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Machining Processes Used to
Produce Round Shapes
Chapter 22
Various Cutting Operations
• Turing – produces straight, conical, curved, or grooved
workpieces
• Facing – produces a flat surface at the end of the part
• Boring – to enlarge a hole
• Drilling - to produce a hole
• Cutting off – to cut off a workpeiece
• Threading – to produce threads
• Knurling – produces a regularly shaped roughness
Cutting Operations
Fig : Various cutting
operations that
can be performed
on a late. Not that
all parts have
circular symmetry
Tool Geometry
– Rake angle
• controls direction of chip flow
• Strength of the tool
• Side rake angle –
• Bake rake angle – controls direction of chip flow
– Cutting edges – affects surface finish and tool-tip
strength
– Nose radius – affects surface finish
• Material Removal Rate – (MRR) is the volume of
material removed per unit time
Turning Parameters
•
Forces in turning
– Cutting force: acts downward on the tool tip
– Thrust force: acts in the longitudinal direction
– Radial force: acts in the radial direction
•
Roughing and Finishing Cuts
– Rough cut: high speed cut with little regard for dimensional tolerance
– Finishing cut: lower feed rate and depth of cut
Tool Materials, Feeds, and Cutting Speeds
– See table 22.4
Cutting Fluids
– See table 22.5
•
•
Components of a Lathe
Fig : Components of Lathe
Lathes and Lathe Operations
• Lathes are the oldest machine tools
• Lathe Components
– Bed: supports all major components
– Carriage: slides along the ways and consists of the
cross-slide, tool post, apron
– Headstock – Holds the jaws for the work piece,
supplies power to the jaws and has various drive
speeds
• Tailstock – supports the other end of the workpiece
• Feed Rod and Lead Screw – Feed rod is powered by a
set of gears from the headstock
Lathe Specifications
• A lathe is specified by its
– Swing – maximum diameter of the workpiece
– Distance from headstock and tailstock centers
– Length of the bed
• Lathes are available in a variety of styles and types of construction
power
• Types of lathes
– Bench lathe:
• Placed on a bench
• Low power
• Hand feed operated
– Toolroom lathes: High precision
– Engine lathes
• Available in a wide variety of sizes
• Used for a variety of turning operations
Right Hand Cutting Tool
Dimensions
Abbreviation
8
Back Rake Angle
BR
14
Side Rake Angle
SR
6
End Relief Angle
ER
12
End Clearance Angle
…….
6
Side Relief Angle
SRF
12
Side Clearance Angle
……
20
End Cutting-Edge Angle
ECEA
15
Side Cutting-edge angle
SCEA
3/4
Nose radius
NR
Tool
Signatur
e
Fig : (a) Designations and symbols for a right-hand cutting tool; solid high-speed-steel tools have a similar
designation. Right-hand means that the tool travels from right to left.
Workholding Devices
Chucks
usually equipped with 3
or 4 jaws
3 jaw chucks generally
are self centering.
Used for round work
pieces.
Can be centered within
.025mm
independently.
4 jaw chucks are for
square, rectangular,
or odd-shaped
workpieces
– Can be power
actuated
Fig : (a) and (b) Schematic illustrations of a draw-in-type collets. The workpiece is placed in the collet
hole, and the conical surfaces of the collet are forced inward by pulling it with a draw bar into the
sleeve. (c) A push-out type collet. (d) Workholding of a part on a face plate.
Mandrels
Fig : Various types of mandrels to hold workpieces for turning. These mandrels are usually mounted between centers
on a lathe. Note that in (a) both the cylindrical and the end faces of the workpiece can be machined, whereas in
(b) and (c) only the cylindrical surfaces can be machined.
Tracer Lathes
• Machine tools with attachments
• Capable of turning parts with various contours
• A tracer finger follows the template and guides the cutting tool
Automatic Lathes
• Increasingly being automated
• Automatic Lathes are suitable for medium to high volume
production
Automatic Bar Machines
• Formerly called automatic screw machines
• Designed for high-production-rate machining of screws and
other threaded parts
• All operations are preformed automatically
• Equipped with single or multiple spindles
Turret Lathes
Capable of performing multiple
cutting operations on the
same workpiece
– Turning
– Boring
– Drilling
– Thread cutting
– Facing
Turret lathes are very versatile
Types of turret lathes
– Ram-type: ram slides
in a separate base on
the saddle
– Saddle type:
• more heavily
constructed
• Used to machine
large
workpeiceces
Computer Numerically Controlled Lathes
•
•
•
•
Computer Numerical Controls (CNC)
Equipped with one or more turrets
Each turret is equipped with a variety of tools
Performs several operations on different surfaces of the workpiece
Fig : A computer numerical control lathe. Note
the two turrets on this machine.
Turning Process Capabilities
• Production rates
– See Table 22.8
• Surface finish and
dimensional accuracy
Fig : The range of dimensional
tolerances obtained in various
machining processes as a
function of workpiece size. Note
that there is an order of
magnitude difference between
small and large workpieces.
Design Considerations for Turning Operations
• Parts should be designed so that can be fixtured and
clamped in the work holding devices
• Dimensional accuracy and surface finish specified
should be as wide as possible
• Avoid sharp corners, tapers, and major dimensional
variations in the part
• Use near-net-shape forming
• Cutting tools should be able to travel across workpiece
without obstruction
• Standard cutting tools, inserts, and toolholders should be
used
• Materials should be selected for their machineability
Guidelines for Turning Operations
• Minimize tool overhang
• Support workpiece rigidly
• Use machine tools with high stiffness and high damping
capacity
• When tools begin to vibrate and chatter, modify one or more of
the process parameters, such as tool geometry, cutting speed,
feed rate, depth of cut, or use of cutting fluid
Chip Collection Systems
•
•
•
•
•
Drop them on a conveyor belt
Dragging the chips from a setting tank
Using augers with feed screws
Magnetic conveyors
Vacuum methods
High-Speed Machining, Ultraprecision Machining, and
Hard Turning
• High-Speed Machining
–
–
–
–
High speed: 600 - 1,800 m/min
Very high speed: 1,800 - 1,800 m/min
Ultrahigh speed: > 18,000
Important factors
•
•
•
•
•
•
•
Power and stiffness of the tools
Stiffness of tool holder
Spindle design
Inertia of the machine-tool components
Fast feed drives
Level of automation
Selection of appropriate cutting tool
• Ultraprecision Machining – uses a single-crystal diamond, also known as
•
diamond turning
Hard turning
– When hardness increases, machinability decreases
– Uses polycrystalline cubic boron nitride, cermit, or ceramic cutting tools
– Competes successfully with the grinding process
Cutting Screw
Threads
• Screw-Thread
Nomenclature
– Standardization
of screw threads
began in the
middle 1880’s
Fig : (a) Standard nomenclature for screw
threads, (b) Unified National thread
and identification of threads, (c)
ISO metric thread and identification
of threads.
Cutting Screw Threads
• Design Considerations for Screw-Thread Cutting
– Should allow for the termination of threads before
they reach a shoulder
– Eliminate shallow, blind tapped hole
– Chamfers should be specified at the ends
– Threaded sections should not be interrupted with
slots, holes, or other discontinuities
– Use standard tooling for threads
– Operations should be completed in one step
Cutting Screw Threads
Fig : (a) Cutting screw threads on a lathe with a single-point cutting tool. (b) Cutting screw threads
with a single-point tool in several passes, normally utilized for large threads. The small arrows in
the figures show the direction of feed, and the broken lines show the position of the cutting tool
as time progresses. (c) A typical carbide insert and toolholder for cutting screw threads. (d)
Cutting internal screw threads with a carbide insert.
Types of Screw threads
Fig : Various types of screw threads
Boring
•
•
•
•
•
Boring produces circular internal profiles in hollow workpieces
Boring mills are used for large workpieces
Holes can be bored up to 20M if needed
See fig. 22.20
Machines are available with a variety of features
– Horizontal boring machines
– Jig borers
Drilling And Drills
• Drills
– Have high lenth to diameter ratio
– Capable of producing deep holes
– Some what fexable
– Flutes: two spiral grooves that run the length
of the drill and allow the chips to escape
– Small changes in drill geometry can have a
significant effect on the drill’s performance
Drilling And Drills
Types of drills
– Twist drill: most
common drill
– Step drill:
produces holes
of two or more
different
diameters
– Core drill: used
to make an
existing hole
bigger
Drilling And Drills
• Drilling operations
– Counterboring & countersinking: produce depressions on the
surface to accommodate the heads of screws
– Center drill: is a short and is used to produce the hole at the
end of a piece of stock
– Spot drill: is used to spot (start) a hole at the desired location
• Gun Drilling - used fro drilling gun barrels and deep holes
• Thrust Force and Torque
– Thrust force acts perpendicular to the hole axis
– Can cause the drill to bend or break if excessive
• Drilling Practice
– Held in drill chucks
– “walking” can be a problem when starting a hole
• The drill should be guided
• Used a center drill to start a hole
• Drills can be reconditioned
• Drill life is measured by the number of holes drilled.
Drill point Geometries
Fig : Standard chisel-point drill indicating various features. The function of the pair of margins is to provide a bearing surface for
the drill against walls of the hole as it penetrates into the workpiece; drills with four margins (double-margin) are available
for improved drill guidance and accuracy. Drills with chip-breaker features are also available. (b) Crankshaft-point drill. (c)
Various drill points and their manufacturers: 1. Four-facet split point, by komet of America. 2. SE point, by Hertel. 3.New
point, by Mitsubishi materials. 4. Hosoipoint, by OSG Tap and Die. 5. Helical point.
Reaming And Reamers
•
•
Operation used to make an existing hole dimensionally more accurate
than by drilling alone
The hole making sequence is
– Centering
– Drilling
– Boring
– reaming
Fig : various types of drilling and reaming operations.
Gun Drilling
Fig : (a) A gun drill
showing various
features. (b)
method of gun
drilling.
Trepanning
Fig : (a) trepanning tool (b) trepanning with
drill mounted single cutter.
Drilling Machines
Fig : Schematic illustration of
the components of (a) a
vertical drill press and (b)
a radial drilling machine.
CNC Milling Machine
Fig : A three axis
computer numerical
control drilling
machine. The turret
holds as much as
eight different tools,
such as drills, taps,
and reamers.
Reamers
Fig : Various types of drilling
and
Tapping And Taps
• Internal threads in workpiceces can be produced
by “tapping”
• A tap is a chip-producing threading tool
• Tapping may be done by hand or
–
–
–
–
Drilling machines
Lathes
Automatic screw machines
Vertical CNC milling machines
Tapping and Taps
Fig : (a) Terminology for a Tap (b) Tapping of steel nuts in production
THE END