Fundamental Machining & Cutting Tools Flashcards
Define compression ratio
Reciprocal of cutting ratio, measures how thick the chip has become compared to the depth of cut
What is the thrust force (F_t)?
Acts in the direction normal to the cutting velocity V i.e. perpendicular to the workpiece
Fundamentals of machining - Define depth of cut
How deep the tool penetrates into the workpiece
mm, inch
What is the formula for cutting ratio (r)?
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
What are the solutions to continuous chips?
Use chip breakers
Change machining parameters e.g. cutting speed, feed, cutting fluids
Describe build-up edge chips
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)
If the thrust force is too high or the machine tool is not sufficiently stiff, what will happen?
Tool will be pushed away from the surface
Reducing the depth of cut
Describe the process of chip formation
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
What are large shear strains associated with?
Small shear angles
Small or negative rake angles
What is the formula for shear strain (γ)?
γ = cot(Φ) + tan(Φ-α)
What does the rate of tool wear depend on?
The tool and the workpiece material
Tool shape
Cutting fluids
Process parameters i.e. speed, feed, and depth of cut
Fundamentals of machining - Define speed
The primary cutting motion that relates velocity of a cutting tool to a workpiece (represented as solid arrows)
Metre/min, metre/sec, rev/min
Define machining
A process of removing unwanted material from a workpiece in the form of chips
Describe adhesive wear
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
What is the cutting force (F_c)?
Acts in the direction of the cutting speed V and supplies the energy required for cutting
What do chips influence?
Surface finish produced
Overall cutting operations i.e. tool life, vibration, and chatter
What characteristics to cutting tools need?
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
Fundamentals of machining - Define feed/feed rate
The distance a tool travels per unit revolution of a workpiece (represented as dashed arrows)
mm/rev, inch/rev
How can BUE chips be minimised?
Decrease depth of cut
Increase rake angle
Use a sharper tool
Describe Crater Wear
Occurs on the rake face of a tool, changes the chip-tool interface geometry, thus, affecting the cutting process
What is the main disadvantage of continuous chips?
For automated machine tools, the chips tangle around a tool holder, fixture, etc. and operations need to be halted to remove the chips
What are the equations relating the resultant force of the cutting force and thrust force to friction and normal force?
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
For discontinuous chips, what will happen if the tool holder isn’t stiff?
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
What are the 2 crucial factors that influence crater wear?
- Temperature at tool-chip interface
2. Chemical affinity between tool & workpiece material
Describe a discontinuous chip
Consists of segments that may be firmly or loosely attached to each other
What are the 4 types of chips?
Continuous
Build-up edge
Serrated
Discontinuous
Where does shearing take place?
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
What is broaching used for?
To remove certain amount of materials and finish it off at the same time.
Roughing teeth and finishing teeth
What is flank wear attributed to?
(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
Where does deformation in cutting take place?
Generally within a very narrow deformation zone
What are the formation conditions for discontinuous chips?
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
Describe abrasive wear
Material is worn away from the tool (it is abraded)
What is the F.W.Taylor tool-life equation?
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
What factors make the process of metal cutting complex?
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
Where does flank wear occur
Occurs on the relief face of a tool
What is reaming used for?
To enlarge a hole, to provide a better tolerance on its diameter, and to improve surface finish
How can the forces in cutting operations be measured?
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
Why does crater wear increase as temperature increases?
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
What is the formula relating velocity, depth of cut, velocity of the chip and chip thickness?
V x t_o = V_c x t_c
Therefore V_c = V x r
Are build-up edge chips desirable?
Usually undesirable
But thin and stable BUE is desirable because it protects the tool’s surface and reduces wear
Describe serrated chips
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
What is planing used for?
To produce flat surfaces and large machining components
What may continuous chips develop?
Secondary shear zone at tool-chip interface, caused y friction
Good surface finish, but, not always desirable
What are the 2 basic regions of wear in a cutting tool?
Flank wear
Crater wear
When are continuous chips formed and where does deformation take place?
Ductile materials at high cutting speeds and/or large rake angles
Deformation of material takes place along primary shear zone
What does shear angle influence?
Has a great influence in the mechanics of cutting operations
Influences chip thickness, force and power requirements and temperature
What is facing used for?
To produce a flat surface at the end of a part (which can then be attached to other components)
