80- 1 - PowerPoint Presentation

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Grinding

Characteristics of an abrasive must be:

Harder than material being ground

Strong enough to withstand grinding pressures

Heat-resistant so that it does not become dull at grinding temperatures

Friable (capable of fracturing) so when cutting edges become dull, they will break off and present new sharp surfaces to material being groundSlide2

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Abrasive Classes

Natural abrasives

Sandstone, garnet, flint, emery, quartz, corundum

Used prior to early part of 20

th

century

Almost totally replaced by manufactured abrasives

Best natural abrasives is diamond (high cost)

Manufactured abrasives

Used because grain size, shape and purity can be closely controlled

Aluminum oxide, silicon, carbide, boron carbide, cubic boron nitride and manufactured diamondSlide3

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Aluminum Oxide

Most important abrasive

Make up 75% of grinding wheels

Used for high-tensile-strength materials

Manufactured with various degrees of purity

Hardness and brittleness increase as purity increasesSlide4

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Aluminum Oxide Purities

Regular aluminum oxide (Al

2

O

3

) at 94.5%

Tough abrasive capable of withstanding abuse

Grayish in color

Used for grinding steel, tough bronzes, etc.

Aluminum oxide at 97.5%

Not as tough as regular but still gray in color

Used in manufacture of grinding wheels for centerless, cylindrical, and internal grinding of steel and cast iron

Purest form of aluminum oxide

White material that produces sharp cutting edge

Used for grinding hardest steels and stelliteSlide5

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Manufacture of

Aluminum Oxide

Made from bauxite ore

Mined by open-pit method in Arkansas and Guyana, Suriname, and French Guiana

Calcined (powered form) in large furnace

Mixed with coke screenings, iron borings

Coke used to reduce impurities

Electrodes lowered, coke heated and fusion of bauxite starts then more bauxite and coke added

When furnace full, shut off, let cool

Material broken up and fed into crushersSlide6

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Silicon Carbide

Suited for grinding materials that have low tensile strength and high density

Harder and tougher than aluminum oxide

Color varies from green to black

Green used mainly for grinding cemented carbides and other hard materials

Black used for grinding cast iron and soft nonferrous metals (also ceramics)Slide7

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Manufacture of Silicon Carbide

Mixture of silica sand and high-purity coke heated in electric resistance furnace

Sawdust added to produce porosity and to permit gas to escape during operation

Salt added to assist in removing impurities

Time required for operation is 36 hours

Cool for 12 hours, remove sidewalls where unfused mixture falls to floor leaving silicon carbide ingot

Ingot crushed; resultant silicon carbide treated, screened and gradedSlide8

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Zirconia-Aluminum Oxide

First alloy abrasive produced

Made by fusing zirconium oxide and aluminum oxide at extremely high temperatures

Contains about 40% zirconia

Used for heavy-duty rough and finish grinding in steel mills, for snagging in foundries and for rapid rough and finish grinding of welds

Performance superior to standard aluminum oxide (last 2 to 5 times longer)Slide9

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Advantages of Zirconia-alumina Over Standard Abrasives

Higher grain strength

Higher impact strength

Longer grain life

Maintains its shape and cutting ability under high pressure and temperature

Higher production per wheel or disk

Less operator time spent changing wheels or disksSlide10

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Boron Carbide

Hardest material manufactured with exception of diamond

Not suitable for use in grinding wheels

Used only as loose abrasive and as cheap substitute for diamond dust

Manufacture of precision gages and sand blast nozzles

Used in ultrasonic machining applicationsSlide11

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Manufacture of Boron Carbide

Produced by dehydrated boric acid being mixed with high-quality coke

Mixture heated in horizontal steel closed cylinder, hole for graphic electrode and hole for escaping gases

To remove air, dampen mixture with kerosene to volatize and expel air

High current at low voltage applied for about 24 hours, then cooled

Resultant product is hard, black lustrous materialSlide12

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Cubic Boron Nitride (CBN)

Synthetic abrasive has hardness properties between silicon carbide and diamond

Developed by General Electric Company in 1969

Capable of withstanding grinding temperatures up to 2500

ºF

Cool-cutting and chemically resistant to all inorganic salts and organic compounds

Capable of maintaining very close tolerancesSlide13

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Manufacture of CBN

Synthesized in crystal form from hexagonal boron nitride with aid of catalyst, extreme heat (2725

ºF) and tremendous pressure

Strong, hard, blocky crystalline structures with sharp corners

Two types

Borozon CBN: uncoated abrasive used for general-purpose grinding

Boraxon Type II CBN: nickel-plated grains used in resin bonds for general-purpose dry and wet grindingSlide14

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Manufactured Diamonds

1954, General Electric Company produced Man-Madey diamonds in laboratory

1957, General Electric Company began commercial production of diamonds

First success involved carbon and iron sulfide in granite tube closed with tantalum disks were subjected to pressure of 66,536,750 psi and temperatures between 2550

ºF

Temperatures must be high enough to melt metal saturated with carbon and start diamond growth

Industrial diamonds referred to as bortSlide15

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Diamond Types

Real Value Grinding Diamond

Elongated

, friable crystal with rough edges

Letters indicate it can be used with

resinoid

or vitrified bond and used for grinding

ultrahard

materials

Tungsten carbide

Silicon carbide

Space-age alloys

Used for wet or dry grindingSlide16

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Abrasive Products

After abrasives produced, formed into products

Grinding wheel

Most important

Abrasive material held together with suitable bond

Material components are abrasive grain and bond; other physical characteristics such as grade and structure

Coated abrasives

Polishing and lapping powders

Abrasive sticksSlide17

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Basic Functions of

Grinding Wheels

Generation of cylindrical, flat and curved surfaces

Removal of stock

Production of highly finished surfaces

Cutting-off operations

Production of sharp edges and pointsSlide18

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Abrasive Grain

Aluminum oxide or silicon carbide abrasive used in most grinding wheels

Each grain on working surface of grinding wheel acts as separate cutting tool

Removes small metal chip as passes over surface of work

As grain becomes dull, fractures and presents new sharp cutting edge to material

Fracturing action reduces heat of friction, producing relatively cool cutting actionSlide19

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Grain Size

Abrasive ingot (pig) removed from electric furnace, crushed, grains cleaned and then sized by passing them through screens

Contain certain number of meshes or openings per inch

8-grain

24-grain

60-grain

Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.Slide20

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Grain Sizes

General applications for various grain sizes

8 to 54 for rough grinding operations

54 to 400 for precision grinding processes

320 to 2000 for ultra precision processes to produce 2 to 4

µ (micron) finish or fineSlide21

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Factors Affecting Selection of Grain Sizes

Type of finish desire

Type of material being ground

Amount of material to be removed

Area of contact between wheel and workpieceSlide22

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Bond Types

Function of bond is to hold abrasive grains together in form of wheel

Six common bond types used in grinding wheel manufacture:

Vitrified

Resinoid

Rubber

Shellac

Silicate

MetalSlide23

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Vitrified Bond

Used on most grinding wheels

Made of clay or feldspar

Fuses at high temperature and when cooled forms glassy bond around each grain

Strong but break down readily on wheel surface to expose new grains during grinding

Bond suited for rapid removal of metal

Not affected by water, oil, or acidSlide24

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Resinoid Bond

Synthetic resins used as bonding agents

Generally operate at 9500 sf/min

Wheels are cool-cutting and remove stock rapidly

Used for cutting-off operations, snagging, and rough grinding, as well as for roll grindingSlide25

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Rubber Bond

Produce high finishes

Ball bearing races

Used for thin cutoff wheels because of its strength and flexibility

Used also as regulating wheels on centerless grindersSlide26

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Shellac Bond

Used for producing high finishes on parts such as cutlery, cam shafts, and paper-mill rolls

Not suitable for rough or heavy grindingSlide27

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Silicate Bond

Not used to any extent in industry

Used principally for large wheels and for small wheels where necessary to keep heat generation to minimum

Bond (silicate of soda) releases abrasive grains more rapidly than does vitrified bondSlide28

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Metal Bond

Generally nonferrous

Used on diamond wheels and for electrolytic grinding operations where current must pass through wheelSlide29

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Grade

Defined as degree of strength with which bond holds abrasive particles in bond setting

Hard grade

When bond posts very strong (retain abrasive grains during grinding operation)

Soft grade

Grains released rapidly during grinding operation

Wheel grade symbols indicated alphabetically, from A (softest) to Z (hardest)Slide30

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Factors that Determine the Grade Selected for Particular Job

Hardness of material

Area of contact

Condition of machine

Speed of grinding wheel and workpiece

Rate of feed

Operator characteristicsSlide31

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Structure

Space relationship of grain and bonding material to the voids that separate them

Density of wheel

Dense structure has close grain spacing

Open structure has relatively wide spacing

Selection of wheel structure depends on type of work required

Indicated by numbers ranging from

1 (dense) to 15 (open)Slide32

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Factors Affecting the Selection of the Proper Wheel Structure

Type of material being ground

Soft material require greater chip clearance, therefore open wheel

Area of contact

Greater area of contact, more open structure

Finish required

Dense wheels give better, accurate finish

Method of cooling

Open-structure wheels provide better supply of coolantSlide33

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Grinding Wheel Manufacture

Most grinding wheels used for machine shop operations are manufactured with vitrified bonds

Main operations in manufacture of vitrified grinding wheels:

Mixing

Molding

Shaving

Firing (Burning)

Truing

Bushing

Balancing

Speed TestingSlide34

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Mixing

Correct proportions of abrasive grain and bond carefully weighed and thoroughly mixed in

rotary power mixing machine

Certain percentage of water added to moisten mix

Molding

Proper amount of mixture placed in steel mold

of desired wheel shape and compressed in

hydraulic press to form wheel slightly larger

than finished size

Amount of pressure used varies with size of wheel and structure requiredSlide35

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Shaving

Some machines requires special wheel shapes and recesses

Shaped or shaved to size in green, or unburned, state on shaving machine which resembles potter's wheel

Firing (Burning)

Green wheels carefully stacked on cars and moved slowly through long kiln 250 to 300 ft long with temperature held at ~2300

ºF

Takes about 5 days

Causes bond to melt and form glassy case around each grain producing hard wheelSlide36

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Truing

Cured wheels mounted in special lathe

Turned to required size and shape by hardened-steel conical cutters, diamond tools, or special grinding wheels

Bushing

Arbor hole in grinding wheel fitted with lead or plastic-type bushing to fit specific spindle size

Edges of bushing are trimmed to thickness of wheelSlide37

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Balancing

Remove vibration that may occur while wheel revolving

Small, shallow holes drilled in "light" side of wheel and filled with lead to ensure proper balance

Speed Testing

Wheels rotated in heavy, enclosed case and revolved at speeds at least 50% above normal operating speeds

Ensures wheel will not break under normal operating speeds and conditionsSlide38

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Standard Grinding

Wheel Shapes

Nine standard grinding wheel shapes established by:

United States Department of Commerce

Grinding Wheel Manufacturers

Grinding Machine Manufacturers

Dimensional sizes for each of the shapes have also been standardized

Table 80.3 in textbookSlide39

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Mounted Grinding Wheels

Driven by steel shank mounted in wheel

Produced in variety of shapes for use with jig grinders, internal grinders, portable grinders, toolpost grinders, and flexible shafts

Manufactured in both aluminum oxide and silicon carbide typesSlide40

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Grinding Wheel Markings

Standard marking system chart used by manufacturers to identify grinding wheels

Information found on blotter of all small and medium-size grinding wheels

Stenciled on side of larger wheels

Six positions in standard sequence

Prefix is manufacturer's symbol and not always used by all grinding wheel producers

Marking system used only for aluminum oxide and silicon carbide wheels, not diamond wheelsSlide41

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Characteristics That Indicate Wheel Too Soft

Breaks down too fast

Poor surface finish

Cuts freely

Sparks out quickly

Difficult to maintain size

Scratches (fishtails)Slide42

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Characteristics That Indicate Wheel Too Hard

Wheel glazes quickly

Loading (material ground fills voids)

Burned work surface

Squealing noise

Doesn't cut freely

Inaccurate work dimensions

Surface finish get progressively better

Won't spark out

Heat checksSlide43

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Three Types of Bonds for Diamond Wheels

Resinoid-bonded

Give maximum cutting rate and require little dressing

Remain sharp for long time and well suited to grinding carbides

New development has been to coat diamond particles with nickel plating before mixed with resin

Reduces tendency to chip and results in cooler-grinding, longer lasting wheelsSlide44

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Metal bonds

Generally nonferrous

Particularly suited to offhand grinding and cutting-off operations

Holds form extremely well and does not wear on radius work on small areas of contact

Vitrified-bonded wheels

Remove stock rapidly but require frequent cleaning with boron carbide abrasive stick to prevent loading

Suited for offhand and surface grinding of cemented carbidesSlide45

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Cubic Boron Nitride Wheels

Recognized as superior cutting tools for grinding difficult-to-machine metals

Have more than twice hardness of conventional abrasives

Also toughness to match so cutting edges stay sharp longer with much slower wear rates

Prolonged cutting capacity and high thermal conductivity help prevent uncontrolled heat buildup (reduce chances of glazing)

Thermally and chemically stable at temperatures above 1832

ºFSlide46

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Properties of Cubic Boron Nitride Wheels

Contain four main properties necessary to grind extremely hard or abrasive materials at high metal-removal rates:

Hardness

Abrasion resistance

Compressive strength

Thermal conductivitySlide47

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Wheel Selection

Type of wheel selected and how used will affect metal-removal rate (MRR) and life of grinding wheel

Generally affected by:

Type of grinding operation

Grinding conditions

Surface finish requirements

Shape and size of workpiece

Type of workpiece materialSlide48

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Coated Abrasives

Consist of flexible backing (cloth or paper) to which abrasive grains have been bonded

Two purposes

Metal grinding and polishing

Coarse-grit used for rapid removal of metal; fine grits used for polishing

Two types

Natural: garnet, flint, and emery

Manufactured: aluminum oxide, silicon carbideSlide49

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Selection of Coated Abrasives

Emery (natural abrasive)

Black in appearance

Used to manufacture emery cloth and emery paper

Grains not as sharp as artificial abrasives

Generally used for polishing metal by handSlide50

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Aluminum Oxide (manufactured abrasive)

Gray in appearance

Used for high-tensile-strength materials

Characterized by long life of cutting edges

Hand operations use 60-80 grit for roughing and 120 to 180 grit for finishing

Machine operations use 36-60 grit for roughing and 80-120 grit for finishing operations

Silicon Carbide (manufactured abrasive)

Bluish-black in appearance

Used for low-tensile-strength materials

Selection of grit size same as aluminum oxideSlide51

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Coated Abrasive Machining

Over past few years, coated abrasive machining has become widely used in industry

Improved abrasives and bonding material, better grain structure, more uniform belt splicing, and new polyester belt backing, abrasive belt machining

Now capable of grinding to less than .001in tolerance and surface finish of to to 20

µin.Slide52

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Objectives

Set up and grind work on a cylindrical grinder

Internal-grind on a universal cylindrical grinder

Identify and state the principles of three methods of centerless grindingSlide53

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Cylindrical Grinder

Grinds accurately to size and high surface finish the diameter of a workpiece

Two types of machines

Center type

Plain – Generally manufacturing type of machine

Universal – More versatile since wheelhead and headstock may swivel

Centerless typeSlide54

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Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

Universal Cylindrical GrinderSlide55

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Parts of the Universal Cylindrical Grinder

Base

Heavy cast-iron construction for rigidity

Top of base machined to form ways for table

Wheelhead

Mounted on cross-slide at back of machine

Mounted on ways right angle to table way

May be swiveled to permit grinding of steep tapers by plunge grindingSlide56

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Parts of the Universal Cylindrical Grinder

Table

Mounted on ways, driven back and forth by hydraulic or mechanical means

Lower table rest on the ways

Upper table may be swiveled

Headstock and footstock used to support work held between centers

Trip dogs

Control reversal of tableSlide57

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Parts of the Universal Cylindrical Grinder

Headstock

Mounted on left end of table

Contains motor for rotating work

Dead center mounted in headstock spindle and used to overcome any spindle inaccuracies

Two dead centers rotate work mounted between centers by means of dog and driveplate

Footstock

Supports right end of work (adjustable)Slide58

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Parts of the Universal Cylindrical Grinder

Backrest (steadyrest)

Provides support for long, slender work and prevents it from springing

May be positioned anywhere along table length

Center rest

Resembles lathe steadyrest

Mounted at any point on table

Used to support right end of work when grinding confined to end of workpieceSlide59

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Parts of the Universal Cylindrical Grinder

Internal grinding attachment

Mounted on wheelhead for internal grinding

Usually driven by separate motor

Diamond wheel dresser

Clamped to table to dress grinding wheel

Coolant system

Provide dust control, temperature control and better surface finish on workpieceSlide60

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Procedure To True

and Dress the Wheel

Start grinding wheel to allow spindle bearings to warm up

Mount proper diamond in holder and clamp it to table

Diamond should be mounted at angle of 10

º to 15º to wheelface and held on centerline of wheel

Adjust wheel until diamond almost touches high point of wheelSlide61

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Turn on coolant if it is to be used for grinding operation

Feed wheel into diamond about .001 in. per pass

Move it back and forth across wheelface at medium rate until wheelface has been completely dressed

Finish-dress wheel by using infeed of .0005 in. and slow traverse feedSlide62

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Plunge Grinding

Feeding wheel into revolving work with table remaining stationary

Grinding wheel fed automatically to setting on feed index

Dwells for suitable time to permit "spark out" and retracts automatically

Length of surface to be ground must be no longer than width of grinding wheel faceSlide63

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Internal Grinders

Designed for accurate finishing of holes in workpiece by grinding wheel

Originally designed for hardened workpieces

Now used for finishing holes in soft material

Wheel fed into work automatically until hole reaches required diameter

Wheel withdrawn and automatically dressedSlide64

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Internal Grinding on a Universal Cylindrical Grinder

Used in toolrooms for this purpose

Attachment mounted on wheelhead column

Swung into place when required

Outside and inside diameters of workpiece may be finished in one setup

Grade of wheel will depend on type of work and rigidity of machineSlide65

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Wheels Used for

Internal Grinding

Generally softer than those for external grinding

Larger area of contact between wheel and workpiece

Requires less pressure to cut than hard wheel

Reduced spindle pressure and springSlide66

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Centerless Grinders

Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

Work not supported on centers but by work rest blade, a regulating wheel, and a grinding wheelSlide67

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How Centerless Grinders Work

Work supported on work rest blade equipped with suitable guides

Rotation of grinding wheel forces work onto rest blade against regulating wheel

Regulating wheel controls speed of work and longitudinal feed movement

Set at slight angle (angle controls rate of feed)

Centers fixed – diameter of work controlled by distance between wheels and height of work rest bladeSlide68

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Methods of Centerless Grinding

Three methods

Thru-feed

Infeed

EndfeedSlide69

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Thru-Feed Centerless Grinding

Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

Consists of feeding work between grinding and regulating wheels

Work fed by regulating wheel past grinding wheel

Speed of feeding work controlled by speed and angle of regulating wheelSlide70

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Infeed Centerless Grinding

Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

Form of plunge grinding

Used when work being ground has shoulder or head

Several diameters of workpiece may be finished simultaneously

Work rest blade and regulating wheel clamped in fixed relation to each otherSlide71

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Endfeed Centerless Grinding

Copyright © The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

Used for grinding tapered work

Grinding wheel, regulating wheel, and work rest

remain in fixed position

Work fed in from front up to fixed stop

Grinding wheel and regulating wheel often dressed to required taperSlide72

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Advantages of Centerless Grinding

No limit to length of work being ground

No axial thrust on workpiece

Permits grinding of long workpieces that would be distorted by other methods

Less stock required on workpiece for truing purposes

Less wheel wear and less grinding time required because there is less stock to be removed