion. Stability of the housing is extremely critical because
any expansion would change the tool location relative to the column.
The
spindle is hardened, ground, and lapped. Preloaded ball bearings also help to
eliminate lost motion of the tool and its driving mechanism. Spindle speeds
range from 30 to 1,500 rpm (revolutions per minute) on an average machine. A
digital readout (DRO) system is used to provide a continuous numerical readout
of the table position. Jig boring machines may also be computer numerically
controlled (CNC). CNC control permits many additional jobs that would be
impossible with a manually operated machine. One example would be to produce
precise, irregularly curved forms to be generated on cams or master templates
without operator involvement.
Planers
Planers
remove metal in a series of straight cuts by reciprocating (moving back and
forth) the workpiece as the single-point tool feeds. The fixed tool is rigidly
supported while the workpiece moves on precision ways for the full length of
the cut, thus ensuring maximum accuracy. The rigidity of the tool allows the
use of powerful motors, up to 150 hp (horsepower), which permits higher
production speeds and the use of multiple tooling with extremely heavy cuts and
feeds. Planers are typically big machines used for handling the largest and
heaviest work that can be supported on the machine table, as much as 75 tons
(68 tonnes). Planers may be fitted with hydraulic tracing attachments to enable
them to cut curved surfaces.
There
are two distinct types of planers, the single-housing, or open-side planer, and
the double-housing planer. Double-housing planers are the most widely used and
provide the greatest tool support rigidity. The major components of a
double-housing planer are the bed, table, housings, arch, cross rail, and heads
(side and rail). The bed is the foundation to which the housings are attached.
The bed is provided with precision ways over its entire length and supports the
reciprocating table.
The
table supports the workpiece and reciprocates along the ways of the bed. The
table is slightly less than half the length of the bed and its travel
determines the dimensional capacity of the machine in length of stroke. The
housings are rigid box-type columns placed on each side of the bed and table.
They are heavily braced and ribbed to absorb the large cutting forces
encountered in planing. The arch joins the housings at the top for greater
rigidity of construction and, also, houses the drive mechanism for tool
feeding. The cross rail is a rigid horizontal beam mounted above and across the
table on the vertical ways of the columns. It supports the rail heads and
provides for horizontal feeding of the cutting tools.
The
heads carry the cutting tools and are equipped with clapper blocks that lift
the tools clear of the work on the return stroke of the table. Single-housing
or open-side planers support the cross rail from a single column. This permits
wide workpieces to overhang the table on the open side if necessary.
Planers
require many strokes of the workpiece to complete a cutting operation.
Horizontal and vertical mills are much more efficient at metal removal than
planers and have replaced planers for production work.
Shapers
Shapers
utilize a reciprocating single-point tool with the workpiece clamped on the
machine table. The workpiece position and feeding are controlled to produce the
desired shape or surface as the tool passes back and forth along a fixed path
taking a series of straight cuts. Horizontal shapers are used for machining
flat surfaces, which may be horizontal, vertical, or angular. Vertical shapers
or slotters are used for machining slots, keyways, and splines. Shapers may be
fitted with hydraulic tracing attachments to enable them to cut curved
surfaces. The size of a shaper is designated by the maximum length of stroke or
cut it can take.
There
are many different types of shapers, but the most common is the horizontal
plain shaper, which consists of a bed, column, cross rail, table, ram, and the
head. The bed is the rigid base of the machine that supports the column and
sometimes an outrigger table support, which is used to increase the rigidity of
the workpiece mounting. The column houses the motor and drive mechanisms, and
it is equipped with two sets of precision ways that support the ram and cross
rail. The cross rail is a horizontal member that travels vertically on the ways
of the column to be adjusted, and clamps in place in the desired position. The
cross rail supports the table on precision ways. The table supports the
workpiece and feeds along the cross rail. The ram is the tool driving member
and reciprocates on precision ways on top of the column. The length of stroke,
rate of reciprocation, and overhang at the extreme end of the ram travel are
all adjustable. The head, which is mounted on the forward end of the ram,
supports the toolholder and provides for vertical feeding or swiveling of the
tool 30° either way from vertical.
Shapers
require many strokes of the tool to complete a cutting operation. Horizontal
and vertical mills are much more efficient at metal removal than shapers and
have replaced shapers for production work.
4)Drilling machines
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| Drilling machines |
Drilled
holes are required in the manufacture of almost every product and drilling is
one of the most common machining operations. Drilling machines are similar in
construction to milling machines except they are used exclusively for making
holes.
All
drilling machines are characterized by a rotating cutting tool that advances
along its axis into a stationary workpiece producing a hole. Six common
operations that can be performed on a drill press are drilling, reaming,
boring, counterboring, countersinking, and tapping. Drilling machine capacity
is determined by the size of the largest workpiece over which the spindle can
be centered, the maximum clearance under the spindle, and the maximum drill
diameter that can be fed at a practical feed rate through mild steel. The five
major classifications of drilling machines are uprights, radials, horizontals,
turret drills, and multiple-spindle machines. Each classification represents a
family of machines that is further subdivided.
Upright
drills comprise the largest group and are characterized by a single vertical
spindle rotating in a fixed position and supported in a modified C-frame
structure. The major components of the upright drill include the base, column,
spindle, motor, head, table, feed mechanism, and quill.
Radial
drills are designed to accommodate large work. These machines are arranged so
that the spindle can be positioned to drill anywhere within reach of the
machine by means of movement provided by the head, the arm, and the rotation of
the arm about the column. Some types of radials and portable horizontal
machines allow the entire machine to be moved to the workpiece.
Horizontal
drills are characterized by the position of the spindle. Way-type and
spindle-feed horizontals are self-contained units consisting of motor drive,
gearing, and spindle which may be mounted at any predetermined drill angle and
are used extensively to meet high production needs.
Turret
drilling machines provide a number of tools mounted in a turret designed to
handle a sequence of operations. The turret drilling machine is also available
as a computer numerically controlled machine.
Multiple-spindle
drilling machines include those designed with fixed spindles for single-purpose
production and those where the spindles are adjustable, either by means of
universal joints or by traversing along a worm or spiral drive in a straight
line. Multiple-spindle drilling machines are primarily used for high production
rate workpieces.
5)Sawing machines
Sawing
machines are primarily used to part material such as rough-cutting excess material
away before machining or cutting curved patterns in sheetmetal. Sawing machines
substitute mechanical or hydraulic powered motion for arm motion to achieve the
speed necessary for production operations. The cutoff operation is usually one
of the first requirements in any production process before any machining,
welding, or forging is done. The saw blade has individual teeth that track
through the workpiece, each tooth deepening the cut made by the preceding tooth
in the direction of feed. The saw or work may be fed and by controlling the
direction of feed, either straight or curved cuts can be made. The width of the
cut (also known as kerf) is approximately equal to the thickness of the saw
blade and because of this saw blades are made as thin as possible but with
adequate tool strength and rigidity.
There
are three common types of sawing machines, reciprocating or hack saws, band
saws, and circular saws. These machines all perform the same operation but vary
in capability, capacity, and application. Power hacksaws use a reciprocating
stroke where on the cutting stroke the saw blade teeth are forced into the
metal either by gravity or hydraulic pressure while on the return stroke the
pressure is automatically removed to prolong saw blade life. Most of the
machines come equipped with a chip tray and a cabinet base that contains the
coolant reservoir and its circulating pump. Heavy-duty power hacksaws come with
automatic bar feeds where the stock is loaded on a carriage that automatically
moves forward the necessary distance when the cutting is finished. Hydraulic
pressure automatically operates the vise jaws, gauges the material, and raises
and lowers the saw blade.
After
being set up for cutting material to a specified length, the power hacksaw will
operate automatically without need for an operator until all the material
loaded on the carriage has been cut. Horizontal band saws are one of the most
widely used sawing machines for cutoff operations. These band saws range from
small manually operated machines to large, fully automatic production machines.
Vertical band saws are also used but are primarily manually controlled machines
used in tool rooms and shops for maintenance and low production work.
Band
saws have several advantages over other kinds of cutoff machines. The saw blade
cutting width, or kerf, is 1/16 in (0.16 cm) compared to 1/8 in (0.33 cm) for
power hacksaws and abrasive disc circular saws, and 1/4 in (0.64 cm) for cold
saws. This can represent a sizable savings especially when cutting large or
expensive material. The thinner saw blades also require less power to cut
through material making them more economical to operate. Because bandsaws have
endless blades (band saw blades are welded together to create an endless loop)
that cut continuously, the cutting rates are much higher.
Two
of the most popular circular saws are the cold saw and the abrasive disc cutoff
saw. Cold saws are low rpm circular saws for metal cutting. These saws range in
size from hand-operated bench-top models with 8 in (20 cm) blades to fully
automatic machines with blades of 3 in (7.6 cm) diameter and larger. Light duty
manual or automatic machines are sometimes equipped with a swivel head that
enables cuts to be made at different angles. These saws are mostly used for cutting
structural shapes such as I-beams, angles, and channel sections because the
circular blades can complete their cuts with less travel than straight blades.
Heavy-duty machines are available with bar feeds and can be used for cutting
solid bars up to 10 in (25 cm). Material larger than this size would require
excessively large blade diameters, which must be more than double the cutting
capacity, which would become too costly along with the machine necessary to
drive them. Different speed ranges are provided for cutting metals of different
hardness and toughness, and built-in coolant systems help produce better
finishes and prolong blade life.
Abrasive
cutoff saws utilize an abrasive disc to separate material by using a grinding
action. Abrasive cutoff saws are built for either manual operation or with
power feeds, with either fixed or oscillating wheel heads. Oscillating wheel
heads are used when cutting thick sections of tough materials such as titanium,
nickel-based superalloys, and other high alloy steels. Sizes range from small
bench-top machines with 8 in (20 cm) wheels to bigger machines with 20 in (50
cm) or larger wheels. Abrasive cutoff saws are very useful for rapidly cutting
small sizes of bar stock, tubing, and structural shapes and also for cutting
tough or hardened materials that cannot be cut efficiently with other types of
saws.
6)Grinding machines
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| Grinding Machine |
KEY TERMS
Accuracy —How close
measurements are to the true value.
Carousel —A rotary tool
holder used to hold many tools as part of an automatic tool changer on a CNC
mill.
Dies —High precision
tooling primarily used in production presses.
Gages —Extremely
accurate tooling used for measuring.
Jigs —Tooling which
is used for locating parts and also for guiding cutting tools such as in a
drill jig.
Precision —How close
repeated measurements are to each other.
Quill —Rotating
tool holder.
Spindle —Assembly that
contains a flange mount housing, bearings, and a tapered nose tooling holder.
The
major types of grinding machines available are cylindrical grinders, internal
and chucking grinders, universal grinders, centerless grinders, surface
grinders, face grinders, disc grinders, and tool and cutter grinders.
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