press working operations

8/4/2019 Press Working Operations

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A simple cutting die used for punching

and blanking operation as shown:


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Terminology

6: Punch :

This is male component of a die assembly, which is directly or indirectly moved byand fastened to the press ram or slide.

7: Upper Shoe:

This is the upper part of the die set which contains guide post bushings.

8: Punch Plate :

The punch plate or punch retainer fits closely over the body of the punch andholds it in proper relative position.

9: Back up Plate:

Back up plate or pressure plate is placed so that intensity of pressure does

not become excessive on punch holder. The plate distributes the pressure

over a wide area and the intensity of pressure on the punch holder is reduced

to avoid crushing.

10: Stripper:

It is a plate which is used to strip the metal strip from cutting a non-cutting

Punch or die. It may also guide the sheet.


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The main features of a press are:

A frame which support a ram or a slide and a bed,

a source of mechanism for operating the ram in line with and normal

to the bed.

The ram is equipped with suitable punch/punches and a die block is

attached to the bed.

A stamping is produced by the downward stroke of the ram

when the punch moves towards and into the die block.

The punch and die block assembly is generally termed as a die set

or simple as the die


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Press working operations

Cutting Operations

Forming Operations


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Cutting operations

In cutting operations the work piece is

stressed by its ultimate strength. The stresses

caused in the metal the applied forces will be

shear stresses. The cutting operations include:

(a) Blanking (b) Punching (c) Notching

(d) Perforating (e) Trimming (f) Shaving

(g) Slitting (h) Lancing


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Cutting operations 1: Blanking:

Blanking is the operation of cutting a flat shape from sheet metal. The product punched out is called theblank and the required product of the operation the hole and the metal left behind is discarded aswaste.

2: Punching or Piercing:

It is a cutting operation by which various shaped holes are made in sheet metal. Punching is similar toblanking except that in punching, the hole is the desired product. The material punched out from the holebeing waste.

3: Notching:

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

4: Perforating:

This is a process by which multiple holes are very small and close together are cut in a flat sheet metal.

5: Trimming:

This operation consists of cutting unwanted excess of material from the periphery of a previously formedcomponent.

6: Shaving:

The edge of a blanked part are generally rough, uneven and un square. Accurate dimensions of the partare obtained by removing a thin strip of metal along the edges.

7: Slitting:

It refers to the operation of making incomplete holes in a work piece.

8: Lancing:

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

9: Nibbling:

The nibbling operation , which is used for only small quantities of components, is designed for cutting outflat parts from sheet metal. The flat parts from simple to complex contours. This operation is generallysubstituted for blanking. The part is usually moved and guided by hand as the continuously operatingpunch cutting away at the edge of the desired contour.


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Forming operations

In forming operations , the stresses are below the

ultimate strength of the metal , in this operation ,

there is no cutting of the metal but only the

contour of the work piece is changed to get thedesired product.

The forming operations include:

(a) Bending (b) Drawing (c) Squeezing


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Forming operations

1: Bending:

In this operation; the material in the form of flat sheet orstrip is uniformly strained around a linear axis which lies inthe neutral plane and perpendicular its the length wise

direction of the sheet or metal. 2: Drawing:

This is a process of forming a flat work piece into a hollowshape by means of a punch which cause the blank into adie cavity.

3: Squeezing: Under the operation, the metal is caused to flow to all

portions of a die cavity under the action of compressiveforces


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THEORY OF CUTTING


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THEORY OF CUTTING

When cutting sheet metal in a die the forces applied to the

sheet by the punch and die are basically shear forces, that is,

equal and opposite forces spaced at a small distance apart

on the metal and produce the cutting.

The cutting or separation of the sheet metal is effected

through the following stages of shearing

Roll over

Penetration

Fracture

From both punch side and Die side simultaneously Thespacing or the small distance of the two shearing planes is

called clearance. It will be generally about 8 - 10% of metal

thickness for MS Sheets.


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BLANKING It is the operation of cutting or shearing a piece out of sheet to a predetermined

contour

PIERCING It is the operation of producing the holes of round or contoured shape in the

blanks or in formed parts.

NOTCHING In Notching operation a small piece or pieces of metal are removed from theedges/ corners of a Strip or a Blank

TRIMMING It is the operation of cutting off the excess metal after the Drawing or Forming

operation

LANCING It is the operation of slitting on three sides and opening out by bending the lug about

an axis on the uncut fourth side with in the blank.


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Die

Clearance

The difference in dimensions between the mating membersof a die set is called clearance. This clearance is applied infollowing manner:

1: when the hole has to be held to size i.e. the hole in thesheet metal is to be accurate (punching operation) , andslug is to be discarded. The punch is made to the size of thehole and the die opening size is obtained by addingclearance to the punch size.

2: In blanking operation , where the slug or blank is thedesired part and has to be held to size, the die opening sizeequals the blank size and the punch size is obtained bysubtract.


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Cutting forces

Cutting Forces

In cutting operation, as the punch in its downwardmovement enters the material, it need not penetrate thethickness of the stock in the order to offset completerupture of the part. The distance which the punch entersinto the work material to cause rupture to take place iscalled penetrable and is usually given as the %age of thestock thickness

The percent penetration depends on the material being cutand also on the stock thickness. When hard and strongmaterial is being cut ,a very little penetration of the punchis necessary to cause feature. With soft material thepenetration will be greater.


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Strippers

After a blank has been cut by the punch on itsdownward stroke, the scrap strip has the tendency toexpand. On the return stroke of the punch the scrapstrip has the tin deny to adhere to the punch and be

lifted by it. This action interface with the feeding of thestroke through the die and some device must be usedto strip the scrap material from the punch as it clearsup the die block. Such a device is called stripper orstripper plate.

Stripper plate are of two types: 1: fixed or stationary

2: spring loaded or movable


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Strippers

(a): fixed or stationary strippers:

This stripper is attached at a fixed height over the die block. Theheight should be sufficient to permit the sheet metal to be fedfreely between the upper die surface and the under surface of thestripper plate. The stripper plate thickness is determining by the

formula: Ts =1/8(w/3+16t)

Where w and t are width and thickness of the stock strip. The fixedstripper is also known as channel stripper

(b): Spring loaded Stripper:

This type is used on large blanking operations and also on very thinand highly ductile materials. As the punch travels downward forblanking operation the stripping force is determined with the helpof following relation. Fs=spt kn

Where p and t are in mm and s is the stripping constant.


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Pilots

Pilots are used in progressive dies. In the design ofprogressive dies, the first step is to establish thesequence of operations. In this sequence , the piercingoperations are placed first. After the holes have been

pierced , these holes are used for piloting the blankingpunches so that the blank formed is truly concentric tothe already punched hole. This piloting is achieved bymeans of pilots secured under the blanking punch. Tobe effective the pilot must be strong enough to align

the stock without bending. Pilots are made of goodgrade of tool steel heat treated to maximum toughnessand to a hardness of 56 to 60 Rockwell C.


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Types of Pilot

(a ): Direct pilot:

Pilots which are mounted on the face of a punch are calleddirect pilots. The pilot holder is generally a block of steelwhich can be fastened to the punch holder.

(b) Indirect pilot: Such pilots are well guided through the hardened bushesin the stripper plate.

5: Stock Stop:

The strip of sheet metal is fed and guided through a slot in

the stock guide or through a slot in the stripper plate aftereach blanking. The strip has to be advanced a correctdistance, the device used to achieve this is called stockstop

press working operations

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