Fundamental Machining & Cutting Tools

Question 1

Define compression ratio

Answer 1

Reciprocal of cutting ratio, measures how thick the chip has become compared to the depth of cut

Question 2

What is the thrust force (F_t)?

Answer 2

Acts in the direction normal to the cutting velocity V i.e. perpendicular to the workpiece

Question 3

Fundamentals of machining - Define depth of cut

Answer 3

How deep the tool penetrates into the workpiece

mm, inch

Question 4

What is the formula for cutting ratio (r)?

Answer 4

r = t_o / t_c Reciprocal = compression ration
= sin(Ф) / cos(Ф-α)
t_o = Depth of cut, t_c = Chip thickness, Ф = Shear angle, α = Rake angle

Question 5

What are the solutions to continuous chips?

Answer 5

Use chip breakers

Change machining parameters e.g. cutting speed, feed, cutting fluids

Question 6

Describe build-up edge chips

Answer 6

Formed at the tip of a tool during cutting
Consists of layers of material from the workpiece that are gradually deposited on the tool (hence build-up)
As the chip becomes larger, the BUE becomes unstable and eventually breaks up (some chips are carried away from the tool and some are randomly deposited on the workpiece, continuous process)

Question 7

If the thrust force is too high or the machine tool is not sufficiently stiff, what will happen?

Answer 7

Tool will be pushed away from the surface

Reducing the depth of cut

Question 8

Describe the process of chip formation

Answer 8

The tool has a cutting velocity of V and is tilted at a relief angle to ease the cutting operation
During cutting, shearing takes place
Material underneath the shear zone do not deform
Everything above the shear zone is converted into chips

Question 9

What are large shear strains associated with?

Answer 9

Small shear angles

Small or negative rake angles

Question 10

What is the formula for shear strain (γ)?

Answer 10

γ = cot(Φ) + tan(Φ-α)

Question 11

What does the rate of tool wear depend on?

Answer 11

The tool and the workpiece material
Tool shape
Cutting fluids
Process parameters i.e. speed, feed, and depth of cut

Question 12

Fundamentals of machining - Define speed

Answer 12

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

Question 13

Define machining

Answer 13

A process of removing unwanted material from a workpiece in the form of chips

Question 14

Describe adhesive wear

Answer 14

Material (from the workpiece) adheres to the tool
After time, these small additions break off, taking parts of the tool with them
It’s wear, as the tool is being worn down, BUT, it is caused by adhesion

Question 15

What is the cutting force (F_c)?

Answer 15

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

Question 16

What do chips influence?

Answer 16

Surface finish produced

Overall cutting operations i.e. tool life, vibration, and chatter

Question 17

What characteristics to cutting tools need?

Answer 17

Hardness - especially at elevated temperatures so that the hardness and strength of the cutting tool are maintained
Toughness - so that impact forces on a tool in interrupted cutting operations do not fracture the tool
Wear resistance - so that an acceptable tool life is obtained before the tool is replaced
Chemical inertness - so that any adverse reactions between tool and workpiece that could contribute to tool wear are avoided

Question 18

Fundamentals of machining - Define feed/feed rate

Answer 18

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

Question 19

How can BUE chips be minimised?

Answer 19

Decrease depth of cut
Increase rake angle
Use a sharper tool

Question 20

Describe Crater Wear

Answer 20

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

Question 21

What is the main disadvantage of continuous chips?

Answer 21

For automated machine tools, the chips tangle around a tool holder, fixture, etc. and operations need to be halted to remove the chips

Question 22

What are the equations relating the resultant force of the cutting force and thrust force to friction and normal force?

Answer 22

Friction force F = Rsin(β)
Normal force N = Rcos(β)
R = Resultant force of F_c and F_t, β = angle between normal force and R

Question 23

For discontinuous chips, what will happen if the tool holder isn’t stiff?

Answer 23

Machine tool may vibrate and chatter and affect the surface finish as well as the dimensional accuracy of a workpiece
Due to the discontinuous nature of the chips, forces continually vary during cutting

Question 24

What are the 2 crucial factors that influence crater wear?

Answer 24

1. Temperature at tool-chip interface

2. Chemical affinity between tool & workpiece material

Question 25

Describe a discontinuous chip

Answer 25

Consists of segments that may be firmly or loosely attached to each other

Question 26

What are the 4 types of chips?

Answer 26

Continuous
Build-up edge
Serrated
Discontinuous

Question 27

Where does shearing take place?

Answer 27

Along the shear plane at a velocity V_s

The plane is at an angle θ, called shear angle, with the surface of the workpiece

Question 28

What is broaching used for?

Answer 28

To remove certain amount of materials and finish it off at the same time.
Roughing teeth and finishing teeth

Question 29

What is flank wear attributed to?

Answer 29

(1) Rubbing of a tool along a machined surface, hence, causing adhesive and/or abrasive wear.
Adhesive wear is incurred when a tangible force is applied and causes a shearing force between two contacted surfaces.
Abrasive wear is caused by a hard and rough surface that slides across another surface
(2) High temperatures (caused by friction) affecting tool-material properties and workpiece surface

Question 30

Where does deformation in cutting take place?

Answer 30

Generally within a very narrow deformation zone

Question 31

What are the formation conditions for discontinuous chips?

Answer 31

Brittle workpiece materials - do not have capacity to undergo high shear strains developed in cutting
Workpiece materials that contain hard inclusions and impurities
Very low or very high cutting speed
High depth of cut and small rake angle
Lack of effective cutting fluid
Low stiffness of a machine tool

Question 32

Describe abrasive wear

Answer 32

Material is worn away from the tool (it is abraded)

Question 33

What is the F.W.Taylor tool-life equation?

Answer 33

V x T^n = C
V = cutting speed (ft/min)
T = time (min) that takes to develop Flank Wear
n = an exponent that depends on tool and workpiece materials, and cutting conditions
C = a constant. It is the cutting speed at T = 1

Each combination of workpiece, tool material, and cutting condition gives different n and C

Question 34

What factors make the process of metal cutting complex?

Answer 34

The selected machine tool
The selected cutting tool
The properties and parameters of the workpiece
The cutting parameters e.g. speed, feed, depth of cut
The device holding the workpiece i.e. fixtures or jigs

Question 35

Where does flank wear occur

Answer 35

Occurs on the relief face of a tool

Question 36

What is reaming used for?

Answer 36

To enlarge a hole, to provide a better tolerance on its diameter, and to improve surface finish

Question 37

How can the forces in cutting operations be measured?

Answer 37

Dynamometers
Force transducers (e.g. piezoelectric crystals)
(Both mounted on the machine tool)
Or can be computed from power consumption during cutting (e.g. with power monitor) if the efficiency of the machine tool is known

Question 38

Why does crater wear increase as temperature increases?

Answer 38

Crater wear is described as a diffusion mechanism i.e. the movement of atoms across the tool-chip interface
Diffusion increases when temperature increases, hence, crater wear increases as temp increases

Question 39

What is the formula relating velocity, depth of cut, velocity of the chip and chip thickness?

Answer 39

V x t_o = V_c x t_c

Therefore V_c = V x r

Question 40

Are build-up edge chips desirable?

Answer 40

Usually undesirable

But thin and stable BUE is desirable because it protects the tool’s surface and reduces wear

Question 41

Describe serrated chips

Answer 41

Semi-continuous chips with zones of low and high shear strain
Chips have a saw-tooth-like appearance
Workpiece metals with low thermal conductivity and strength that decrease sharply with temperature (e.g. titanium) exhibit this behaviour

Question 42

What is planing used for?

Answer 42

To produce flat surfaces and large machining components

Question 43

What may continuous chips develop?

Answer 43

Secondary shear zone at tool-chip interface, caused y friction
Good surface finish, but, not always desirable

Question 44

What are the 2 basic regions of wear in a cutting tool?

Answer 44

Flank wear

Crater wear

Question 45

When are continuous chips formed and where does deformation take place?

Answer 45

Ductile materials at high cutting speeds and/or large rake angles
Deformation of material takes place along primary shear zone

Question 46

What does shear angle influence?

Answer 46

Has a great influence in the mechanics of cutting operations

Influences chip thickness, force and power requirements and temperature

Question 47

What is facing used for?

Answer 47

To produce a flat surface at the end of a part (which can then be attached to other components)