Lec6 Flashcards
4 types of chips
continuous
built up edge
serrated
discontinous
Types of wear
Flank: adhesive
Abrasive; Crater
What is machining
Machining is a process of removing unwanted
material from a workpiece in the form of chips
Basic terms: speed
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.
Feed or feed rate
The distance a tool travels per unit revolution of a workpiece (represented as dashed arrows) mm/rev or inch/rev
Depth of cut
mm or in pretty self explanatory
Diagram of two dimensional cutting process
chip rake angle tool face tool shear plane shear angle workpiece flank relief or clearance angle
Purpose of relief angle
ease cutting operation
What happens during cutting
shearing takes place
material underneath the shear zone not removed
everything above the shear zone converted into chips
Where does shearing take place
Along the shear plane which is at angle theta called the shear angle with the workpiece
What happens above the shear plane
Chip is already formed and moving up the face of the tool as cutting progresses
What can chip thickness (tc)be determined from
by knowing depth of cut (to) rake angle (alpha) and shear angle (theta)
What velocity does shearing take place at
Vc
What is the cutting ratio
r = to/ tc = sin(theta) / cos (theta - alpha) theta = shear angle alpha = rake angle
What is the compression ratio
reciprocal of r, measures how thick the chip has become compared to the depth of cut
What is shear strain
shear strain material undergoes = cos (theta) + tan (theta - alpha)
theta = shear angle
alpha = rake angle
Effect of shear angle
influences chip thickness, force and power requirements and temperature
Large shear strains are due to?
small shear angles and small or negative rake angles
equation relating cutting velocity and chip velocity
V (cutting velocity) * depth of cut (to) = Vc (chip velocity) * tc (chip thickness)
or Vc = V * r
Vc = V * sin(theta)/cos(theta - alpha)
What is the cuttting force
Acts in the direction of the cutting speed V and supplies energy req for cutting
What is the thrust forcce
acts in the direction normal to the cutting velocity (perpendicular to the workpiece)
Importance of thrust force and the balance between machine tool
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
What are forces measured
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
Importance of chips
Influence the surface finish produced and overall cutting operations (tool life, vibrations and chatter)
how are continuous chips formed
formed with ductile materials at high cutting speeds and or large rake angles
deformation takes place along primary shear zone
Adv and disadv of continuous chips
Produce good surface finish but not always desirable for automated machines operations need to be stopped to remove chips
Solutions to continuous chips
chip breakers, change machine parameters cutting speed feed cutting fluids
If continous chips produce secondary shear plane where would this be and due to what
zone at tool chip interface caused by friction
Where are build up edge chips (BUE) formed
tip of tool during cutting
What do build up edge chips (BUE) consist of
Layers of material from the workpiece that are gradually deposited on the tool
What happens as BUS becomes larger
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
How to minimise formation of BUE
decrease depth of cut
increase rake angle
use a sharper tool
BUE desirable or undesirable
Generally undesirable but thin and stable BUE protects the tools surface and reduces wear
What are serrated chips
semi continuous chis with zones of low and high shear strain
saw tooth like appearance
when do serrated chips appear
workpiececs with low thermal conductivity and strength that decreases sharply with temperature ie titanium
What do discontinuous chips consist of
segments that may be firmly or loosely attached to each other
conditions for discontinuous chip formation
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
Discontinuous chips tool holder and fixture stiffness
vital if insuffecient machine tool will vibrate and chatter - affect the SA and dimensional accuracy
forces vary during operation
What does tool wear depend on
tool and workpiece material
tool shape
cutting fluids
process parameters ie speed feed and depth of cut
Two basic regions of wear in cutting tool
flank wear
crater weak
Flank weak occurs?
Relief face of a tool
Tool wear due to
rubbing of a tool along a machined surface causing adhesive and or abrasive wear
Types of wear
Adhesive wear
abrasive wear
Adhesive wear occurs when?
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 tool is worn down by hard
work-piece materials
Adhesive wear
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
Taylor tool life equation
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
Where does crater wear occur
occurs on the rake face of a tool, changes the chip tool interface geometry thus affecting cutting process
Two factors that affect crater wear
temperature at tool chip interface
chemical affinity between tool and workpiece material
Type of mechanism crater wear
described as diffusion mechanism movement of atoms across tool chip interface (as temp increases diffusion increases crater wear increases)
Characteristics of cutting tool
Hardness
toughness
wear resistance
chemical inertness
Hardness in cutting rool
esp at elevated temp so hardness and strength of tool maintained in process
toughness in cutting tool
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 adverse reactions between tool and workpiece that could contribute to tool wear are avoided
