Lathe Machine and Processes

Turning And Related Operations

• Turning is a machining process in which a single-point tool removes
material from the surface of a rotating workpiece.

• The tool is fed linearly in a direction parallel to the axis of rotation to

generate a cylindrical geometry.

• Turning is carried out on a machine tool called a lathe, which provides

power to turn the part at a given rotational speed and to feed the tool at a

specified rate and depth of cut.

Cutting Conditions In Turning
• The rotational speed in turning is related to the desired cutting speed at the
surface of the cylindrical workpiece by the equation

• The turning operation reduces the diameter of the work from its original
diameter Do to a final diameter Df, as determined by the depth of cut d:
• The feed in turning is generally expressed in mm/rev (in/rev).

• This feed can be converted to a linear travel rate in mm/min (in/min) by the
formula

• The time to machine from one end of a cylindrical workpart to the other is
given by
• A more direct computation of the machining time is provided by the
following equation:
• The volumetric rate of material removal can be most conveniently
determined by the following equation:
Operations Related To Turning
 Facing:

• The tool is fed radially into the rotating work on one end to create a flat
surface on the end.
 Taper turning:

• Instead of feeding the tool parallel to the axis of rotation of the work, the
tool is fed at an angle, thus creating a tapered cylinder or conical shape.
 Contour turning:

• Instead of feeding the tool along a straight line parallel to the axis of
rotation as in turning, the tool follows a contour that is other than straight,
thus creating a contoured form in the turned part.
 Form turning:

• In this operation, sometimes called forming, the tool has a shape that is
imparted to the work by plunging the tool radially into the work.
 Chamfering:

• The cutting edge of the tool is used to cut an angle on the corner of the
cylinder, forming what is called a ‘‘chamfer.’’
 Cutoff:

• The tool is fed radially into the rotating work at some location along its
length to cut off the end of the part. This operation is sometimes referred to
as parting.
 Threading:

• A pointed tool is fed linearly across the outside surface of the rotating
workpart in a direction parallel to the axis of rotation at a large effective
feed rate, thus creating threads in the cylinder.
 Boring:

• A single-point tool is fed linearly, parallel to the axis of rotation, on the

inside diameter of an existing hole in the part.
 Drilling:

• Drilling can be performed on a lathe by feeding the drill into the rotating
work along its axis.

• Reaming can be performed in a similar way.

 Knurling:

• This is not a machining operation because it does not involve cutting of

material. Instead, it is a metal forming operation used to produce a regular
crosshatched pattern in the work surface.
The Engine Lathe
• The basic lathe used for turning and related operations is an engine lathe.

• It is a versatile machine tool, manually operated, and widely used in low

and medium production.

• The term engine dates from the time when these machines were driven by
steam engines.
• The headstock contains the drive unit to rotate the spindle, which rotates
the work.

• The headstock is clamped on the left-hand side of the bed.

• The headstock spindle, a hollow, cylindrical shaft supported by bearings,

provides a drive through gears from the motor to work-holding devices.

• A live center, a faceplate, or a chuck can be fitted to the spindle nose to

hold and drive the work.

• Headstock spindles can be driven either by a stepped pulley and a belt, or

by transmission gears in the headstock.

• The lathe with a stepped pulley drive is generally called a belt-driven lathe;
the gear driven lathe is referred to as a geared-head lathe.
• The quick-change gearbox, containing a number of different-size gears,
provides the feed rod and lead screw with various speeds for turning and
thread-cutting operations.

• The feed rod advances the carriage for turning operations when the
automatic feed lever is engaged.

• The lead screw advances the carriage for thread-cutting operations when
the split-nut lever is engaged.
• Opposite the headstock is the tailstock, in which a center is mounted to
support the other end of the workpiece.
• The cutting tool is held in a tool post fastened to the cross-slide, which is
assembled to the carriage.

• The carriage is designed to slide along the ways of the lathe in order to feed
the tool parallel to the axis of rotation.

• The ways are like tracks along which the carriage rides, and they are made
with great precision to achieve a high degree of parallelism relative to the
spindle axis.
Tool Post
• The ways are built into the bed of the lathe, providing a rigid frame for the
machine tool.

• The carriage is driven by a lead screw that rotates at the proper speed to
obtain the desired feed rate.

• The cross-slide is designed to feed in a direction perpendicular to the

carriage movement.

• Thus, by moving the carriage, the tool can be fed parallel to the work axis
to perform straight turning; or by moving the cross-slide, the tool can be
fed radially into the work to perform facing, form turning, or cutoff
operations.
Lathe Bed
• The conventional engine lathe is horizontal turning machines; that is, the
spindle axis is horizontal.

• This is appropriate for the majority of turning jobs, in which the length is
greater than the diameter.

• For jobs in which the diameter is large relative to length and the work is
heavy, it is more convenient to orient the work so that it rotates about a
vertical axis; these are vertical turning machines.
Vertical Turning Machines
Methods of Holding the Work in a Lathe
• There are four common methods used to hold workparts in turning.

• These workholding methods consist of various mechanisms to grasp the

work, center and support it in position along the spindle axis, and rotate it.

The methods shown in figure next are:

(a) Mounting the work between centers

(b) Chuck

(c) Collet

(d) Face plate

• Holding the work between centers refers to the use of two centers, one in
the headstock and the other in the tailstock.

• This method is appropriate for parts with large length-to-diameter ratios.

• At the headstock center, a device called a dog is attached to the outside of

the work and is used to drive the rotation from the spindle.

• The tailstock center has a cone-shaped point which is inserted into a

tapered hole in the end of the work.
A variety of 60° lathe centers
Revolving Dead Center

Long Point Center

Changeable Point Center

Types of Changeable Points

• The tailstock center is either a ‘‘live’’ center or a ‘‘dead’’ center.

• A live center rotates in a bearing in the tailstock, so that there is no relative

rotation between the work and the live center, hence, no friction between
the center and the workpiece.

• In contrast, a dead center is fixed to the tailstock, so that it does not rotate;
instead, the workpiece rotates about it.

• Because of friction and the heat buildup that results, this setup is normally
used at lower rotational speeds.

• The live center can be used at higher speeds.

• The chuck is available in several designs, with two, three or four jaws to
grasp the cylindrical workpart on its outside diameter.

• The jaws are often designed so they can also grasp the inside diameter of a
tubular part.

• A self-centering chuck has a mechanism to move the jaws in or out

simultaneously, thus centering the work at the spindle axis.

• Other chucks allow independent operation of each jaw.

• Chucks can be used with or without a tailstock center.

• For parts with low length-to-diameter ratios, holding the part in the chuck
in a cantilever fashion is usually sufficient to withstand the cutting forces.

• For long workbars, the tailstock center is needed for support.

• A collet consists of a tubular bushing with longitudinal slits running over
half its length and equally spaced around its circumference.

• The inside diameter of the collet is used to hold cylindrical work such as
barstock.

• Owing to the slits, one end of the collet can be squeezed to reduce its
diameter and provide a secure grasping pressure against the work.
• A face plate is a workholding device that fastens to the lathe spindle and is
used to grasp parts with irregular shapes.

• Because of their irregular shape, these parts cannot be held by other

workholding methods.

• The face plate is therefore equipped with the custom-designed clamps for
the particular geometry of the part.
• Magnetic chucks are used to hold iron or steel parts that are too thin or that
may be damaged if held in a conventional chuck.

• These chucks are fitted to an adapter mounted on the headstock spindle.

• This type of chuck is used only for light cuts and for special grinding
applications.
Other Lathes And Turning Machines
 Toolroom Lathe:

• The toolroom lathe and speed lathe are closely related to the engine lathe.

• The toolroom lathe is smaller and has a wider available range of speeds and
feeds.

• It is also built for higher accuracy, consistent with its purpose of fabricating
components for tools, fixtures, and other high-precision devices.
 Speedlathe:

• The speed lathe is simpler in construction than the engine lathe.

• It has no carriage and cross-slide assembly, and therefore no lead screw to

drive the carriage.

• The cutting tool is held by the operator using a rest attached to the lathe for
support.

• The speeds are higher on a speed lathe, but the number of speed settings is
limited.

• Applications of this machine type include wood turning, metal spinning,

and polishing operations.
 Turret Lathe:

• A turret lathe is a manually operated lathe in which the tailstock is replaced

by a turret that holds up to six cutting tools.

• These tools can be rapidly brought into action against the work one by one
by indexing the turret.

• In addition, the conventional tool post used on an engine lathe is replaced

by a four-sided turret that is capable of indexing up to four tools into
position.

• Hence, because of the capacity to quickly change from one cutting tool to
the next, the turret lathe is used for high-production work that requires a
sequence of cuts to be made on the part.
 Chucking Machine:

• A chucking machine (nicknamed chucker) uses a chuck in its spindle to

hold the workpart.

• The tailstock is absent on a chucker, so parts cannot be mounted between

centers.

• This restricts the use of a chucking machine to short, lightweight parts.

• The setup and operation are similar to a turret lathe except that the feeding
actions of the cutting tools are controlled automatically rather than by a
human operator.

• The function of the operator is to load and unload the parts.

 Barmachine:

• A barmachine is similar to a chucking machine except that a collet is used

(instead of a chuck), which permits long bar stock to be fed through the
headstock into position.

• At the end of each machining cycle, a cutoff operation separates the new
part.
• Bar machines can be classified as single spindle or multiple spindle.

• A single spindle bar machine has one spindle that normally allows only one
cutting tool to be used at a time on the single workpart being machined.

• Multispeindle machines have more than one spindle, so multiple parts are
machined simultaneously by multiple tools.

• For example, a six spindle automatic bar machine works on six parts at a
time.
 Numerically Controlled Lathe:

• The modern form of control is computer numerical control (CNC), in

which the machine tool operations are controlled by a ‘‘program of
instructions’’ consisting of alphanumeric code.

• This has led to the development of machine tools capable of more complex
machining cycles and part geometries, and a higher level of automated
operation.

• The CNC lathe is an example of these machines in turning.

• Automatic chuckers and bar machines are implemented by CNC.