Lecture 6 - Fundamental machining and cutting tools

Question 1

Machining definition

Answer 1

process of removing unwanted material from worpiece in form of chips

Question 2

What variables make metal cutting a complex process

Answer 2

Selected machine tool
Selected cutting tool
Properties and parameters of workpiece
Cutting parameters - depth, feed, speed
Device holding workpiece ie. fixtures or jigs

Question 3

Speed

Answer 3

measured in metre/min or metre/sec or rev/min

Primary cutting motion that relates velocity of cutting tool relative to workpiece (solid arrows)

Question 4

Feed or feed rate

Answer 4

measured in mm/rev or inch/rev. Distance tool travels per unit revolution of workpiece (represented by dashed arrows)

Question 5

Depth of cut

Answer 5

measured in mm or inches

Question 6

Planing

Answer 6

produces flat surface and large machine components

Question 7

Broaching

Answer 7

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

Question 8

Reaming

Answer 8

enlarges holes providing better tolerance on diameter and improve surface finish

Question 9

Facing

Answer 9

produces flat surface at end of part (can be attached to other components)

Question 10

Chip formation

Answer 10

Tool has cutting velocity V tilted at relief angle to ease cutting operation
During cutting - shearing
Material under shear zone not deformed
Everything above shear zone converted to chips (shear angle = phi)
Chip moves up face of cutting tool
Chip thickness determined by depth of cut, rake angle, shear angle

Question 11

Cutting ratio

Answer 11

r = to/tc = sin(phi) / cos(phi - alpha)

Reciprocal of r = compression ratio (how thick chip has become compared to depth of cut

Question 12

Mechanics of chip formation

Answer 12

Large shear strains associated with small shear angles and small or -ve rake angles
Shear angle influences dchip thickness, force and power requirements and temp

Question 13

cutting velocity x depth of cut equation

Answer 13

cutting velocity x depth of cut = velocity of chip x chip thickness

Question 14

Forces

Answer 14

Cutting force acts in direction of cutting speed V supplies energy required for cutting
Thrust force acts in direction of normal to cutting velocity (perpendicular to workpiece)
Combining thrust and cutting force produces resultant force R

Question 15

Cutting forces and power

Answer 15

Knowledge of thrust force vital
Thrust force is too high or machine tool is not sufficiently stiff -> tool pushed away from surface -> depth of cut decreases

Question 16

Measuring forces

Answer 16

Forces measured by dynamometers and force transducers

Can also be computed from power consumption during cutting if efficiency is known

Question 17

Types of chip

Answer 17

Influences surface finish and overall cutting operations ie tool life, vibration and chatter

Continuous
Build-up edge
Serrated
Discontinuous

Question 18

Continuous chips

Answer 18

Formed with ductile materials at high cutting speeds and/or large rake angles
Deformation takes place along primary shear zone
Can develop secondary shear zone at tool-chip interface, caused by friction
Produces good surface finish, but, not always desirable
Entanglement of chip - solutions are chip breakers or changing cutting speed, feed, cutting fluids

Question 19

Build-up edge chips

Answer 19

Formed at tip of tool during cutting - layers of material from workpiece gradually deposited on tool
As chips become larger they become unstable and break up - some carried away by tool and rest are deposited randomly on workpiece. Process repeated during cutting operation
Generally undesirable but thin and stable BUE desirable as it protects tool surface and reduces wear
To minimise BUE: decrease depth of cut, increase rake angle and use sharper tool

Question 20

Serrated chips

Answer 20

Semi-continuous chips with zones of low and high shear strain
Saw tooth appearance
Occurs on low thermal conductivity workpieces and low strength that decreases sharply with temp (eg. titanium) NOTE Machining creates heat

Question 21

Discontinuous chips

Answer 21

Segments that may be firmly or loosely attached to each other
Forces continually vary during cutting

Question 22

Discontinuous chip formation conditions

Answer 22

Brittle workpiece materials - no high shear strains developed in cutting
Workpiece materials contain hard inclusions and impurities
Very low/Very high cutting speeds
High depth of cut and small rake angle
Lack of effective cutting fluid

Stiffness of cutting tool holder and fixture also important
If stiffness is insufficient, machine tool may vibrate and chatter and affect surface finish and dimensional accuracy

Question 23

What does rate of tool wear depend on?

Answer 23

Tool and workpiece material
Tool shape
Cutting fluids
Process parameters ie. speed, feed and depth of cut

Question 24

Flank wear

Answer 24

Occurs on relief face of tool

Rubbing of tool along machined surface, causes adhesive and/or abrasive wear

Question 25

Adhesive wear definition

Answer 25

When tangible force is applied and causes shearing force between 2 contacting forces

Question 26

Abrasive wear definition

Answer 26

Caused by hard and rough surface sliding across another surface

Question 27

Tool life equation

Answer 27

cutting speed (ft/min) * time (min) ^ n = constant cutting speed at T=1

Different n and C for each workpiece, tool material and cutting condition combination

Question 28

Crater wear

Answer 28

On rake face of tool, changes chip-tool interface geometry and therefore affects cutting process

Described as diffusion mechanism ie. movement of atoms across tool-chip interface

Question 29

What factors influence crater wear?

Answer 29

Temperature at tool-chip interface

Chemical affinity between tool and workpiece material

Question 30

Cutting tool characteristics

Answer 30

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