Unit 4 - ABRASIVE PROCESSES AND BROACHING

G.Ravisankar, Asst Prof , Mechanical, Sri Eshwar college of Engineering , Coimbatore .

UNIT IV - ABRASIVE PROCESSES AND BROACHING

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Abrasive processes: grinding wheel – specificationsand selection, types of grinding process–cylindricalgrinding, surface grinding, centreless grinding andinternal grinding- Typical applications –concepts ofsurface integrity, broaching machines: broachconstruction – push, pull, surface and continuousbroaching machines

1. Hajra Choudhury, "Elements of Workshop Technology", Vol.II., Media Promoters2. Rao. P.N “Manufacturing Technology - Metal Cutting and Machine Tools", Tata McGraw-Hill, New Delhi, 2003.

Abrasive processes - Outline

• Abrasives• Grinding▫ Grinding Wheels▫ Grinding Process

• Coated Abrasives▫ Belt Grinding

• Honing• Lapping• Other Finishing Operations• Deburring Processes

Abrasive Machining

Abrasive machining is a material removal process thatinvolves the use of abrasive cutting tools.There are three principle types of abrasive cuttingtools according to the degree to which abrasivegrains are constrained,

•Bonded abrasive tools• Coated abrasive tools• Free abrasives

Abrasive Machining

Abrasive Machining

Why a smooth surface?• Reduction in Friction▫ Heat - Bearings

• Reduction in Wear▫ Bushings/Bearings

• Appearance▫ Car Body, Furniture

• Clearance▫ Disk Head

• Sharpness▫ Cutting Tools

Abrasive MachiningHow do we get a smooth surface?

• Remove Material▫ Abrasive Machining

• Flatten▫ Burnishing – polish the metal by rubbing.

• Fill in Voids▫ Add material▫ Paint▫ Finish▫ Wax

Abrasives• Small, hard nonmetallic particles with sharp edges

and irregular shapes

• Can remove small amounts of material, producing tiny chips

• Abrasive processes can produce fine surface finishes and accurate dimensional tolerances

Types of AbrasivesConventional Abrasives

a. Aluminum oxide (Al2O3)

b. Silicon carbide (SiC)

Super abrasives

c. Cubic Boron Nitride (cBN)

d. Diamond

Natural abrasives (sand stone, emery, diamond)

Abrasives are harder than conventional tool materials

Abrasive Factors

- Grain size

- Grain shape

- Hardness

- Friability (tendency to fracture)

Abrasive Material

Abrasive Hardness and Thermal Conductivity

Grinding

• Grinding is the most common form of abrasivemachining.

• It is a material cutting process which engages anabrasive tool whose cutting elements are grains ofabrasive material known as grit.

• These grits are characterized by sharp cuttingpoints, high hot hardness, chemical stability andwear resistance.

• The grits are held together by a suitable bondingmaterial to give shape of an abrasive tool.

Grinding

• Cutting action of abrasive grains

Grinding

Example of a Grinding Machine

Major advantages of grinding

A grinding wheel requires two types of specification

• dimensional accuracy

• good surface finish

• good form and locational accuracy applicable to both hardened and unhardened material

Major applications of grinding

Applications

• surface finishing

• slitting and parting

• descaling, deburring

• stock removal (abrasive milling) finishing of flat as well as cylindrical surface

• grinding of tools and cutters and resharpening of the same.

Types of Grinding

- Surface Grinding

- Cylindrical Grinding

- Internal Grinding

- Centerless Grinding

- Others- Tool and cutter grinders

- Tool-post grinding

- Swing-frame grinders

- Bench grinders

- Creep-Feed Grinding

Floor stand, Bench grinding

Portable grinding

Abrasive belt, Swing frame

Surface Grinding

Horizontal spindle reciprocating table

Surface Grinding• Surface grinding is an abrasive machining process in

which the grinding wheel removes material from the plain flat surfaces of the work piece.

Cylindrical Grinding

Cylindrical Grinding

PRECISION GRINDING

1. Cylindrical grinding

Types of cylindrical grinding

• Plunge grinding (form tool method)

Plain center type cylindrical grinding

Center type universal grinding

Cylindrical Grinding

• Three types of feed motion are possible according to the direction of feed motion,

Internal Grinding

Internal Grinding

1.Chucking type internal grinding

2.Planetary type

Centerless grinders

Centerless Grinding• Centerless grinding is a process for continuously

grinding cylindrical surfaces in which the workpiece is supported not by centers or chucks but bya rest blade.

• The work piece is ground between two wheels.

• The larger grinding wheel does grinding, while thesmaller regulating wheel, which is tilted at anangle i, regulates the velocity Vf of the axialmovement of the work piece.

Centerless Grinding

Centerless Grinding

Center less grinding

Creep-Feed Grinding• Grinding has traditionally been associated with small

rates of material removal and finishing operations.

• However, grinding can also be used for large-scale metal removal operations similar to milling, shaping, and planing.

• In creep-feed grinding, the depth of cut d is as much as 6mm, and the workpiece speed is low.

• The wheels are mostly softer grade resin bonded with open structure to keep temperatures low.

• Creep-feed grinding can be economical for specific applications, such as grinding cavities, grooves, etc.

Creep-Feed Grinding

Special purpose grinding machine

1.Form Tool, Gear Teeth Grinding 2. Cam grinding

3. Tool and cutter grinder

Bonded Abrasives/ Grinding Wheels

Bonded Abrasives• Most grinding wheels are made of abrasive grains

held together by a bonding material

Types of bonding material:• Vitrified (glass)• Resinoid (thermosetting resin)• Rubber• Metal (the wheel itself is metal; the grains are

bonded to its surface

Grinding Wheel Components

Grinding Wheel - Structure

Grinding Wheel - Shapes

Grinding Process

Grinding

- Grains have irregular shapes and random spacing

- Average rake angle is very negative (about -60° or lower)

- Radial positions of grains vary

- Cutting speed is very high (ca. 600 ft/min)

Grinding Process

Grinding Process

Grain force ((v/V)√(d/D))(material strength)

Temperature rise D1/4d3/4(V/v)1/2

Effects caused by grinding temperature increase:- Sparks- Tempering- Burning- Heat Checking

Grinding Wheel – Standard marking system

• Grade

Grinding Wheel - Loading

Grinding Wheel - Wear

Types:• Attritious Grain Wear

Grains develop a wear flat

• Grain FractureNecessary to produce sharp grain edges

• Bond FractureAllows dull grains to be dislodged from the

wheel

Grinding Wheel - Parameters• The way the abrasive grains, bonding material, and

the air gaps are structured, determines theparameters of the grinding wheel,

which are

• Abrasive material,

• Grain size,

• Bonding material,

• Wheel grade,

• Wheel structure.

Truing and Dressing

Cutting Fluids

- Remove heat

- Remove chips, grain fragments and dislodged grains

- Are usually water-based emulsions

- Are added by flood application

Grinding Ratio

G = Volume of material removed

Volume of wheel wear

Vary greatly (2-200 or higher) depending on the type of wheel, grinding fluid, and process parameters

Higher forces decrease the grinding ratio

GrindingDesign Considerations:- Design parts so that they can be held securely- Avoid interrupted surfaces if high dimensional

accuracy is required because they can causevibrations

- Ensure cylindrical parts are balanced and thickenough to minimize deflections

- Short pieces may be difficult to grind accurately incenterless grinding because of limited support by theblade

- Parts requiring high accuracy form grinding should bekept simple to prevent frequent wheel dressing

- Avoid small deep or blind holes or include a relief

Coated Abrasives

Coated Abrasives

Abrasive grains are deposited on flexible backing; they are more pointed than those in grinding wheels

Common examples: sandpaper, emery

Coated Abrasives

Coated Abrasives

Belt Grinding

Uses coated abrasives in the form of a belt; cutting speeds are about 2500-6000 ft/min

Microreplication

Abrasives with a pyramid shape are placed in a predetermined regular pattern on the belt

Belt Grinding

BROACHING

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Broaching

• Broaching is the process of removing metal with atool which has “teeth” arranged in a row. Eachtooth is successively higher than the previoustooth and removes more material. In broaching,one stroke or cycle of the machine produces afinished part.

• Broaching is used to produce both internal andexternal features. Production rates are high andtolerances of +/- .0005” are possible.

BroachingBroaching machine is a process of machining surface with a special multipoint cutting tool

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Tool Feed DirectionGullet

Workpiece

Depth ofcut per tooth

Tool

Broaching

Specification of broaching machine

• Max length of the stoke

• Max force developed by the slide in tonnes

• Types of drives

• Power rating of electrical motor in HP

• Speed and feed

• Weight of the machine

• Floor space required

Advantages & Disadvantages

▫ Advantages Rough to finish in one pass Production rates are high Cutting time is quick Rapid load and unload of parts External and internal features Any form that can be produced on a broaching tool can

be produced Production tolerances are excellent Surface finishes are equal to milling Operator skill is low

Advantages & Disadvantages

▫ Disadvantages Tooling cost can be high

In some cases--not suited for low production rates

Parts to be broached must be strong enough to withstand the forces of the process

Surface to be broached must be accessible

TYPES OF BROACHING MACHINE

According to the nature and direction of primary cutting motion▫ Horizontal broaching machine

▫ Vertical broaching machine

▫ Continuous broaching machine

According to the purpose

▫ Internal broaching machine

▫ External surface broaching machine

According to method operation

▫ Pull broaching machine

▫ Push broaching machine

According to the construction of the broach tool

▫ Solid broaching machine

▫ Inserted tooth broaching machine

▫ Progressive cut broaching machine

▫ Built-up broaching machine

• According to the function

▫ Keyway broaching machine

▫ Burnishing broaching machine

▫ Spline broaching machine

▫ Round hole broaching machine

▫ Surface broaching machine

• According to the number of main slides or stations

▫ Single broaching machine

▫ Double broaching machine

▫ Multiple slides broaching machine

• According to the motion of the broach tool relative to the work

▫ Straight line motion broaching machine

▫ Stationary broach tool broaching machine

Horizontal broaching machine

Vertical broaching machine

Continuous broaching machine

Continuous broaching machine

Nomenclature of Broaching tool

Types of broaching toolAccording to the method of operation

▫ Push Broach

▫ Pull Broach

According to the kind of operation

▫ Internal Broach

▫ External Broach

According to their construction

▫ Solid Broach

▫ Built up of replaceable section

▫ Inserted tooth Broach

▫ Overlapping teeth Broach

▫ Progressive cut Broach

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