turning mfg chapter23

Manufacturing Engineering Technology in SI Units, Manufacturing Engineering Technology in SI Units,

66thth Edition Edition Chapter 23: Chapter 23:

Machining Processes: Turning and Hole Machining Processes: Turning and Hole MakingMaking

Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Outline

1. Introduction

2. The Turning Process

3. Lathes and Lathe Operations

4. Boring and Boring Machines

5. Drilling, Drills, and Drilling Machines

6. Reaming and Reamers

7. Tapping and Taps


Introduction

Machining processes has the capability of producing parts that are round in shape

Such as miniature screws for the hinges of eyeglass frames and turbine shafts for hydroelectric power plants

Most basic machining processes is turning where part is rotated while it is being machined

Turning processes are carried out on a lathe or by similar machine tools

Highly versatile and produce a wide variety of shapes


Introduction


Introduction

Turning is performed at various:

1. Rotational speeds, N, of the workpiece clamped in a spindle

2. Depths of cut, d

3. Feeds, f, depending on the workpiece materials, cutting-tool materials, surface finish, dimensional accuracy and characteristics of the machine tool


The Turning Process

Majority of turning operations use simple single-point cutting tools, which is a right-hand cutting tool

Important process parameters have a direct influence on machining processes and optimized productivity

Tool Geometry Rake angle control both the direction of chip flow and

the strength of the tool tip Side rake angle controls the direction of chip flow Cutting-edge angle affects chip formation, tool

strength and cutting forces


The Turning Process

Tool Geometry Relief angle controls interference and rubbing at the

tool–workpiece interface Nose radius affects surface finish and tool-tip strength


The Turning Process

Tool Geometry

Material-removal Rate The material-removal rate (MRR) is the volume of

material removed per unit time (mm3/min)


The Turning Process

Material-removal Rate The average diameter of the ring is

Since there are N revolutions per minute, the removal rate is

Since the distance traveled is l mm, the cutting time is

The cutting time does not include the time required for tool approach and retraction


20 f

avg

DDD

dfVMMRdfNDMMR avg toreduceor

fN

lt

The Turning Process

Material-removal Rate


The Turning Process

Forces in Turning The 3 principal forces acting on a cutting tool are

important in the design of machine tools, deflection of tools and workpieces for precision-machining operations

Cutting force acts downward on the tool tip and deflect the tool downward and the workpiece upward

Thrust force (or feed force) acts in the longitudinal direction


The Turning Process

Roughing and Finishing Cuts First practice is to have one or more roughing cuts at

high feed rates and large depths of cut Little consideration for dimensional tolerance and

surface roughness Followed by a finishing cut, at a lower feed and depth

of cut for good surface finish


The Turning Process

Tool Materials, Feeds, and Cutting Speeds The range of applicable cutting speeds and feeds for a

variety of tool materials is shown


The Turning Process

Tool Materials, Feeds, and Cutting Speeds


The Turning Process

Cutting Fluids Recommendations for cutting fluids appropriate to

various workpiece materials


The Turning Process

EXAMPLE 23.1

Material-removal Rate and Cutting Force in Turning

A 150-mm-long, 12.5-mm-diameter 304 stainless steel rod is being reduced in diameter to 12.0 mm by turning on a lathe. The spindle rotates at N 400 rpm, and the tool is travelling at an axial speed of 200 mm/min. Calculate the cutting speed, material-removal rate, cutting time, power dissipated, and cutting force.


The Turning Process

Solution


The maximum cutting speed is

The cutting speed at the machined diameter is

The depth of cut is


mm 25.02

0.125.12

d

m/min 7.15

1000

4005.120

NDV

m/min 1.15

1000

4000.120

NDV

The Turning Process

Solution


The feed is

The material-removal rate is

The actual time to cut is


mm 75.04005.0

150t

mm/rev 5.0400

200f

/minm 102/min mm 19244005.025.025.12 363 MMR

The Turning Process

Solution


The power dissipated is

Since W=60 N•m/min, power dissipated is 7680 N m/min.

Also, power is the product of torque:

Since , we have Copyright © 2010 Pearson Education South Asia Pte Ltd

2avgcDFT

W128

60

19244Power

Nm 1.34002

7680

T

N 506

2/25.12

10001.3cF

Lathes and Lathe Operations Lathes are considered to be the oldest machine tools Speeds may range from moderate to high speed

machining Simple and versatile But requires a skilled machinist Lathes are inefficient for repetitive operations and for

large production runs


Lathes and Lathe Operations:Lathe ComponentsBed Supports all major components of the lathe

Carriage Slides along the ways and consists of an assembly of

the cross-slide, tool post, and apron

Headstock Equipped with motors, pulleys, and V-belts that supply

power to a spindle at various rotational speeds


Lathes and Lathe Operations:Lathe ComponentsTailstock Slide along the ways and be clamped at any position,

supports the other end of the workpiece

Feed Rod and Lead Screw Powered by a set of gears through the headstock

Lathe Specifications1. Max diameter of the workpiece that can be machined2. Max distance between the headstock and tailstock

centers3. Length of the bed


Lathes and Lathe Operations:Lathe ComponentsLathe Specifications


Lathes and Lathe Operations:Workholding Devices and Accessories Workholding devices must hold the workpiece securely Chuck is equipped with three or four jaws Three-jaw chucks used for round workpieces Four-jaw (independent) chucks used for square,

rectangular, or odd-shaped workpieces Power chucks are used in automated equipment for

high production rates Chuck selection depends on the type and speed of

operation, workpiece size, production and dimensional accuracy requirements and the jaw forces required


Lathes and Lathe Operations:Workholding Devices and Accessories


Lathes and Lathe Operations:Workholding Devices and Accessories A collet is a longitudinally-split, tapered bushing Used for round workpieces and shapes like square or

hexagonal It can grips the entire circumference of the part,

suitable for parts with small cross sections Face plates are used for clamping irregularly shaped

workpieces Mandrels used to hold workpieces that require

machining on both ends or on their cylindrical surfaces


Lathes and Lathe Operations:Lathe Operations Cutting tool removes material by travelling along the

bed Facing operations are done by moving the tool radially

with the cross-slide and clamping the carriage for better dimensional accuracy

Form tools are used to produce various shapes on solid, round workpieces by moving the tool radially inward while the part is rotating

Boring on a lathe is similar to turning Drilling can be performed on a lathe by mounting the

drill bit in a chuck in the tailstock quill


Lathes and Lathe Operations:Types of LathesBench Lathes Have low power, operated by hand feed, and are used

to machine small workpieces

Special-purpose Lathes Used for applications such as railroad wheels, gun

barrels, and rolling-mill rolls

Tracer Lathes Cutting tool follows a path that duplicates the contour of

a template


Lathes and Lathe Operations:Types of LathesAutomatic Lathes For fully automatic lathe, parts are fed and removed

automatically For semiautomatic machines, functions are performed

by the operator

Automatic Bar Machines Designed for high-production-rate machining of screws

and similar threaded parts


Lathes and Lathe Operations:Types of LathesTurret Lathes Perform multiple cutting operations, such as turning,

boring, drilling, thread cutting, and facing


Lathes and Lathe Operations:Types of LathesComputer-controlled Lathes Movement and control of the machine tool and its

components can be achieved


Lathes and Lathe Operations:Types of LathesComputer-controlled Lathes Each turret is equipped with a variety of tools and

performs several operations on different surfaces of the workpiece


Lathes and Lathe Operations:Types of LathesEXAMPLE 23.2

Typical Parts Made on CNC Turning Machine Tools

The capabilities of CNC turning-machine tools are illustrated as shown. The material and number of cutting tools used and the machining times are indicated for each part. These parts also can be made on manual or turret lathes, although not as effectively or consistently.



Typical Parts Made on CNC Turning Machine Tools



Machining of Complex Shapes

The capabilities of CNC turning are shown



Machining of Complex Shapes


Lathes and Lathe Operations:Turning-process Capabilities Relative production rates in turning are important on

productivity in machining operations


Lathes and Lathe Operations:Turning-process Capabilities High-removal-rate machining has two requirements: (a)

high machine tool and (b) high power The surface finish and dimensional accuracy depend

on the characteristics of the machine tool, stiffness, vibration and chatter, process parameters, tool geometry and wear, the use of cutting fluids, the machinability of the workpiece material, and operator skill


Lathes and Lathe Operations:Turning-process Capabilities


Lathes and Lathe Operations:Design Considerations and Guidelines for Turning Operations

General design guidelines:

1. Parts should be designed to be fixtured and clamped easily into work-holding devices

2. Dimensional accuracy and surface finish should be as wide as permissible for the part to still function properly

3. Sharp corners should be avoided

4. Blanks should be as close to final dimensions as possible

5. Parts should be designed so that cutting tools can travel across the workpiece without obstruction

6. Design features should be commercially available


Lathes and Lathe Operations:Design Considerations and Guidelines for Turning Operations

Guidelines for Turning Operations Some guidelines have to be implemented on a trial-

and-error basis:

1. Minimize tool overhang

2. Support the workpiece rigidly

3. Use machine tools with high stiffness and high damping capacity

4. When tools begin to vibrate and chatter, modify one or more of the process parameters


Lathes and Lathe Operations:Chip Collection Systems Chips produced during machining must be collected

and disposed properly Chip management involves collecting chips from their

source in the machine tool in an efficient manner and removing them from the work area

Chips can be collected by:

1. Gravity drop

2. Dragging the chips from a settling tank

3. Using augers with feed screws

4. Using magnetic conveyors

5. Employing vacuum methods of chip removal


Lathes and Lathe Operations:Cutting Screw Threads Screw thread defined as a ridge of uniform cross

section that follows a helical or spiral path on the outside or inside of a cylindrical (straight thread) or tapered surface (tapered thread)

Threads can be machined externally or internally with a cutting tool called thread cutting or threading

Screw-thread Cutting on a Lathe The cutting tool, the shape depends on the type of

thread to be cut


Lathes and Lathe Operations:Cutting Screw ThreadsScrew-thread Cutting on a Lathe A number of passes are required to produce threads

with good dimensional accuracy and surface finish


Lathes and Lathe Operations:Cutting Screw ThreadsScrew-thread Cutting on a Lathe The production rate in cutting screw threads can be

increased with tools called die-head chasers


Lathes and Lathe Operations:Cutting Screw ThreadsDesign Considerations for Screw Thread Machining Design considerations must be taken into account:

1. Designs should allow for the termination of threads

2. Attempts should be made to eliminate shallow, blind tapped holes

3. Chamfers should be specified at the ends of threaded sections

4. Threaded sections should not be interrupted

5. Standard threading tooling and inserts should be used

6. Thin-walled parts should have sufficient thickness and strength


Boring and Boring Machines

The cutting tools are mounted on a boring bar to reach the full length of the bore

Boring bars have been designed and built with capabilities for damping vibration

Large workpieces are machined on boring mills



In horizontal boring machines, the workpiece is mounted on a table that can move horizontally in both the axial and radial directions

A vertical boring mill is similar to a lathe, has a vertical axis of workpiece rotation



Design Considerations for Boring:

1. Through holes should be specified

2. Greater the length-to-bore-diameter ratio, the more difficult it is to hold dimensions

3. Interrupted internal surfaces should be avoided


Drilling, Drills, and Drilling Machines

Holes are used for assembly with fasteners, for design purposes or for appearance

Hole making is the most important operations in manufacturing

Drilling is a major and common hole-making process


Drilling, Drills, and Drilling Machines: Drills

Drills have high length-to-diameter ratios, capable of producing deep holes



Drills are flexible and should be used with care in order to drill holes accurately and to prevent breakage

Drills leave a burr on the bottom surface upon breakthrough, necessitating deburring operations



Twist Drill The most common drill is the conventional standard-

point twist drill The geometry of the drill point is such that the normal

rake angle and velocity of the cutting edge vary with the distance from the center of the drill

Main features of this drill are:

1. Point angle

2. Lip-relief angle

3. Chiseledge angle

4. Helix angle



Twist Drill Drills are available with a chip-breaker feature ground

along the cutting edges Other drill-point geometries have been developed to

improve drill performance and increase the penetration rate

Other Types of Drills



Gun Drilling Gun drilling is used for drilling deep holes and requires

a special drill



Trepanning The cutting tool produces a hole by removing a disk-

shaped piece (core) from flat plates Trepanning can be carried out on lathes, drill presses,

or other machine tools using single-point or multipoint tools


Drilling, Drills, and Drilling Machines:Material-removal Rate in Drilling The material-removal rate (MRR) in drilling is the

volume of material removed per unit time


fND

MMR

4

2

Drilling, Drills, and Drilling Machines:Thrust Force and Torque Thrust force acts perpendicular to the hole axis Excessive thrust force can cause the drill to break,

distort the workpiece and cause the workpiece to slip into the workholding fixture

The thrust force depends on:

1. Strength of the workpiece material

2. Feed

3. Rotational speed

4. Drill diameter

5. Drill geometry

6. Cutting fluid Copyright © 2010 Pearson Education South Asia Pte Ltd

Drilling, Drills, and Drilling Machines:Thrust Force and TorqueTorque A knowledge of the torque in drilling is essential for

estimating the power requirement Due to many factors involved, it is difficult to calculate Torque can be estimated from the data table


Drilling, Drills, and Drilling Machines:Thrust Force and TorqueEXAMPLE 23.4

Material-removal Rate and Torque in Drilling

A hole is being drilled in a block of magnesium alloy with a 10-mm drill bit at a feed of 0.2 mm/rev and with the spindle running at N = 800 rpm. Calculate the material-removal rate and the torque on the drill.


Drilling, Drills, and Drilling Machines:Thrust Force and TorqueSolution

Material-removal Rate and Torque in Drilling

The material-removal rate is

The power required is

The torque is


/smm 210min/mm 570,12)800)(2.0(4

)10( 332

MMR

W1055.0210 Power

Nm 25.18.83

105T

Drilling, Drills, and Drilling Machines:Drill Materials and Sizes Drills are made of high-speed steels and solid carbides

or with carbide tips Drills are coated with titanium nitride or titanium carbon

nitride for increased wear resistance Standard twist-drill sizes consist of:

1. Numerical

2. Letter

3. Fractional

4. Millimeter


Drilling, Drills, and Drilling Machines:Drilling Practice Drill does not have a centering action, tends to “walk”

on the workpiece surface at the beginning of the operation

A small starting hole can be made with a center drill before drilling

Or the drill point may be ground to an S shape which has a self-centering characteristic and produces accurate holes with improved drill life


Drilling, Drills, and Drilling Machines:Drilling PracticeDrilling Recommendations The speed is the surface speed of the drill at its

periphery


Drilling, Drills, and Drilling Machines:Drilling PracticeDrilling Recommendations The feed in drilling is the distance the drill travels into

the workpiece per revolution Chip removal during drilling can be difficult for deep

holes in soft and ductile workpiece materials


Drilling, Drills, and Drilling Machines:Drilling PracticeDrill Reconditioning Drills are reconditioned by grinding them either

manually or with special fixtures Hand grinding is difficult and requires considerable skill

in order to produce symmetric cutting edges Grinding on fixtures is accurate and is done on special

computer controlled grinders


Drilling, Drills, and Drilling Machines:Drilling PracticeMeasuring Drill Life Drill life is measured by the number of holes drilled

before they become dull and need to be re-worked or replaced

Drill life is defined as the number of holes drilled until this transition begins


Drilling, Drills, and Drilling Machines:Drilling Machines Drilling machines are used for drilling holes, tapping,

reaming and small-diameter boring operations The most common machine is the drill press


Drilling, Drills, and Drilling Machines:Drilling Machines The types of drilling machines range from simple bench

type drills to large radial drills The drill head of universal drilling machines can be

swiveled to drill holes at an angle Numerically controlled three-axis

drilling machines are automate in the desired sequence using turret

Drilling machines with multiple spindles (gang drilling) are used for high-production-rate operations


Drilling, Drills, and Drilling Machines:Design Considerations for Drilling Basic design guidelines:

1. Designs should allow holes to be drilled on flat surfaces and perpendicular to the drill motion

2. Interrupted hole surfaces should be avoided

3. Hole bottoms should match standard drill-point angles

4. Through holes are preferred over blind holes

5. Dimples should be provided

6. Parts should be designed with a minimum of fixturing

7. Blind holes must be drilled deeper


Reaming and Reamers

Reaming is an operation used to:

1. Make existing hole dimensionally more accurate

2. Improve surface finish Most accurate holes in workpieces are produced by:

1. Centering

2. Drilling

3. Boring

4. Reaming For even better accuracy and surface finish, holes may

be burnished or internally ground and honed


Reaming and Reamers

Hand reamers have a tapered end in the first third of their length

Machine reamers are available in two types: Rose reamers and Fluted reamers

Shell reamers are used for holes larger than 20 mm Expansion reamers are adjustable for small variations

in hole size Adjustable reamers can be set for specific hole

diameters and therefore are versatile


Tapping and Taps

Internal threads in workpieces can be produced by tapping

A tap is a chip-producing threading tool with multiple cutting teeth

Tapered taps are designed to reduce the torque required for the tapping of through holes

Bottoming taps are for tapping blind holes to their full depth

Collapsible taps are used in large-diameter holes


Tapping and Taps

Tapping may be done by hand or with machines:

1. Drilling machines

2. Lathes

3. Automatic screw machines

4. Vertical CNC milling machines One system for the automatic tapping of nuts is shown


Tapping and Taps

CASE STUDY 23.1

Bone Screw Retainer A cervical spine implant


turning mfg chapter23

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