Manufacturing Engineering Technology in SI Units, 6 th Edition PART III: Forming and Shaping Processes and Equipment Copyright © 2010 Pearson Education.

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



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




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


Introduction

Rolling is the process of reducing the cross section of a long workpiece by compressive forces applied through a set of rolls


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


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


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


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.



Solution

Roll-strip contact length is calculated through geometry,

Absolute true strain of the strip is

Average true stress is

The roll force is


223.020

25ln

mm 7.3820253000 fo hhRL

MN 4.171801000250

10007.38 avgLwYF

MPa 1802/28080


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


W705000,66

100

1000

7.381074.12

000,66

2 6 FLNPower


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: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


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


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


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


Flat-rolling Practice


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


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


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


Rolling Mills

Automated mills produce close-tolerance, low cost and high quality plates and sheets at high production rates


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


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


Various Rolling Processes and Mills

Shape Rolling Straight and long structural shapes are formed at

elevated temperatures by shape rolling



Roll Forging Cross section of a round bar is shaped by passing it

through a pair of rolls with profiled grooves



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



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



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



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



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



Tube Rolling Diameter and thickness of pipes and tubing can be

reduced by tube rolling, which utilizes shaped rolls


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


Manufacturing Engineering Technology in SI Units, 6 th Edition PART III: Forming and Shaping Processes and Equipment Copyright © 2010 Pearson Education.

Documents

flat rolling

flat strip

strip slide

process flat

introduction rolling

roll radius

strip surfaces roll

equipment copyright