report 4 machining

7/28/2019 report 4 machining

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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.

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report 4 machining