nptel metal cutting and tool design

Module 1

Classification of Metal Removal Processes and

Machine tools

Version 2 ME IIT, Kharagpur

Lesson 1

Introduction to

Manufacturing and Machining


Instructional objectives At the end of this lesson, the student would be able to : (i) Identify the necessity of manufacturing (ii) Define with examples the concept of manufacturing (iii) List the main classifications of the manufacturing processes with examples (iv) State the main purposes of machining (v) Define with examples the concept of machining (vi) State with example the principles of machining (vii) State with examples the main requirements for machining (viii) State with examples the main functions of Machine tools (ix) Define the concept of machine tools (i) Manufacturing Need and concept The progress and the prosperity of human civilization are governed and judged mainly by improvement and maintenance of standard of living through availability or production of ample and quality goods and services for mens material welfare (MMW) in all respects covering housing, clothing, medicine, education, transport, communication and also entertainment. The successful creation of mens material welfare (MMW) depends mainly on

availability of natural resources (NR) exertion of human effort (HE); both physical and mental development and use of power tools and machines (Tools),

This can be depicted in a simple form, MMW = NR(HE)TOOLSwhere, NR: refers to air, water, heat and light, plants and animals and solid and liquid minerals TOOLS: refers to power plants, chemical plants, steel plants, machine tools etc. which magnify human capability. This clearly indicates the important roles of the components; NR, HE and TOOLS on achieving MMW and progress of civilization. Production or manufacturing can be simply defined as value addition processes by which raw materials of low utility and value due to its inadequate material properties and poor or irregular size, shape and finish are converted into high utility and valued products with definite dimensions, forms and finish imparting some functional ability. A typical example of manufacturing is schematically shown in Fig. 1.1.


A lump of mild steel of irregular shape, dimensions and surface, which had almost no use and value, has been converted into a useful and valuable product like bolt by a manufacturing process which imparted suitable features, dimensional accuracy and surface finish, required for fulfilling some functional requirements.

Value added

Fig. 1.1 Value addition by manufacturing.

Production Engineering covers two domains:

(a) Production or Manufacturing Processes (b) Production Management

(a) Manufacturing Processes This refers to science and technology of manufacturing products effectively, efficiently, economically and environment-friendly through

Application of any existing manufacturing process and system Proper selection of input materials, tools, machines and environments. Improvement of the existing materials and processes Development of new materials, systems, processes and techniques

All such manufacturing processes, systems, techniques have to be Technologically acceptable Technically feasible Economically viable Eco-friendly

Manufacturing Science and technology are growing exponentially to meet the growing demands for;

(i) Increase and maintenance of productivity, quality and economy specially in respect of liberalisation and global competitiveness


(ii) Making micro and ultra precision components for the modern electronics, computers and medical applications

(iii) Processing exotic materials, coming up with rapid and vast advent of science and technology like aerospace and nuclear engineering.

(b) Production Management This is also equally important and essential in the manufacturing world. It mainly refers to planning, coordination and control of the entire manufacturing in most profitable way with maximum satisfaction to the customers by best utilization of the available resources like man, machine, materials and money. It may be possible to manufacture a product of given material and desired configuration by several processes or routes as schematically indicated in Fig. 1.2.

Processes

Input

(raw material)

Output

(product)

Fig. 1.2 Possibility of manufacturing in number of routes. The various process routes may be different in respect of principle, technique, quality of products and time requirement and cost of manufacture. The best one is to be selected based on some criteria. Achieving the goal in manufacturing requires fulfillment of one or more of the following objectives:

reduction of manufacturing time increase of productivity reduction of manufacturing cost increase in profit or profit rate

The most significant and ultimate objective, i.e., Increase in Profit, Pr, can be attained by

(i) reducing the overall manufacturing cost, Cm (ii) increase in revenue, R by increasing quality and reliability of the

products (iii) enhancement of saleable production

As has been indicated in Fig. 1.3


CmProfit

Q*

R

Volume of production, Q

(i)

(ii)

(iii)

Man

ufac

turin

g co

st, C

T

Fig. 1.3 Strategies of increasing profit. Production management integrates and accomplishes all such essential activities leading to maximum benefits by best utilization of the resources and strategies. (ii) Broad classification of Engineering Manufacturing

Processes. It is extremely difficult to tell the exact number of various manufacturing processes existing and are being practiced presently because a spectacularly large number of processes have been developed till now and the number is still increasing exponentially with the growing demands and rapid progress in science and technology. However, all such manufacturing processes can be broadly classified in four major groups as follows:

(a) Shaping or forming Manufacturing a solid product of definite size and shape from a given material taken in three possible states:

in solid state e.g., forging rolling, extrusion, drawing etc. in liquid or semi-liquid state e.g., casting, injection moulding

etc. in powder form e.g., powder metallurgical process.

(b) Joining process Welding, brazing, soldering etc.

(c) Removal process Machining (Traditional or Non-traditional), Grinding etc.


(d) Regenerative manufacturing

Production of solid products in layer by layer from raw materials in different form:

liquid e.g., stereo lithography powder e.g., selective sintering sheet e.g., LOM (laminated object manufacturing) wire e.g., FDM. (Fused Deposition Modelling)

Out of the aforesaid groups, Regenerative Manufacturing is the latest one which is generally accomplished very rapidly and quite accurately using CAD and CAM for Rapid Prototyping and Tooling. (iii) Machining Purpose, Principle and Definition (a) Purpose of Machining Most of the engineering components such as gears, bearings, clutches, tools, screws and nuts etc. need dimensional and form accuracy and good surface finish for serving their purposes. Preforming like casting, forging etc. generally cannot provide the desired accuracy and finish. For that such preformed parts, called blanks, need semi-finishing and finishing and it is done by machining and grinding. Grinding is also basically a machining process. Machining to high accuracy and finish essentially enables a product

fulfill its functional requirements improve its performance prolong its service

(b) Principle of Machining The basic principle of machining is typically illustrated in Fig. 1.4.

PRODUCT BLANK

Vc

Feed,so

TOOL

t

Fig. 1.4 Principle of machining (turning)


A metal rod of irregular shape, size and surface is converted into a finished rod of desired dimension and surface by machining by proper relative motions of the tool-work pair. (c) Definition of Machining: Machining is an essential process of finishing by which jobs are produced to the desired dimensions and surface finish by gradually removing the excess material from the preformed blank in the form of chips with the help of cutting tool(s) moved past the work surface(s). (iv) Machining requirements The essential basic requirements for machining work are schematically illustrated in Fig. 1.5

Machining Process

Blank

Machine

Fixture

Tools Environment

Correction

Power

Product

Analysis

Fig. 1.5 Requirements for machining The blank and the cutting tool are properly mounted (in fixtures) and moved in a powerful device called machine tool enabling gradual removal of layer of material from the work surface resulting in its desired dimensions and surface finish. Additionally some environment called cutting fluid is generally used to ease machining by cooling and lubrication. (v) Basic functions of Machine Tools Machine Tools basically produce geometrical surfaces like flat, cylindrical or any contour on the preformed blanks by machining work with the help of cutting tools. The physical functions of a Machine Tool in machining are:

firmly holding the blank and the tool transmit motions to the tool and the blank


provide power to the tool-work pair for the machining action. control of the machining parameters, i.e., speed, feed and depth of cut.

(vi) Machine Tool - definition A machine tool is a non-portable power operated and reasonably valued device or system of devices in which energy is expended to produce jobs of desired size, shape and surface finish by removing excess material from the preformed blanks in the form of chips with the help of cutting tools moved past the work surface(s). A. Quiz Test: Select the correct answer from the given four possible answers: - 1. Machining is a

(a) shaping process (b) removal process (c) regenerative process (d) joining process.

2. An object is machined to

(a) fulfill its functional requirement (b) provide desirably good performance (c) render longer service life (d) all of the above.

3. Feed rate is expressed in turning operation by

(a) mm/revolution (b) mm/stroke (c) mm per min (d) none of the above.

4. Rapid prototyping is a

(a) joining process (b) removal process (c) regenerative manufacturing process (d) finishing process.


B. Exercises: 1. What should be the aims and objectives in manufacturing of any product? 2. Justify Machining is a value addition process. 3. Why even a battery operated pencil sharpener cannot be accepted as a machine tool? 4. Why is making profit must for any industry ? Answers of the given questions. A. 1. (b)

2. (d) 3. (a) 4. (c)

B. Ans. 1 Aim enhance profit rate and job opportunity Objectives

reduce manufacturing time increase rate of production reduce cost of manufacturing raise profit and profit rate

Ans.2 Preformed Machining Product

Poor quality Less utility Less value

Dimensionalaccuracy

Good finish High quality High utility High value

Ans. 3 Inspite of having all other major features of machine tools, the sharpener is of low value. Ans. 4 For Maintenance, repair & replacement Modernisation Increase salary / incentive Expansion


Module 1

Classification of Metal

Removal Processes and Machine tools


Lesson 2

Basic working

principle, configuration,

specification and classification of

machine tools Version 2 ME IIT, Kharagpur

Instructional Objectives At the end of this lesson, the students should be able to : (a) Describe the basic functional principles of machine tools

(i) Illustrate the concept of Generatrix and Directrix (ii) Demonstrate Tool work motions (iii) Give idea about machine tool drives

(b) Show configuration of basic machine tools and state their uses (c) Give examples of machine tools - specification (d) Classify machine tools broadly.

Basic functional principles of machine tool operations Machine Tools produce desired geometrical surfaces on solid bodies (preformed blanks) and for that they are basically comprised of;

Devices for firmly holding the tool and work Drives for providing power and motions to the tool and work Kinematic system to transmit motion and power from the sources to

the tool-work Automation and control systems Structural body to support and accommodate those systems with

sufficient strength and rigidity. For material removal by machining, the work and the tool need relative movements and those motions and required power are derived from the power source(s) and transmitted through the kinematic system(s) comprised of a number and type of mechanisms. (i) Concept of Generatrix and Directrix

Generation of flat surface The principle is shown in Fig. 2.1 where on a flat plain a straight line called

Generatrix (G) is traversed in a perpendicular direction called Directrix (D) resulting a flat surface.

Generation of cylindrical surfaces The principles of production of various cylindrical surfaces (of revolution) are shown in Fig. 2.2, where,

A long straight cylindrical surface is obtained by a circle (G) being traversed in the direction (D) parallel to the axis as shown in Fig. 2.2(a)

A cylindrical surface of short length is obtained by traversing a straight line (G) along a circular path (D) as indicated in Fig. 2.2(b)

Form cylindrical surfaces by rotating a curved line (G) in a circular path (D) as indicated in Fig. 2.2 (c and d).


G

D

D

G

(a) (b)

Fig. 2.1 Generation of flat surfaces by Generatrix and Directrix.

Fig. 2.2 Generation of cylindrical surfaces (of revolution)

(ii) Tool work motions The lines representing the Generatrix and Directrix are usually produced

by the locus of a point moving in two different directions and are actually obtained by the motions of the tool-tip (point) relative to the work surface. Hence, for machining flat or curved surfaces the machine tools need relative tool work motions, which are categorized in following two groups:

Formative motions namely Cutting motion (CM) Feed motion (FM)

Auxiliary motions such as Indexing motion Additional feed motion Relieving motion

The Generatrix and Directrix, tool and the work and their motions generally remain interconnected and in different way for different machining work. Such interconnections are typically shown in Fig. 2.3 for straight turning and in Fig. 2.4 for shaping.


D

G

FM

CM

Feed motion

Cutting motion

G D

(a) longitudinal turning (b) transverse turning

Fig. 2.3 Principle of turning (cylindrical surface)

The connections in case of straight longitudinal turning shown in Fig. 2.3 (a) are: Generatrix (G) Cutting motion (CM) Work (W) Directrix (D) Feed motion (FM) Tool (T) tool

work

Desired flat surface

G

FM CM D

Fig. 2.4 Principle of producing flat surface in shaping machine In case of making flat surface in a shaping machine as shown in Fig. 2.4 the connections are:

G CM T D FM W which indicates that in shaping flat surfaces the Generatrix is provided by the cutting motion imparted to the cutting tool and the Directrix is provided by the feed motion of the work.


Flat surfaces are also produced by planning machines, mainly for large jobs, where the cutting motion is imparted to the work and feed motion to the tool and the connections will be: G CM Work D FM Tool The Genratrix and Directrix can be obtained in four ways:

Tracing (Tr) where the continuous line is attained as a trace of path of a moving point as shown in Fig. 2.3 and Fig. 2.4.

Forming (F) where the Generatrix is simply the profile of the cutting edge as indicated in Fig. 2.2 (c and d)

Tangent Tracing (TTr) where the Directrix is taken as the tangent to the series of paths traced by the cutting edges as indicated in Fig. 2.5.

Generation (G): Here the G or D is obtained as an envelope being tangent to the instantaneous positions of a line or surface which is rolling on another surface. Gear teeth generation by hobbing or gear shaping is the example as can be seen in Fig. 2.6.

Fig. 2.5 typically shows the tool-work motions and the corresponding Generatrix (G) and Directrix (D) while producing flat surface by a plain or slab milling cutter in a conventional horizontal arbour type milling machine. The G and D are connected here with the tool work motions as G x T F D FM W T.Tr CM T Here G and D are independent of the cutting motion and the G is the line of contact between the milling cutter and the flat work surface. The present cutter being of roller shape, G has been a straight line and the surface produced has also been flat. Form milling cutters will produce similar formed surfaces as shown in Fig. 2.7 where the G is the tool-form.

Fig. 2.5 Directrix formed by tangent tracing in plain milling


Fig. 2.6 Generatrix (or Directrix) in gear teeth cutting by generation.

For making holes in drilling machines both the cutting motion and the feed

G CM T Tr

Fig. 2.7 Tool-work motions and G & D in form milling

motion are imparted to the cutting tool i.e., the drill bit whereas the workpiece remains stationary. This is shown in Fig. 2.8. The G and D are linked with the tool-work in the way: D FM W Tr


Fig. 2.8 Tool-work motions and G & D in drilling.

oring machines are mostly used for enlargement and finishing of existing

Vertical boring machine low or medium duty and high precision, e.g., g machine medium or heavy duty.

respect of tool-work motions and G and D, vertical boring and drilling are

ii) Machine tool drives

or the desired tool-work motions with power, machine tools are driven by

achine tools essentially need wide ranges of cutting speed and feed rate to

achining different jobs (material and size) try and size)

o low speed

desired.

achine tool drives may be

CM

FM

G

D

D

G

Bcylindrical holes. Boring machines are of two types:

Jig boring machine Horizontal axis borin

Insame. In horizontal boring machine the feed motion is imparted to the work to provide the Directrix by Tracing. (i Felectric motors and use of some mechanisms like belt-pulley, gears etc. In some machine tools, the tool-work motions are provided by hydraulic drive also. Menable

M Using different cutting tools (material, geome Various machining operations like high speed turning t

thread cutting in lathes Degree of surface finish

Mo Stepped drive o Stepless drive


Stepped drives are very common in conventional machine tools where a discrete number of speeds and feeds are available and preferably in G.P. (Geometric Progression) series. Whereas the modern CNC machine tools are provided with stepless drives enabling optimum selection and flexibly automatic control of the speeds and feeds. Stepped drive is attained by using gear boxes or cone pulley (old method) along with the power source. Stepless drive is accomplished usually by

Variable speed AC or DC motors Stepper or servomotors Hydraulic power pack

Configuration of Basic Machine Tools and their use

Centre lathes - configuration

Fig. 2.9 shows the general configuration of center lathe. Its major parts are:

o Head stock: it holds the blank and through that power and rotation are transmitted to the job at different speeds

o tailstock: supports longer blanks and often accommodates tools like drills, reamers etc for hole making.

o carriage: accommodates the tool holder which in turn holds the moving tools

o bed: headstock is fixed and tailstock is clamped on it. Tailstock has a provision to slide and facilitate operations at different locations

carriage travels on the bed o columns: on which the bed is fixed o work-tool holding devices

uses of center lathes

Centre lathes are quite versatile being used for various operations: external straight

turning taper internal stepped facing, centering, drilling, recessing and parting thread cutting; external and internal knurling.

Some of those common operations are shown in Fig. 2.10. Several other operations can also be done in center lathes using suitable attachments.

Shaping machine Fig. 2.11 shows the general configuration of shaping machine. Its major parts are:

o Ram: it holds and imparts cutting motion to the tool through reciprocation

o Bed: it holds and imparts feed motions to the job (blank) o Housing with base: the basic structure and also accommodate

the drive mechanisms


o Power drive with speed and feed change mechanisms. Shaping machines are generally used for producing flat surfaces, grooving, splitting etc. Because of poor productivity and process capability these machine tools are not widely used now-a-days for production. tool post job

tool headstock tailstock

saddle

leadscrew

rack feedrod bed

Fig. 2.9 Schematic view of a center lathe Fig. 2.10 Some common machining operations done in center Lathes.

turning facing grooving forming threading

External

Internal

Fig. 2.10 Some common machining operations done in center lathes.


clapperbox

tool Job Vice

ram

housing

bed

Power drive

base

Fig. 2.11 Schematic view of a shaping machine

Planing machine The general configuration is schematically shown in Fig. 2.12. This machine tool also does the same operations like shaping machine but the major differences are:

o In planing the job reciprocates for cutting motion and the tool moves slowly for the feed motions unlike in shaping machine.

o Planing machines are usually very large in size and used for large jobs and heavy duty work.

Drilling machine

Fig. 2.13 shows general configuration of drilling machine, column drill in particular. The salient parts are

o Column with base: it is the basic structure to hold the other parts o Drilling head: this box type structure accommodates the

power drive and the speed and feed gear boxes. o Spindle: holds the drill and transmits rotation and axial

translation to the tool for providing cutting motion and feed motion both to the drill. Drilling machines are available in varying size and configuration such as pillar drill, column drill, radial drill, micro-drill etc. but in working principle all are more or less the same.

Drilling machines are used: o Mainly for drilling (originating or enlarging cylindrical holes)


o Occasionally for boring, counter boring, counter sinking etc. o Also for cutting internal threads in parts like nuts using suitable attachment.

frame

tool Job table

bed power drive

base

Fig. 2.12 Schematic view of a planning machine

Column

Spindle

Job

bed

Feed change lever Speed

change lever

Drill

base


Fig. 2.13 Schematic view of a drilling machine

Milling machine The general configuration of knee type conventional milling machine with horizontal arbour is shown in Fig. 2.14. Its major parts are

o Milling arbour: to hold and rotate the cutter o Ram: to support the arbour o Machine table: on which job and job holding devices are

mounted to provide the feed motions to the job. o Power drive with Speed and gear boxes: to provide power

and motions to the tool-work o Bed: which moves vertically upward and downward and accommodates the various drive mechanisms o Column with base: main structural body to support other

parts.

ram

Cutter job

Speed Gear Box Feed

GB

base

MOTOR

Fig. 2.14 Schematic view of a milling machine Milling machines are also quite versatile and can do several operations like

o making flat surfaces o grooving, slitting and parting o helical grooving


o forming 2-D and 3-D contoured surfaces

Fig. 2.15 shows some of the aforesaid milling operations.

surfacing slotting slitting grooving forming

Fig. 2.15 Some common milling operation

Specification of Machine Tools. A machine tool may have a large number of various features and characteristics. But only some specific salient features are used for specifying a machine tool. All the manufacturers, traders and users must know how are machine tools specified. The methods of specification of some basic machine tools are as follows:

o Centre lathe Maximum diameter and length of the jobs that can be

accommodated Power of the main drive (motor) Range of spindle speeds Range of feeds Space occupied by the machine.

o Shaping machine

Length, breadth and depth of the bed Maximum axial travel of the bed and vertical travel of the bed /

tool Maximum length of the stroke (of the ram / tool) Range of number of strokes per minute Range of table feed Power of the main drive Space occupied by the machine

o Drilling machine (column type)

Maximum drill size (diameter) that can be used Size and taper of the hole in the spindle Range of spindle speeds


Range of feeds Power of the main drive Range of the axial travel of the spindle / bed Floor space occupied by the machine

o Milling machine (knee type and with arbour)

Type; ordinary or swiveling bed type Size of the work table Range of travels of the table in X-Y-Z directions Arbour size (diameter) Power of the main drive Range of spindle speed Range of table feeds in X-Y-Z directions Floor space occupied.

Broad classification of Machine Tools Number of types of machine tools gradually increased till mid 20th century and after that started decreasing based on Group Technology. However, machine tools are broadly classified as follows:

According to direction of major axis : o horizontal center lathe, horizontal boring machine etc. o vertical vertical lathe, vertical axis milling machine etc. o inclined special ( e.g. for transfer machines).

According to purpose of use :

o general purpose e.g. center lathes, milling machines, drilling machines etc. o single purpose e.g. facing lathe, roll turning lathe etc. o special purpose for mass production.

According to degree of automation

o non-automatic e.g. center lathes, drilling machines etc. o semi-automatic capstan lathe, turret lathe, hobbinh machine etc. o automatic e.g., single spindle automatic lathe, swiss type

automatic lathe, CNC milling machine etc.

According to size : o heavy duty e.g., heavy duty lathes (e.g. 55 kW), boring mills,

planning machine, horizontal boring machine etc. o medium duty e.g., lathes 3.7 ~ 11 kW, column drilling machines,

milling machines etc. o small duty e.g., table top lathes, drilling machines, milling

machines. o micro duty e.g., micro-drilling machine etc.

According to precision :


o ordinary e.g., automatic lathes o high precision e.g., Swiss type automatic lathes

According to number of spindles :

o single spindle center lathes, capstan lathes, milling machines etc. o multi-spindle multispindle (2 to 8) lathes, gang drilling machines etc.

According to blank type :

o bar type (lathes) o chucking type (lathes) o housing type

According to type of automation :

o fixed automation e.g., single spindle and multispindle lathes o flexible automation e.g., CNC milling machine

According to configuration :

o stand alone type most of the conventional machine tools. o machining system (more versatile) e.g., transfer machine,

machining center, FMS etc.

Exercise 2

1. Show the tool-work motions and the Generatrix and Directrix in external thread cutting in centre lathe. Also state how those G & D are obtained. 2. In which conventional machine tools flat surface can be produced ? 3. State the major differences between shaping machine and planing machine. 4. In which machine tools both the cutting motion & the feed motion are imparted to the tool ? 5. How is feed expressed in turning, shaping, drilling and milling ?

Answers

Ans. Q 1

CM

FM

G

D


G x T F D (CM+FM) (T+W) - T

Ans. Q. 2 Flat surfaces can be produced in centre lathes e.g., facing shaping, slotting and planing machines milling machines Ans. Q. 3 Shaping machine Planing machine

o for small and medium size jobs o for medium and large size jobs o tool reciprocates and provide

CM o job on table reciprocates and

provide CM o feed motion is given to the job o feed motion is given to the tool o G CM T Tr D FM W Tr

o G CM W Tr D FM T Tr

Ans. Q. 4 Both CM and FM are imparted to the tool in drilling machine vertical boring machine Ans. Q. 5 turning mm/rev shaping mm/stroke drilling machine mm/rev milling machine mm/min


Module 2

Mechanics of Machining (Metal

Cutting)


Lesson 3

Geometry of single point cutting tools


Instructional objectives At the end of this lesson, the student should be able to : (a) conceive rake angle and clearance angle of cutting tools (b) classify systems of description of tool geometry (c) demonstrate tool geometry and define tool angles in :

Machine Reference System Orthogonal Rake System and Normal Rake System

(d) designate cutting tool geometry in ASA, ORS and NRS Geometry of single point turning tools Both material and geometry of the cutting tools play very important roles on their performances in achieving effectiveness, efficiency and overall economy of machining. Cutting tools may be classified according to the number of major cutting edges (points) involved as follows:

Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools

Double (two) point: e.g., drills Multipoint (more than two): e.g., milling cutters, broaching tools, hobs,

gear shaping cutters etc. (i) Concept of rake and clearance angles of cutting tools. The word tool geometry is basically referred to some specific angles or slope of the salient faces and edges of the tools at their cutting point. Rake angle and clearance angle are the most significant for all the cutting tools. The concept of rake angle and clearance angle will be clear from some simple operations shown in Fig. 3.1

rake angle,

Clearance angle,

Flank surface

rake surface Reference plane (R)

Cutting Velocity, VC

Flank surface

Velocity vector,VC

Ref. Plane (R)

VC

Rake face

Fig. 3.1 Rake and clearance angles of cutting tools.


Definition - Rake angle (): Angle of inclination of rake surface from reference plane clearance angle (): Angle of inclination of clearance or flank surface from the finished surface Rake angle is provided for ease of chip flow and overall machining. Rake angle may be positive, or negative or even zero as shown in Fig. 3.2.

+ + + +

- =0VC

VC VC

VC

VCR R R

(a) positive rake (b) zero rake (c) negative rake

Fig. 3.2 Three possible types of rake angles

Relative advantages of such rake angles are: Positive rake helps reduce cutting force and thus cutting power

requirement. Negative rake to increase edge-strength and life of the tool Zero rake to simplify design and manufacture of the form tools.

Clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface which causes loss of energy and damages of both the tool and the job surface. Hence, clearance angle is a must and must be positive (3o ~ 15o depending upon tool-work materials and type of the machining operations like turning, drilling, boring etc.) (ii) Systems of description of tool geometry

Tool-in-Hand System where only the salient features of the cutting tool point are identified or visualized as shown in Fig. 3.3. There is no quantitative information, i.e., value of the angles.

Machine Reference System ASA system Tool Reference Systems

Orthogonal Rake System ORS Normal Rake System NRS

Work Reference System WRS (iii) Demonstration (expression) of tool geometry in :

Machine Reference System This system is also called ASA system; ASA stands for American Standards Association. Geometry of a cutting tool refers mainly to its


several angles or slope of its salient working surfaces and cutting edges. Those angles are expressed w.r.t. some planes of reference. In Machine Reference System (ASA), the three planes of reference and the coordinates are chosen based on the configuration and axes of the machine tool concerned. The planes and axes used for expressing tool geometry in ASA system for turning operation are shown in Fig. 3.4.

rake surface

Auxiliary cutting edge

Tool nose

Auxiliary flank (clearance) surface

principal cutting edge

principal flank (clearance) surface

Fig. 3.3 Basic features of single point tool (turning) in Tool-in-hand system

feed

R XY

Ym

Xm

Ym

Xm

Zm ( )CV

Fig. 3.4 Planes and axes of reference in ASA system


The planes of reference and the coordinates used in ASA system for tool geometry are :

R - X - Y and Xm Ym - Zmwhere,

R = Reference plane; plane perpendicular to the velocity vector (shown in Fig. 3.4) X = Machine longitudinal plane; plane perpendicular to R and taken in the direction of assumed longitudinal feed Y = Machine Transverse plane; plane perpendicular to both R and X [This plane is taken in the direction of assumed cross feed]

The axes Xm, Ym and Zm are in the direction of longitudinal feed, cross feed and cutting velocity (vector) respectively. The main geometrical features and angles of single point tools in ASA systems and their definitions will be clear from Fig. 3.5.

Zm, VC

ZmYm

Xm

Y

X

R

X

y

s

e y

x

X

Y

Ym

Xm

R

R

x

Fig. 3.5 Tool angles in ASA system

Definition of:

Rake angles: [Fig. 3.5] in ASA system x = side (axial rake: angle of inclination of the rake surface from the reference plane (R) and measured on Machine Ref. Plane, X. y = back rake: angle of inclination of the rake surface from the reference plane and measured on Machine Transverse plane, Y.


Clearance angles: [Fig. 3.5] x = side clearance: angle of inclination of the principal flank from the machined surface (or CV ) and measured on X plane. y = back clearance: same as x but measured on Y plane. Cutting angles: [Fig. 3.5] s = approach angle: angle between the principal cutting edge (its projection on R) and Y and measured on Re = end cutting edge angle: angle between the end cutting edge (its projection on R) from X and measured on R Nose radius, r (in inch) r = nose radius : curvature of the tool tip. It provides strengthening of the tool nose and better surface finish.

Tool Reference Systems Orthogonal Rake System ORS

This system is also known as ISO old. The planes of reference and the co-ordinate axes used for expressing the tool angles in ORS are:

R - C - O and Xo - Yo - Zowhich are taken in respect of the tool configuration as indicated in Fig. 3.6

RC

YoXoXo

Yo

Zo ( )CV

Fig. 3.6 Planes and axes of reference in ORS


where,

R = Refernce plane perpendicular to the cutting velocity vector, CV C = cutting plane; plane perpendicular to R and taken along the principal cutting edge O = Orthogonal plane; plane perpendicular to both R and C

and the axes; Xo = along the line of intersection of R and O Yo = along the line of intersection of R and C Zo = along the velocity vector, i.e., normal to both Xo and Yo axes. The main geometrical angles used to express tool geometry in Orthogonal Rake System (ORS) and their definitions will be clear from Fig. 3.7.

o o

Fig. 3.7 Tool angles in ORS system Definition of

Rake angles [Fig. 3.7] in ORS o = orthogonal rake: angle of inclination of the rake surface from Reference plane, R and measured on the orthogonal plane, o


= inclination angle; angle between C from the direction of assumed longitudinal feed [X] and measured on C

Clearance angles [Fig. 3.7] o = orthogonal clearance of the principal flank: angle of inclination of the principal flank from C and measured on oo = auxiliary orthogonal clearance: angle of inclination of the auxiliary flank from auxiliary cutting plane, C and measured on auxiliary orthogonal plane, o as indicated in Fig. 3.8.

Cutting angles [Fig. 3.7] = principal cutting edge angle: angle between C and the direction of assumed longitudinal feed or X and measured on R1 = auxiliary cutting angle: angle between C and X and measured on R Nose radius, r (mm) r = radius of curvature of tool tip

o'

Auxiliary flank

Zo'

C'o'

o' C'

C

YoXo o

C

C' Xo'

Fig. 3.8 Auxiliary orthogonal clearance angle

Normal Rake System NRS This system is also known as ISO new.

ASA system has limited advantage and use like convenience of inspection. But ORS is advantageously used for analysis and research in machining and tool performance. But ORS does not reveal the true picture of the tool geometry when the cutting edges are inclined from the reference plane, i.e., 0. Besides, sharpening or resharpening, if necessary, of the tool by grinding in ORS requires some additional calculations for correction of angles.


These two limitations of ORS are overcome by using NRS for description and use of tool geometry. The basic difference between ORS and NRS is the fact that in ORS, rake and clearance angles are visualized in the orthogonal plane, o, whereas in NRS those angles are visualized in another plane called Normal plane, N. The orthogonal plane, o is simply normal to R and C irrespective of the inclination of the cutting edges, i.e., , but N (and N for auxiliary cutting edge) is always normal to the cutting edge. The differences between ORS and NRS have been depicted in Fig. 3.9. The planes of reference and the coordinates used in NRS are: RN - C - N and Xn Yn Zn where, RN = normal reference plane N = Normal plane: plane normal to the cutting edge and Xn = Xo Yn = cutting edge Zn = normal to Xn and YnIt is to be noted that when = 0, NRS and ORS become same, i.e. oN, YN Yo and ZnZo. Definition (in NRS) of

Rake angles n = normal rake: angle of inclination angle of the rake surface from R and measured on normal plane, Nn = normal clearance: angle of inclination of the principal flank from C and measured on Nn= auxiliary clearance angle: normal clearance of the auxiliary flank (measured on N plane normal to the auxiliary cutting edge.

The cutting angles, and 1 and nose radius, r (mm) are same in ORS and NRS.


Yn

Zn n n

o o

YoZo o (A-A)

n (B-B)

A A

B

B

R C

n

o

Zn o Zn

Fig. 3.9 Differences of NRS from ORS w.r.t. cutting tool geometry.

(b) Designation of tool geometry The geometry of a single point tool is designated or specified by a series of values of the salient angles and nose radius arranged in a definite sequence as follows: Designation (signature) of tool geometry in ASA System

y, x, y, x, e, s, r (inch)

ORS System , o, o, o, 1, , r (mm)

NRS System

, n, n, n, 1, , r (mm)

Xo, Xn

(a) (b)

YoYno

n

R


Exercise 3 Quiz Test: Select the correct answer from the given four options : 1. Back rake of a turning tool is measured on its

(a) machine longitudinal plane (b) machine transverse plane (c) orthogonal plane (d) normal plane

2. Normal rake and orthogonal rake of a turning tool will be same when its

(a) = 0 (b) 1 = 0 (c) = 0 (d) 1 = 90o

3. Normal plane of a turning tool is always perpendicular to its

(a) X plane (b) Y plane (c) C plane (d) none of them

4. Principal cutting edge angle of any turning tool is measured on its

(a) R (b) Y (c) X (d) o

5. A cutting tool can never have its

(a) rake angle positive (b) rake angle negative (c) clearance angle positive (d) clearance angle negative

6. Orthogonal clearance and side clearance of a turning tool will be same if its perpendicular cutting edge angle is

(a) = 30o (b) = 45o (c) = 60o (d) = 90o


7. Inclination angle of a turning tool is measured on its

(a) reference plane (b) cutting plane (c) orthogonal plane (d) normal plane

8. Normal rake and side rake of a turning tool will be same if its

(a) = 0o and = 0o (b) = 90o and = 0o (c) = 90o and = 90o (d) = 0o and = 90o

Answer of the objective questions 1 (b) 2 (c) 3 (c) 4 (a) 5 (d) 6 (d) 7 (b) 8 (b)


Module 2

Mechanics of Machining


Lesson 4

Conversion of tool angles from one system

to another


Instructional objectives At the end of this lesson the students should be able to (i) State the purposes of conversion of tool angles (ii) Identify the four different methods of conversion of tool angles (iii) Employ the graphical method for conversion of

Rake angles clearance angles Cutting angles

From ASA to ORS and ORS to ASA systems (iv) Convert rake angle and clearance angle from ORS to NRS (v) Demonstrate tool angles relationship in some critical conditions.

(i) Purposes of conversion of tool angles from one system to another

To understand the actual tool geometry in any system of choice or convenience from the geometry of a tool expressed in any other systems

To derive the benefits of the various systems of tool designation as and when required

Communication of the same tool geometry between people following different tool designation systems.

(ii) Methods of conversion of tool angles from one system to another

Analytical (geometrical) method: simple but tedious Graphical method Master line principle: simple, quick and popular Transformation matrix method: suitable for complex tool geometry Vector method: very easy and quick but needs concept of vectors

(iii) Conversion of tool angles by Graphical method Master Line principle. This convenient and popular method of conversion of tool angles from ASA to ORS and vice-versa is based on use of Master lines (ML) for the rake surface and the clearance surfaces.

Conversion of rake angles The concept and construction of ML for the tool rake surface is shown in Fig. 4.1.


A

B

C

D

A' B'

C'

D'O'

O' O'

Zm

Xm

Ym

Xm

Xo

O' Zm

Ym

x

o

y

Master line for rake surface

R

X

Y

C

T

T

T

Zo Yo Zo

Yo

O

R

o

R

T

Xo

Fig. 4.1 Master line for rake surface (with all rake angles: positive) In Fig. 4.1, the rake surface, when extended along X plane, meets the tools bottom surface (which is parallel to R) at point D i.e. D in the plan view. Similarly when the same tool rake surface is extended along Y, it meets the tools bottom surface at point B i.e., at B in plan view. Therefore, the straight line obtained by joining B and D is nothing but the line of intersection of the rake surface with the tools bottom surface which is also parallel to R. Hence, if the rake surface is extended in any direction, its meeting point with the tools bottom plane must be situated on the line of intersection, i.e., BD. Thus the points C and A (in Fig. 4.1) obtained by extending the rake surface along o and C respectively upto the tools bottom surface, will be situated on that line of intersection, BD. This line of intersection, BD between the rake surface and a plane parallel to R is called the Master line of the rake surface. From the diagram in Fig. 4.1, OD = TcotX

OB = TcotYOC = TcotoOA = Tcot


Where, T = thickness of the tool shank. The diagram in Fig. 4.1 is redrawn in simpler form in Fig. 4.2 for conversion of tool angles.

A

B

C M

E

F

Ym

Xm

Xo Yo

D

Masterline of Rake surface

OD = cotxOB = cotyOC = cotoOA = cot

for T=unity]

O G

Fig. 4.2 Use of Master line for conversion of rake angles.

Conversion of tool rake angles from ASA to ORS o and (in ORS) = f (x and y of ASA system) costansintantan yxo += (4.1) and sintancostantan yx += (4.2) Proof of Equation 4.1: With respect to Fig. 4.2, Consider, OBD = OBC + OCD Or, OB.OD = OB.CE + OD.CF Or, OB.OD = OB.OCsin + OD.OCcos Dividing both sides by OB.OD.OC,

cosOB1sin

OD1

OC1 +=

i.e. Proved. costansintantan yxo +=


Similarly Equation 4.2 can be proved considering; OAD = OAB + OBD i.e., OD.AG = OB.OG + OB.OD where, AG = OAsin and OG = OAcos Now dividing both sides by OA.OB.OD,

OA1cos

OD1sin

OB1 +=

Proved. sintancostantan yx +=The conversion equations 4.1 and 4.2 can be combined in a matrix form,

(4.3)

=

y

xo

tantan

sincoscossin

tantan

(ORS) (ASA)

where, is the transformation matrix.

sincoscossin

Conversion of rake angles from ORS to ASA system x and y (in ASA) = f(o and of ORS) costansintantan ox = (4.4) and sintancostantan oy += (4.5) The relations (4.4) and (4.5) can be arrived at indirectly using Equation 4.3. By inversion, Equation 4.3 becomes,

(4.6)

=

tantan

sincoscossin

tantan o

y

x

from which equation 4.4 and 4.5 are obtained. The conversion equations 4.4 and 4.5 can also be proved directly from the diagram in Fig. 4.2 Hints To prove equation 4.4, proceed by taking (from Fig. 4.2) OAD = OAC + OCD, [involving the concerned angles o, and x i.e., OC, OA and OD] And to prove Equation 4.5, proceed by taking OAC = OAB + OBC [involving the concerned angles o, and y i.e., OC, OA and OB] Maximum rake angle (max or m) The magnitude of maximum rake angle (m) and the direction of the maximum slope of the rake surface of any single point tool can be easily derived from its geometry specified in both ASA or ORS by using the diagram of Fig. 4.2. The smallest intercept OM normal to the Master line (Fig. 4.2) represents max or m as


OM = cot mSingle point cutting tools like HSS tools after their wearing out are often resharpened by grinding their rake surface and the two flank surfaces. The rake face can be easily and correctly ground by using the values of m and the orientation angle, (visualized in Fig. 4.2) of the Master line. Determination of m and from tool geometry specified in ASA system. In Fig. 4.2, OBD = OB.OD = BD.OM or, OB.OD = OMODOB .22 + Dividing both sides by OB.OD.OM

22 OB1

OD1

OM1 +=

or y2

x2

m tantantan += (4.7) Again from OBD

ODOBtan =

or

=

y

x

tan

tantan (4.8)

m and from tool geometry specified in ORS Similarly from the diagram in Fig. 4.2, and taking OAC, one can prove 2o2m tantantan += (4.9)

=

o tan

tantan (4.10)

Conversion of clearance angles from ASA system to ORS and vice

versa by Graphical method. Like rake angles, the conversion of clearance angles also make use of corresponding Master lines. The Master lines of the two flank surfaces are nothing but the dotted lines that appear in the plan view of the tool (Fig. 4.3). The dotted line are the lines of intersection of the flank surfaces concerned with the tools bottom surface which is parallel to the Reference plane R. Thus according to the definition those two lines represent the Master lines of the flank surfaces. Fig. 4.4 shows the geometrical features of the Master line of the principal flank of a single point cutting tool. From Fig. 4.4,

OD = Ttanx OB = TtanyOC = Ttano


OA = Tcot where, T = thickness of the tool shank.

ML of principal flank ML of auxiliary flank

Fig. 4.3 Master lines (ML) of flank surfaces.

x

y

x

y

o

x

o

yR

O

Ym

Xm

Xo Yo

Yo

Zo

Zo Xo

Zm

Zo Yo

A'

D'

C'

B' B C

D

A

ML for principal flank

T

T

T

o

Fig. 4.4 Master line of principal flank.


The diagram in Fig. 4.4 is redrawn in simpler form in Fig. 4.5 for conversion of clearance angles. The inclination angle, basically represents slope of the rake surface along the principal cutting edge and hence is considered as a rake angle. But appears in the analysis of clearance angles also because the principal cutting edge belong to both the rake surface and the principal flank.

A

B

M C

O

Ym YoXo

Xm

Master line for principal flank

OD = tanxOB = tanyOC = tanoOA = cot ] for T=unity

D

Fig. 4.5 Use of Master line for conversion of clearance angles. Conversion of clearance angles from ASA to ORS Angles, o and in ORS = f(x and y in ASA system) Following the same way used for converting the rake angles taking suitable triangles (in Fig. 4.2), the following expressions can be arrived at using Fig. 4.5: cosycotsinxcotocot += (4.11) and sinycotcosxcottan += (4.12) Combining Equation 4.11 and 4.12 in matrix form


(4.13)

=

y

xo

cotcot

sincoscossin

tancot

Conversion of clearance angles from ORS to ASA system

x and y (in ASA) = f(o and in ORS) Proceeding in the same way using Fig. 4.5, the following expressions are derived cossinocotxcot tan= (4.14) and sincosocotycot tan+= (4.15) The relations (4.14) and (4.15) are also possible to be attained from inversions of Equation 4.13 as indicated in case of rake angles.

Minimum clearance, min or m The magnitude and direction of minimum clearance of a single point tool may be evaluated from the line segment OM taken normal to the Master line (Fig. 4.5) as OM = tanmThe values of m and the orientation angle, (Fig. 4.5) of the principal flank are useful for conveniently grinding the principal flank surface to sharpen the principal cutting edge. Proceeding in the same way and using Fig. 4.5, the following expressions could be developed to evaluate the values of m and

o From tool geometry specified in ASA system yxm 22 cotcotcot += (4.16) and

=

y

x

cot

cottan 1 (4.17)

o From tool geometry specified in ORS 202 tancotcot +=x (4.18) and

=

o cot

tantan 1 (4.19)

Similarly the clearance angles and the grinding angles of the auxiliary flank surface can also be derived and evaluated. o Interrelationship amongst the cutting angles used in ASA and ORS The relations are very simple as follows: (in ORS) = 90o - s (in ASA) (4.20)

and 1(in ORS) = e (in ASA) (4.21)


(iv) Conversion of tool angles from ORS to NRS The geometry of any single point tool is designated in ORS and NRS respectively as,

, o, o, o, 1, , r (mm) ORS , n, n, n, 1, , r (mm) NRS

The two methods are almost same, the only difference lies in the fact that o, o and o of ORS are replaced by n, n and n in NRS. The corresponding rake and clearance angles of ORS and NRS are related as , tann = tanocos (4.22) cotn = cotocos (4.23) and cotn'= coto'cos' (4.24) The equation 4.22 can be easily proved with the help of Fig. 4.6.

o

n

o

n

n

c

ZoZn Yn

Yo

A

B

C A

B A

C

Xo, Xn

Fig. 4.6 Relation between normal rake (n) and orthogonal rake (o) The planes o and n are normal to Yo and Yn (principal cutting edge) respectively and their included angle is when o and n are extended below OA (i.e. R) they intersect the rake surface along OB and OC respectively. Therefore, AOB = o AOC = nwhere, BAC =


Now AC = ABcos Or, OAtann= (OAtano)cos So, tann= tanocos proved The equation (4.23) relating n and o can be easily established with the help of Fig. 4.7.

Fig. 4.7 Relation between normal clearance, n and orthogonal clearance, o From Fig. 4.7, AC = ABcos Or AAcotn= AAcotocos cotn= cotocos proved Similarly it can be proved, cotn= cotocos where is the inclination angle of the auxiliary cutting edge. (v) Tool geometry under some critical conditions

Configuration of Master lines (in graphical method of tool angle conversion) for different tool geometrical conditions.

The locations of the points A, B, C, D and M along the ML will be as shown in Fig. 4.2 when all the corresponding tool angles have some

n

o

o

n

n

o

C

RA

B

C

A

C

A

B A'

A'

Zn

Zo

Zn

Yn

Zo

Yo

C o


positive values. When any rake angle will be negative, the location of the corresponding point will be on the other side of the tool. Some typical configurations of the Master line for rake surface and the corresponding geometrical significance are indicated in Fig. 4.8.

Configuration of ML Tool geometry

f

or ML parallel to C

x = positive y = positive o = positive = 0 m = o for ML parallel to Y x = positive y = 0

o = positive = negative

m = x for ML parallel to X x = 0 y = negative o = negative = negative m = y

(c)

(b)

(a)

O

ML

ML

C B A

O

B at infinity

C

D

A

ML

A at infinity

O

D C

B


Fig. 4.8 Tool geometry and Master line (rake face) in some typical conditions.

Tool angles relations in some critical conditions From the equations correlating the cutting tool angles, the following critical observations are made:

When = 90o; x = o for X = o When = 0 ; n = o

n = o When =0 and = 90o; n=o=x pure orthogonal cutting

(N=o=X)

Exercise 4 A. Quiz test Select the correct answer from the given four options 1. The master line for the rake surface of the turning tool of geometry : - 10o, 0o, 8o, 6o, 15o, 30o, 0.1 (inch) (a) machine longitudinal plane (b) machine transverse plane (c) cutting plane (d) orthogonal plane 2. If the approach angle of a turning tool be 30o, the value of its principal cutting edge angle will be (a) O deg. (b) 30o deg. (c) 60o deg. (d) 90o deg. 3. The value of side rake of the turning tool of geometry : - 0o, 10o, 8o, 6o, 20o, 60o, 0 (mm) will be (a) 0o deg. (b) 10o deg. (c) 8o deg.

(d) 6o deg.

4. The values of orthogonal clearance and normal clearance of a turning tool will be same if, (a) =0 (b) X = Y


(c) = 0 (d) none of the above 5. The angle between orthogonal plane and normal plane of a turning tool is (a) o (b) (c) n (d) B. Problem 1. Determine the values of normal rake of the turning tool whose geometry is designated as : 10o, - 10o, 8o, 6o, 15o, 30o, 0 (inch)? 2. Determine the value of side clearance of the turning tool whose geometry is specified as 0o, - 10o, 8o, 6o, 20o, 60o, 0 (mm) ? Solutions of Exercise 4 A. Quiz test 1 (a) 2 (c) 3 (b) 4 (c) 5 (d) B. Problems Ans. 1 Tool geometry given : 10o, - 10o, 8o, 6o, 15o, 30o, 0 (inch) y, x, y, x, e, s, r ASA tann = tanocos where, tano = tanxsin + tanycos = tan(-10o)sin(90o- 30o) + tan(10o)cos(90o- 30o) = - 0.065 So, o = - 3.7oAnd tan = - tanxcos + tanysin = - tan(-10o)cos(90o- 30o) + tan(10o)sin(90o- 30o) = 0.2408 So, = 13.54o tann = tanocos = tan (-3.7)cos(13.54) = - 0.063 So, n = - 3.6o Ans.


Ans. 2 Tool geometry given : 0o, - 10o, 8o, 6o, 20o, 60o, 0 (mm) , o, o, o', 1, , r (mm) cotx = cotosin- tancos = cot 8.sin60 tan0.cos60 = cot8.sin60 = 6.16 So, x = 9.217o Ans.


Module 2



Lesson 5

Mechanism of chip formation


Instructional Objectives At the end of this lesson, the student would be able to (i) describe with illustration the mechanism of chip formation in machining

ductile materials and brittle materials

(ii) illustrate and assess geometrical characteristics of ductile chips : chip reduction coefficient & cutting ratio shear angle and cutting strain

(iii) Identify and state the causes, characteristics and effects of built up edge (BUE) formation. (iv) Classify chips and identify the condition for different chip forms. (i) Mechanism of chip formation in machining Machining is a semi-finishing or finishing process essentially done to impart required or stipulated dimensional and form accuracy and surface finish to enable the product to

fulfill its basic functional requirements provide better or improved performance render long service life.

Machining is a process of gradual removal of excess material from the preformed blanks in the form of chips. The form of the chips is an important index of machining because it directly or indirectly indicates :

Nature and behaviour of the work material under machining condition Specific energy requirement (amount of energy required to remove unit

volume of work material) in machining work Nature and degree of interaction at the chip-tool interfaces.

The form of machined chips depend mainly upon :

Work material Material and geometry of the cutting tool Levels of cutting velocity and feed and also to some extent on depth of

cut Machining environment or cutting fluid that affects temperature and

friction at the chip-tool and work-tool interfaces.


Knowledge of basic mechanism(s) of chip formation helps to understand the characteristics of chips and to attain favourable chip forms. Mechanism of chip formation in machining ductile materials During continuous machining the uncut layer of the work material just ahead of the cutting tool (edge) is subjected to almost all sided compression as indicated in Fig. 5.1.

Work

a1: chip thickness (before cut) a2: chip thickness (after cut)

Vf

VC

Tool

o

F N

R

Fig. 5.1 Compression of work material (layer) ahead of the tool tip The force exerted by the tool on the chip arises out of the normal force, N and frictional force, F as indicated in Fig. 5.1. Due to such compression, shear stress develops, within that compressed region, in different magnitude, in different directions and rapidly increases in magnitude. Whenever and wherever the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region and the plane of maximum shear stress. But the forces causing the shear stresses in the region of the chip quickly diminishes and finally disappears while that region moves along the tool rake surface towards and then goes beyond the point of chip-tool engagement. As a result the slip or shear stops propagating long before total separation takes place. In the mean time the succeeding portion of the chip starts undergoing compression followed by yielding and shear. This phenomenon repeats rapidly resulting in formation and removal of chips in thin layer by layer. This phenomenon has been explained in a simple way by Piispannen [1] using a card analogy as shown in Fig. 5.2. In actual machining chips also, such serrations are visible at their upper surface as indicated in Fig. 5.2. The lower surface becomes smooth due to further plastic deformation due to intensive rubbing with the tool at high pressure and temperature. The pattern of shear deformation by lamellar sliding, indicated in the model, can also be seen in actual chips by proper mounting, etching and polishing the side surface of the machining chip and observing under microscope.


The pattern and extent of total deformation of the chips due to the primary and the secondary shear deformations of the chips ahead and along the tool face, as indicated in Fig. 5.3, depend upon [1] Piispannen V., Theory of formation of metal chips, J. Applied Physics, Vol. 19, No. 10, 1948, pp. 876.

work material tool; material and geometry the machining speed (VC) and feed (so) cutting fluid application

VC

chip

Tool

(a) Shifting of the postcards by partial (b) Chip formation by shear in sliding against each other lamella.

Fig. 5.2 Piispanen model of card analogy to explain chip formation in machining ductile materials

primary deformation (shear) zone

secondary deformation zone

CHIPWORK

VC

Fig. 5.3 Primary and secondary deformation zones in the chip.

The overall deformation process causing chip formation is quite complex and hence needs thorough experimental studies for clear understanding the phenomena and its dependence on the affecting parameters. The feasible and popular experimental methods [2] for this purpose are:

Study of deformation of rectangular or circular grids marked on the side surface as shown in Fig. 5.4


[2] Bhattacharya, A.., Metal cutting Theory and Practice, Book, New Central Book Agency, Kolkata.

Microscopic study of chips frozen by drop tool or quick stop apparatus Study of running chips by high speed camera fitted with low

magnification microscope.

(a) rectangular grids (b) circular grids

Fig. 5.4 Pattern of grid deformation during chip formation. It has been established by several analytical and experimental methods including circular grid deformation that though the chips are initially compressed ahead of the tool tip, the final deformation is accomplished mostly by shear in machining ductile materials. However, machining of ductile materials generally produces flat, curved or coiled continuous chips. Mechanism of chip formation in machining brittle materials The basic two mechanisms involved in chip formation are

Yielding generally for ductile materials Brittle fracture generally for brittle materials

During machining, first a small crack develops at the tool tip as shown in Fig. 5.5 due to wedging action of the cutting edge. At the sharp crack-tip stress concentration takes place. In case of ductile materials immediately yielding takes place at the crack-tip and reduces the effect of stress concentration and prevents its propagation as crack. But in case of brittle materials the initiated crack quickly propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path as indicated in Fig. 5.5. Machining of brittle material produces discontinuous chips and mostly of irregular size and shape. The process of forming such chips is schematically shown in Fig. 5.6.


crack propagation in case of brittle materials

Initial minute crack

VC

Fig. 5.5 Development and propagation of crack causing chip separation.

(a) separation (b) swelling (c) further swelling (d) separation (e) swelling again

Fig. 5.6 Schematic view of chip formation in machining brittle materials. (ii) Geometry and characteristics of chip forms The geometry of the chips being formed at the cutting zone follow a particular pattern especially in machining ductile materials. The major section of the engineering materials being machined are ductile in nature, even some semi-ductile or semi-brittle materials behave ductile under the compressive forces at the cutting zone during machining. The pattern and degree of deformation during chip formation are quantitatively assessed and expressed by some factors, the values of which indicate about the forces and energy required for a particular machining work.


Chip reduction coefficient or cutting ratio The usual geometrical features of formation of continuous chips are schematically shown in Fig. 5.7. The chip thickness (a2) usually becomes larger than the uncut chip thickness (a1). The reason can be attributed to

compression of the chip ahead of the tool frictional resistance to chip flow lamellar sliding according to Piispannen

so

(a) (b)

Fig. 5.7 Geometrical features of continuous chips formation.

The significant geometrical parameters involved in chip formation are shown in Fig. 5.7 and those parameters are defined (in respect of straight turning) as:

t = depth of cut (mm) perpendicular penetration of the cutting tool tip in work surface so = feed (mm/rev) axial travel of the tool per revolution of the job b1 = width (mm) of chip before cut b2 = width (mm) of chip after cut a1 = thickness (mm) of uncut layer (or chip before cut) a2 = chip thickness (mm) thickness of chip after cut A1 = cross section (area, mm2) of chip before cut

The degree of thickening of the chip is expressed by

1

2

aa= > 1.00 (since a2 > a1) (5.1)

where, = chip reduction coefficient


a1= sosin (5.2) where = principal cutting edge angle Larger value of means more thickening i.e., more effort in terms of forces or energy required to accomplish the machining work. Therefore it is always desirable to reduce a2 or without sacrificing productivity, i.e. metal removal rate (MRR). Chip thickening is also often expressed by the reciprocal of as,

2

11aar == (5.3)

where, r = cutting ratio The value of chip reduction coefficient, (and hence cutting ratio) depends mainly upon

tool rake angle, chip-tool interaction, mainly friction,

Roughly in the following way [3]

)

2( oe

= [for orthogonal cutting] (5.4) /2 and o are in radians The simple but very significant expression (5.4) clearly depicts that the value of can be desirably reduced by

Using tool having larger positive rake Reducing friction by using lubricant

The role of rake angle and friction at the chip-tool interface on chip reduction coefficient are also schematically shown in Fig. 5.8.

Rake angle,

(friction coefficient)

Chi

p re

duct

ion

coef

ficie

nt,

Fig. 5.8 Role of rake angle and friction on chip reduction coefficient Chip reduction coefficient, is generally assessed and expressed by the ratio of the chip thickness, after (a2) and before cut (a1) as in equation 5.1. But can also be expressed or assessed by the ratio of


[3] Kronenberg, M., A new approach to some relationships in the Theory of Metal Cutting, J. Applied Physics, Vol.6, No. 6, 1945.

Total length of the chip before (L1) and after cut (L2) Cutting velocity, VC and chip velocity, Vf

Considering total volume of chip produced in a given time, a1b1L1 = a2b2L2 (5.5) The width of chip, b generally does not change significantly during machining unless there is side flow for some adverse situation. Therefore assuming, b1=b2 in equation (5.5), comes up to be,

2

1

1

2

LL

aa =

= (5.6)

Again considering unchanged material flow (volume) ratio, Q Q = (a1b1)VC = (a2b2)Vf (5.7) Taking b1=b2,

f

C

VV

aa =

=

1

2 (5.8) Equation (5.8) reveals that the chip velocity, Vf will be lesser than the cutting

velocity, VC and the ratio is equal to the cutting ratio,

=

1r

Shear angle It has been observed that during machining, particularly ductile materials, the chip sharply changes its direction of flow (relative to the tool) from the direction of the cutting velocity, VC to that along the tool rake surface after thickening by shear deformation or slip or lamellar sliding along a plane. This plane is called shear plane and is schematically shown in Fig. 5.9. Shear plane: Shear plane is the plane of separation of work material layer in the form of chip from the parent body due to shear along that plane. Shear angle: Angle of inclination of the shear plane from the direction of cutting velocity [as shown in Fig. 5.9].

o (o - o)

o o

a1

a2

VC'

VC

A B

C O

Shear plane

Fig. 5.9 Shear plane and shear angle in chip formation


The value of shear angle, denoted by o (taken in orthogonal plane) depends upon

Chip thickness before and after cut i.e. Rake angle, o (in orthogonal plane)

From Fig. 5.9, AC = a2 = OAcos(o-o) And AB = a1 = OAsinoDividing a2 by a1

o

oo

aa

sin)cos(

1

2 == (5.9)

or, o

oo

sin

costan = (5.10)

Replacing chip reduction coefficient, by cutting ratio, r, the equation (5.10) changes to

o

oo r

r

sin1

costan = (5.11)

Equation 5.10 depicts that with the increase in , shear angle decreases and vice-versa. It is also evident from equation (5.10) as well as equation (5.4) that shear angle increases both directly and indirectly with the increase in tool rake angle. Increase in shear angle means more favourable machining condition requiring lesser specific energy. Cutting strain The magnitude of strain, that develops along the shear plane due to machining action, is called cutting strain (shear). The relationship of this cutting strain, with the governing parameters can be derived from Fig. 5.10. Due to presence of the tool as an obstruction the layer 1 has been shifted to position 2 by sliding along the shear plane. From Fig. 5.10,

Cutting strain (average), ONPM

Ys ==

or,ONNM

ONPN

ONNMPN +=+=

or, )tan(cot ooo += (5.12) (iii) Built-up-Edge (BUE) formation Causes of formation In machining ductile metals like steels with long chip-tool contact length, lot of stress and temperature develops in the secondary deformation zone at the chip-tool interface. Under such high stress and temperature in between two clean surfaces of metals, strong bonding may locally take place due to adhesion similar to welding. Such bonding will be encouraged and


accelerated if the chip tool materials have mutual affinity or solubility. The weldment starts forming as an embryo at the most favourable location and thus gradually grows as schematically shown in Fig. 5.11.

s

o

o

o s

s

P

M

N

O

Y

Y

shear strain

Fig. 5.10 Cutting strain in machining Built-up-edge

F

VC

Fig. 5.11 Scheme of built-up-edge formation


With the growth of the BUE, the force, F (shown in Fig. 5.11) also gradually increases due to wedging action of the tool tip along with the BUE formed on it. Whenever the force, F exceeds the bonding force of the BUE, the BUE is broken or sheared off and taken away by the flowing chip. Then again BUE starts forming and growing. This goes on repeatedly. Characteristics of BUE Built-up-edges are characterized by its shape, size and bond strength, which depend upon:

work tool materials stress and temperature, i.e., cutting velocity and feed cutting fluid application governing cooling and lubrication.

BUE may develop basically in three different shapes as schematically shown in Fig. 5.12.

(a) positive wedge (b) negative wedge (c) flat type

Fig. 5.12 Different forms of built-up-edge.

In machining too soft and ductile metals by tools like high speed steel or uncoated carbide the BUE may grow larger and overflow towards the finished surface through the flank as shown in Fig. 5.13 .

VC

chip

Fig. 5.13 Overgrowing and overflowing of BUE causing surface roughness


While the major part of the detached BUE goes away along the flowing chip, a small part of the BUE may remain stuck on the machined surface and spoils the surface finish. BUE formation needs certain level of temperature at the interface depending upon the mutual affinity of the work-tool materials. With the increase in VC and so the cutting temperature rises and favours BUE formation. But if VC is raised too high beyond certain limit, BUE will be squashed out by the flowing chip before the BUE grows. Fig. 5.14 shows schematically the role of increasing VC and so on BUE formation (size). But sometime the BUE may adhere so strongly that it remains strongly bonded at the tool tip and does not break or shear off even after reasonably long time of machining. Such detrimental situation occurs in case of certain tool-work materials and at speed-feed conditions which strongly favour adhesion and welding.

Siz

e of

BU

E

Cutting velocity, VC

S03 > S02S02 > S01

S01

Fig. 5.14 Role of cutting velocity and feed on BUE formation.

Effects of BUE formation Formation of BUE causes several harmful effects, such as:

It unfavourably changes the rake angle at the tool tip causing increase in cutting forces and power consumption

Repeated formation and dislodgement of the BUE causes fluctuation in cutting forces and thus induces vibration which is harmful for the tool, job and the machine tool.

Surface finish gets deteriorated May reduce tool life by accelerating tool-wear at its rake surface by

adhesion and flaking Occasionally, formation of thin flat type stable BUE may reduce tool wear at the rake face.


(iv) Types of chips and conditions for formation of those chips Different types of chips of various shape, size, colour etc. are produced by machining depending upon

type of cut, i.e., continuous (turning, boring etc.) or intermittent cut (milling)

work material (brittle or ductile etc.) cutting tool geometry (rake, cutting angles etc.) levels of the cutting velocity and feed (low, medium or high) cutting fluid (type of fluid and method of application)

The basic major types of chips and the conditions generally under which such types of chips form are given below:

o Discontinuous type of irregular size and shape : - work material brittle like grey

cast iron of regular size and shape : - work material ductile but hard and work hardenable - feed large

- tool rake negative - cutting fluid absent or inadequate

o Continuous type Without BUE : work material ductile Cutting velocity high Feed low Rake angle positive and large Cutting fluid both cooling and lubricating With BUE : - work material ductile - cutting velocity medium - feed medium or large - cutting fluid inadequate or absent.

o Jointed or segmented type - work material semi-ductile - cutting velocity low to medium - feed medium to large - tool rake negative - cutting fluid absent

Often in machining ductile metals at high speed, the chips are deliberately broken into small segments of regular size and shape by using chip breakers mainly for convenience and reduction of chip-tool contact length.


Exercise - 5 A. Quiz Test Identify the correct one out of the four given answers 1.In turning mild steel the value of will be (a) > 1.0 (b) < 1.0 (c) = 1.0 (d) none of the above 2.The value of shear angle, o depends upon (a) tool rake angle (b) friction at chip-tool interface (c) built up edge formation (d) all of the above 3.Shaping grey cast iron block will produce (a) continuous chip with BUE (b) continuous chip without BUE (c) discontinuous chip of irregular size & shape

(d) discontinuous chip of regular size & shape 4.The value of chip reduction coefficient, does not depend upon (a) cutting velocity (b) depth of cut (c) cutting tool material (d) tool rake angle B. Numerical Problem 1.During plain turning mild steel by a tool of geometry, 0o, 0o, 8o, 7o, 15o, 90o, 0 (mm) at so= 0.2 mm/rev, the chip thickness was found to be 0.5 mm. Determine the values of and o in the above case.


Answers A. Quiz Test 1 (a) 2 (d) 3 (c) 4 (b) B. Numerical Problem

5.290sin2.0

5.0sin

2

1

2 ==== oo xs

aaa

]0[0sin5.2

0cossin

costan oooo

o

oo ===

oo 8.21)4.0(tan

4.05.2

11

1 =====


Module 2


Version 2 ME IIT, Kharapur

Lesson 6

Orthogonal and oblique cutting


Instructional Objectives At the end of this lesson, the student would be able to (i) define and distinguish, with illustrations, between orthogonal cutting and oblique cutting (ii) identify the causes of oblique cutting and chip flow deviation (iii) determine angle of chip flow deviation. (iv) illustrate and deduce effective rake angle (v) state the effects of oblique cutting (i) Orthogonal and oblique cutting It is appears from the diagram in Fig. 6.1 that while turning ductile material by a sharp tool, the continuous chip would flow over the tools rake surface and in the direction apparently perpendicular to the principal cutting edge, i.e., along orthogonal plane which is normal to the cutting plane containing the principal cutting edge. But practically, the chip may not flow along the orthogonal plane for several factors like presence of inclination angle, , etc.

VC

Yo

Xo

t

so

C Orthogonal plane o

chip flow direction

Fig. 6.1 Ideal direction of chip flow in turning

The role of inclination angle, on the direction of chip flow is schematically shown in Fig. 6.2 which visualises that,

when =0, the chip flows along orthogonal plane, i.e, c = 0 when 0, the chip flow is deviated from o and c = where c is chip

flow deviation (from o) angle


chip

Co o

VC

VBf

tool

90o90o

o n

VBf

tool

o C

VC

c=

=0

(a) = 0 (b) 0

Fig. 6.2 Role of inclination angle, on chip flow direction

Orthogonal cutting: when chip flows along orthogonal plane, o, i.e., c = 0 Oblique cutting : when chip flow deviates from orthogonal plane, i.e. c0 But practically c may be zero even if = 0 and c may not be exactly equal to even if 0. Because there are some other (than ) factors also which may cause chip flow deviation. Pure orthogonal cutting: This refers to chip flow along o and =90o as typically shown in Fig. 6.3 where a pipe like job of uniform thickness is turned (r

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