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MEC130 Other > Lec6 > Flashcards

Lec6 Flashcards

1
Q

4 types of chips

A

continuous
built up edge
serrated
discontinous

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2
Q

Types of wear

A

Flank: adhesive

Abrasive; Crater

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3
Q

What is machining

A

Machining is a process of removing unwanted

material from a workpiece in the form of chips

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4
Q

Basic terms: speed

A

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.

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5
Q

Feed or feed rate

A

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

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6
Q

Depth of cut

A

mm or in pretty self explanatory

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7
Q

Diagram of two dimensional cutting process

A 
chip
rake angle
tool face
tool
shear plane
shear angle
workpiece
flank
relief or clearance angle
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8
Q

Purpose of relief angle

A

ease cutting operation

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9
Q

What happens during cutting

A

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

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10
Q

Where does shearing take place

A

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

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11
Q

What happens above the shear plane

A

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

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12
Q

What can chip thickness (tc)be determined from

A

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

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13
Q

What velocity does shearing take place at

A

Vc

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14
Q

What is the cutting ratio

A 
r = to/ tc = sin(theta) / cos (theta - alpha)
theta = shear angle
alpha = rake angle
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15
Q

What is the compression ratio

A

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

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16
Q

What is shear strain

A

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

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17
Q

Effect of shear angle

A

influences chip thickness, force and power requirements and temperature

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18
Q

Large shear strains are due to?

A

small shear angles and small or negative rake angles

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19
Q

equation relating cutting velocity and chip velocity

A

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

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20
Q

What is the cuttting force

A

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

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21
Q

What is the thrust forcce

A

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

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22
Q

Importance of thrust force and the balance between machine tool

A

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

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23
Q

What are forces measured

A

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

24
Q

Importance of chips

A

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

25
Q

how are continuous chips formed

A

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

26
Q

Adv and disadv of continuous chips

A

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

27
Q

Solutions to continuous chips

A

chip breakers, change machine parameters cutting speed feed cutting fluids

28
Q

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

A

zone at tool chip interface caused by friction

29
Q

Where are build up edge chips (BUE) formed

A

tip of tool during cutting

30
Q

What do build up edge chips (BUE) consist of

A

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

31
Q

What happens as BUS becomes larger

A

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

32
Q

How to minimise formation of BUE

A

decrease depth of cut
increase rake angle
use a sharper tool

33
Q

BUE desirable or undesirable

A

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

34
Q

What are serrated chips

A

semi continuous chis with zones of low and high shear strain

saw tooth like appearance

35
Q

when do serrated chips appear

A

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

36
Q

What do discontinuous chips consist of

A

segments that may be firmly or loosely attached to each other

37
Q

conditions for discontinuous chip formation

A

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

38
Q

Discontinuous chips tool holder and fixture stiffness

A

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

39
Q

What does tool wear depend on

A

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

40
Q

Two basic regions of wear in cutting tool

A

flank wear

crater weak

41
Q

Flank weak occurs?

A

Relief face of a tool

42
Q

Tool wear due to

A

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

43
Q

Types of wear

A

Adhesive wear

abrasive wear

44
Q

Adhesive wear occurs when?

A

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

45
Q

abrasive wear

A

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

46
Q

Adhesive wear

A

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

47
Q

Taylor tool life equation

A

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

48
Q

Where does crater wear occur

A

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

49
Q

Two factors that affect crater wear

A

temperature at tool chip interface

chemical affinity between tool and workpiece material

50
Q

Type of mechanism crater wear

A

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

51
Q

Characteristics of cutting tool

A

Hardness
toughness
wear resistance
chemical inertness

52
Q

Hardness in cutting rool

A

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

53
Q

toughness in cutting tool

A

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

54
Q

Wear resistance

A

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

55
Q

Chemical inertness

A

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

MEC130 Other (13 decks)

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