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Unit 5 - Turning

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CA1 (35 %) Assignments -15% (review, sketching,

case study)

2 BB Quizzes -10% (turning and milling)

General Performance -10% (attitude,attendance..)

CA2 (40%) Practical (machining tape holder -

individual)

CA3 (25%) Mini Project (machining car chassis and

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Learning Objectives:

List the components of a centre lathe

Discuss the safety rules Discuss the use of tool holding devices Discuss the characteristics of cutting tool

materials and inserts Watch and discuss video VC2495/8

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What is the primary function of a centrelathe?

Can you name the components of a lathe?

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What 2 movements are necessary beforeturning process can take place?

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a) Saddle

b) cross slide

c) compound slide

d) tool post

e) apron

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Knurling toolPartingtool Chamferingtool

Facing andturning tool

Knife

turning tool

toolRadius forming

Undercuttingtool

Facing tool

Boringtool

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Can you give other examples ofturning besides facing and parallel

turning?

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Compare and contrast the differences of a4-way tool-post (above) and quick changetool-post?

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On Centre good finish

Above Centre -not cutting

Below Centre poor finish

= Front Clearance

= Wedge Angle

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Locking screw

Insert

Insert holder

Chip

Breaker

The carbide insert is locked by a lockingscrew or cam. Shape may be triangular(as above) or diamond, square or round.

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2. Define Turning Operation.

3. State the effects of improper tool setting.

4. List 10 safety precautions in the machine shop

and how would you categorised them?5. State the 3 purposes of facing operation.

6. What is the main advantage of quick changetool post?

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Compare and contrast 3-jaw and 4-jaw

chucks Explain the use of work holding support used

in between centres Discuss the use of other work holding

devices like faceplate, mandrels andsteadies.

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Normally used for holding round orhexagonal stock.

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Suitable for round, square, octagonal andodd-shaped workpieces.

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Long Workpiece

Normally used for turning a longworkpiece supported betweenbetween centres. It requires 2centres, a drive plate and a lathe dog.

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Drive Plates

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Compare and contrast live and plaincentres.

Morse standard taper

Plain

Live

Sleeve

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Plain solid mandrel one

size of bore

Gang mandrel manyworkpieces with sameInternal diameter

Expansion mandrel slight difference (up to2mm variation) indiameter

Cone mandrel largevariation in diameterallowed.

Workpiece

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Fixed

Steady

Travelling

Steady

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1. How would you hold a hollow thin wall roundpipe for turning?

2. When is a faceplate used?

3. Explain the reason why the 3-jaw chuck is

self-centred.4. State the advantages of a 4-jaw chuck.

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Distinguish between Orthogonal and obliquecutting.

Discuss the tool geometry of a single-pointedtool.

Discuss the factors that influence metal cuttingoperations.

Discuss the types of chips

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Cutting edge is set at 90 to the direction of movement. Chips in the form of a clock spring or a flat spiral Chips disposal problem & damage to workpiece surface.

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Cutting edge is set at any angle other than 90 to

the direction of tool movement. Chips able to move freely away from workpiece Taking deeper cuts is possible surface finish is better with a nose radius

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Side Rake angle :provides a cutting edgeto allow chips to escapeduring cutting.

Back Rake angle :

promotes smooth chipflow and good finishing

Front Relief angle :prevents tool end fromrubbing

Side Relief angle: allowsthe tool to feed into thework

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Minor cutting edge

Minor cutting edge

angle

Major cutting edge

angle

Major cutting

edge

Nose Radius

Side (Major) cutting edge angle - Mainly responsible for the chipremoval

End (Minor) Cutting edge angle - Mainly responsible for producingthe finished workpiece surface

Nose radius - Influences the surface finish of the workpiece

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Continuous ------------ Discontinuous Chips

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1. Compressive stress cause movement ofmetal.

2. Then, compression increases until plasticflow or rupture (or fracture) occurs.

Ductile material - continuous chip is formed

Brittle materials - rupture takes place withsmall discontinuous fractured chip

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Brittle material, small rake angle, largedepth of cut or feedrate and no cutting fluid

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Ductile materials, large rake angle, smalldepth of cut or feedrate and efficient use ofcutting fluid.

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2. Compare and contrast orthogonal and obliquecutting.

3. With simple sketches, show the tool geometry

of single-pointed cutters.4. List the three types of chip formation.

5. State the conditions for the formation ofcontinuous chip.

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Describe 3 cutting forces acting on the tool Discuss the relationship of cutting forces and the

following:

rake angle

depth of cut

feedrate

cutting speed, and

plan approach angle

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Tangential and Axial Forces

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Besides Tangential and Axial Forces, there isalso a Radial Force.

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Tangential Force (Ft) tangential to thework surface, main power-consuming.

Axial Force (Fa) parallel to the work axisand opposing the lengthwise feed direction.

Radial Force (Fr) present only in oblique

cutting and opposing the crosswise feeddirection.

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Use a Force Dynamometer - calibrate before use. Units - Newton

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A Test pieceFt

Rake angle ()

Dry

With coolant

Uniform thickness

Side Rake angle varies from 0 to 40 in steps of 5 Lower Tangential force when Rake angle is smaller.

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Ft Ft

Fa

Test piece with

stepped-wall

Depth of cut (mm)

Depth of Cut varies from 2 to 10 mm. Bigger Tangential and Axial forces

encountered when Depth of cut increases.

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Test piece with

uniformed wall thickness

Ft Fa

Fa

Ft

Feed rate (mm)

Feedrate varies from 0.1 to 1.0 mm/rev Axial force increases exponentially when Feedrate

increases.

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Ft

Breakage point of cutter

Cutting speeds (m/min)

Cutting Speed varies from 30 to 80 of allowablespeed value

No change in tangential force before tool break-up.

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Plan Approach Angle varies from 45 to 90. Axial force equals to Radial force at 45. Radial force equals to zero at 90 and

tangential force is at maximum.

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2. Describe the forces acting on the cutterduring orthogonal and oblique cutting.

3. Sketch the graph and explain test resultswhen the rake angle was varied.

4. Explain the effects of depth of cut andfeedrate on the following:

Chip thickness

Cutting force

5. Explain the effects of the cutting forceswhen plan approach angle is set at 45.

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Learning Objectives: Discuss the differences of cutting speed, spindle

speed and feedrate. Calculate the machining time, cutting power,

feed power and specific metal removal rate. Discuss the different types of cutting tool

materials. Describe the effects of positive and negative

back rake angle Explain the functions of cutting fluid.

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WORKPIECE

MATERIALS

FEED (mm/rev) CUTTING

SPEED (m/min)

Aluminium 0.2 1.0 70 100

Brass (Alpha,Ductile)

0.2 1.0 50 80

Brass (free-cutting) 0.2 1.5 70 100

Bronze (phosphor) 0.2 1.0 35 70

Cast iron (grey) 0.15 1.0 25 40

Copper 0.2 1.0 35 - 70

Steel (mild) 0.2 1.0 35 - 70

Steel (medium

carbon)0.15 0.7 30 - 35

Steel (alloy, high-tensile)

0.08 0.3 5 10

Thermo-settingplastic

0.2 1.0 35 - 50

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0

sec

1

min

1 rev = d

The rate at which a point in the circumference of the worktravels past the cutting tool (V=m/min).

Depends on work material, tool material, type of operation,machine condition, coolant type, type of cuts.

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The spindle speed is the number orrevolutions turned in one min.

N = 1000V (rev/min) d

The Spindle Speed, N depends on:

3. the cutting speed, V and4. the diameter, d of the workpiece.

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Feedrate is the distance the tool bit advances alongthe work for each revolution of the spindle(F=mm/rev)

Depends on work material, tool material, type ofcut, cutting speed.

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Calculate the spindle speed, to the nearest rev/min,for turning a 50 mm diameter bar at a cutting speedof 40 m/min.

1000 * V where, N = spindle speed

N = ---------------- V = 40 m/min D D = 50 mm1000 * 40

= ------------------ = 255 (rev / min)

3.142 * 50

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Calculate the time taken to turn a brass component 65mm diameter by 95 mm long, if the cutting speed is45 m/min and the feed is 0.6 mm/rev. Only one cut isto be taken.

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Use Ft

Work done = force * dist moved

= force * Circumference * N

= F * 2 r N= F * d NPower is the rate of doing work

= work done time taken

= F * d N Nm or watts 1000*60 sec

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D NWORKDONE = Ft (Newtons) x ---------- (Nm/min) or (Joules /

min)

1000

D N 1CUTTING POWER (watts) = Ft x ------------- x ------ (Nm / sec)

1000 60

Where, Ft = tangential force; = 3.142D = diameter of workpiece (mm);N = rotational speed in rpm

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Feed x NWORKDONE = Fa (Newtons) x --------- (Joules / min)

1000

Fa x Feed x NFEED POWER = ------------------------ (Joules / s or Watts)

1000 x 60

Where, Fa = Axial force;Feed = mm / rev;

N = spindle speed in rpm

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During turning on the lathe, the workpiece has adiameter of 75 mm & the spindle speed was set at 200rpm. If the feed rate was 0.33 mm / rev & the

dynamometer readings for the tangential force and theaxial force were 1,600 N & 900 N respectively, find thecutting & feed power.

3.142 x 75 mm x 200 rpm

Cutting power = 1600 N x ---------------------------------------1000 x 60

= 1,257 watts

900 N x 0.33 mm / rev x 200 rpmFeed power = ----------------------------------------------

1000 x 60

= 0.99 Watts

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V=25m/min

D=3mmF=1.5mm/rev

The volume of metal removed perminute is

= V * D * F

= 25000 * 3 * 1.5

= 112500 mm3/min = 112.5 cm3/min

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= Volume removed per min

Power consumed

= (mm/min)/watt

N.B. Power consumed is the sum of the

power at the cutting tool and including thepower to drive the mechanical systems.

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High Carbon steel HSS

Cemented carbide(contains 90% carbidepowder and 10% cobalt as binder) Cemented ceramic(contains 95% Al2O3)

Diamond

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Ability to: Retain hardness at high temperature

Resist Shock i.e. toughness Resist Wear Reasonably cheap

Acceptable mechanical properties

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Good abrasion resistance Slow rate of wear Hot Hardness properties

Vibration and chatter must be avoided Very brittle Unable to withstand shock and bending

loads

Used it as a throwaway tip or insert.

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Normal force, N,on unsupportedpart

Tendency to break

Lower cutting

forces

positive rake

Cutting tool

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Normal force, N, onsupported part

Less likely to break

Operate at higherspeed Higher compressive

forces

Nnegative rake

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Functions: Cooling and lubrication Better surface finish and accuracy Flushing away chips Longer Tool life Preventing chip welding, corrosion

Permits higher speed and feedrate

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1. Explain the difference between cuttingspeed and spindle speed.

2. What are the factors that determinecutting speed?

3. What is SMRR used for?

4. Explain the toughness of cutting toolmaterial.

5. Compare and contrast positive andnegative back rake angle.

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