Manufacturing Engineering Technology in SI Units, 6 th Edition PART III: Forming and Shaping...
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Transcript of Manufacturing Engineering Technology in SI Units, 6 th Edition PART III: Forming and Shaping...
Manufacturing Engineering Technology in SI Units, Manufacturing Engineering Technology in SI Units,
66thth Edition Edition PART III:PART III:
Forming and Shaping Processes and Forming and Shaping Processes and EquipmentEquipment
Copyright © 2010 Pearson Education South Asia Pte Ltd
PART III: Forming and Shaping Processes PART III: Forming and Shaping Processes and Equipmentand Equipment
“Forming” indicates changing the shape of an existing solid body
Copyright © 2010 Pearson Education South Asia Pte Ltd
PART III: Forming and Shaping Processes PART III: Forming and Shaping Processes and Equipmentand Equipment
For forming processes, the starting material may be in the shape of a plate, sheet, bar, rod, wire, or tubing of various cross sections
Shaping processes involve the molding and casting of molten materials and the finished product is near the final desired shape
Molten metalis cast into individual ingots or continuously cast into slabs, rods, or pipes
Cast structures are converted to wrought structures by plastic-deformation processes
Copyright © 2010 Pearson Education South Asia Pte Ltd
PART III: Forming and Shaping Processes PART III: Forming and Shaping Processes and Equipmentand Equipment
Copyright © 2010 Pearson Education South Asia Pte Ltd
Manufacturing Engineering Technology in SI Units, Manufacturing Engineering Technology in SI Units,
66thth Edition Edition Chapter 13: Metal-Rolling Processes and Chapter 13: Metal-Rolling Processes and
EquipmentEquipment
Copyright © 2010 Pearson Education South Asia Pte Ltd Copyright © 2010 Pearson Education South Asia Pte Ltd
Chapter Outline
Introduction The Flat-rolling Process Flat-rolling Practice Rolling Mills Various Rolling Processes and Mills
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Introduction
Rolling is the process of reducing the cross section of a long workpiece by compressive forces applied through a set of rolls
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The Flat-rolling Process
Flat-rolling process is shown Friction forces act on strip surfaces Roll force, F, and torque, T, acts on the rolls
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The Flat-rolling Process
As the surface speed of the rigid roll is constant, there is relative sliding between the roll and the strip along the arc of contact in the roll gap, L
At neutral point or no-slip point, the velocity of the strip is the same as that of the roll
The maximum possible draft is defined as the difference between the initial and final strip thicknesses
From the relationship, higher the friction and the larger the roll radius, the greater the maximum possible draft becomes
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Rhh fo2
The Flat-rolling Process:Roll Force, Torque, and Power Requirements Rolls apply pressure on the flat strip to reduce its
thickness, resulting in a roll force, F Roll force in flat rolling can be estimated from
Total power (for two rolls) is
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avgLwYF L = roll-strip contact length w = width of the stripYavg = average true stress of the strip
000,33
2hp)(in Power
000,60
2kW)(in Power
FLN
FLN
The Flat-rolling Process:Roll Force, Torque, and Power Requirements
EXAMPLE 13.1
Calculation of Roll Force and Torque in Flat-rolling
An annealed copper strip 228 mm wide and 25 mm thick is rolled to a thickness of 20 mm in one pass. The roll radius is 300 mm, and the rolls rotate at 100 rpm. Calculate the roll force and the power required in this operation.
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The Flat-rolling Process:Roll Force, Torque, and Power Requirements
Solution
Roll-strip contact length is calculated through geometry,
Absolute true strain of the strip is
Average true stress is
The roll force is
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223.020
25ln
mm 7.3820253000 fo hhRL
MN 4.171801000250
10007.38 avgLwYF
MPa 1802/28080
The Flat-rolling Process:Roll Force, Torque, and Power Requirements
Solution
With 100 rpm, the total power is calculated from
Force and the power requirements is difficult to obtained due to (a) the exact geometry between the roll and the strip and (b) coefficient of friction and the strength of the material in the roll gap
Copyright © 2010 Pearson Education South Asia Pte Ltd
W705000,66
100
1000
7.381074.12
000,66
2 6 FLNPower
The Flat-rolling Process:Roll Force, Torque, and Power Requirements
Reducing Roll Force Roll forces can cause deflection and flattening of the
rolls The columns of the roll stand may deflect under high
roll forces Roll forces can be reduced by:
1. Reducing friction at the roll–workpiece interface
2. Using smaller diameter rolls
3. Reduce the contact area
4. Rolling at elevated temperatures
5. Applying front and/or back tensions to the strip Copyright © 2010 Pearson Education South Asia Pte Ltd
The Flat-rolling Process:Roll Force, Torque, and Power Requirements
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The Flat-rolling Process:Geometric Considerations
Roll forces will bend the rolls elastically during rolling When the roll bends, the strip has a constant thickness
along its width The heat generated by plastic deformation cause the
rolls to be slightly barrel shaped (thermal camber) Roll forces also tend to flatten the rolls elastically
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The Flat-rolling Process:Geometric Considerations
Spreading Increase in width is called spreading Spreading increases with:
1. Decreasing width-to-thickness ratio of the entering strip
2. Increasing friction
3. Decreasing ratio of the roll radius to the strip thickness
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The Flat-rolling Process:Vibration and Chatter
Vibration and chatter have effects on product quality and the productivity of metalworking operations
Chatter defined as self-excited vibration Occur in rolling and in extrusion, drawing, machining,
and grinding operations Chatter results from interactions between the structural
dynamics of the mill stand and the dynamics of the rolling operation
Chatter can be reduced by increasing the roll radius, strip-roll friction and incorporating dampers in the roll supports
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Flat-rolling Practice
Initial rolling steps (breaking down) of the material is done by hot rolling
A cast structure is dendritic and is brittle and porous Hot rolling converts the cast structure to a wrought
structure with finer grains and enhanced ductility Product of the first hot-rolling operation is called a
bloom, a slab, or a billet To improve flatness, the rolled strip goes through a
series of leveling rolls
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Flat-rolling Practice:Defects in Rolled Plates and Sheets
Defects may be present on the surfaces or there may be internal structural defects
They are undesirable as they compromise surface appearance and adversely affect strength, formability, and other manufacturing characteristics
Surface defects may be caused by inclusions and impurities in the original cast material
Wavy edges on sheets are the result of roll bending
Cracks are due to poor material ductility at the rolling temperature
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Flat-rolling Practice:Other Characteristics of Rolled Metals
Residual Stresses Residual stresses develop in rolled plates and sheets
due to nonuniform deformation of materials in roll gap
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Flat-rolling Practice:Other Characteristics of Rolled Metals
Dimensional Tolerances Thickness tolerances for cold-rolled sheets range from
±0.1~0.35 mm Flatness tolerances are within ±15 mm/m for cold
rolling and ±55 mm/m for hot rolling
Surface Roughness Cold rolling can produce a very fine surface finish Cold-rolled sheets products may not require additional
finishing operations
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Rolling Mills
Automated mills produce close-tolerance, low cost and high quality plates and sheets at high production rates
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Rolling Mills
Two-high rolling mills are used for hot rolling in initial breakdown passes (cogging mills) on cast ingots or in continuous casting
In tandem rolling, the strip is rolled continuously through a number of stands to thinner gages with each pass
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Rolling Mills
Roll Materials Basic requirements for roll materials are strength and
resistance to wear Forged-steel rolls have higher strength, stiffness, and
toughness than cast-iron rolls Rolls made for cold rolling should not be used for hot
rolling as they may crack from thermal cycling (and spalling
Lubricants Hot rolling of ferrous alloys do not need lubricants Water-based solutions are used to cool the rolls
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Various Rolling Processes and Mills
Shape Rolling Straight and long structural shapes are formed at
elevated temperatures by shape rolling
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Various Rolling Processes and Mills
Roll Forging Cross section of a round bar is shaped by passing it
through a pair of rolls with profiled grooves
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Various Rolling Processes and Mills
Skew Rolling Similar to roll forging and used for making ball bearings Another method is to shear pieces from a round bar
and then upset them in headers between two dies with hemispherical cavities
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Various Rolling Processes and Mills
Ring Rolling A thick ring is expanded into a large-diameter thinner
one Thickness is reduced by bringing the rolls closer
together as they rotate Short production times, material savings and close
dimensional tolerances
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Various Rolling Processes and Mills
Thread Rolling Thread rolling is a cold-forming process by which
straight or tapered threads are formed on round rods or wire
Threads are formed with rotary dies at high production rates
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Various Rolling Processes and Mills
Thread Rolling Thread-rolling process has the advantages of
generating threads with good strength without any loss of material
Internal thread rolling can be carried out with a fluteless forming tap, produces accurate internal threads with good strength
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Various Rolling Processes and Mills
Rotary Tube Piercing Also known as the Mannesmann process It is a hot-working operation for making long, thick-
walled seamless pipe and tubing The round bar is subjected to radial compressive
forces while tensile stresses develop at the center of the bar
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Various Rolling Processes and Mills
Tube Rolling Diameter and thickness of pipes and tubing can be
reduced by tube rolling, which utilizes shaped rolls
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Various Rolling Processes and Mills:Various MillsIntegrated Mills Integration process starts from production of hot metal
to the casting and rolling of finished products to shipping
Minimills Scrap metal is:
1. Melted in electric-arc furnaces
2. Cast continuously
3. Rolled directly into specific lines of products
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