report 4 machining
TRANSCRIPT
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Q1. Discuss the parameters that could control the
machining time.
Answer:
Total machining time is being controlled by controlling thefollowing essential parameters:
Workpiece machining time: that should be calculated exactly according
to the type of the material of the cutting edge and the work piece to
avoid the damage that could occur to any of them.
Time taken to load and unload the part: should be minimized as
possible.
Machining Time
Reproductive Time Non-Reproductive Time
Time in which the workpiece
is being machined (time spent
in removing metal.
Time taken to load and
unload the part (workpiece.
Time taken to change the
cutting tool when its edge is
worn out.
Time taken to move andposition the cutting tool
between cuts.
Time taken for the cutting
tool to retrieve its starting
position.
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Time taken to position and move the cutting tool and attach it to the
workpiece: that depends on the experience and the accuracy of the
labor.
Time taken to change the cutting tool when its edge warn out. Time taken for the cutting tool to retrieve its position: after completing
cutting process for each part the tool must retrieve to its starting
position, this time must be as minimum as possible.
Worth mentioning that the using ofNCand CNCmachines
helped in controlling the previous parameters and minimizing all
the consumed time which helped in improving the industrial
economics and quality.
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Q2. Derive an equation to actually estimate the total
machining time (turning machining time). Solve the
given example to calculate the total time.
Answer:
The total time, ttotal to machine a part by turning has three
contributions:
1. The time tload taken to load and unload the part to and from
a machine tool.
2. The time tactive in the machine tool.
3. A contribution to the time taken to change the turning tool
when its edge is worn out.
Tactive is longer than the actual machining time tmach, because the
tool spends some time moving and being positioned between
cuts.
tactive may be written tmach/fmach where fmach is the fraction of the
time spent in removing metal.
If machining N parts results in the tool edge being worn out, thetool change time tct allocated to machining one part is tct/N.
So, total machining time will be:
(1
It is easy to show that as the cutting speed of a process isincreased, ttotal passes through a minimum value. This is because,
although the machining time decreases as speed increases, tools
wear out faster and N also decreases. Suppose the volume of
material to be removed by turning is written Vvol, so:
(2
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The machining time for N parts is N times this. If the time for N
parts is equated to the tool life time T in the equations:
(3
(generalized to VTn =C , N may be written in terms of n and C,
f, d, Vvol and V, as
(4
Substituting equation 4 and 2 in 1, we get that:
An example, to show how the time to reduce the diameter of
the tube stock from 100mm to 50mm,over the length of50mm,depends on both what tool material (the influence of n
and C and how advanced a machine technology is being used
(the influence offmach and tct.
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In this example, Vvol = 2.95105 mm3.
It is supposed that turning is carried out at a feed and depth of
cut of 0.25mm and 4mm respectively.
tloadis 1min (an appropriate value for a component of this size,
according to Boothroyd and Knight,1989.
Times have been estimated for high speed steel, cemented
carbide and an alumina ceramic tool material, in solid, brazed or
insert form, used in mechanical or simple CNC lathes or in
machining centers.
n and C values have been taken from equation (3. Thefmach and tct values are listed in the following Table. The
variation offmach with machine tool development has been based
on active non-productive time changes.
tctvalues for solid or brazed and insert cutting tools have been
taken from Table 1.1 on page 95.
Results are shown in the following Figure:
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The major influence of tool material on minimum
manufacturing time:
From around 30min to 40min for high speed steel, to 5min
to 8min for cemented carbide, to around 3min for aluminaceramic.
The time saving comes from the higher cutting speeds
allowed by each improvement of tool material, from
20m/min for high speed steel, to around 100m/min for
carbide, to around 300m/min for the ceramic tooling.
For each tool material, the more advanced the
manufacturing technology, the shorter the time. Changing
from mechanical to CNC control reduces minimum timefor the high speed steel tool case from 40min to 30min.
Changing from brazed to insert carbide with a simple CNC
machine tool reduces minimum time from 8min to
6.5min,while using insert tooling in a machining centre
reduces the time to 5min.
Only for the ceramic tooling are the times relatively
insensitive to technology: this is because, in this example,
machining times are so small that the assumed workload/unload time is starting to dominate.
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Q1. In details discuss the hot and cold metal working
processes for metals.Answer:
Cold MetalworkingHot MetalworkingPoints
Deformation carried out
under conditions where
recovery processes are noteffective.
Deformation under
conditions of temperature
and strain rate such that
recrystallisation process take
place simultaneously with the
deformation.
Definition
rolling, forging, extrusion,
wire/tube drawing,
swaging, coiningrolling, forging, extrusionExamples
Normally performed at room
temperature, where recovery islimited and recrystallisation
does not occur.
Work hardening occurs
(strength and hardness increase
but ductility decreases.
The extent of deformation is
rather limited if cracks are to be
avoid, therefore intermediate
anneals that enablerecrystallisation are frequently
used afterwards.
The materials suitable for cold
working should have a relatively
low yield stress and a relatively
high work hardening rate
(determined primarily by its
tensile properties.
Involves deformation at
temperatures where
recrystallisation can occur.
The minimum temperature atwhich reformation of the
crystals occurs is called
Recrystallisation Temperature.
Recrystallisation takes place at
higher temperatures than
recovery which leads to a new
formation of grains.
Above the recrystallisation
temperature the kinetic energyof atoms increases and
therefore they are able to attach
themselves to the newly
formed nuclei which in turn
begin to grow into crystals.
This process continues until all
the distorted crystals have been
transformed.
Hot working results in grainrefining.
Process
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Provide work hardening,
materials are stronger.
Provide fine grain size
and good surface finish.
Dimension tolerance is
better than in hot
working.Easier handling (low
operating temperatures.
Higher ductility more
deformation without
cracking.
Lower flow stress lessmechanical energy required
for deformation.
Pores seal up.
Smaller grain size.
Micro segregation is much
reduced or removed due to
atomic diffusion, which is
higher at hightemperatures.
Stronger, tougher and more
ductile than as-cast metals
due to breaking down and
refinement of coarse
columnar grains in the cast
ingot.
Advantages
Use high amount of
deformation due to low
operating temperatures,therefore, require soft
materials.
Equipment (rolls, dies,presses is big and
expensive.
Reduced ductility,
therefore, require
subsequent annealing
treatments.
Surface reactions between
the metal and the furnace
atmosphere, i.e., oxidation
(oxide scales,
decaburisation in steels.
Hot shortness, when the
working temperature
exceeds the melting
temperature of constituent
at grain boundaries such as
FeS.
Dimension tolerance is
poor due to thermal
expansion at high
temperatures.
Handling is more difficult
(from furnace to machine.
Dis-advantages
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Q2. Briefly discuss different types rolling.
Answer:
Shape RollingShape rolling is a broad term for a range of metal rolling operations, that involve
forming the work with rolls of a certain geometry. The rolls form the part to a specific
shape. Most shape rolling involves passing the material through several steps. Two
very common examples of continuous shape rolled product are the I beam for
structural purposes and the rail for railroad track.
Ring RollingRing rolling is a particular category of
metal rolling in which a ring of smaller
diameter is rolled into a precise ring of
larger diameter and a reduced cross
section.
This is accomplished by the use of two
rollers one driven and one idle acting on
either side of the ring's cross section.
Edging rollers are typically used inindustrial metal rolling manufacture to
ensure that the part will maintain a
constant width throughout the forming
operation.
The work will essentially retain the same
volume, therefore the geometric reduction
in thickness will be compensated for
entirely by an increase in the ring's
diameter.
A significant advantage of parts produced by
this metal rolling process is that the forming
of the material will impart the ring with a grain
orientation that gives it enhanced strength relative
to most applications.
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Thread Rolling
Thread rolling is a metal rolling process
used extensively in manufacturing
industry to produce screws, bolts and otherfasteners.
A common thread rolling process used in
industry to manufacture threaded parts
involves forming the threads into the metal
of a blank by a pressing and rolling action
between two die.
The die surfaces hold the shape and the
force of the action forms the threads into the
material. A similar metal forming processhas been developed for the production of
gears.
Thread rolling in manufacturing today
has an extremely high productivity rate,
significantly higher than producing
threaded parts by machining.
Forming will harden the metal through
cold working, does not waste material by
cutting, and produces a favorable grain
structure to strengthen the part with
respect to its function.
Rotary Tube Piercing
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Roll forming
Roll forming, roll bending or plate
rolling is a continuous bending
operation in which a long strip of
metal (typically coiled steel is
passed through consecutive sets of
rolls, or stands, each performing only
an incremental part of the bend, until
the desired cross-section profile is
obtained.
Roll forming is ideal for producing
parts with long lengths or in large
quantities.
There are 3 main processes:
4 rollers,3 rollers and 2 rollers,
each of which has as different
advantages according to the desired
specifications of the output plate.
Foil rolling
http://en.wikipedia.org/wiki/File:Zg-prof.jpg -
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Aluminum foil is the most
commonly produced product via
pack rolling.
This is evident from the two
different surface finishes; the shiny
side is on the roll side and the dullside is against the other sheet of foil.
Q3. Make a comparison between roll bending and rollforming processes
Answer
Roll FormingRoll BendingPoints
A continuous manufacturing process
that uses rolls to bend a sheet metal
cross section into a certaingeometry.
Roll bending provides a technique
that is useful for relatively thick
work. Although sheets of various
sizes and thicknesses may be used,this is a major process for the
bending of large pieces of plate.
Definition
(working idea(
Sketch
Often several rolls may be employed
in series to continuously bend stock.
Roll bending uses three rolls to feed
and bend the plate to the desired
curvature.
Number of
rolls
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Roll forming usually involves
bending of the work in sequential
steps.
Each roll will form the sheet metal
to a certain degree, in preparation
for the next roll.
The final roll completes the
geometry.
The arrangement of the rolls
determines the exact bend of the
work.
Different curves are obtained by
controlling the distance and anglebetween the rolls.
A moveable roll provides the
ability to control the curve. The
work may already have some
curve to it, often it will be
straight.
Rolls
arrangementand working
techniques
Q4. Discuss the requirements which should beprovided with the rolling mills
Answer:
Work rolls.
Backup rolls: are intended to provide rigid support required by the
working rolls to prevent bending under the rolling load.
Rolling balance system: to ensure that the upper work and back up
rolls are maintain in proper position relative to lower rolls. Roll changing devices: use of an overhead crane and a unit
designed to attach to the neck of the roll to be removed from or
inserted into the mill.
Mill protection devices: to ensure that forces applied to the backup
roll chocks are not of such a magnitude to fracture the roll necks or
damage the mill housing.
Roll cooling and lubrication systems.
Pinions: gears to divide power between the two spindles, rotating
them at the same speed but in different directions. Gearing: to establish desired rolling speed.
Drive motors: rolling narrow foil product to thousands of
horsepower.
Electrical controls: constant and variable voltages applied to the
motors.
Coilers and uncoilers: to unroll and roll up coils of metal.
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Q5. In details discuss the different types of mills
which can be classified into two categories, classical
and modern.Answer:
Mills can be classified into: Blooming, cogging and slabbing mills, being the preparatory mills to rolling
finished rails, shapes or plates, respectively. If reversing, they are from 34 to
48 inches in diameter, and if three-high, from 28 to 42 inches in diameter.
Billet mills, three-high, rolls from 24 to 32 inches in diameter, used for the
further reduction of blooms down to 1.5x1.5-inch billets, being the
preparatory mills for the bar and rod
Beam mills, three-high, rolls from 28 to 36 inches in diameter, for the
production of heavy beams and channels 12 inches and over.
Rail mills with rolls from 26 to 40 inches in diameter.
Shape mills with rolls from 20 to 26 inches in diameter, for smaller sizes of
beams and channels and other structural shapes. Merchant bar mills with rolls from 16 to 20 inches in diameter.
Small merchant bar mills with finishing rolls from 8 to 16 inches in
diameter, generally arranged with a larger size roughing stand.
Rod and wire mills with finishing rolls from 8 to 12 inches in diameter,
always arranged with larger size roughing stands.
Hoop and cotton tie mills, similar to small merchant bar mills.
Armour plate mills with rolls from 44 to 50 inches in diameter and 140 to
180-inch body.
Plate mills with rolls from 28 to 44 inches in diameter.
Sheet mills with rolls from 20 to 32 inches in diameter.
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Universal mills for the production of square-edged or so-called universal
plates and various wide flanged shapes by a system of vertical and horizontal
rolls
Q6. Discuss the following paragraph: "Roll formed
sections have an advantage over extrusion of a similar
shape."Answer:
Roll formed parts are generally much lighter and
stronger, having been work hardened in a cold state.
Another advantage is that the part can be made having afinish or already painted.
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Labor is greatly reduced since volume is a major
consideration for choosing the roll forming process.
Roll forming machines are now being produced so that
for similar products such as stud and track profiles, anew set of profile rolls is not required.
This is achieved by the mill being split along its center line
and the web, flange and ear sizes are set using a control
panel which moves the mill rafts centrally to increase or
decrease the aforementionedfeatures.