Lecture 6 - Fundamental machining and cutting tools Flashcards
Machining definition
process of removing unwanted material from worpiece in form of chips
What variables make metal cutting a complex process
Selected machine tool Selected cutting tool Properties and parameters of workpiece Cutting parameters - depth, feed, speed Device holding workpiece ie. fixtures or jigs
Speed
measured in metre/min or metre/sec or rev/min
Primary cutting motion that relates velocity of cutting tool relative to workpiece (solid arrows)
Feed or feed rate
measured in mm/rev or inch/rev. Distance tool travels per unit revolution of workpiece (represented by dashed arrows)
Depth of cut
measured in mm or inches
Planing
produces flat surface and large machine components
Broaching
used to remove certain amount of materials and finish it off at same time. Roughing teeth and finishing teeth
Reaming
enlarges holes providing better tolerance on diameter and improve surface finish
Facing
produces flat surface at end of part (can be attached to other components)
Chip formation
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
Cutting ratio
r = to/tc = sin(phi) / cos(phi - alpha)
Reciprocal of r = compression ratio (how thick chip has become compared to depth of cut
Mechanics of chip formation
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
cutting velocity x depth of cut equation
cutting velocity x depth of cut = velocity of chip x chip thickness
Forces
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
Cutting forces and power
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
Measuring forces
Forces measured by dynamometers and force transducers
Can also be computed from power consumption during cutting if efficiency is known
Types of chip
Influences surface finish and overall cutting operations ie tool life, vibration and chatter
Continuous
Build-up edge
Serrated
Discontinuous
Continuous chips
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
Build-up edge chips
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
Serrated chips
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
Discontinuous chips
Segments that may be firmly or loosely attached to each other
Forces continually vary during cutting
Discontinuous chip formation conditions
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
What does rate of tool wear depend on?
Tool and workpiece material
Tool shape
Cutting fluids
Process parameters ie. speed, feed and depth of cut
Flank wear
Occurs on relief face of tool
Rubbing of tool along machined surface, causes adhesive and/or abrasive wear
Adhesive wear definition
When tangible force is applied and causes shearing force between 2 contacting forces
Abrasive wear definition
Caused by hard and rough surface sliding across another surface
Tool life equation
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
Crater wear
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
What factors influence crater wear?
Temperature at tool-chip interface
Chemical affinity between tool and workpiece material
Cutting tool characteristics
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
