Lec6

Question 1

4 types of chips

Answer 1

continuous
built up edge
serrated
discontinous

Question 2

Types of wear

Answer 2

Flank: adhesive

Abrasive; Crater

Question 3

What is machining

Answer 3

Machining is a process of removing unwanted

material from a workpiece in the form of chips

Question 4

Basic terms: speed

Answer 4

The primary cutting motion that relates velocity of a cutting tool relative to a workpiece (represented as solid arrows)
metre/min. or metre/sec. or rev./min.

Question 5

Feed or feed rate

Answer 5

The distance a tool travels per unit revolution of a workpiece (represented as dashed arrows) mm/rev or inch/rev

Question 6

Depth of cut

Answer 6

mm or in pretty self explanatory

Question 7

Diagram of two dimensional cutting process

Answer 7

chip
rake angle
tool face
tool
shear plane
shear angle
workpiece
flank
relief or clearance angle

Question 8

Purpose of relief angle

Answer 8

ease cutting operation

Question 9

What happens during cutting

Answer 9

shearing takes place
material underneath the shear zone not removed
everything above the shear zone converted into chips

Question 10

Where does shearing take place

Answer 10

Along the shear plane which is at angle theta called the shear angle with the workpiece

Question 11

What happens above the shear plane

Answer 11

Chip is already formed and moving up the face of the tool as cutting progresses

Question 12

What can chip thickness (tc)be determined from

Answer 12

by knowing depth of cut (to) rake angle (alpha) and shear angle (theta)

Question 13

What velocity does shearing take place at

Answer 13

Vc

Question 14

What is the cutting ratio

Answer 14

r = to/ tc = sin(theta) / cos (theta - alpha)
theta = shear angle
alpha = rake angle

Question 15

What is the compression ratio

Answer 15

reciprocal of r, measures how thick the chip has become compared to the depth of cut

Question 16

What is shear strain

Answer 16

shear strain material undergoes = cos (theta) + tan (theta - alpha)
theta = shear angle
alpha = rake angle

Question 17

Effect of shear angle

Answer 17

influences chip thickness, force and power requirements and temperature

Question 18

Large shear strains are due to?

Answer 18

small shear angles and small or negative rake angles

Question 19

equation relating cutting velocity and chip velocity

Answer 19

V (cutting velocity) * depth of cut (to) = Vc (chip velocity) * tc (chip thickness)
or Vc = V * r
Vc = V * sin(theta)/cos(theta - alpha)

Question 20

What is the cuttting force

Answer 20

Acts in the direction of the cutting speed V and supplies energy req for cutting

Question 21

What is the thrust forcce

Answer 21

acts in the direction normal to the cutting velocity (perpendicular to the workpiece)

Question 22

Importance of thrust force and the balance between machine tool

Answer 22

too high tool will be pushed away from the surface - reduced depth of cut
machine tool, tool holder work holding devices must be sufficiently stiff to minimise deflections caused by force or same thing will happen

Question 23

What are forces measured

Answer 23

Using dynamometeres or force transducers mounted on the machine tool or
forces can be computer from power consumption during cutting if efficiency of the machine tool is known

Question 24

Importance of chips

Answer 24

Influence the surface finish produced and overall cutting operations (tool life, vibrations and chatter)

Question 25

how are continuous chips formed

Answer 25

formed with ductile materials at high cutting speeds and or large rake angles deformation takes place along primary shear zone

Question 26

Adv and disadv of continuous chips

Answer 26

Produce good surface finish but not always desirable for automated machines operations need to be stopped to remove chips

Question 27

Solutions to continuous chips

Answer 27

chip breakers, change machine parameters cutting speed feed cutting fluids

Question 28

If continous chips produce secondary shear plane where would this be and due to what

Answer 28

zone at tool chip interface caused by friction

Question 29

Where are build up edge chips (BUE) formed

Answer 29

tip of tool during cutting

Question 30

What do build up edge chips (BUE) consist of

Answer 30

Layers of material from the workpiece that are gradually deposited on the tool

Question 31

What happens as BUS becomes larger

Answer 31

Becomes unstable and eventually breaks off carried away by the rool and rest is randomly deposited on the workpiece formation and destruction repeated continuously during cutting operation

Question 32

How to minimise formation of BUE

Answer 32

decrease depth of cut increase rake angle use a sharper tool

Question 33

BUE desirable or undesirable

Answer 33

Generally undesirable but thin and stable BUE protects the tools surface and reduces wear

Question 34

What are serrated chips

Answer 34

semi continuous chis with zones of low and high shear strain | saw tooth like appearance

Question 35

when do serrated chips appear

Answer 35

workpiececs with low thermal conductivity and strength that decreases sharply with temperature ie titanium

Question 36

What do discontinuous chips consist of

Answer 36

segments that may be firmly or loosely attached to each other

Question 37

conditions for discontinuous chip formation

Answer 37

Brittle workpiece - cannot undergo high shear strain workpiece material contains hard inclusion or impurities very low or very high cutting speed high depth of cut and small rake angle lakc of effective cutting fluid low stiffness of machine tool

Question 38

Discontinuous chips tool holder and fixture stiffness

Answer 38

vital if insuffecient machine tool will vibrate and chatter - affect the SA and dimensional accuracy forces vary during operation

Question 39

What does tool wear depend on

Answer 39

tool and workpiece material tool shape cutting fluids process parameters ie speed feed and depth of cut

Question 40

Two basic regions of wear in cutting tool

Answer 40

flank wear | crater weak

Question 41

Flank weak occurs?

Answer 41

Relief face of a tool

Question 42

Tool wear due to

Answer 42

rubbing of a tool along a machined surface causing adhesive and or abrasive wear

Question 43

Types of wear

Answer 43

Adhesive wear | abrasive wear

Question 44

Adhesive wear occurs when?

Answer 44

incurred when a tangible force is applied and causes a shearing force between two contacted surfaces

Question 45

abrasive wear

Answer 45

is caused by a hard and rough surface that slides across another surface tool is worn down by hard work-piece materials

Question 46

Adhesive wear

Answer 46

material from the work-piece bonds to the tool in the form of micro-welds, this bonded micro-weld then fractures taking part of the tool with it

Question 47

Taylor tool life equation

Answer 47

VT^n = C V = cutting speed T = time that takes to develop flank wear n = exponent that depens on tool workpiece material and cutting conditions C= a constant cutting speed at T = 1 n and c must be determined empirically

Question 48

Where does crater wear occur

Answer 48

occurs on the rake face of a tool, changes the chip tool interface geometry thus affecting cutting process

Question 49

Two factors that affect crater wear

Answer 49

temperature at tool chip interface | chemical affinity between tool and workpiece material

Question 50

Type of mechanism crater wear

Answer 50

described as diffusion mechanism movement of atoms across tool chip interface (as temp increases diffusion increases crater wear increases)

Question 51

Characteristics of cutting tool

Answer 51

Hardness toughness wear resistance chemical inertness

Question 52

Hardness in cutting rool

Answer 52

esp at elevated temp so hardness and strength of tool maintained in process

Question 53

toughness in cutting tool

Answer 53

impact forces on a tool in interrupted cutting operations do not fracture the tool

Question 54

Wear resistance

Answer 54

so that an acceptable tool life is obtained before the tool is replaced

Question 55

Chemical inertness

Answer 55

So that adverse reactions between tool and workpiece that could contribute to tool wear are avoided