Introduction to Manufacturing

Chapter 14: Forging

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Forging• Metal-forming process in which the

workpiece is shaped by compressive forces

applied through various dies and tools.

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Example – Blacksmith

Hammer and anvil

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• Forgeability• Capability of a material to undergo

deformation without cracking.

• Forging is a discrete (individual) parts process

• Forging is a near net shape process

Forging

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Metal flow and grain structure controlled – parts have good strength• Landing gear• Jet engine shafts• Bolts• Connecting rods• Hand tools

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Forging classified cold or hot

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Cold Forging• Requires greater forces and workpiece

materials must have sufficient ductility at room temperature; good surface finish and dimensional accuracy.

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Hot Forging• Requires smaller forces, not as good

finish or dimensional accuracy; usually require additional machining.

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Open-Die Forging• Upsetting or flat-die forging• Simplest forging process.• Solid workpiece is set between two flat

dies and reduced (height) through compression.

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Barreling• outward bowing of material • Two causes• 1.) by friction at die-work piece

interface• minimized by use of lubricants.

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Barreling• (2) by hot work piece between cold dies• Material cools at die interface and

resists deformation

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Cogging:• Drawing out• Operation where the thickness of a bar is

reduced by successive forging steps at certain intervals.

• Typical for parts such as I-beams which must be pre-shaped before rolling

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Fullering• Distribute material away from die

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Edging• Gather material into die

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• Work piece takes shape of die cavities (impressions).

• Flash: material which flows out from dies (frictional resistance from cooling flash causes material to fill the inside of the die cavity).

Impression-Die Forging

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Closed-Die Forging• Also called precision forging• flashless forging (flash does not form).• workpiece completely fills the die cavity.

Volume of material is precise.• Economical - eliminates trimming flash

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Closed die forging (cont.)• Greater forces to obtain fine detail• Al and magnesium good because

malleable• Less material wasted• Gears, connecting rods, etc.

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Coining:• Another closed die process• Produces fine details• No lubricants used – can be trapped in

die cavities – don’t compress• pressures five to six times the strength

of material.

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Related Forging Operations

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(1) Heading• upsetting operation, usually performed at

the end of a round rod or wire to produce a large cross section

• heads of bolts, screws nails, rivets, etc.

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(2) Piercing• indenting, without breaking, the surface of

a workpiece with a punch to produce a cavity or an impression

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(4) Roll Forging• the cross section of a bar is reduced or

shaped by passing it through a pair of rolls with shaped grooves

• leaf springs, hand tools, discrete parts

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(5) Skew Rolling• similar to roll forging• Ball bearing production

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(6) Orbital Forging• upper die moves along an orbital path

above the part which is also spinning.

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(7) Incremental Forging• process where a blank is forged into a

shape in several small steps (the die penetrates the blank at different depths across the surface). Similar to cogging.

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(8) Isothermal Forging• Hot die forging• Dies heated to same temp as

workpiece• Good for complex parts with

dimensional accuracy

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Forging die design• Most important rule in die design

• Work piece material must flow in direction of least resistance

• Machining allowance used

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Die inserts • Separate sections• Reduce cost of making dies

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Die lubricants• Slow rate of cooling• Improve metal flow• Serve as parting agent• Not for coining – precision forging

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Forging Machines

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Presses• Hydraulic• Mechanical • Screw

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Hammers• Gravity drop – free falling ram• Power drop – stroke accelerated by

steam, air, or hydraulic

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Hammers (cont.) • Counterblow (two hammers

simultaneously approach each other• High-energy-rate machines - ram

accelerated by high pressure – part forged in one blow