RTU Paper Solution Branch Mechanical Engineering Paper ... · Global Institute of Technology,...
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Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
RTU Paper Solution
Branch – Mechanical Engineering
Subject Name – Manufacturing Technology
Paper Code –5ME4-03
Date of Exam – 20/11/2019
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year Part-A
1. Cutting tools can be classified in various ways; however the most common way is based on
the number of main cutting edges that participates in cutting action at a time. On this basis,
cutting tools can be classified into three groups as given below.
Single point cutting tool—Such cutters have only one main cutting edge that
participate in cutting action at a time. Examples include turning tool, boring tool, fly
cutter, slotting tool, etc.
Double point cutting tool—As the name implies, these tools contain two cutting
edges that simultaneously participate in cutting action at a pass. Example includes
drill (common metal cutting drill that has only two flutes).
Multi-point cutting tool—These tools contain more than two main cutting edges that
can simultaneously remove material in a single pass. Examples include milling cutter,
broach, gear hobbing cutter, grinding wheel, etc.
2. Systems of description of tool geometry
Tool-in-Hand System – where only the salient features of the cutting tool point are
identified or visualized. There is no quantitative information, i.e.,value of the angles.
Machine Reference System – ASA system
Tool Reference Systems
Orthogonal Rake System – ORS
Normal Rake System – NRS
Work Reference System – WRS
3. Built-up-Edge (BUE)
In machining ductile metals like steels with long chip-tool contact length, lot of stress and
temperature develops in the secondary deformation zone at the chip-tool interface. Under
such high stress and temperature in between two clean surfaces of metals, strong bonding
may locally take place due to adhesion similar to welding. Such bonding will be encouraged
and accelerated if the chip tool materials have mutual affinity or solubility. The weldment
starts forming as an embryo at the most favourable location and thus gradually grows as
schematically shown in Fig
4. Zero rake – to simplify design and manufacture of the form tools. Clearance angle is
essentially provided to avoid rubbing of the tool (flank) with the machined surface which
causes loss of energy and damages of both the tool and the job surface. Hence, clearance
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year angle is a must and must be positive (3
о ~ 15
о depending upon tool-work materials and type
of the machining operations like turning, drilling, boring etc.)
5. Machinability index is used to compare the machinability of different materials in the
various cutting process. It is an attempt to quantify the relative machinability of different
material. The rated machinability may vary for different cutting operation such as turning,
milling, forming etc. In order to find the machinability index, factors like tool material, tool
geometry, tool life and other cutting conditions are fixed except the speed. Then find the
speed at which tool cut the material for a pre-determined tool life. Then it is compared with a
standard material. Here machinability of standard steel is arbitrarily fixed as 100%. The
slower speed indicates, low metal removal rate and hence poor machinability.
6. Factors Affecting Tool Life
The life of tool is affected by many factors such as: cutting speed, depth of cut, chip
thickness, tool geometry, material or the cutting fluid and rigidity of machine. Physical and
chemical properties of work material influence tool life by affecting form stability and rate of
wear of tools. The nose radius tends to affect tool life.
1. Cutting speed: Cutting speed has the greatest influence on tool life. As the cutting
speed increases the temperature also rises. The heat is more concentrated on the tool
than on the work and the hardness of the tool metrix changes so the relative increase
in the hardness of the work accelerates the abrasive action. The criterion of the wear is
dependent on the cutting speed because the predominant wear may be wear for flank
or crater if cutting speed is increased.
2. Feed and depth of cut: The tool life is influenced by the feed rate also. With a fine
feed the area of chip passing over the tool face is greater than that of coarse feed for a
given volume of swarf removal, but to offset this chip will be greater hence the
resultant pressure will nullify the advantage.
3. Tool Geometry: The tool life is also affected by tool geometry. A tool with large rake
angle becomes weak as a large rake reduces the tool cross-section and the amount of
metal to absorb the heat.
4. Tool material: Physical and chemical properties of work material influence tool life by
affecting form stability and rate of wear of tool.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year 5. Cutting fluid: It reduces the coefficient of friction at the chip tool interface and
increases tool life.
7. Cast iron is commonly used for machinery housings or bases due to the stable structure of
the material. It is also known for holding its shape when it is subjected to contraction and
expansion due to temperature fluctuations. This is ideal for a lathe bed
This material offers good damping, it is easy to machine, and can be made in various sizes.
Aging of the material can take up to a year and it can be more expensive than other materials
available.
8.
9. Grinding wheel is generally made from silicon carbide or aluminium oxide. It is
generally made up of particles of hard substance called the abrasive and is embedded
in a matrix called the bond. These abrasives form the cutting points in a wheel and are
termed as grains. The abrasives are of generally two types namely natural and
artificial. Emery and corundum are two natural abrasives, while carborundum and
aloxite are artificial abrasives. The hardness or softness of the wheel is dependent on
the amount and kind of the bonding material. Generally, hard wheels of aloxite are
used for grinding soft materials and soft wheels of carborundum for grinding hard
materials using various types of grinding machines known as grinders.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year 10. Truing is done when a new wheel is installed and before it’s used for the first time and is
necessary for precision grinding. The purpose of truing is to bring every point of the grinding
surface concentric with the machine spindle (to establish concentricity) and to introduce a
form (shape) into a wheel. No matter how precisely manufactured, there will be a slight gap
between the wheel bore and the machine spindle. Even if the gap is one thousand of a
millimeter, problems like chatter marks will occur if the wheel is not trued to the center of the
spindle. Conventional grinding wheels can be trued easily with a diamond cutter that is
harder than the wheel, while the superabrasives cannot be cut and must be ground to size.
This is done by using a sintered diamond roller or by traversing a conventional grinding
wheel.
Dressing is the process that comes after truing (especially in the case of superabrasives) and it
represents grinding wheel sharpening by exposing abrasive grits above the bond. In other
words, removing small chips of workpiece lodged in the wheel surface or removing dull
abrasives which returns the wheel to its original dimensions and provides crystal exposure.
The wheel surface after dressing is open with grits exposed.
Part-B
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Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
1.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year
Merchant’s Circle Diagram with cutting forces.
The significance of the forces displayed in the Merchant’s Circle Diagram is:
PS– the shear force essentially required to produce or separate the chip from the parent body
by shear
Pn– inherently exists along with PS
F – Friction force at the chip tool interface
N – Force acting normal to the rake surface
PZ– main force or power component acting in the direction of cutting velocity
The magnitude of PS provides the yield shear strength of the work material under the cutting
condition.
The values of F and the ratio of F and N indicate interaction like friction at the chip-tool
interface. The force components PX, PY, PZ are generally obtained by direct measurement.
Again
PZ helps in determining cutting power and specific energy requirement. The force
components are also required to design the cutting tool and the machine tool.
Advantages of using Merchant Circle Diagram (MCD)
Proper use of MCD enables the followings:
• Easy, quick and reasonably accurate determination several other forces from a few known
forces involved in machining
• Friction at chip-tool interface and dynamic yield shear strength can be easily determined
• Equation relating to different forces can be easily developed
2. Mechanisms and pattern (geometry) of cutting tool wear
For the purpose of controlling tool wear one must understand the various mechanisms of
wear
that the cutting tool undergoes under different conditions. The common mechanisms of
cutting
tool wear are:
1. Mechanical wear
a. thermally insensitive type; like abrasion, chipping and de-lamination
b. Thermally sensitive type; like adhesion, fracturing, flaking etc.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year 2. Thermo chemical wear
a. macro-diffusion by mass dissolution
b. micro-diffusion by atomic migration
3. Chemical wear
4. Galvanic wear
In diffusion wear the material from the tool at its rubbing surfaces, particularly at the rake
surface gradually diffuses into the flowing chips either in bulk or atom by atom when the tool
material has chemical affinity or solid solubility towards the work material. The rates of such
tool wear increases with the increase in temperature at the cutting zone.
Diffusion wear becomes predominant when the cutting temperature becomes very high due to
high cutting velocity and high strength of the work material. Chemical wear, leading to
damages like grooving wear may occur if the tool material is not enough chemically stable
against the work material and/or the atmospheric gases.
Galvanic wear, based on electrochemical dissolution, seldom occurs when both the work tool
materials are electrically conductive, cutting zone temperature is high and the cutting fluid
acts as an electrolyte. The usual pattern or geometry of wear of turning and face milling
inserts are typically shown in Fig
In addition to ultimate failure of the tool, the following effects are also caused by the growing
tool-wear:
increase in cutting forces and power consumption mainly due to the principal
flank wear
increase in dimensional deviation and surface roughness mainly due to wear of the tool-
tips and auxiliary flank wear (Vs)
odd sound and vibration
worsening surface integrity
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year Mechanically weakening of the tool tip.
3.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year 4.
Capston lathe
S.no Capstan Lathe Turret Lathe
1 It is a Light weight machine. It is a heavy weight machine.
2
In capstan lathe the turret
tool head is mounted over the
ram and that is mounted over
the saddle.
In turret lathe the turret tool
head is mounted over the
saddle like a single unit
3
For providing feed to the
tool, ram is moved.
For providing feed to the tool,
saddle is moved.
4
Because of no saddle
displacement, Movement of
turret tool head over the
longitudinal direction of bed
is small along with the ram.
Turret tool head move along
with the saddle over the entire
bed in the longitudinal
direction.
5
Used for shorter workpiece
because of limited ram
movement.
Used for longer workpiece
because of saddle movement
along the bed.
6
Its working operations are
fast because of lighter in
constructions.
Its working operations are
slower because of heavier in
constructions.
7
Heavy cuts on the workpiece
cannot be given because of
non-rigid construction.
Heavy cuts on the workpiece
can be given because of rigid
construction of machine.
8
For indexing turret tool head,
the hand wheel of the ram is
reversed and turret tool index
automatically.
For indexing turret tool head,
turret is rotated manually after
releasing clamping lever.
9
The turret head cannot be
moved in the lateral direction
of the bed.
The turret head can be moved
crosswise i.e. in the lateral
direction of bed in some turret
lathe.
10
In capstan lathe, Collet is
used to grip the Job.
In turret lathe, power Jaw
chuck is used to grip the Job.
11
Used for machining
workpiece up to 60 mm
diameter.
Used for machining workpiece
up to 120 mm diameter.
12
These are usually horizontal
lathes.
Turret lathes are available in
horizontal and vertical lathes.
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Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
Turret lathe
5. CUTTTING SPEED- The milling cutter is circular and a large number of cutting edges (or
teeth) are arranged along its circumference. The cutter is rotated at a speed of N r.p.m. If the
cutter diameter is D, then cutting speed at the tip of teeth can be calculated as ΠDN
metres/minute.
FEED-Feed of the work piece is measured in terms of mm/minute. Actually, the correct
measure of feed is movement of work piece per revolution of cutter per teeth. If a milling
cutter has z number of teeth and if the table feed is ‘f’ mm/minute, feed per rev per teeth will
be f/NZ mm. It should therefore be clear that metal removal rate in milling operation is much
higher than in shaping or planing operations.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year 6. Grinding, honing, and lapping are three finishing processes that can be used on hardened
gears. Gear grinding can be based on either of two methods. The first is form grinding, in
which the grinding wheel has the exact shape of the tooth spacing (similar to form milling),
and a grinding pass or series of passes are made to finish form each tooth in the gear. The
other method involves generating the tooth profile using a conventional straight-sided
grinding wheel. Both of these grinding methods are very time consuming and expensive.
Honing and lapping, are two finishing processes that can be adapted to gear finishing using
very fine abrasives. The tools in both processes usually possess the geometry of a gear that
meshes with the gear to be processed.Gear honing uses a tool that ismade of either plastic
impregnated with abrasives or steel coated with carbide. Gear lapping uses a cast iron tool
(other metals are sometimes substituted), and the cutting action is accomplished by the
lapping compound containing abrasives.
Honing, which is used to remove material from internal cylindrical surfaces to improve the
geometry of a part or produce a finer surface finish, is performed at a slow speed — much
slower than the cutting speeds typically used in precision grinding. The cutting action is
achieved by a rotating hone, which consists of bonded abrasive sticks or stones mounted on a
metal mandrel. The workpiece, rather than being clamped in place, is fixed to allow floating
and prevent distortion that would result in an oval rather than round hole.
The hone is rotated in a controlled path over the surface of the part. In some cases, a
machinist might move the workpiece back and forth over the rotating hone, ensuring that the
part is floating rather than being pressed against the hone, to avoid an oval hole. With a
horizontal honing machine, the workpiece may be held in a self-aligning fixture while the
machine controls the speed and length of the stroke; the hone is hydraulically or mechanically
expanded until the desired hole diameter is achieved. With honing, a cutting fluid must also
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year be used to clean small chips from the work area, cool the workpiece and hone, and lubricate
the cutting action.
Lapping is used to achieve surfaces that are very flat and smooth, as well as to finish round
work, such as precision plug gages, to very tight tolerances. The process is more gentle than
honing and removes much less surface material. Therefore, the workpiece should be as close
as possible to final size — achieved, for example, through double disk grinding — because
lapping typically removes only 0.0005” to 0.005” (0.0127 mm to 0.127 mm) of material.
Unlike honing, lapping uses fine-grained, loose abrasive particles suspended in a viscous or
liquid base rather than a bonded abrasive stick or stone. The lapping process involves passing
the workpiece between one or two large, very flat lap plates along with the abrasive
suspension. Close attention is paid to controlling every detail, including the speed of the
plate(s), the pressure on the workpiece, the size and type of abrasive used, the feeding
method, and the plate temperature.
7. High Velocity Forming
The concept of high velocity forming of metal is one of the newest technological advantages
in manufacturing. These processes have proved to be very useful in solving many fabrication
processes where conventional processes are find more difficult and more costly. Increase in
size of the work piece highly heat resistant materials, deep recessing, shallow recessing and
bulging operations are the examples which led to the development of high velocity forming
methods. A major advantage of high velocity forming is the ability to form one piece
complex part shapes in single operation, where as conventional methods require several
operations and result in a welded structure.
The variety of energy sources and techniques for applying the energy to accomplish
deformation of work piece makes the scope of high velocity forming as broad as the field of
metal working operation like draw forming, cupping, bulging, swaying, flanging joining. The
other application is die forming cutting, welding and surface hardening. The variety of
materials that have been fabricated with velocity methods includes magnesium, aluminum,
beryllium, titanium, zirconium, carbon and stainless steel, superalloy and the refractory
metals and alloys.
The process is based on the principle of deformation of metal by using very high velocities,
provided on the movements of rams and dies. Since the kinetic energy is proportional to the
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year square of the velocity, high energy is delivered to the metal with relatively small weight (ram
or die). It reduces the cost and size of the machine. Since accelerations are high, high
velocities are obtained by using short stroked of the ram. This increases the rate of
production.
There are three main high energy rate forming processes:
1. Explosive forming,
2. Magnetic forming,
3. Electro hydraulic forming.
Explosive Forming
Explosive forming, is distinguished from conventional forming in that the punch or
diaphragm is replaced by an explosive charge. The explosives used are generally high –
explosive chemicals, gaseous mixtures, or propellants. There are two techniques of high –
explosive forming: stand – off technique and the contact technique.
Standoff Technique . The sheet metal work piece blank is clamped over a die and the
assembly is lowered into a tank filled with water. The air in the die is pumped out. The
explosive charge is placed at some predetermined distance from the work piece. On
detonation of the explosive, a pressure pulse of very high intensity is produced. A gas bubble
is also produced which expands spherically and then collapses. When the pressure pulse
impinges against the work piece, the metal is deformed into the die with as high velocity as
120 m/s.
Applications. Explosive forming is mainly used in the aerospace industries but has also found
successful applications in the production of automotive related components. The process has
the greatest potential in limited – production prototype forming and for forming large size
components for which conventional tooling costs are prohibitively high.
Electro Magnetic Forming
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year The process is also called magnetic pulse forming and is mainly used for swaging type
operations, such as fastening fittings on the ends of tubes and crimping terminal ends of
cables. Other applications are blanking, forming, embossing, and drawing. The work coils
needed for different applications vary although the same power source may be used.
To illustrate the principle of electromagnetic forming, consider a tubular work piece. This
work piece is placed in or near a coil. A high charging voltage is supplied for a short time to a
bank of capacitors connected in parallel. (The amount of electrical energy stored in the bank
can be increased either by adding capacitors to the bank or by increasing the voltage). When
the charging is complete, which takes very little time, a high voltage switch triggers the
stored electrical energy through the coil. A high – intensity magnetic field is established
which induces eddy currents into the conductive work piece, resulting in the establishment of
another magnetic field. The forces produced by the two magnetic fields oppose each other
with the consequence that there is a repelling force between the coil and the tubular work
piece that causes permanent deformation of the work piece.
Applications
Electromagnetic forming process is capable of a wide variety of forming and assembly
operations. It has found extensive applications in the fabrication of hollow, non – circular, or
asymmetrical shapes from tubular stock. The compression applications involve swaging to
produce compression, tensile, and torque joints or sealed pressure joints, and swaging to
apply compression bands or shrink rings for fastening components together. Flat coils have
been used on flat sheets to produce stretch (internal) and shrink (external) flanges on ring and
disc – shaped work pieces.
Electromagnetic forming has also been used to perform shearing, piercing, and rivettting.
Electro Hydraulic Forming
Electro hydraulic forming (EHF), also known as electro spark forming, is a process in which
electrical energy is converted into mechanical energy for the forming of metallic parts. A
bank of capacitors is first charged to a high voltage and then discharged across a gap between
two electrodes, causing explosions inside the hollow work piece, which is filled with some
suitable medium, generally water. These explosions produce shock waves that travel radially
in all directions at high velocity until they meet some obstruction. If the discharge energy is
sufficiently high, the hollow work piece is deformed. The deformation can be controlled by
applying external restraints in the form of die or by varying the amount of energy released.
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Advantages
1. EHF can form hollow shapes with much ease and at less cost compared to other forming
techniques.
2. EHF is more adaptable to automatic production compared to other high energy rate forming
techniques.
3. EHF can produce small – to intermediate sized parts that don't have excessive energy
requirements.
Accuracy of parts produced Accuracy of electro hydraulically formed parts depends on the control of both the magnitude
and location of energy discharges and on the dimensional accuracy of the dies used. With the
modern equipment, it is now possible to precisely control the energy within specified limits,
therefore the primary factor is the dimensional accuracy of the die. External dimensions on
tubular parts are possible to achieve within ± 0.05 mm with the current state of technology.
Part-C
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Subject Code_5ME4-03 V Semester/3rd Year 2.
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Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
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Subject Code_5ME4-03 V Semester/3rd Year 3. Selection of Cutting Fluid
The benefit of application of cutting fluid largely depends upon proper selection of the type
of the cutting fluid depending upon the work material, tool material and the machining
condition. As for example, for high speed machining of not-difficult-to-machine materials
greater cooling
type fluids are preferred and for low speed machining of both conventional and difficult-to
machine materials greater lubricating type fluid is preferred. Selection of cutting fluids for
machining some common engineering materials and operations are presented as follows :
• Grey cast iron: Generally dry for its self lubricating propertyAir blast for cooling and
flushing
chips.Soluble oil for cooling and flushing chips in high speed machining and grinding
• Steels : if machined by HSS tools, sol. Oil (1: 20 ~30) for low carbon and alloy steels and
neat oil with EPA for heavy cuts .If machined by carbide tools thinner sol. Oil for low
strength
steel, thicker sol.Oil ( 1:10 ~ 20) for stronger steels and staright sulphurised oil for heavy and
low speed cuts and EP cutting oil for high alloy steel. Often steels are machined dry by
carbide tools for preventing thermal shocks.
•Aluminium and its alloys: Preferably machined dry Light but oily soluble oil Straight neat
oil
or kerosene oil for stringent cuts.
• Copper and its alloys: Water based fluids are generally used Oil with or without inactive
EPA for tougher grades of Cu-alloy.
• Stainless steels and Heat resistant alloys: High performance soluble oil or neat oil with
high concentration with chlorinated EP additive.
The brittle ceramics and cermets should be used either under dry condition or light neat oil in
case of fine finishing.
Grinding at high speed needs cooling ( 1:50 ~ 100) soluble oil. For finish grinding of metals
and
alloys low viscosity neat oil is also used.
4. Marking system for conventional grinding wheel:
The standard marking system for conventional abrasive wheel can be as follows:
51 A 60 K 5 V 05, where
The number ‘51’ is manufacturer’s identification number indicating exact kind of
abrasive used.
The letter ‘A’ denotes that the type of abrasive is aluminium oxide. In case of silicon
carbide the letter ‘C’ is used.
The number ‘60’ specifies the average grit size in inch mesh. For a very large size grit
this number may be as small as 6 where as for a very fine grit the designated number may
be as high as 600.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year The letter ‘K’ denotes the hardness of the wheel, which means the amount of force
required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can
range between ‘A’ and ‘Z’, ‘A’ denoting the softest grade and ‘Z’ denoting the hardest
one.
The number ‘5’ denotes the structure or porosity of the wheel. This number can assume
any value between 1 to 20, ‘1’ indicating high porosity and ‘20’ indicating low porosity.
The letter code ‘V’ means that the bond material used is vitrified. The codes for other
bond materials used in conventional abrasive wheels are B (resinoid), BF (resinoid
reinforced), E(shellac), O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate)
The number ‘05’ is a wheel manufacturer’s identifier.
Marking system for superabrasive grinding wheel:
Marking system for superabrasive grinding wheel is somewhat different as illustrated
below
R D 120 N 100 M 4, where
The letter ‘R’ is manufacture’s code indicating the exact type of superabrasive used.
The letter ‘D’ denotes that the type of abrasive is diamond. In case of cBN the letter ‘B’ is
used.
The number ‘120’ specifies the average grain size in inch mesh. However, a two number
designation (e.g. 120/140) is utilized for controlling the size of superabrasive grit.
Like conventional abrasive wheel, the letter ‘N’ denotes the hardness of the wheel.
However, resin and metal bonded wheels are produced with almost no porosity and
effective grade of the wheel is obtained by modifying the bond formulation.
The number ‘100’ is known as concentration number indicating the amount of abrasive
contained in the wheel. The number ‘100’ corresponds to an abrasive content of 4.4
carats/cm3
. For diamond grit, ‘100’ concentration is 25% by volume. For cBN the
corresponding volumetric concentration is 24%.
The letter ‘M’ denotes that the type of bond is metallic. The other types of bonds used in
superabrasive wheels are resin, vitrified or metal bond, which make a composite structure
with the grit material. However, another type of superabrasive wheel with both diamond
and cBN is also manufactured where a single layer of superabrasive grits are bonded on a
metal perform by a galvanic metal layer or a brazed metal layer.
5. Hobbing machines provide gear manufacturers a fast and accurate method for cutting
parts. This is because of the generating nature of this particular cutting process. Gear
hobbing is not a form cutting process, such as gashing or milling where the cutter is a
conjugate form of the gear tooth. The hob generates a gear tooth profile by cutting several
facets of each gear tooth profile through a synchronized rotation and feed of the work piece
and cutter.
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year Figure 1
As the hob feeds across the face of the work piece at a fixed depth, gear teeth will gradually
be generated by a series of cutting edges, each at a slightly different position. The number of
cuts made to generate the gear tooth profile will correspond to the number of gashes of the
hob. Simply put, more gashes produce a more accurate profile of the gear tooth.
The hobs several cutting edges will be working simultaneously, which provide significant
potential for fast cutting speeds and/or short cycle times. With this realization, one can see
the hobbing process’s advantage over other cutting processes.
All gear hobbing machines, whether mechanical or CNC, consist of five common elements,
which are listed below and shown in Figure 2.
1. A work spindle to rotate the work piece (shown in blue)
2. A cutter spindle to rotate the cutting tool, the hob (shown in yellow)
3. A means to rotate the work spindle and cutter spindle with an exact ratio, depending on
the number of teeth of the gear and the number of threads of the hob (shown in red)
4. A means to traverse the hob across the face of the work piece (shown in green)
5. A means to adjust the center distance between the hob and work piece for different size
work pieces and hobs
figure 2
Global Institute of Technology, Jaipur ITS-1, IT Park, EPIP, Sitapura Jaipur 302022 (Rajasthan)
Solution V sem University Examination 2019
Subject Code_5ME4-03 V Semester/3rd Year
While the hob and work piece are rotating, the hob normally feeds axially across the
gear face at the gear’s tooth depth to cut and produce the gear. In conventional
hobbing, the direction of feed matches the direction of the cutting motion.
Alternatively, in climb feeding, the feed is opposite to the direction of the cutting
motion. Generally, conventional hobbing produces a better finish, whereas climb
hobbing yields better tool life. For either method, the cutting forces of the hob should
be directed towards the work spindle and not the tailstock.
Figure 3
To cut a helical gear, a standard hob cutter can be used. Mechanical hobbing
machines provide a differential motion through a series of change gears to generate a
gear tooth helix. Today, CNC hobbing machines electronically provide this
necessary differential to produce helical gears.