Presses & Press Work

Prof. Sahil B. ShethDepartment of Mechanical Engineering

Alpha College of Engg & Tech.

CHAPTERE-6PRESSES & PRESS WORK

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IntroductionPress working is a Chipless mfg. process, which is used to form sheet metal parts.

Material thickness : upto 1 mm = Foil (Aluminum foils for Food packaging)

1 to 5 mm = sheet metal

above 5 mm = plate

It’s a Cold working process & Also called Cold stamping.

Plastic deformation takes place at room temp.

Merits & DemeritsFastest & most efficient way to form parts from sheetmetal in mass production.

Unique parts can b made.

New tool sets are needed for every dissimilar part.

Materials & ApplicationsLow carbon steel ( 0.05-0.15 % C ) is used mostly as sheetmetal bcoz of low cost, good strength & good formability(Cold work).

Aluminum & Titanium are used as sheetmetal for Aircraft & Aerospace, weight)

Aluminum & Stainless steel are used for Automobile bodies, Cold-drink canes.

G.I. sheets are used for making Roofs, Cabinets, Kitchen appliances etc

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Press working/Sheetmetal Operations

The sheet metal operations done on a press may be grouped into two categories, cutting (shearing) operations and forming operations.

In cutting (shearing) operations operation, the workpiece is stressed beyond its ultimate strength.

The stresses caused in the metal by the applied forces will be shearing stresses.

The cutting operations include:

Punching (Piercing)

Blanking

Notching

Perforating

Slitting

Lancing

Shaving

Trimming

Lecture-05: Sheet Metal Forming Processes 34/4

Shearing Operations

Punching (Piercing): It is a cutting operation by which various shaped holes are made in sheet metal. Punching is similar to blanking except that in punching, the hole is the desired product, the material punched out to form the hole being waste.

Blanking: Blanking is the operation of cutting a flat shape sheet metal. The article punched out is called the blank and is the required product of the operation. The hole and metal left behind is discarded as waste.


Notching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip or blank.

Perforating: This is a process by which multiple holes which are very small and close together are cut in flat work material.

Slitting: It refers to the operation of making incomplete holes in a workpiece.

Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab. Since no metal is actually removed, there will be no scrap.

Parting: Parting involves cutting a sheet metal strip by a punch with two cutting edges that match the opposite sides of the blank.


Shaving: The edge of blanked parts is generally rough, uneven and unsquare. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges.

Trimming: This operation consists of cutting unwanted excess material from the periphery of previously formed components.

Schematic illustrations of shaving on a sheared edge.

(a) Shaving a sheared edge.

(b) Shearing and shaving, combined in one stroke.

Classification of Presses

According to the Power source :

1) Manually operated---Hand, ball or fly press →For thin sheetmetal work

Single sided Fly press &

Double sided Fly press

2) Power press----Mechanical, Hydraulic press etc → Heavy production work

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Fly press

Single sided Fly Press

Robust Cast iron Frame.

Top side of Frame is Nut, in which vertical Screw

rotates.

Screw is joined with Arm at the top that carries two

Cast iron Balls at both ends & also Handle.

These balls store Kinetic energy for further

downward movement of Screw.

Mid portion of Frame is Guide, in which ram works

attached at screw end.

Punch is attached at Ram end.

Die is rigidly fixed at Press platen(Bolster)

Stop collar is to adjust Ram stroke length, when

it touches the top surface of the Nut, prevents

further downward movement of Screw.

This Press is used for Rough Pressing work.

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Fly press

Single sided Fly Press

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Double sided Fly Press

Power press

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Ram is driven by Crank-

Connecting Rod (Pitman).

Flywheel is rotated by motor

through Belt-pulley system.

Flywheel carries a Clutch

through which, it is

connected to the Crankshaft.

Crankshaft is held steady by

Brake when Ram is at the

top position.

Most commonly used Power

Press for all press work.

Power press

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1) Accd. To type & design of Frame :

Mostly used

Main feature is the Ability to tilt back

Can be locked in Inclined position.

Open back is for scrap removal & produced parts by Gravity.

So called OBI (open back inclinable) press

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i) Inclinable frame Press ii) Gap frame Press

Frame is integral with the Base.

So called Solid /Rigid/Stable frame

Always fixed , Cant be locked Inclined .

Back may b opened/closed

Longer Bed & larger Frame opening & so can accommodate larger dies than former.

Shut height is more than former.

Used for large Presses having

large bed area & high tonnage.

Great Rigidity, longer strokes.

Has straight sides(vertical)

It consists : 1)Bed, to support

Die, 2) Vertical columns on

both sides, 3) Crown , carries

whole Press mechanism.

1) Single press unit formed by

Integrating Bed, Columns &

Crown

2) Frame has Arch shape to

give large bed area

1 2

iii) Straight side Press

.


Straight side Press

3All parts are held together by 4 Tie Rods as shown.

These parts can b disassembled when needed.

Most common in straight side types.

Available in different sizes.

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Called open frame type..

Consists 4 strong Pillars, bed, Crown

These parts r fastened by Nuts

Pillars act as a Guide for Ram

4

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It’s a vertical frame

Top of the Frame overhangs towards the Frame.

This top acts as housing for ram & Crankshaft.

Horn(cylindrical post ) is fitted as shown

Top face of Horn is flat which supports the Die

Horn is used for Tubular work.

5

34/19

Has Knee type bed, so called Knee press.

Knee can be adjusted at desired height by screw.

So this gives variable Shut height.

Lacks rigidity, so not used for heavy work

Solid vertical frame with all sides open.

Gears & flywheel are mounted at back.

Ram works at front.

Used for light press work

6 7

2) Accd. to Frame positions

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3) Accd. To Actions

Inclinable : > Frame can be inclined & adjusted at different angles

Vertical : > Frame can’t be adjusted

> It remains in vertical position only

> For examples : Gap type, Knee type, Straight side type, Open end type ,

Horning type.

Inclined : >Frame is fixed once & remains at that inclination always,

> Purpose for this inclination is to facilitate removal of Scraps & parts by Gravity.

Horizontal : > Fixed horizontal position, operation at high speed.

It denotes number of Rams on press

Single action press has single Ram

Double action press has two Rams

Tripple action press has three Rams

4) Accd. to the Mechanism of applying Power to Ram

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Crankshaft is driven by flywheel & is supported in bearing

Working is same is in Power press.

Rotary motion to reciprocating motion.

Carries Eccentric drive mechanism for its Ram.

One end of Connecting rod is with eccentric housing & other is with ram.

Stroke length is smaller than Crank drive


In Cam drive press, cam –follower are used for reciprocating action of Ram

It is spring back as in I.C. engine valve mechanism.

Very limited sizes & rarely used.

Toggle mechanism is for Double &

tripple actions presses for driving the

outer Ram.

Crankshaft drive is to drive Inner Ram.

Mostly used for large drawing Dies.

In that, mechanism actuates the blank

holder & punch is operated by crank

driven inner Ram.

This design makes maintainance &

repair easy.

Also called inverted drive


In Screw press/Power screw, a vertical screw is used for reciprocating action of Ram.

Frame is same as in Arch type press & working is like in Doubled sided Fly press.

Die height can b adjusted.

Very small movement of Ram.

Used for long stroke length of ram

Ram is joined at the bottom of the vertical

Rack.

Rack is driven by pinion.

Press tonnage is limited bcoz entire load is

on teeth of Rack-pinion.

Ram movement is slow.

Rarely used now.


Carries a Knuckle joint mechanism to drive ram

Consists two knuckle levers & a connecting lever

One lever is pivoted in Crown & other on ram.

Both levers r pivoted with connecting lever as shown

by wrist pin.

connecting lever is joined to crankshaft.

Crankshaft rotates in clockwise direction.

Main advantage is that back thrust from die goes to

crown through knuckle levers instead of crankshaft.

So preferable for heavy work with high intensity

blow.

Coining ,extruding ,squeezing ,sizing , embossing.

Stroke length is limited


Longer strokes than mechanical press

Blow intensity can b adjusted

Double acting piston cylinder

Ram is directly attached to piston rod.

Used for Deep drawing & other forming

operations.

More maintainance required.

5) Accd. to Number of Drive Gears


6) Accd. to Number of Crankshafts used

Gears attatched at the crankshaft end are called Drive gears

They drive the crankshaft

Very small Presses carry only 1 Drive gear at 1 end called Single Drive.

If 1 drive gear at each end ,then called Twin drive presses

Large presses having 2 Crankshafts & each having a Twin drive ,so called Quadruple drive

press.

Single crank : 1 Crankshaft

Double crank : 2 crankshafts

7) Accd. to Power Transmission method : Motor→Crankshaft

1) DIRECT DRIVE

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Power is transferred from Motor > Gears > Crankshaft

Pinion is mounted on Motor shaft.

Large gear is mounted on Crankshaft & acts like

Flywheel.

Clutch is to engage & disengage

Quicker & shorter strokes so used for small presses.

Fastest Ram movement

Carries no Gear

Power transmission is from Motor to

Crankshaft through endless ‘V’ belts.

‘V’ grooved pulley is on Motor shaft

Flywheel has ‘V’ grooves.

Clutch is to engage-disengage the

Crankshaft

Used for light work, as strokes are

quicker but short


2) NON-GEARED DRIVE

Power transmission is from motor to

Counter shaft, then to Pinion-gear &

then to Crankshaft.

Can b converted to two in drive by

putting same Pinion-gear on other

sides of both shaft.

For further heavy work with long

strokes.


3) SINGLE GEARED DRIVE

Intermediate shaft as introduced between

Flywheel shaft & Crankshaft.

Still longer strokes & high mechanical

advantage

Used for Drawing

If made two in Drive then can b used for

Forming-cutting.

Slowest Ram movement.


4) DOUBLE GEARED DRIVE

Details of a Die set


Classification of Dies

Single operation Dies : single operation in each Ram stroke

1) Cutting Dies : Shearing Operations

, 2) Forming Dies : Bending, Drawing & Squeezing operations

Multi operation Dies :

1) Compound Dies

2) Combination Dies

3) Progressive Dies


Compound Dies


Blanking & piercing operations can

be done at 1 station.

Considered as Cutting dies

Washer is made by simultaneously

Blanking & piercing operations & in 1

stroke

More accurate & economical in

mass production

Combination Dies


Here also Two or more

operations can be done at 1

station.

Cutting action is combined

with Bending & Drawing

Die shoe has pad,die ring &

drawing punch

Die ring is counterbored for up-

down movement of Pad ,is

connected with spring.

Strippers strip the skeleton from

blanking punch.

Knockout bar acts as ejecting

pins


Progressive Dies


Has series of station.

At each station, w/p is operated

during a press stroke.

Piercing,blanking,Forming, Drawing

etc. are done in each Stations.

Between stroke,metal sheet piece

is transferred to next station.

Die –punch blocks can be easily

replacable.

Highest production rates compared

to other Dies.


Transfer Dies: In a transfer die setup, the sheet metal undergoes different operations at different stations, which are arranged along a straight line or a circular path. After each operation, the part is transfer to the next operation for additional operations.

Cutting actions in Dies


Punch is smaller than Die in all

sides by an amount called

clearance.

Punch pushes w/p into Die

cavity, so w/p is subjected to

tensile & compressive forces.

Highest stress will b at die-

punch edges & w/p will start

cracking there.

Steps: Stretching w/p beyond

its elastic limit, Plastic

deformation, reduction in area,

fracturing starts & ends.

Clearance


Punch is smaller than Die in all sides

by an amount called clearance.

Clearance angle depends on

stock material & w/p thickness.

Mostly Clearance angle is kept

2˚





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