MP- II Lecture No 2

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MACHINE POCESSES FOR

PRODUCING VARIOUS SHAPES

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Milling

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Milling

Milling is a process of removing material by amulti-point rotating cutter.

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Milling operations

Milling is one of the basic machiningprocesses.

The basic function of milling machines is toproduce flat surfaces in any orientation.

Milling is a very versatile process capable ofproducing simple two dimensional flat shapes

to complex three dimensional surface s.

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The Milling machines are widely used for the followingpurposes :

Flat surface in vertical, horizontal and inclined planes Making slots or ribs of various sections

Slitting or parting

Often producing surfaces of revolution

Making helical grooves like flutes of the drills Long thread milling on large lead screws, power

screws, worms etc and short thread milling for small

size fastening screws, bolts etc.

2-D contouring like cam profiles, clutches etc and 3-D

contouring like die or mould cavities

Cutting teeth in piece or batch production of spur

gears, straight toothed bevel gears, worm wheels,

sprockets, clutches etc.

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The Milling Process

The milling process:

Typically uses a multi-toothcutter

Work is fed into the rotatingcutter

Capable of high MRR

Well suited for mass productionapplications

Cutting tools for this process arecalled milling cutters

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Milling Processes

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Milling Cutters

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Milling Operations Milling operations are classified into two major categories:

Peripheral Milling

Generally in a plane parallel to the axis of the cutter Cross section of the milled surface corresponds to the

contour of the cutter

Face Milling

Generally at right angles to the axis of

rotation of the cutter Milled surface is flat and has no

relationship to the contour of the cutter

Combined cutting action of the side and

face of the milling cutter

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Related Operations

Side milling - machining a plane surfaceperpendicular to the milling machine arbor withan arbor mounted tool. This tool is called a side

mill. Straddle milling - milling two parallel surfaces

using two cutters spaced apart on an arbor.

Gang milling - milling multiple surface

simultaneously using multiple cutters mounted onan arbor.

Thread milling - milling treads using the capabilityof a three axis contouring CNC machine.

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Operating Parameters (Feed direction) Conventional Milling

Conventional milling

Most common method of feed

Feed work against the rotation of the cutter

Climb milling

Load of the cutter tends to pull the work into the cutter This results in a small feed force and about 20% less Hp

than conventional milling

Downward motion increases the load on the table ways

This method can pull the work into the cutter and scrapthe work and/or damage the fixture and tool.

Machine must be very ridged to safely utilize climb

milling(CNC machines)

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Operating Parameters

Conventional vs. Climb Milling

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Types of Millng Machines

The Milling machines exist in two basic forms:i) Horizontal Milling machineii) Vertical Milling Machine

These terms refer to the orientation of the cuttingtool spindle.

The primary difference in milling with respect tothe lathe cutting is that the cut is always interrupted.

As the cutter rotates and crosses across the part,

each insert will enter and exit the cut at least one timeper revolution. Not only does this cause a repeatedimpact situation on the cutting point, it also causesrepeated heating and cooling of the insert itself

lending it to be susceptible to thermal cracking.

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Classification of milling machine

Milling machines can bebroadly classified;

(a) According to nature of purposes ofuse :

general purpose

single purpose e.g., threadmilling machines, cam millingmachines and slitting machine which

are generally used for batch or lotproduction.

Special purpose these are usedfor lot or mass production.

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(b) According to configurationand motion of the work-holding table / bed

i) Knee type

ii) Bed type

iii)Planer type

iv) Rotary table type

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(c) According to the orientationof the spindle(s).

i) Plain horizontal knee type

ii) Vertical spindle typeiii) Universal head milling

machine

iv) Computer Numerical

Controlled (CNC)

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Variation of Machines

CNC Horizontal, Vertical,and Planner (up to 5 axis)

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Process Accuracy

Accuracy of milling machines depend uponthe following factors.

Fixture

Rigidity of machine tool

Accuracy of the spindle

Cutter condition

Coolant

Type

Delivery method

Material condition

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Planning and shaping

Operations

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Planning and shaping operation- In Planning nad shaping operation a

single point tool moves relative to the wrkpart.

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Planning & Shaping

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Broaching

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Broaching is a milling process in which amultiple tooth cuting tool moves linearlyrelative to the work.

hi

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Broaching

Broaching is a machining operation which uses a toothed tool,called a broach, to remove material. The broach is used in abroaching machine, which is also sometimes shortened to broach. Itis used when precision machining is required, especially for oddshapes. Broaching finishes a surface in a single pass, which makes itvery efficient. There are two main types ofbroaching: linearand rotary. In linear broaching, which is the more

common process, the broach is run linearly against a surface of theworkpiece to effect the cut. Linear broaches are used in a broachingmachine, which is also sometimes shortened to broach. In rotarybroaching, the broach is rotated and pressed into the workpiece tocut an axis symmetric shape. A rotary broach is used ina lathe or screw machine. In both processes the cut is performed in

one pass of the broach, which makes it very efficient. Commonly machined surfaces include circular and non-circular

holes, spline and flat surfaces. Even though broaches can beexpensive, broaching is usually favorable to other processes whenused for high-quantity production runs.

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Internal broach

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B hi P

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Broaching Process Two types of broaching processes are in common:-

i) Surface broaching

It is very simple as either the work piece is moved against astationary surface broach, or the work piece is held stationary whilethe broach is moved against it.

ii) Internal broaching

This is more commonly used process. The process begins by

either clamping the work piece into the work holder of thebroaching machine or the broach is held in the clamp and the workpiece is moved. Through relative motion (pull or push) the broach ispassed through the work piece completely leaving behind thefinished work piece.

The broach usually only moves linearly, but sometimes it is alsorotated to create a spiral spline or gun-barrel rifling. Most broachesare made from High speed steel (HSS) or an alloy steel or tungstencarbide. Tin coatings are common on HSS to prolong life.

Except when broaching Cast iron, tungsten carbide is rarely usedas a tooth material because the cutting edge will crack on the first

pass.

T f f b h B h

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Types of surface broach Broaches i) The slab broach

It is the simplest surface broach. It is a general purpose tool for cutting flatsurfaces.

ii) Slot broache sThese are cut slots of various dimensions at high production rates. Slot

broaching is much quicker than milling when more than one slot needs to bemachined, because the broach can produce both slots at the same time.

iii) Contour broachesThse are designed to cut concave, convex, cam-, contoured, and irregular

shaped surfaces.

iv) Pot broaches These are cut the inverse of an internal broach; they cut the outside diameter

of a cylindrical workpiece. The broach is held stationary while the workpiece is

pushed or pulled through it.

This has replaced hobbing for some involute gearsand cutting external splines and slots.

v) Straddle broaches They use two slab broaches to cut parallel surfaces on opposite sides of a

workpiece in one pass. This type of broaching holds closer tolerances than if thetwo cuts were done independently.

T f i t l b h B h

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Types of internal broach Broaches

Hollow or shellbroaches

These are internal cutting broaches for cutting large

diameters. They are designed to mount on an arbor.

This is cheaper to produce than a solid broach.

Keywaybroach

This a common type of internal broach. It uses aspecial fixture called a horn to support the broach and

properly locate the part with relations to the broach.

Concentricity broach

It is a is a special type of spline cutting broach whichcuts both the minor diameter and the spline form to

ensure precise concentricity.

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Broaching Machines A broaching machine, or broach, uses tall chisel points to apply

linear shearing and scraping motions to the given material. Broachesare often used to create noncircular shapes out of holes that havebeen previously punched in the metal. They also cut splines andkeyways on gears and pulleys. Rotary broaches are a uniquesubsection of broaching machines, used in conjunction with a lathe

to create a simultaneous horizontal and vertical cutting motion. Broaching machines are relatively simple as they only have to move

the broach in a linear motion at a predetermined speed and provide ameans for handling the broach automatically. Most machines arehydraulic , but a few specialty machines are mechanically driven.

The machines are distinguished by whether their motion ishorizontal or vertical. The choice of machine is primarily dictated bythe stroke required. Vertical broaching machines rarely have a strokelonger than 60 in (1.5 m).

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Types of Broaching Machines

These are classified as vertical and horizontal

i) Vertical broaching

These machines can be designed for pushbroaching, pull-down broaching, pull-upbroaching, or surface broaching. Pushbroaching machines are similar to an arbor

press with a guided ram; typical capacities are5 to 50 tons.

Most surface broaching is done on a verticalmachine.

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l b h

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ii) Horizontal broaching These machines are designed for pull

broaching, surface broaching, continuousbroaching, and rotary broaching. Surface stylemachines hold the broach stationary while theworkpieces are clamped into fixtures that aremounted on a conveyor system. Continuous stylemachines are similar to the surface style machinesexcept adapted for internal broaching.

Horizontal machines used to be much more

common than vertical machines, however todaythey represent just 10% of all broaching machinespurchased. Vertical machines are more popularbecause they take up less space.

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Hydraulic Cylinderical of a

Broaching Machine

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iii) Rotary Broaching

To create a spiral spline or gun-barrel rifling rotarybroaching is used. The Rotary broaching requires twotooling components: a tool holder and a broach. Thebroach is free to rotate within the tool holder. If the

work piece rotates, the broach is pressed against it, isdriven by it, and rotates synchronously with it. If thetool holder rotates, the broach is pressed against thework piece, but is driven by tool holder rotation.

In general, a rotary broach will not cut asaccurately as a push or pull broach. However, theability to use this type of cutting tool on high-production machinery such as a screw machine, andeliminate secondary operations, makes this a

desirable manufacturing method.

Usage

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Usage Broaching was originally developed for machining internal keyways.

However, it was soon discovered that broaching is very useful formachining other surfaces and shapes for high volume workpieces.

A customized broach is usually only viable with high volumeworkpieces, because the broach can easily cost $15,000 to $30,000to produce.

Broaching speeds vary from 20 surface feet per minute (SFPM) to120 SFPM. This results in a complete cycle time of 5 to 30 seconds.

Most of the time is consumed by the return stroke, broach handling,and work piece loading and unloading.

Broaching works best on softer materials, such as brass,bronze,copper alloys, aluminium, graphite, hard rubbers, wood,composites, and plastic. However, it still has a good machinabilityrating on mild steels and free machining steels. When broaching themachinability rating is closely related to the hardness of thematerial. For steels the ideal hardness range is between 16 and 24Rockwell C (HRC); a hardness greater than HRC 35 will dull thebroach quickly. Broaching can also be used on harder materials, likestainless steel and titanium, but it is tougher.

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Usage

The only limitations on broaching are that the workpiece is strongenough to withstand the forces involved. Specifically for internalbroaching a hole must first exist in the workpiece so the broach canenter.Also, there are limits on the size of internal cuts. Common internalholes can range from 0.125 to 6 in (3.2 to 150 mm) in diameter but it ispossible to achieve a range of 0.05 to 13 in (1.3 to 330 mm). Surface

broaches' range is usually 0.075 to 10 in (1.9 to 250 mm), although thefeasible range is 0.02 to 20 in (0.51 to 510 mm).

Broaching works best on softer materials, such asbrass, bronze, copperalloys, aluminium, graphite, hard rubbers, wood, composites, andplastic. However, it still has a good machinability rating on mild steelsand free machining steels. When broaching the machinability rating is

closely related to the hardness of the material. For steels the idealhardness range is between 16 and 24 Rockwell C (HRC); a hardnessgreater than HRC 35 will dull the broach quickly. Broaching can also beused on harder materials, like stainless steel and titanium, but it istougher.

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Broached part (spline)

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Gear manufacturing

Gear

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Gear

A gear is a rotating machine having cut teeth, orcogs, which mesh with another toothed part in

order to transmit torque. Two or more gearsworking in tandem are called a transmission andcan produce a mechanical advantage through agear ratio and thus may be considered a simple

machine. Geared devices can change the speed,magnitude, and direction of a power source. Themost common situation is for a gear to meshwith another gear, however a gear can also mesh

a non-rotating toothed part, called a rack,thereby producing translation instead ofrotation. An advantage of gears is that the teethof a gear prevent slipping.

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Two meshing gears transmitting rotational motion.

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Types Of Gears

i) External vs. internal gearsAn external gearis one with the teeth

formed on the outer surface of a cylinder or

cone. Conversely, an internal gearis one withthe teeth formed on the inner surface of acylinder or cone.

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Types of Gears

ii) Spur gear

Spur gears or straight-cut gears are the simplesttype of gear. They consist of a cylinder or disk, andwith the teeth projecting radially, and althoughthey are not straight-sided in form, the edge of

each tooth thus is straight and aligned parallel tothe axis of rotation. These gears can be meshedtogether correctly only if they are fitted to parallelaxles. In spur gears teeth suddenly meet at a linecontact across their entire width causing stress

and noise. Spur gears are used for low speedand less torque applications and thosesituations where noise control is not a problem

iii) H li l

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iii) Helical gears Helical gears offer a refinement over spur gears. The

leading edges of the teeth are not parallel to the axis of

rotation, but are set at an angle. Since the gear is curved,this angling causes the tooth shape to be a segment of ahelix. Helical gears can be meshed in aparallelor crossedorientations. The former refers to when the shafts areparallel to each other; this is the most common

orientation. In the latter, the shafts are non-parallel. Theangled teeth engage more gradually than do spur gearteeth causing them to run more smoothly and quietly.The use of helical gears is indicated when the applicationinvolves high speeds, large power transmission, or where

noise abatement is important. A disadvantage of helical gears is a resultant thrust along

the axis of the gear, which needs to be accommodated byappropriate thrust bearings, and a greater degree ofslidingfriction between the meshing teeth, often addressed with

additives in the lubricant.

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Helical gears

Top: parallel configuration

Bottom: crossed configuration

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iv) Double helical Double helical gears, or herringbone gears,

overcome the problem of axial thrust presentedby "single" helical gears by having two sets of

teeth that are set in a V shape. Each gear in adouble helical gear can be thought of as twostandard mirror image helical gears stacked. Thiscancels out the thrust since each half of the gear

thrusts in the opposite direction. Double helicalgears are more difficult to manufacture due totheir more complicated shape.

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Double Helical Gears

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v) Bevel gears A bevel gear is shaped like a right circular

cone with most of its tip cut off. When two bevelgears mesh their imaginary vertexes mustoccupy the same point. Their shaft axes alsointersect at this point, forming an arbitrary non-

straight angle between the shafts. The anglebetween the shafts can be anything except zeroor 180 degrees. Bevel gears with equal numbersof teeth and shaft axes at 90 degrees are called

miter gears. The teeth of a bevel gear may be straight-cut

as with spur gears, or they may be cut in a varietyof other shapes.

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Bavel Gear

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Types of Gears

Hypoid Hypoid gears resemble spiral bevel gears except the shaft

axes do not intersect. The pitch surfaces appear conical but,to compensate for the offset shaft, are in fact hyperboloids

of revolution. Hypoid gears are almost always designed tooperate with shafts at 90 degrees. Depending on which sidethe shaft is offset to, relative to the angling of the teeth,contact between hypoid gear teeth may be even smootherand more gradual than with spiral bevel gear teeth. Also,the pinion can be designed with fewer teeth than a spiral

bevel pinion, with the result that gear ratios of 60:1 andhigher are feasible using a single set of hypoid gears. Thisstyle of gear is most commonly found in mechanicaldifferentials.

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Hypoid Gear

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vi) Crown

Crown gears or contrate gears are a particularform of bevel gear whose teeth project at right

angles to the plane of the wheel; in their

orientation the teeth resemble the points on a

crown. A crown gear can only mesh accuratelywith another bevel gear, although crown gearsare sometimes seen meshing with spur gears. A

crown gear is also sometimes meshed with an

arrangement such as found in mechanical clocks.

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Crown gear

ii) Worm

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vii) Worm Worm gears resemble screws. A worm gear is usually meshed with

an ordinary looking, disk-shaped gear, which is called thegear,wheel, or worm wheel.

Worm-and-gear sets are a simple and compact way to achieve ahigh gear ratio. For example, helical gears are normally limited togear ratios of less than 10:1 while worm-and-gear sets vary from10:1 to 500:1. A disadvantage is the potential for considerablesliding action, leading to low efficiency.

Worm gears can be considered a species of helical gear, but its helixangle is usually somewhat large (close to 90 degrees) and its bodyis usually fairly long in the axial direction; and it is these attributeswhich give it its screw like qualities. The distinction between aworm and a helical gear is made when at least one tooth persists

for a full rotation around the helix. If this occurs, it is a 'worm'; ifnot, it is a 'helical gear'. A worm may have as few as one tooth.

In a worm-and-gear set, the worm can always drive the gear.However, if the gear attempts to drive the worm, it may or may notsucceed.

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Worm Gear

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Types of Gears

viii) Non-circular gears.

Non-circular gears are designed for special

purposes. While a regular gear is optimized to

transmit torque to another engaged memberwith minimum noise and wear and maximum

efficiency, a non-circular gear's main objective

might be ratio variations, axle displacement

oscillations and more. Common applications

include textile machines, potentiometers and

continuously variable transmissions.

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Non-Circular Gear

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ix) Rack and pinion

A rack is a toothed bar or rod that can bethought of as a sector gear with an infinitelylarge radius of curvature. Torque can beconverted to linear force by meshing a rack

with a pinion: the pinion turns; the rackmoves in a straight line. Such a mechanism isused in automobiles to convert the rotationof the steering wheel into the left-to-right

motion of the tie rod(s).

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Rack and pinion

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Types of Gears

Epicyclic

In epicyclic gearing one or more of the gearaxes moves. Examples are sun and planetgearing (see below) and mechanicaldifferentials

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x) Sun and planet

Sun and planet gearing

was a method ofconverting reciprocalmotion into rotarymotion in steam

engines. It played animportant role in theIndustrial Revolution.The Sun is yellow, the

planet red, thereciprocating crank isblue, the flywheel isgreen and the driveshaftis grey

Sun (yellow) and planet (red) gearing

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Materials for gear manufacturing

The majority of gears are composed ofcarbon and low-alloy steels, includingcarburised steels. Among the carburised

steels used in gears are 1018, 1524, 4026,4118, 4320, 4620, 4820, 8620 and 9310. Theintended gear use will dictate the materialused in its creation. For example gears to be

used in food processing are made of stainlesssteels or nickel-base alloys because of theircorrosion resistance.

Gear mfg.processes

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g p There are multiple ways in which gear blanks can be shaped through the

cutting and finishing processes.

Blanking

Gear Cutting Processes Broaching

Hobbing

Shaping

Milling

Gear Finishing Processes Grinding

Honing

Shaving

Lapping

Gear Forming (non-cutting) Processes Plastic Injection Molding

Powder Metal Sintering

Forging

Casting

Roll-Forming (Spline Rolling)

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Blanking

Blanking refers to initial forming ormachining operations that produce a semifinished part ready for gear cutting, starting

from a piece of raw material.

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Gear Cutting Processes

i) BroachingBroaching is a machining operation which

uses a toothed tool, called a broach, toremove material.

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ii) Hobbing Hobbing is a machining process for making

gears, splines, and sprockets on a hobbingmachine, which is a special type ofmillingmachine. The teeth or splines are progressivelycut into the workpiece by a series of cuts made

by a cutting tool called a hob. Compared to othergear forming processes it is relativelyinexpensive but still quite accurate, thus it isused for a broad range of parts and quantities.[1]

It is the most widely used gear cutting processfor creating spur and helical gears[2] and moregears are cut by hobbing than any other processsince it is relatively quick and inexpensive.

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A hob the cutter used for

hobbing

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iii) Shaping

The cutter is a circular pinion-shaped cutterwith the necessary rake angles to cut asshown. Both the gear blank and cutter are setin a vertical plane and rotated such as thatthe two are like gears in mesh. Gear shapingis faster than gear planing because thecutting process is continuous and the cutter

does not have to be stepped back.

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Gear Shaping

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iv) Milling Milling is a form-cutting process limited to

making single gears for prototype or very smallbatches of gears as it is a very slow anduneconomical method of production. A involute

form-milling cutter, which has the profile of thespace between the gears, is used to remove thematerial between the teeth from the gear blankon a horizontal milling machine.

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Gear Milling

Gear Finishing Processes

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g Grinding

A grinding machine is a machine tool used for

grinding, which is a type ofmachining using an abrasivewheel as the cutting tool. Each grain of abrasive on thewheel's surface cuts a small chip from the workpiece viashear deformation.

The grinding machine consists of a power drivengrinding wheel spinning at the required speed and a bedwith a fixture to guide and hold the work-piece. Thegrinding head can be controlled to travel across a fixed

work piece or the workpiece can be moved whilst thegrind head stays in a fixed position. Very fine control ofthe grinding head or tables position is possible using avernier calibrated hand wheel, or using the features of

numerical controls.

ii) Honing

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ii) HoningHoningis an abrasive machining process that

produces a precision surface on a metal workpiece byscrubbing an abrasive stone against it along a controlledpath. Honing is primarily used to improve the geometricform ofa surface, but may also improve the surfacetexture.

Typical applications are the finishing ofcylinders forinternal combustion engines, air bearingspindles andgears. Types of hone are many and various but allconsist of one or more abrasive stones that are heldunder pressure against the surface they are working on.

In everyday use, a honing steel is used to honeknives, especially kitchen knives, and is a fine process,there contrasted with more abrasive sharpening.

Other similar processes are lapping and superfinishing.

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iii) Shaving

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iii) Shaving Gear shaving is the most commonly used method for

finishing spur and helical gear teeth prior to hardening.

The gear is run in contact with the shaving tool. Such atool is very hardened, accurate and ground gear thatcontains a number of peripheral serration, thusforming a series of cutting edges on each tooth. The

gear and shaving cutters are run in mesh with theiraxes crossed at a small angle usually about 10 degrees.As they rotate, the gear is reciprocated longitudinallyacross the shaving tool (or vice versa). During this

action, which usually requires less than one minute,very fine chips are shaved from the gear tooth faces,thus eliminating any high spots and producing a veryaccurate tooth profile.

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Gear shaving

iv)Lapping

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pp g Lapping is a machining operation, in which two

surfaces are rubbed together with an abrasivebetween them, by hand movement or by way of amachine.

This typically involves rubbing a brittle material

such as glass against a surface such as iron or glassitself with an abrasive such as aluminum oxide,emery, silicon carbide, diamond, etc., in between

them. This produces microscopic conchoidalfractures as the abrasive rolls about between thetwo surfaces and removes material from both.