Unit-II• Machine tools and machining operations for

TurningBoring ShapingMillingGrinding

– Calculation of MRR, Power required and Cutting time

•Finishing operations•Surface treatment•Coating and cleaning•Design of Jigs and Fixtures•Design of press working tools

Turning: Lathe Machine

LATHE MACHINE

LATHE MACHINE

LATHE MACHINE: OPERATIONS

LATHE MACHINE:

OPERATIONS

Notations

1. A cylindrical stainless steel rod with length L=150 mm,diameter Do = 12 mm is being reduced in diameter to Df =11mm by turning on a lathe. The spindle rotates at N = 400rpm, and the tool is travelling at an axial speed of υ=200mm/min

Calculate:a. The cutting speed V (maximum and minimum)b. The material removal rate MRRc. The cutting time td. The power required if the unit power is estimated to 4

w.s/mm3

SOLUTION:a. The maximum cutting speed is at the outer diameter Do

and is obtained from the expression V = π DoNThus,Vmax = (π) (12) (400) = 15072 mm/minThe cutting speed at the inner diameter Df isVmin = (π) (11) (400) = 13816 mm/minb. From the information given, the depth of cut isd = (12 – 11) / 2 = 0.5 mm , and the feed is f = υ / Νf = 200 / 400 = 0.5 mm/revthus the material removal rate is calculated asMRR = (π) (Davg) (d) (f) (N) = (π) (11.5) (0.5) (0.5) (400) = 3611 mm3

/min = 60.2 mm3 /sc. The cutting time ist = l / (f. N) = (150) / (0.5) (400) = 0.75 mind. The power required isPower = (4) (60.2) = 240,8 W

2. The part shown below will be turned in two machining steps.In the first step a length of (50 + 50) = 100 mm will be reducedfrom Ø100 mm to Ø80 mm and in the second step a length of50 mm will be reduced from Ø80 mm to Ø60 mm. Calculatethe required total machining time T with the following cuttingconditions:

Cutting speed V=80 m/min,Feed is f=0.8 mm/rev,Depth of cut = 3 mm per pass.

SOLUTION:

V=80 m/min

f=0.8 mm/rev

The turning will be done in 2 steps. In first step a

length of (50 + 50) = 100 mm will be reduced from

Ø100 mm to Ø80 mm and in second step a lengthof 50 mm will be reduced from Ø80 mm to Ø60 mm

3. The shaft shown in the figure below is to bemachined on a lathe from a Ø 25mm bar.Calculate the machining time if speed V is 60m/min., turning feed is 0.2mm/rev, drillingfeed is 0.08 mm/rev and knurling feed is 0.3mm/rev.

Step 2Turn Ø20 mm from Ø25 mmTm = L/FN

= 45/0.2 x 764= 0.29 min

4. A batch of 800 workpieces is to be produced on aturning machine. Each workpiece with lengthL=120mm and diameter D=10mm is to bemachined from a raw material of L=120mm andD=12mm using a cutting speed V=32 m/min and afeed rate f=0.8 mm/rev. How many times we haveto resharpen or regrind the cutting tool?

In the Taylor’s expression, use constants as n=1.25 and C=175.

Milling: Milling Machine

Vertical milling machine

Horizontal milling machine

Horizontal milling operations

Slab-milling operation, in which a rotating cuttingtool removes a layer of material from the surfaceof workpiece.

End-milling operation, in which a rotating cuttertravels along a certain depth in the workpiece andproduces a cavity.

Horizontal milling operations

Vertical milling operations

Vertical milling operations

Side milling cutter / HSS

Up milling and down milling• When the feed and the cutting

action is in the opposite directionthe operation is called up milling.

• When the feed and cuttingaction are in the samedirection the operation iscalled down milling.

• In down milling there istendency of the job beingdragged into the cutter, upmilling is safer and commonlydone.

• However down milling resultsin a better surface finish andlonger tool life.

The scheme of chip formation during plain slab milling using a straight cutter is shown in the figure.D= Diameter of the cutter N= rpm of the cutterd=depth of the cut f= feed velocity

Since all the cuttingedges take part inmachining, a study of theprocess is facilitated byconsidering the action ofonly a single tooth.

Iff= feed velocity of a

table in mm/min

Then effective feed pertooth in mm is f/NZ

N= cutter rpmZ= number of teeth inthe cutter

Material removal rate per unit width= f d

λ

λ

It is seen from the figurethat the thickness of theuncut material in front ofthe cutting edgeincreases gradually,reaching a maximumnear the surface, andagain drops to zeroquickly.If feed velocity is small ascompared with thecircumferential velocityof the cutter then,

AB=AC Sin λ ort1max = (f/NZ) Sin λ

Where λ =angle included by the contact arc at the cutter center O in radians

λ

λ

Consider triangle OATCos λ = OT/OA = (D/2 – d)/ D/2

Hence Sin λ = (1- Cos2 λ)1/2

= [1- (1-2d/D)2]1/2

= 2 (d/D) 1/2

Neglecting the higher order terms in d/D as it is normally very small.Using this value of Sin λ in the expression of maximum uncut thickness we get

t1max = (f/NZ) Sin λ

t1max = 2f/NZ (d/D) 1/2

λ

λ

The cutting forcecomponents FC and FT notonly change in directionbut also in magnitude asthe cutting edge movesalong the cutting surface.

When cutting with straightcutter, there is nocomponent of the cuttingforce along the cutter axis.

The average uncutthickness can be taken ashalf of the maximum value.t1max = 2f/NZ (d/D) 1/2

t1av = f/NZ (d/D) 1/2

The average values FC

and FT can beapproximately foundusing the average valueof uncut chip thickness.

Since FT acts in theradial direction , it doesnot produce any torqueand the arbor toque isdue to the componentFC.

Mmax = FC (D/2).

M= torque due to onecutting tooth and variesapproximately as FC.

• The above figure shows the variation of arbor torque with arbor rotation for the action of only one single tooth.

• Now to get the over all torque (M ), the moments due to all the teeth should be properly superimposed. This leads to three different possibilities.

1) λ<2π/Z 2) λ =2π/Z 3) λ >2π/Z

λ

λ

The figure shows the threedifferent possibilities, thearbor torque correspondingto each of the these areshown.

The machining power canbe calculated by taking theproduct of the arbor speedand the average overallarbor torque.

The average thrust force canbe considered to be actingalong the midradial line ofthe work-cutter contact arc.

λλ

λ

λ

λ

λ

tm=Machining time = (lw +a)/f

lw = Length of the workpiece

a= approach length= [d(D-d)] 1/2

• Q: A mild steel block of 20mm width is being milledusing a straight slab milling cutter with 20 teeth,50mm diameter and 10 degree radial rake. The feedvelocity of the table is 15mm/min and the cutterrotates at 60 rpm. If the depth of cut of 1mm is used,what will be power consumption. (Assumecoefficient of friction=0.5, and shear stress=400N/mm2 .

• Q: Estimate the power required during the upmilling of a mild steel block of 20mm width usingstraight slab milling cutter with 10 teeth, 75mmdiameter, and 10 degree radial rake. The feedvelocity of the table is 100mm/min, the cutterrotates at 60 rpm and the depth of cut is 5mm.

Shaping: Shaper machine

The cutting operationin shaper isintermittent in natureand takes place duringthe forward stroke.

During the return ofthe tool the feedmotion is is providedwhen there is nocutting action.

In an actual cuttingoperation the majorparameters are:N= Stroke per unittime

S= Stroke lengthR= Quick return ratio (displacement/stroke)D= depth of cut and the tool angles.

The uncut thickness and width of the cut is given by the relations

t1 = f cosψ

w =d / cosψ

Ψ = primary principal cutting edge angle

α = normal rake angle

The figure shows thecutting and thrustcomponents of theforce.

The cuttingcomponents Fc

acts against v and FT

acts perpendicular tothe transient surface.

FT can be resolved intotwo components Ff

(feed component) andFn (component normalto the machinedsurface)

Ff =FT cosψFn =FT sinψ

MRR= LdfNL= length of the job N= number of cutting strokes per unit time

The cutting time is given byTm H/d x B/f x 1/N

B=breadth of the jobH= the total depth by which the work surface to be loweredd= depth of cutf=feed N=cutting stroke per unit time

Since the cutting speed changes during the cutting stroke, the average cutting speed v is given by

v=[NS(1+R)]/2whereS=stroke lengthR= quick return ratioN= number of strokes per unit time

• Q: Determine the three components of themachining force when shaping a cast iron blockwith depth of cut = 4mm, feed= 0.25 mm/stroke,normal rake angle of the tool= 10 degree,principal cutting edge angle=30 degree,coefficient of friction between chip and tool=0.6,and ultimate shear stress of cast iron=340N/mm2

.If the operation takes place with 60stroke/min,what will be the power consumption ifthe length of the job is 200mm.

Grinding: Grinding Machine

Grinding

• It is the process of removing material by the abrasive action ofrevolving wheel from the surface of the work piece, in order toachieve required dimension and surface finish.

• The wheel used for this purpose is called grinding wheel.• Grinding wheel consists of sharp grains called abrasives held

together by bonding material.• The grains actually taking part in the material removal process

are called the active grains.• Generally the sharp edges of the grains wear out and become

blunt.• This results in large forces on the active grains during

machining.• When the cutting edge is too blunt and the force is sufficiently

high, the grain may either get fractured or break away from thewheel.

Mechanics of grinding

• In the analysis of grinding process, all grains are assumed to beidentical.

• Two different types of grinding operations namely plunge grindingand surface grinding will be considered for analysis.

• In plunge grinding operation a job of rectangular cross-section isbeing fed radially at the feed rate f (mm/min).

• The uncut chip thickness per grit ( t1 ) can be expressed as

t1 = f/ZN.

Z=number of active grains per revolution in one line

N=rpm of the wheel

b‘ = average grain width of cut in mm

t1 = uncut thickness per grit

The number of grains /revolution/line (Z) is given by:

Z=πDC b'

D = diameter of the grinding wheel

b‘ = average grain width of cut in mm

C = the surface density of active grains (mm-2

Alsot1 = f/πDC b‘ N

t1 = [f/πDNC rg]1/2

rg can take the value between 10 and 20

The uncut sections have approximately triangular cross section. The ratio rg =b‘/ t1 since b‘= t1 rg

Once t1 is estimated, the value of specific energy UC can be determined and the power consumption is

W= A f UC /60

• A= cross section area of the job (mm2)

Force per single grit

Fc = 60000 W/ πDACN

Fc = [1000 f UC / πDCN] N

Uc = Uo t1-0.4 Uo depends on materials, for

steel it is 1.4.

MRR= lxbxf mm3

• Q: Estimate the power requirement during plunge

grinding of a mild steel prismatic bar (20mmx15mm)

using a grinding wheel with 3 grits/ mm2. The

diameter of the wheel is 250 mm and the wheel

rotates at 2000 rpm. The plunge feed rate is

5mm/min.

Surface grinding The uncut thickness and width vary and the maximum value are: t1max and

b'max

The average values may be taken as one-half of these.

λ

The average length of the chip is given as:l=(D/2) λBut Cos λ = (D/2 –d)/D/2Cos λ =1-(2d/D)

d=depth of cut

λ

Cos λ can be expanded (keeping only two terms since λ is generally small) asCos λ =1- λ2 /2Henceλ = 2 (d/D)1/2

Substituting this in the value of l we obtainl= (dD)1/2

The total volume of material removed per unit time = fdBWhere: f=feed, D=depth of cut, b=width of cut in mm

The average volume per chip can be approximately taken as: 1/6 (l t1max b’ max )

The number of chips produced per unit time is: πNDBCTaking rg = b’ max / t1max

We have

(πNDBC) x (1/6 rg l t 21max ) = fdB

ort1max = [6f/πNDCrg(d/D)1/2 ] 1/2

One half of this value is to be taken as the mean uncut thickness.

The power consumption can be taken as:

W = (BfDUc )/60 W

Total Tangential Force

Fc = 60000 W/ π DN

Fc = 60000 BfdUc / πDN 60

Fc = 1000 BfdUc/ πDN

N= wheel RPM

The number of grit actively engaged is: CBl=CB(Dd)1/2

The average force per grit is given by:

F’c = 60000 W/ π DNCB(Dd)1/2

Q. Estimate the grinding force during surface grinding of a25mm wide mild steel block with a depth of cut of 0.05mm. The diameter of the wheel is 200 mm and the wheelrotates at 3000 rpm. The number of grits/mm2 is measuredand found to be 3. The feed velocity of the table is100mm/min.

Boring: Boring Machine

Horizontal Boring Machine

Vertical Boring Machine

Boring operation

Finishing operations

Honing operation– The honing operation is used for finishing the inside surface

of a hole.

– The honing tool consists of a set of aluminum–oxide orsilicon- carbide bonded abrasives called stones.

– Abrasives in the form of sticks are mounted on the mandrelwhich is then given reciprocating (along the hole axis)superimposed on a uniform rotary motion.

– The grit size varies from 80 mesh to 600 mesh.

– Depending on the work material, the honing speed may varyfrom 15 m/min to 60 m/min, and the honing pressure lies inthe range 1-3 N/mm2.

• The tolerance and finish achieved in this operation are of theorder of 0.0025mm and 0.25μm respectively.

• Honing is also done external cylindrical or flat surfaces and toremove sharp edges on cutting tools and inserts.

• A fluid is used to remove chips and to keep temperatures.

• If not done properly, honing can produce holes that areneither straight nor cylindrical , but with shapes that arebellmouthed wavy or tapered.

Lapping operation• Lapping is another operation for improving the accuracy and

finish.

• It is accomplished by abrasives in the range of 120-1200 mesh.

• A lap is made of material softer than the work material.

• In this process straight, narrow groves are cut at 90 degree onthe lap surface and this surface is charged by sprinkling theabrasive powder.

• The workpiece is then held against the lap and moved inunrepeated paths.

• The material removal about 0.0025 mm and the lappingpressure is generally kept in the range of 0.01-02 N/mm2

depending upon the hardness of the work material

Polishing

• Polishing is a process that produces a smooth, lustrous surface finish.

• Two basic mechanism are involved in the polishing process.

• a) Fine-scale abrasive removal and b) softening and smearing of the surfacelayers by frictional heating during polishing.

• Polishing is done with disks or belts made of fabric, leather that are coatedwith fine powder of aluminum oxide or diamond.

• Buffing is similar to polishing , with the exception that very fine abrasivesare used on soft disks made of cloth.

• The abrasives are supplied externally from the stick of abrasive compound.

• Polished parts may subsequently be buffed to obtain an even finer surfacefinish.

Electropolishing

• Mirror like finishes can be obtained on metal surface byelectropolishing, a process that that is the reverse ofelectroplating.

• Because there is no mechanical contact with the workpiece,this process is particularly suitable for polishing irregularshapes.

• The electrolyte attacks projections and peaks on the workpiecesurface at a higher rate than the rest of the surface, producing asmooth surface.

Surface treatment, coating and cleaningSurface treatments are performed in order to:

• Improve resistance to wear, erosion, and indentation.• Control friction• Reduce adhesion (electrical contacts)• Improve lubrication• Improve resistance to corrosion and oxidation• Improve fatigue resistance• Rebuild surfaces on worn components• Modify surface texture• Impart decorative features (color)

Several techniques are used to impart these characteristics tovarious types materials.

Mechanical surface treatment and coating

• Several techniques are used to improve the properties offinished components.

Shot peening: • The workpiece surface is hit repeatedly with a large large

number of cast steel glass or ceramic balls.

• This action causes plastic surface deformation, at a depth up to1.25mm using ball size ranging from 0.125mm to 5mm indiameter.

• Shot peening causes compressive residual stresses on thesurface, thus improving the fatigue life of the component.

• This process is used extensively on shafts, gears, springs etc.

Water jet peening• A water jet of water at a pressure as high as 400MPa

impinges on the surface of the workpiece, inducingcompressive residual stresses.

• The water jet peening process has been used successfullyon steels and aluminum alloys.

Laser peening• In laser peening the workpiece surface is subjected to laser

shocks from high powered lasers.• This surface treatment process produces compressive

residual stress layers that are typically 1 mm.• Laser peening has been applied successfully to jet engine

fan blades and material such as titanium and nickel alloys.• The laser intensities necessary for this process are on the

order of 100 to 300 J/cm2

Roller burnishing• Also called surface rolling, the surface of the component is cold worked

by a hard and highly polished roller or rollers.

• This process is used on various flat , cylindrical or conical surfaces.

• Roller burnishing improves surface finish by removing scratches, toolmarks and pits.

• All types of metals, soft or hard can be roller burnished.

Cladding• In cladding , metals are bonded with a thin layer of corrosion-resistant

metal through the application of pressure, using rolls.

• Atypical application is cladding of aluminum (Alcad), in which a corrosion–resistant layer of aluminum alloy is clad over aluminum alloy body,usually in sheet or tubular form.

• Other applications are steel clad with stainless steel or nickel alloys.

Case hardening and Hard facing

Case hardening• Case hardening processes induce residual stress on surface.• The formation of martensite during case hardening cause s

compressive residual stress on surface.• Such stress are desirable, because they improve the fatigue life

of components by delaying the initiation of fatigue cracks. Etc.• Some of the case harding process are carburizing,

carbonnitriding, cyaniding, nitriding, flame hardeningHard facing• In hard facing, a relatively thick layer , edge or point wear

resistant hard metal is deposited on the surface using any of thewelding technique.

• Hard coting of tungsten, carbide or chromuium andmolybdenum carbide can be deposited by using electric arc.

• Hard facing alloys can be used as electrode, rod wire or powder.• Typical application for these alloys are valve seats, oil well

drilling tools, and dies for hot metalworking.

Thermal spraying

• In thermal spraying processes coating (various metalsand alloys, carbides and ceramics) are applied to metalsurfaces by a spray gun with a stream of oxyfuel flame,electric arc plasma arc.

• The coating material can be in the form of wire, rod, orpowder and the droplets or particles impact thesurface at speeds in the range of 100 to 1200 m/s.

• Typical application includes aircraft enginecomponents,, structures, storage tanks, andcomponents which require resistance to wear andcorrosion.

Vapor deposition

• Vapor deposition is a process in which the worksurface is subjected to chemical reaction by gasesthat contain chemical compounds of the materialto be deposited.

• The coating thickness is usually a few μm.

• The deposited material can consist of metals,alloys, carbides, nitrides, borides, ceramics, oroxides.

• The surface may be metals, plastic, glass or paper.

• Typical application of vapor deposition are thecoating of cutting tools , drills, milling cutters,punches, dies and wear surface.

Anodizing

• Anodizing is an oxidation process in which theworkpiece surface are converted to hard and porousoxide that provides corrosion resistance and decorativefinish.

• The workpiece is an anode in an electrolytic cellimmersed in an acid bath, which results in a chemicaladsorption of oxygen from the bath.

• Organic dyes of various color can be used to producestable, durable surface finish.

• Typical application of for anodizing are aluminumfurniture, and utensils, architectural shapes, pictureframes, etc.

Diffusion coating

• Diffusion coating is a process in which an alloyingelement is diffused in the surface thus altering itsproperties.

• The alloying elements can be supplied in solid,liquid, or gaseous state.

Electroplating, electroless plating and electroforming

• In electroplating, the workpiece (cathode) is plated with differentmetal (anode)while both are suspended in a bath containing awater base electrolyte solution.

• All metals can be electroplated; electrolyte thickness can rangefrom few atomic layers to a maximum of about 0.05mm.

• Chromium, nickel, cadmium, copper, zinc, and tin are the commonplating materials.

• Electroplating is used copper plating aluminum wire, chromeplating hardware, tin plating copper electric terminals.

• Electroless plating is done by chemical reaction and without theuse of an external source of electricity.

• The most common application utilizes nickel although copper isalso used.

• In electroless nickel plating, nickel chloride is reduced usingsodium hypophosphite as a reducing agent, to nickel metal, whichis then deposited on the workpiece.

• Cavities, recesses, and the inner surfaces of tubes can be platedsuccessfully.

• A variation of electroplating is electroforming, which actually is ametal forming process.

• Metal is electrodeposited on a mandrel, also called mould ormatrix , which is then removed, thus coating itself becomes theproduct.

• The electroforming process is particularly suitable for lowproduction quantities and is suitable for aerospace, electronicapplications.

Painting • Because of its decorative and functional properties (such as

environmental protection, low cost, relative ease of application and therange of available colors), paint is widely used as a surface coating.

• The engineering application of painting range from machinery toautomobile parts.

Paints are classified as:• i) Enamels: produces a smooth coat and dry with glossy or semi glossy

appearance.• Ii) Lacquers: which form a film by evaporation of a solvent• iii) Water based paints: which are easily applied but have a porous

surface and absorb water, making them more difficult to clean then thefirst two.

• Selection of particular paints depends on resistance to mechanicalaction (abrasion, impact etc.) or to chemical actions ( acids, solvents,detergents etc.)

• Common methods of applying paints are dipping, brushing andspraying).

Cleaning Surfaces• Cleaning involves the removal of solid, semi solid or liquid

contamination from a surface.

• Basically there are two types of cleaning processesmechanical and chemical.

• Mechanical cleaning consists of physically disturbing thecontaminants, often with wire or fibre brushing, abrasiveblasting or steam jets.

• Many of these processes are particularly effective inremoving rust, scale and other solid contamination.

• Chemical cleaning usually involves the removal oil andgreese from the surfaces

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Jigs and fixtures

127

Accuracy of Machining process depends upon:

• The precision of mounting of– Work– Tool

• Their accurate movement

For repeated identical work (in mass production)• Reduction in

– Set-up time– Clamping time

• Minimizes production time• Jigs and fixtures

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A jig or fixtures needs to provide the following functionalityto be an effective production device:

• Location

• Clamping

• Support

• Resistance to cutting forces

• Safety

Jigs and fixtures both

• Hold the work

• Support the work

• Locate the work

Jigs in addition

• Guide the cutting tool

Fixtures have

• Reference point for setting the cutting tool with reference to the workpiece

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• Jigs are designed for specific operations.• Jigs are commonly used for making parts that contains

holes.• Jigs are used for operations like drilling, reaming,

counter boring and tapping.• They are light in weight.

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133

134

135

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• Fixtures are workpiece supporting devices• The are used for holding and locating the workpiece but

not for guiding the tool.• The are designed on the basis of machines on which the

operations are to be performed.• They are heavy in weight.

• Turning• Milling• Grinding• Shaping• Planning etc

Fixtures

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90

140

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Functional surfaces

• It is necessary to understand the functional surfacespresent in a component and their utility from thestandpoint of its manufacture.

• The essential reason for machining is that these surfacesare to be mating with surfaces machined in the otherpart.

• It is always necessary to consider the fact that machiningincreases the final cost of the component and henceshould be minimized based on the following as far aspossible:

• Location surfaces

• Support surfaces

• Clamping (holding) surfaces

Location surfaces

– Required to be correctly identified

– Generally identified through

• Baselines in dimensioning

• Already finished surface

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Support surfaces

– Surface in the end

– Not necessary to provide support for all operations

– May be provided through clamping at critical points

– Surface where maximum deflection under action

– Should not disturb the location/locators

– Should not interfere with loading/unloading

Clamping (holding) surfaces

– Surfaces should provide

• Easy clamping

• Shortest possible time

– Generally opposite to locating surface

– If not possible, alternate surfaces can be chosen

– Machined surfaces should be avoided

– Clamping surface area should be large

– Surfaces should have enough rigidity

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Press working tools

• Press working has been defined as chiplessmanufacturing process by which component are madefrom sheet metal.

• Press working operations are caused out with help of ametal forming machine called press which shear orforms the component by applying force.

• The main features of the press include a stationary bedand a powered ram can be driven towards the bed oraway from the bed to apply force or required pressurefor various metal forming operations.

• The ram is equipped with a punch or a set of puncheswhich have the shape of the job to be produced whilethe die block is attached to the bed.

• Workpiece are produces or formed as the punchdescends onto the die block.

• Die – punch combination is used for the process toimpart the desired shape to the blank.

• Press tool operations of sheet metal is by far thecheapest and fastest method to completemanufacturing of component.

• Press working is used in large number of industrieslike automobile industry, aircraft industry, lockindustry, telecommunication, electrical appliance,utensils etc.

Classification of presses.

Classification on the basis of source of power.• Manual Presses. These are either hand or foot operated

through levers, screws or gears. A common press of thistype is the arbor press used for assembly operations.

• Mechanical presses. These presses utilize flywheelenergy which is transferred to the work piece by gears,cranks, eccentrics, or levers.

• Hydraulic Presses. These presses provide working forcethrough the application of fluid pressure on a piston bymeans of pumps, valves, intensifiers, and accumulators.

• Pneumatic Presses. These presses utilize air cylinders toexert the required force. These are generally smaller insize and capacity than hydraulic or mechanical presses,and therefore find use for light duty operations only.

Manual Press

Power Press

Classification on the basis of number of slides• Single Action Presses. A single action press has one

reciprocation slide that carries the tool for the metalforming operation. It is the most widely used press foroperations like blanking, coining, embossing, and drawing.

• Double Action Presses. A double action press has twoslides moving in the same direction against a fixed bed. Itis more suitable for drawing operations, especially deepdrawing, than single action press.

• Triple Action Presses. A triple action press has threemoving slides. Two slides move in the same direction as ina double – action press and the third or lower slide movesupward through the fixed bed in a direction opposite tothat of the other two slides. This action allows reverse –drawing, forming or bending operations against the innerslide while both upper actions are dwelling.

Classification on the basis of frame and construction• Arch – Frame Presses.

These presses have theirframe in the shape of anarch. These are notcommon.

• Gap Frame Presses. Thesepresses have a C-shapedframe. These are mostversatile and common inuse, as they provide unobstructed access to thedies from three sides andtheir backs are usually openfor the ejection ofstampings and / or scrap.

Straight Side Presses. Thesepresses are stronger since theheavy loads can be taken in avertical direction by the massiveside frame and there is littletendency for the punch and diealignment to be affected by thestrain.

Horn Presses. These pressesgenerally have a heavy shaftprojecting from the machine frameinstead of the usual bed. This pressis used mainly on cylindrical partsinvolving punching, riveting,embossing, and flanging edges.

Classification of dies

There is a broader classification of single operation dies and multi-operation dies.

• (a) Single operation dies are designed to perform only a singleoperation in each stroke of ram.

• (b) Multi operation dies are designed to perform more than oneoperation in each stroke of ram.

Single operation dies are further classified as described below.

Cutting Dies

• These dies are meant to cut sheet metal into blanks. Theoperation performed so is named as blanking operation.

Forming Dies

• These dies are used to change two shape of workpiece material bydeforming action. No cutting takes place in these dies.

Compound Dies

• In these dies two or more cutting actions (operations) can beexecuted in a single stroke of the ram.

Combination Dies

• These dies are meant to do combination of two or moreoperations simultaneously. This may be cutting action followed byforming operation. All the operations are done in a single action ofram.

Progressing Dies

• These dies are able to do progressive actions (operations) on theworkpiece like one operation followed by another operation andso on. An operation is performed at one point and then workpieceis shifted to another working point in each stroke of ram.

Press operations: Shearing• Shearing is a cutting operation used to remove a blank of required

dimensions from a large sheet.

• A metal being sheared between a punch and a die.

• Shearing begins with formation of cracks on both sides of the

blank, which propagates with application of shear force.

• The fracture progresses downwards with the movement of upper

shear and finally results in separation from parent strip.

• Shearing a blank involves plastic deformation due to shear stress.

Therefore, the force required for shearing is theoretically equal to

the shear strength of blank material.

The shearing operations are:

• Punching• Blanking• Notching• Piercing

• Perforating• Parting• Nibbling• Trimming• Shaving

Punching/BlankingPunching or blanking is a process inwhich the punch removes aportion of material from the largerpiece or a strip of sheet metal.

If the small removed piece isdiscarded, the operation is calledpunching, whereas if the smallremoved piece is the useful partand the rest is scrap, the operationis called blanking.

Notching

• Punching the edge of a sheet,

forming a notch in the shape of

a portion of the punch.

• It usually removes a small

portion from edge or side of a

sheet.

Piercing

The typical operation,

in which a cylindrical

punch pierces a hole

into the sheet.

Sometimes called

Punching.

Identical to Blanking,

only the punched out

portion which is coming

out through die is

scrap.

Normally Blanking

follows a piercing

operation

• Trimming:

When parts are produced by die casting or drop forging, asmall amount of extra metal gets spread out at the partingplane. This extra metal, called flash, is cut – off before thepart is used, by an operation called trimming.

• Shaving:

Shaving operation is a finishing operation where a smallamount of metal is sheared away from an already blankedpart. Its main purpose is to obtain better dimensionalaccuracy.

The forming operations are:

• Lancing • Drawing• Bending• Embossing

Drawing

It is a cold drawing operation.

A process of making utensils, pressure

vessels, gas cylinders, cans, shells,

kitchen sinks, etc from blanks.

Similar to blanking except that the

punch and die are provided with the

necessary rounding at the corners to

allow smooth flow of metal during

drawing & to avoid shearing.

One of the widely used sheet metal

forming operations.

Cupping and Deep Drawing are two

operations.

Press operations: Forming

• Proper selection of a press is necessary for successful andeconomical operation.

• Press is a costly machine, and the return on investment dependsupon how well it performs the job.

• There is no press that can provide maximum productively andeconomy for all application.

• Hence when a press is required to be used for varying jobs,compromise is generally made between economy andproductivity.

Important factors affecting the selection of a press are:

Size

• Bed and slide areas of the press should be of enough size so as toaccommodate the dies to be used.

• Stroke requirements are related to the height of the parts to beproduced.

Force and Energy

• Press selected should have the capacity to providethe force and energy necessary for carrying out theoperation.

Press Speed

• Fast speeds are generally desirable, but they arelimited by the operations performed.

• Size, shape and material of workpiece, die life,maintenance costs, and other factors should beconsidered while attempting to achieve the highestproduction rate at the lowest cost per piece.

180

Press selection

• Required force f for: blanking, piercing, lancing, etc., is given by

» h = gage thickness, m» ls = length to be sheared, m» U= ultimate tensile strength

181

Press selectionExample:

Circular disks 50 cm in diameter are to be blanked from No. 6 gage commercial-quality, low-carbon steel.

– thickness of 6 gage steel = 5.08 x 10-3 m

– ultimate tensile strength, U = 330 x103 kN/m2

required blanking force

f = 0.5 x (330 x 103) x (5.08 x 10-3) x (π x 50 x 10-2) = 1316.6kN

Table 9.3: 1750kN press

182

The clearance between the die and punch can be determined as

c = 0.003 t. pwheret is the sheet thicknessp is the shear strength of sheet material

For blanking operation, die size = blank size, and the punch is made smaller, by considering the clearance.

The maximum force, F required to be exerted by the punch to shear out a blank from the sheet can be estimated as

F = t. L. pwhere t is the sheet thickness, L is the total length sheared p is the shear strength of the sheet material.

• Stripping force.

Two actions take place in the punching process–punching and stripping. Stripping means extracting thepunch. A stripping force develops due to the springback of the punched material that grips the punch. Thisforce is generally expressed as a percentage of the forcerequired to punch the hole, although it varies with thetype of material being punched and the amount ofclearance between the cutting edges. The followingsimple empirical relation can be used to find this force.

SF = 0.02 L.twhereSF = stripping force, kNL = length of cut, mmt = thickness of material, mm

Example: A circular blank of 30 mm diameter is to be cut from 2 mmthick 0.1 C steel sheet. Determine the die and punch sizes. Alsoestimate the punch force and the stripping force needed. You mayassume the following for the steel : Tensile strength: 410 MPa ; shearstrength : 310 MPa

Solution:- For cutting a blank, die size = blank size = 30mmClearance = c = 0.003 t. p = 0.003 x t x p = 0.003 x 2 x 310 = 1.86 mm

Punch size = blank size – 2 clearance= 30 – 2 x 1.86 = 26.28 mm

Punch force needed = L. t. p = π x 30 x 2 x 310 (L= πD)= 58.5 kN

Stripping force needed = 0.02 L. t

= 0.02 x p x 30 x 2

= 3.77 kN

Important consideration for design of a die set

The following important points should be considered while designing a die set:

• Cost of manufacturing depends on the life of die set, soselection of material should be done carefully keepingstrength and wear resistant properties in mind.

• Die is normally hardened by heat treatment so designshould accommodate all precautions and allowances toovercome the ill effects of heat treatment.

• Accuracy of production done by a die set directlydepends on the accuracy of die set components.

• Standardized components should be used as much aspossible.

• Easy maintenance should be considered. Replacementof parts should be easy.

End of 2nd Unit