SHRI GURU GOBIND SINGHJI INSTITUTE OF ENGG & TECHNOLOGY

DEPARTMENT OF PRODUCTION ENGINEERING

SUBJECT:MECHANICAL WORKING OF METALS EXPERIMENT NO: 1

AIM: DESIGN OF CUTTING DIE, BENDING DIE & DRAWING DIE

AIM: Design of Cutting Die, Bending Die, & Drawing Die.

OBJECTIVES:

A Die is a specialized tool used in manufacturing industries to cut or shape material using press.

Like moulds, dies are generally customized to the item they are used to create. Products made with dies

range from simple paper clips to complex pieces used in advanced technology. After completing this

Experiment, the students will be able to:

Understand the basic Press operations.

Basic cutting operation in die.

Understand different parts of a die.

Classify & understand different types of dies.

THEORY:

1) PRESS OPERATIONS:

The Different sheet-metal operations done on a press can be grouped into two categories:

1. Cutting operations.

2. Forming operations.

In cutting operations, the work piece is stressed beyond its ultimate strength. The stresses caused in

the metal by the applied forces will be shearing stresses. In forming operations, the stresses are below the

ultimate strength of the metal, and there is no cutting of the metal but only the contour of the work piece is

changed to get the desired product. The various cutting operations are as Blanking, Punching/Piercing,

Notching, Perforating, Trimming, Shaving, Slitting, and lancing.

a) BLANKING:

Blanking is the operation of cutting a flat shape from sheet metal. The article punched out is

called ‘blank’ and is the required product of the operation. The hole and metal left behind is discarded as

waste. Figure shows a blanking operation.

b) PUNCHING/PIERCING:

Punching is a cutting operation by which various shaped holes are made in sheet metal. It is

similar to Blanking, except that in punching, the hole is the desired product, and the material punched out to

form the hole being waste.

FIG.1. BLANKING OPERATION FIG.2. PUNCHING OPERATION

c) NOTCHING:

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

blank. It is a low-cost process, particularly for its low tooling costs with a small range of standard punches.

This operation removes metal from either or both edges of the strip. Figure shows a Notching operation.

FIG.3. NOTCHING OPERATION FIG.4. PERFORATING OPERATION

d) PERFORATING:

This is processes by which multiple holes, which are very small and close together are cut in

flat work material. It is similar to punching, except that, the holes are smaller in size and closer to each

other, as shown in the figure.

e) TRIMMING:

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

formed component and provides a smooth edge.

f) SHAVING:

The edges of a blanked part are generally rough, uneven and square. Accurate dimensions of

the part are obtained by removing a thin strip of metal along the edges. This operation is termed as

‘shaving’. It is a secondary operation, which is usually followed by punching.

g) SLITTING:

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

h) LANCING:

This is a cutting process in which a hole is partially cut and then one side is bent down to form

a sort of tab or louver. Since no metal is removed, there will be no scrap.

FIG.5. SLITTING OPERATION FIG.6. LANCING OPERATION

The forming operations include Bending, Drawing, Redrawing, and Squeezing.

i) BENDING:

In this operation, the material in the form of flat sheet or strip, is uniformly strained around a

linear axis which lies in the neutral plane and perpendicular to the lengthwise direction of the sheet or metal.

FIG.7. BENDING OPERATION

j) DRAWING:

This is a process of forming a flat work piece into a hollow shape by means of punch which

causes the blank to flow into a die cavity. Figure shows an Drawing operation.

Where,

C= clearance

Db = Blank Diameter

Dp = Punch Diameter

Rd = Die corner Radius

Rp = Punch corner Radius

F= Drawing Force

Fh = Holding Force

FIG.8. DRAWING OPERATION

k) CUTTING OFF & PARTING:

A cut-off operation separates the work material along a straight line in a single-line cut. When

the operation separates the work material along a straight-line cut in a double-line cut, it is known as

‘Parting’. Cutting off and parting operations are used to separate the workpiece from the scrap strip, as

shown in figure 9.

FIG .9.CUTTING & PARTING OFF OPERATION

2) SHEAR OF METAL IN CUTTING OPERATION:

During any metal-cutting operation, the material is compressed between the punch and die until

parted by the act of shearing. These forces against the material are not the only acting forces encountered. In

parallel with the law of action/reaction, the material puts forth forces against the tooling as well.

When the punch hits the sheet-metal material, it first elastically extends the grain of the metal,

forcing it to swell up, while pulling a portion of it from underneath the punch. Some of this swelling

progresses downward too, and it remains tightened around the walls of die opening. The upper swelling

wraps around the punch, impairing its withdrawal, sometimes breaking the tool where too thin a punch is

used to penetrate heavier material. For this reason, the minimum diameter of the punched/pierced opening

should be at least 1.5 of material thickness with regular punches, and 1.1 to 1.2 thickness with guided

tooling.

FIG.10. STRESSES IN SHEAR OPERATION

Following the penetration of the metal, the development of tensile and compressive stresses

accompanied by subsequent changes of the part’s edges, causes the material to separate, as shown in fig. 10.

In Fig. 11-a, clearance between the punch and die is clearly visible, and its amount is crucial to the

success of the metal-cutting process. Clearance is the space between the two cutting edges, those of the

punch and those of the die. Clearance not only allows for the body of a punch to be contained in the cavity

of a die; it also provides for the development of fractures during the cutting process.

In Fig. 11-b, the punch moves down and forces its way into the material. Stretching occurs at

points A and B, where the stock is in tension; the remaining material under the punch is compressed.

FIG.11. CUTTING-ACTION PROGRESSION DURING SHEARING OF METAL

However, the material’s elastic limit has not been exceeded yet.

In Fig. 11-c, the punch pushes further down, and fractures begin to form around thecorners of both

punch and die as the elastic limit of the material is being exceeded. The angle of these fractures depends on

the die clearance. If the clearance is either excessive or too small, this angle may not allow for a smooth

connection of the upper and lower fractions, and a rough, jagged-cut may result.

In Fig. 11-d, with further descent of the punch, fractures deepen and finally meet. The Cut-out is

separated from the strip and pushed into the die. There, owing to inner stresses thus created, it swells up; the

strip also tightens around the punch prompted by forces from within.

3) MAIN PARTS OF AN DIE-SET:

The complete die set consists of a punch, die and some other accessories which are described in this

sectionlater. Perfect alignment of punch and die is most important for satisfactory working of punch.

Accessories of die set provides the require alignment and rigidity to the system and improves accuracy of

the system performance. These accessories are the finished parts, removal of waste. The die accessories are

shown in Figure.

FIG.12. CROSS-SECTION OF A BREHM’S SHIMMY DIE

The Different accessories in an die-set consists of the following parts:-

1. BASE:

Base is the one of the parts of a press. It is main supporting member for work piece holding dies and different

controlling mechanisms of press.

2. RAM:

This is main operating part of the press which works directly during processing of a work piece. It reciprocates

to and fro within its guide ways with prescribed stroke length and power. The stroke length and power transferred

can be adjusted as per the requirements. At its bottom end, it carries punch to process work.

3. BOLSTER PLATE:

It is a thick plate attached to the bed or base of the press. It is used for locating and clamping the die

assembly rigidly to support the work piece. It is usually 5 to 12.5 cm thick.

4. DRIVING MECHANISMS:

Different types of driving mechanisms are used in different types of presses like cylinder and piston

arrangement in hydraulic press, crankshaft and eccentric mechanisms in mechanical press, etc. these

mechanisms are used to drive ram by transferring power from motor to ram.

5. PITMAN:

It is a connecting rod which is used to transmit motion from the main drive shaft to the press slide.

6. DIE SET:

It is unit assembly which incorporates a lower and upper shoe, two or more guideposts and guidepost

bushings.

7. DIE SHOE OR LOWER SHOE:

The die-shoe or lower shoe of a die-set is generally mounted on the bolster plate of a press. It

supports the die-block and the guide posts.

8. UPPER SHOE OR PUNCH HOLDER:

The upper part of a die-set which contains the guidepost bushings. Punch holder is clamped to the

ram of press and holds the punch below it.

9. PUNCH PLATE:

Punch plate is also known as punch retainer. This is fixed to the punch holder. Punch plate serves as a

guide way to hold the punch in right position and properly aligned. This makes the replacement of punch

quick and correct.

10. PUNCH:

It is the male component of the die assembly which directly comes in contact of work piece during its

processing. It is directly or indirectly moved by and fastened to the press ram or slide.

11. DIE:

It is the female component of the die assembly, which is used for producing work in a press. It is

also referred to a complete tool consisting of a pair of mating members for producing work in a press.

12. STRIPPER:

Stripper is used to discard the work piece outside the press after the completion of cutting or forming

operation. After the cutting when punch follows upward stroke the blank is stripped off from punch

cutting edge and prevents it from being lifted along with the punch. This action of prevention is

Performed by the stripper.

13. KNOCKOUT:

Knockout is also a type of stripper which is used generally in case of invarted dies. After the

completion of cutting action, the blank is ejected by the knockout plate out of cutting edge.

14. PRESSURE PADS:

Pressure pads are plates which grip the work piece very tightly at the ends when it plastically flows

between the punch and the die. This tight gripping eliminates the chances of wrinkling in the process of

metal forming.

15. GUIDE POSTS:

Accurate alignment between die opening and punch movement is very important. Guide posts are

used for correct alignment of punch and die shoe.

4) CUTTING DIE:

These dies are used to cut the metal. They utilize the cutting or shearing action. The common cutting

dies are blanking dies, punching die, perforating die, notching die, trimming die, shaving die. According

to the method of operation, the cutting-dies can also be classified as:

1. SIMPLE DIE:

This die performs single operation for each stroke of the press slide. The operation may be any

of the operation listed under cutting or forming dies.

2. PROGRESSIVE DIE:

Progressive dies are a mixture of various single dies operating as different stations and grouped

into the same die shoe. These stations are positioned to follow a sequence of operations needed to produce

the required part. Usually, the die sequence is arranged side by side, or horizontally. The vertical

arrangement of operations. Such dies are also called tandem dies, and are used mostly for drawing of shell

types of products.

3. COMPOUND DIE:

Compound dies produce very accurate parts, but their production rate is quite slow. These dies

consist of a single station where the part is most often blanked out and either formed, embossed, pierced, or

otherwise adjusted in a single stroke of the press. No progression of the strip is involved, like progressive

dies, as each stroke of the press produces a single, complete part. There are many variations of compound

dies, all of them having one feature in common: with each stroke of the press, a minimum of one operation

is being performed. They combine at least two operations during each stroke of the press.

4. MULTIPLE DIE:

Multiple dies or gang diesare used where a large amount of simple blanksis required. The die

consists of duplicate punches and dies, which cut as many blanks as there are tools during each stroke of the

press.

In this section, there is a design for following component.

FIG.13. GEOMETRIC DETAILS AND ACTUAL VIEW OF COMPONENT

Material: M.S.

Thickness: 5 mm.

Ultimate shear strength: 350 MPa.

1. CALCULATE PERIMETER.

P = [2(π/2)x60] + [4(π/4)x8] + 2(12) + 4(6)

P = 261.626 mm

2. CUTTING FORCE

F = Shear strength x perimeter x thickness.

F =350 x 261 x 5.

F = 457.845KN.

3. CAPACITY OF PRESS

For MS %penetration factor is 1.2.

Capacity = cutting force x % penetration

C = 457.84 x 1.2

C = 549.416 KN = 56.007 TON

Thus, use the press with capacity ranging from 60 to 80 tonnes.

4. SIZE OF BLANK DIE AND STRIPPER

As this is the blanking operation hence size of die is sameas the component and size of punch is less

by an amount of clearance than die on each side.

Keep the space of 2 mm on each side than size of punch for the stripper plate.

For the MS the clearance is 5% of thickness of sheet

c = 5/100 x 5 = 0.25 mm

Parameter Die Punch Stripper

R 30

29.5

31.5

r

4

4.5

2.5

L 1 16 16.5 18

L 2 6 6.25 7

FIG.14. SIZE OF PUNCH, DIE AND STRIPPER

5. Die block and stripper thickness

As the perimeter of blank is greater than 250mm hence select the die size of 30mm.

Provide a straight land of 5 mm and relief angle 1/2°.

Thickness of stripper = 0.5 x die thickness

= 0.5 x 30 = 15mm.

6. Diameter of screws and dowel pins

d = 0.4 x die block thickness = 0.4x 30 = 12 mm.

5) DRAWING DIE:

Drawing dies force the material to flow in conjunction with the movement of the punch, which

causes plastic deformation to its structure. During the drawing process, the volumnar amount of flat blank

is transformed into a drawn, shell-like shape. In some cases, thinning of the part’s cross section may be

observed. The category of drawing dies consists of drawing, redrawing, ironing, reducing, and bulging

dies.

In drawing operation the punch forces the blank into die cavity, the blank diameter decreases and

causes the blank to become thicker at its outer portion. This is due to circumferential compressive stresses

to which material element in the outer portion is subjected.

The portion of the blank between the die wall and the punch surface is subjected to nearly pure

tension and tends to stretch and become thinner, as shown in fig.

FIG.15. DRAWING OPERATION

The following problem illustrates the design procedure of deep drawing die.

FIG.16. DRAWN COMPONENT

Material thickness = 0.8mm

Shell diameter, d = 50 mm

Radius of bottom corner = 1.6 mm

Height, h = 50 mm

1. SIZE OF BLANK

The ratio d/r = 50/1.6 = 31.25mm

Hence, blank diameter D = (d2+4dh)

1/2

= 111.80 mm

2. NUMBER OF DRAWS

Here, h/d ratio = 50/50 = 1

Thus, Number of draws = 2.

let the first reduction be 45%

therefore, diameter d1= 111.8-0.45(111.8)

= 61.49 mm

The diameter at the end of second draw is 50 mm.

Thus, the reduction of second draw = 100 x (1-{50/61.49})

= 18.68 %

This is less than 30%, the permissible reduction of second draw.

3. PERCENTAGE REDUCTION

% Reduction = 100(1-[d/D])

= 55.27%

4. RADIUS ON PUNCH & DIE

Radius on punch = 4t = 4 x 0.8 = 3.2 mm

Radius on Die = 64t = 6 x 0.8 = 4.8 mm = 5 mm(approx.)

5. DIE CLEARANCE

For first Draw = 1.1t = 0.88 mm

For second Draw = 1.12t = 0.896 mm

Punch Diameter for first Draw = 61.49 – 2t = 59.89 mm

Die opening for first draw = 61.49 mm

Punch diameter for second draw = 50 – 2t = 48.40 mm.

Die opening for second draw = 50 mm

6. DRAWING PRESSURE

F = πdtϭy([D/d]- c)

Assuming, ϭy = 427 MPa and considering c = 0.65

F = π x 50 x 0.8 x 427 (2.37 – 0.65)

F = 92.30 KN

7. CAPACITY OF PRESS

For a Force of 92.30 KN, a press ranging from 100 – 150 KN will be alright.

8. BLANK HOLDING PRESSURE

It is taken as 1/3rd

of the drawing Pressure = 1/3(92.30) = 30.8 KN.

6) BENDING DIE:

Bending dies are used to form, fold, or offset parts without subjecting their material structure to the

flow and plastic deformation. Aside from simple bending dies, this type of tooling includes: curling dies,

twisting dies, and straightening or flattening dies. Forming dies, however, belong into this category only

marginally, as they fit in with the drawing dies as well.

Bending dies deform a flat part into an angular shape. The bend line is straight, with no plastic

deformation present. Forming dies deform a flat part in much the same manner, but the line of a bend may

be curved, with plastic deformation evident in some areas surrounding the curvature.

In the bending operation, the total deformation consists of plastic-deformation and elastic-

deformation. The plastic deformation is permanent, whereas the elastic deformation is recoverable. After the

load on part is released, the elastic recovery takes place and bend angle reduces, this is called as ‘Springback

Effect’. Spring back effect should be considered in the design of die, so that the desired angle is obtained at

final after elastic recovery takes place, that is, springback.

FIG.17. SPRINGBACK EFFECT

The extent of the spring-back depends upon,

Elastic limit of the material formed.

Bending type (air bending or die bending)

Bend radius.

FIG.18. BENDING TERMINOLOGY

Fig. shows the bending terminology

1. BEND RADIUS:

It is defined as the radius of curvature on the inside or concave surface of the bend.

2. BEND ALLOWANCE:

To calculate the blank length for bending, the length of material in the curved section or bend area

has to be calculated. This length in the bend area which will be more than the corresponding

length of blank before bending, is called bend allowance.

3. BEND ANGLE:

The angle through which the sheet is bending about neutral axis is bend angle.

4. BEVEL ANGLE:

The angle between the surfaces of the bend sheet with the vertical is bevel angle.

CONCLUSION:

In this way, we have studied the different types of dies, their working principle and design.

REVIEW QUESTIONS:

1. What are the different types of die?

2. What is the difference between cutting die and forming die?

3. What is the difference between piercing and perforating?

4. What is stripper?

5. What is Pitman?

6. In deep drawing, why the multiple draws is used instead of a single draw?

7. What is springback?