Machinability of Metals - PowerPoint PPT Presentation

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Machinability of Metals

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Machinability of Metals * * * * * * * Beryllium-bronze (copper and beryllium), containing up to about 2% beryllium, is easily formed in the annealed condition. – PowerPoint PPT presentation

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Title: Machinability of Metals


1
Machinability of Metals
2
Machinability
  • Ease or difficulty with which metal can be
    machined
  • Measured by length of cutting-tool life in
    minutes or by rate of stock removal in relation
    to cutting speed employed

3
Grain Structure
  • Machinability of metal affected by its
    microstructure
  • Ductility and shear strength modified greatly by
    operations such as annealing, normalizing and
    stress relieving
  • Certain chemical and physical modifications of
    steel improve machinability
  • Addition of sulfur, lead, or sodium sulfite
  • Cold working, which modifies ductility

4
Results of (Free-Machining) Modifications
  • Three main machining characteristics become
    evident
  • Tool life is increased
  • Better surface finish produced
  • Lower power consumption required for machining

5
Low-Carbon (Machine) Steel
  • Large areas of ferrite interspersed with small
    areas of pearlite
  • Ferrite soft, high ductility and low strength
  • Pearlite low ductility and high strength
  • Combination of ferrite and iron carbide
  • More desirable microstructure in steel is when
    pearlite well distributed instead of in layers

6
High-Carbon (Tool) Steel
  • Greater amount of pearlite because of higher
    carbon content
  • More difficult to machine steel efficiently
  • Desirable to anneal these steels to alter
    microstructures
  • Improves machining qualities

7
Alloy Steel
  • Combinations of two or more metals
  • Generally slightly more difficult to machine than
    low-or high-carbon steels
  • To improve machining qualities
  • Combinations of sulfur and lead or sulfur and
    manganese in proper proportions added
  • Combination of normalizing and annealing
  • Machining of stainless steel greatly eased by
    addition of selenium

8
Cast Iron
  • Consists generally of ferrite, iron carbide, and
    free carbon
  • Microstructure controlled by addition of alloys,
    method of casting, rate of cooling, and heat
    treating
  • White cast iron cooled rapidly after casting
  • hard and brittle (formation of hard iron carbide)
  • Gray cast iron cooled gradually
  • composed by compound pearlite, fine ferrite, iron
    carbide and flakes of graphite (softer)

9
Cast Iron
  • Machining slightly difficult due to iron carbide
    and presence of sand on outer surface of casting
  • Microstructure altered through annealing
  • Iron carbide broken down into graphitic carbon
    and ferrite
  • Easier to machine
  • Addition of silicon, sulfur and manganese gives
    cast iron different qualities

10
Aluminum
  • Pure aluminum generally more difficult to machine
    than aluminum alloys
  • Produces long stringy chips and harder on cutting
    tool
  • Aluminum alloys
  • Cut at high speeds, yield good surface finish
  • Hardened and tempered alloys easier to machine
  • Silicon in alloy makes it difficult to machine
  • Chips tear from work (poor surface)

11
Copper
  • Heavy, soft, reddish-colored metal refined from
    copper ore (copper sulfide)
  • High electrical and thermal conductivity
  • Good corrosion resistance and strength
  • Easily welded, brazed or soldered
  • Very ductile
  • Does not machine well long chips clog flutes of
    cutting tool
  • Coolant should be used to minimize heat

12
Copper/Beryllium
  • Heavy, hard, reddish-colored copper metal with
    Beryllium added
  • High electrical and thermal conductivity
  • Good corrosion resistance and strength
  • Can be welded
  • Somewhat ductile
  • Withstands high temperature
  • Machines well
  • Highly abrasive to HSS Tooling
  • Coolant should be used to lubricate and minimize
    tool wear

13
Copper-Based Alloys Brass
  • Alloy of copper and zinc with good corrosion
    resistance, easily formed, machines, and cast
  • Several forms of brass
  • Alpha brasses up to 36 zinc, suitable for cold
    working
  • Alpha 1 beta brasses Contain 54-62 copper and
    used in hot working
  • Small amounts of tin or antimony added to
    minimize pitting effect of salt water
  • Used for water and gas line fittings, tubings,
    tanks, radiator cores, and rivets

14
Copper-Based Alloys Bronze
  • Alloys of copper and tin which contain up to 12
    of principal alloying element
  • Exception copper-zinc alloys
  • Phosphor-bronze
  • 90 copper, 10 tin, and very small amount of
    phosphorus
  • High strength, toughness, corrosion resistance
  • Used for lock washers, cotter pins, springs and
    clutch discs

15
Copper-Based Alloys Bronze
  • Silicon-bronze (copper-silicon alloy)
  • Contains less than 5 silicon
  • Strongest of work-hardenable copper alloys
  • Mechanical properties of machine steel and
    corrosion resistance of copper
  • Used for tanks, pressure vessels, and hydraulic
    pressure lines

16
Copper-Based Alloys Bronze
  • Aluminum-bronze (copper-aluminum alloy)
  • Contains between 4 and 11 aluminum
  • Other elements added
  • Iron and nickel (both up to 5) increases
    strength
  • Silicon (up to 2) improves machinability
  • Manganese promotes soundness in casting
  • Good corrosion resistance and strength
  • Used for condenser tubes, pressure vessels, nuts
    and bolts

17
Effects of Temperature and Friction
  • Heat created
  • Plastic deformation occurring in metal during
    process of forming chip
  • Friction created by chips sliding along
    cutting-tool face
  • Cutting temperature varies with each metal and
    increases with cutting speed and rate of metal
    removal

18
Effects of Temperature and Friction
  • Greatest heat generated when ductile material of
    high tensile strength cut
  • Lowest heat generated when soft material of low
    tensile strength cut
  • Maximum temperature attained during cutting
    action
  • affects cutting-tool life, quality of surface
    finish, rate of production and accuracy of
    workpiece

19
High Heat
  • Temperature of metal immediately ahead of cutting
    tool comes close to melting temperature of metal
    being cut

20
Friction
  • Kept low as possible for efficient cutting action
  • Increasing coefficient of friction gives greater
    possibility of built-up edge forming
  • Larger built-up edge, more friction
  • Results in breakdown of cutting edge and poor
    surface finish
  • Can reduce friction at chip-tool interface and
    help maintain efficient cutting temperatures if
    use good supply of cutting fluid

21
Factors Affecting Surface Finish
  • Feed rate
  • Nose radius of tool
  • Cutting speed
  • Rigidity of machining operation
  • Temperature generated during machining process

22
Surface Finish
  • Direct relationship between temperature of
    workpiece and quality of surface finish
  • High temperature yields rough surface finish
  • Metal particles tend to adhere to cutting tool
    and form built-up edge
  • Cooling work material reduces temperature of
    cutting-tool edge
  • Result in better surface finish
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