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2 0 1 4 - 2 0 1 5 S E M E S T E R - I

TA201 Manufacturing Processes

2014-15 Semester-I

Advantages of Cold Forming vs. Hot Working

Better accuracy, closer tolerances

Better surface finish

Strain hardening increases strength and hardness

Grain flow during deformation can cause desirable directional properties in product

No heating of work required

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Dr. Shashank Shekhar

2014-15 Semester-I

Disadvantages of Cold Forming

Higher forces and power required

Surfaces of starting workpiece must be free of scale and dirt

Ductility and strain hardening limit the amount of forming that can be done

In some operations, metal must be annealed to allow further deformation

In other cases, metal is simply not ductile enough to be cold worked

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2014-15 Semester-I

Advantages of Hot Working vs. Cold Working

Workpart shape can be significantly altered

Lower forces and power required

Metals that usually fracture in cold working can be hot formed

Strength properties of product are generally isotropic

No strengthening of part occurs from work hardening

- Advantageous in cases when part is to be subsequently processed by cold forming

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2014-15 Semester-I

Disadvantages of Hot Working

Lower dimensional accuracy

Higher total energy required (due to the thermal energy to heat the workpiece)

Work surface oxidation (scale), poorer surface finish

Shorter tool life

Secondary processes required

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2014-15 Semester-I

Advantages of Warm Working

Lower forces and power than in cold working

More intricate work geometries possible

Need for annealing may be reduced or eliminated

Energy requirements are moderate

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2014-15 Semester-I

Basic bulk deformation processes: Rolling

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2014-15 Semester-I TA 201 Dr. Shashank Shekhar

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draft: d = t0 – tf

dmax = 2R

Conservation of mass: t0w0L0 = tfwfLf

Similarly t0w0V0 = tfwfVf

Ideally w should remain constant

Mechanics of Rolling:

)( fo ttRL

Rolls contact along the work along an arc defined by the angle


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true strain: = ln(t0/tf)

Average flow stress can be used to estimate force and power requirements

n

KY

n

f

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Velocity of the rolls are constant Vr

Metal feed is continuous

Velocity of the work between the rolls

change gradually from Vo to Vf

At one point, Vr is same as work velocity

This is called neutral point or no-slip point

On either side there is slipping and friction

2014-15 Semester-I

Force and Torque 11

Torque: τ= Load X moment arm = 2 X (F X a) Typically, a = 0.5L τ = 2 X (0.5 X F X L)

Average flow stress can be used to estimate force and power requirements

TA 201 Dr. Shashank Shekhar

2014-15 Semester-I

Power requirements 12

Work per revolution = 2 X ( 2 X X a) X F Hence, Power = 4 aFN (N is freq of rot.) Power = 2 LFN


2014-15 Semester-I

Thread Rolling

Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies • Important commercial process for mass producing bolts and screws • Performed by cold working in thread rolling machines

Advantages over thread cutting (machining): o Higher production rates o Better material utilization o Stronger threads and better fatigue resistance due to work hardening

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2014-15 Semester-I

Ring Rolling

Deformation process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter • As thick-walled ring is compressed, deformed metal elongates,

causing diameter of ring to be enlarged • Hot working process for large rings and cold working process for

smaller rings • Applications: ball and roller bearing races, steel tires for railroad

wheels, and rings for pipes, pressure vessels, and rotating machinery • Advantages: material savings, ideal grain orientation, strengthening

through cold working

Start of the Process Completion of the Process

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2014-15 Semester-I

Basic bulk deformation processes: Forging

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Work is compressed between two dies and force is applied Pressure can be gradual or

can be impact Components include

engine crankshaft, connecting rods, turbine parts Mostly performed at

elevated temperatures, however cold forging are also common


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true strain: = ln(h0/hf)

Force required to continue deformation at a given height h and cross section A

Ideal Forging Operation

n

KY

n

f

1

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Actual Forging Operation involves Barreling

Friction opposes flow of motion at surface leading to barreling effect It is more pronounced when working on hot workpart with cold dies Hotter metal in the middle flow more freely compared to colder metal near

the surface Effect is more severe when diameter to height ratio is large

2014-15 Semester-I

Example


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A cylindrical workpiece is subjected to a cold upset forging operation. The starting piece is 75mm in dia. It is reduced in the operation to a height of 36mm. The work material has a flow defined by K=350MPa and n=0.17. Assume a coefficient of friction of 0.1. Determine the force as the process begins, at intermediate heights of 62mm, 49mm, and at the final height of 36mm.

245N, 649N, 946N, 1467N


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Something wrong in the plot?

2014-15 Semester-I

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Forging

Dalmadal Canon (West Bengal) Made by forging rings of wrought iron.. Forging operation also welds two side-by rings….Forge welding operation

Mandrel


2014-15 Semester-I

Open die forging

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2014-15 Semester-I

Close die forging (Impression die forging)

Flash

Flash: (1) Safety valve (2) Regulates the escape of extra metal-flow resistance increased for thin flash

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Higher production rates Conservation of metal Greater strength Favorable grain orientation

Week10

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