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Metal Removal Processes. Dr. Ramon E. Goforth Adjunct Professor of Mechanical Engineering Southern Methodist University. Outline of Lecture. Basic information on material removal Factors involved in material removal Independent variables Dependent variables Machining Processes

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Metal removal processes

Metal Removal Processes

Dr. Ramon E. Goforth

Adjunct Professor of Mechanical Engineering

Southern Methodist University


Outline of lecture
Outline of Lecture

  • Basic information on material removal

  • Factors involved in material removal

  • Independent variables

  • Dependent variables

  • Machining Processes

  • Machining Economics

  • Machines

Lecture 10

Lecture 11

Lecture 12


Basic cutting processes
Basic Cutting Processes

  • Rotating part - turning

    • Creates round shapes

  • Stationary part - milling, drilling, sawing, etc


Basic turning
Basic Turning

  • Part of cylindrical cross section clamped in a "chuck" so that it can rotate about its axis

  • Part is rotated at fixed speed

  • A cutting tool is brought to bear on the moving surface of the part cutting of material

  • The "chuck" is a kind of vice which has rotational symmetry



Turning parameters
Turning Parameters

  • Tool Geometry

    • Rake angles

    • Side rake angle - more important than

    • Back rake angle

    • Cutting edge angles


Turning parameters1
Turning Parameters

  • Tool Geometry

  • Tool Materials

  • Feeds and speeds, N,d,f

    • (see table 22.4 for recommendations)

  • Cutting fluids

  • Material Removal rates

    • = p Davg d f N

      • Where Davg is the average diameter, d is the depth of cut, f is the feed rate and N the rotational speed

  • Forces and power used

  • Surface finish (scallops)


Power used
Power used

  • Power used is the material removal rate, MRR, times the specific energy


Feed marks in turning
Feed Marks in Turning

  • Scallops created

  • The depth depends on the feed rate, surface velocity and tool shape

Scallops


Machining processes for round shapes
Machining Processes for Round Shapes

  • Turning

  • Facing

  • Boring

    • Produces circular internal profiles in hollow workpieces

  • Drilling

    • Produces round holes

  • Reaming

    • Produces more accurate holes than drilling

  • Parting

  • Threading

  • Knurling



Turning guidelines
Turning Guidelines

  • Avoid long skinny parts

  • Request wide accuracy and surface finish parameters

  • Avoid sharp corners and tapers

  • Avoid major dimensional changes

  • Design blanks to be as close to final dimensions as possible


Turning guidelines1
Turning Guidelines

  • Allow for travel of tools across surfaces of workpiece

  • Design features so that standard tools can be used

  • Choose machinable materials

  • Minimize overhang of tool

  • Support workpiece

  • Use machines with high rigidity


Non round machining processes
Non Round Machining Processes

  • The operation

    • Clamp the workpiece onto a stationary bed or one that can move in multiple directions slowly

    • Bring a rotating tool to bear on the surface to be shaped

    • Move the rotating tool over the part or move the part past the rotating tool to shape it


Non round machining slab milling
Non Round Machining - Slab Milling

  • Milling

    • Slab/Peripheral

      • Cutter rotation axis parallel to workpiece surface

    • Conventional/up

      • Maximum chip thickness at end of cut

      • Low impact of tool with workpiece

    • Climb/down

      • Maximum chip thickness at beginning of cut

      • High low impact of tool with workpiece


Non round machining face milling
Non Round Machining - Face milling

  • Axis of rotation perpendicular to workpiece surface

  • Large multi-insert cutter


Non round machining face milling1
Non Round Machining - Face Milling

  • Difference between climb and conventional face milling

Action of an insert in face milling

Climb Milling

Conventional milling

Parameters in face milling



Generic milling formula
Generic Milling formula

  • Cutting (peripheral) speed,

    • V = p D N

    • where D is the cutter diameter and N its rotational speed

  • Feed per tooth,

    • f = v/Nn

    • where v is the linear speed or feed rate of the workpiece, and n is the number of teeth

  • Undeformed chip thickness, (chip depth of cut),

    • tc = 2 f (d / D)

    • Where f is the feed per tooth, d is the depth of cut


Generic milling formula1
Generic Milling formula

  • Cutting time, t = (l + 2lc)/ v

    • where v is the feed rate of the workpiece, l is the length of the workpiece and lc is the extent of the cutter’s first contact with the workpiece

  • Material removal rate, MRR

    • MRR = lwd/t = wdv

    • assuming the lc<<l and where w is the width of the cut

    • Power is equal to the MRR times the specific energy



Design guidelines for milling
Design Guidelines for Milling

  • Design for standard cutters

  • Use chamfers instead of radii

  • Avoid internal cavities and pockets with sharp corners

  • Design workpieces with sufficient rigidity


Other non round machining processes
Other Non Round Machining Processes

  • Drilling

  • Straddle milling

  • Planing

  • Broaching

  • Sawing

    • Generally used for cutting off pieces to be worked on by other processes

  • Filing and finishing

  • Gear machining


Drilling practices
Drilling Practices

  • Type of drill bit, drill point geometry

  • Type of machine

    • Drill, press, radial drills, gang drills, NC controlled

    • Capabilities of drilling and boring operations (p 633)

    • HP used = Spec. Energy times MRR (pD2fN/4)



Drilling guidelines
Drilling Guidelines

  • Design holes perpendicular to the surface

  • Do not design interrupted/overlapping holes

  • Design bottoms to match standard drill-point angles

  • Through holes are preferred over blind holes

  • If need large diameter holes design in smaller hole for casting

  • Design to minimize fixturing

  • Avoid reaming blind or intersecting holes


Machining economics
Machining Economics

  • Cost per piece decreases with cutting speed

  • Tool cost increases with cutting speed

  • Tool change time increases with cutting speed

  • Total cost goes through a minimum

  • Time spent removing material usually small fraction (<5%) of total time on machine

Kalpakjian p 775/698





Turning machine components
Turning Machine Components

  • Bed

    • Supports all other major components

    • Top part has two ways

  • Carriage

    • Slides along the ways

    • Consists of the cross-slide, tool post and apron


Turning machine components1
Turning Machine Components

  • Headstock

    • Fixed

    • Contains the motors, pulley and belts to drive the spindle

    • Spindle has fixtures for attaching the workpiece

  • Tailstock

    • Can slide along the ways

    • Supports the other end of the workpiece

  • Feed rod and lead screw

    • Provides motion to the carriage and cross slide



Turning machines
Turning Machines

  • Lathes

    • Tracer

    • Automatic

    • Automatic bar machines

    • Turret

    • Vertical

      • For very large diameters

    • Boring

      • Vertical

      • Horizontal (like a milling machine)

    • Computer controlled





Milling machines
Milling Machines

  • Column and Knee type

    • Horizontal spindle

    • Vertical spindle

  • Bed type

    • Skin mills

  • Other types

    • Planer type

    • Rotary tables

    • Duplicating machines

    • Profiling milling

    • More than three axes



Machining and turning centers
Machining and Turning Centers

  • Combines turning with milling

  • Computer control essential

  • Multiaxis capabilities

  • Replacing simple lathes or milling machines




Drilling machines
Drilling Machines production center

  • Drill presses

  • Radial machines

  • CNC Three axis drilling machine


Trends
Trends production center

  • High speed machining

  • Dry machining

  • Combining milling, drilling and turning operations

  • New, stiffer and highly damped machine tools

    • Graphite epoxy, ceramics (high modulus)

  • Modular machines

  • Multiple loading stations

  • More sensors

  • More and more automation

    • Automated program generation


Summary
Summary production center

  • There are many different types of machining operations

  • That is what makes it so versatile and attractive to industry

  • The basic cutting process is the same in all

  • Must consider the cutting operation as a system

  • Actual cutting time is a small fraction of the total time to create a part by machining


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