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A

PROJECT REPORT

ON

COMMON BENDING TOOL DESIGN

FOR TWO SHEET METAL COMPONENTS (LEFT & RIGHT)

SUBMITTED BY

GAURAV KUMAR SINGH

Branch - Mechanical

Reg. No - 1201298388

UNDER THE GUIDANCE OF

PROF. R.K SAHU (Deptt. Of Mech. Engg.)

SUBMITTED TO Biju Patnaik University of Technology (BPUT), Rourkela

IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGY (B.TECH) IN MECHANICAL ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

GANDHI INSTITUTE FOR TECHNOLOGY

GRAMADHIA, GANGAPADA

BHUBANESWAR, ODISHA

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CONTENTS

CHAPTER NO. PAGES

1. CHAPTER 1 4

ABSTRACT 5

2. CHAPTER 2 6

HISTORY 7

3.CHAPTER 3 8

3.1 Press Tool ……………………………………………………………………………………….9

3.2 Compound Tool………………………………………………………………………………….9

3.3 Basic die components ………………………………………………………………………10

3.4 Part Details of Compound Tool………………………………………………………….11

3.5 Types of Press Tool and Operations ………………………………………………….12

4. CHAPTER 4 16

4.1 Metal-Cutting Process ………………………………………………………………17

4.2 Principle of Metal Cutting………………………………………………………….18

4.3 Forces Involved In the Metal-Cutting Process ……………………………19

4.4 Alignment Of Cutting Tools ………………………………………………………22

4.5 Cutting Clearances …………………………………………………………………...23

4.6 Stripping Pressure……………………………………………………………………25

4.7 Major Operation by Compound Tool………………………………………….26

5. CHAPTER 5 31

Practical blanking and piercing die design5.1 Basic Approach to Die Design…………………………………………………..32

5.2Types of blanking dies …………………………………………………………….32

5.2.1 General Types Of Blanking Dies………………………………..32

5.2.2 Die- Block ……………………………………………………………33

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5.2.3 Die Block Thickness……………………………………………….34

5.2.4 Die Opening…………………………………………………………..35

5.2.5 Punch……………………………………………………………………35

5.2.6 Back Up Plate………………………………………………………...35

5.2.7 Methods Of Holding Punches………………………………….36

5.2.8 Centre Of Pressure…………………………………………………36

5.2.9 Strippers……………………………………………………………….36

5.2.10 Stock Stop……………………………………………………………...37

5.2.11 Strip Feeding…………………………………………………………38

5.2.12 Knockouts……………………………………………………………..38

5.2.13 The Die Shoe………………………………………………………….39

5.2.14 Bolster Plate………………………………………………………….39

5.2.15 Stops……………………………………………………………………..39

5.2.16 Ejecting Of Parts…………………………………………………….40

6. CHAPTER 6 41

STRIP LAYOUT

6.1 Strip layout ………………………………………………………………….42

6.1.1 Economy of material…………………………………………….42

6.1.2 Direction of material grain or fiber………………………..44

6.1.3 Strip or coiled stock……………………………………………...44

6.1.4 Direction of burr…………………………………………………..45

6.1.5 Press used……………………………………………………………45

6.1.6 Production required……………………………………………..45

6.1.7 Die cost………………………………………………………………..46

6.2 Press Capacity…………………………………………………………………....48

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6.3 Cutting Forces……………………………………………………………………49

7.CHAPTER 7…………………………………………………………………………….50

7.1 Advantages………………………………………………………………………………..51

7.2 Disadvantages……………………………………………………………………………51

7.3 Applications………………………………………………………………………………51

8. CHAPTER 8……………………………………………………………………………52

8.1 References & conclusion………………………………………………………………….53

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CHAPTER ONE

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ABSTRACT

ress tools are tools, used to produce a particular component in

large quantity, out of sheet metals by using presses. The most

common group of metal working operations is that which includes

blanking and piercing. More and more industrial sectors today look to

blanking and piercing for production of the components they need, from

small accurate parts to massively rigid ones. To deal with such a broad

range of requirements a considerable knowhow in the technology of both

tool-design and of pressing is necessary. Blanking is a process of producing

flat components. The entire periphery is cut. The cut-out piece is called

blank. This process is called blanking and tool used is called as blanking

tool. For producing any sheet metal components, blanking operation is the

primary process to carry-out. Similarly in case of piercing operation the cut

hole is the requirement, the cut-out piece is the scrap. In this project, a real

time design of a blanking and piercing tool is presented with the help of

Auto-CAD and the hard copy prints out.

P

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CHAPTER TWO

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HISTORY

istorically, metal was shaped by hand using a hammer. Later,

larger hammers were constructed to press more metal at once,

or to press thicker materials. Often a smith would employ a

helper or apprentice to swing the sledgehammer while the smith

concentrated on positioning the work piece.

Adding windmill or steam power yielded still larger hammers such

as steam hammers. Most modern machine presses use a combination of

electric motors and hydraulics to achieve the necessary pressure. Along

with the evolution of presses came the evolution of the dies used within

them.

H

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CHAPTER THREE

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INTRODUCTION

3.1 PRESS TOOL

A machine press, commonly shortened to press, is a machine tool that changes the shape of a work piece.Press tools are commonly used in hydraulic and mechanical presses to produce components at a high productivity rate. Generally press tools are categorized by the types of operation performed using the tool, such as blanking, piercing, bending, forming, forging, trimming etc. The press tool will also be specified as blanking tool, piercing tool, bending tool etc.

3.2 COMPOUND TOOL

A die designed to simultaneously perform more than one operation with each stroke of the press. For example, a compound die may blank and pierce in a single stroke.

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(COPMOUND PRESS)

3.3Basic die components

In this tool two or more operations may be performed at on station. Such dies are considered as cutting tools since, only cutting operations are carried out. Simple compound die in which a washer is made by one stroke of the press. The washer is produced by simultaneous blanking and piercing operations. Compound die are more accurate and economical in mass production as compared to single operation dies.

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3.4 PART DETAILS OF COMPOUND TOOL

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3.5 Types of Press Tool and Operations

3.5.1 Blanking tool

When a component is produced with one single punch and die where the entire outer profile is cut in single stoke is called Blanking.Blanking is the operation of cutting flat shapes from sheet metal.The hole and metal remained after blanking operation is discarded as a waste.Size of blank or product is the size of the die & clearance is given on punch.It is a metal cutting operation.It is fast process and generally used for medium and mass production volumes.It is cheapest process in manufacturing.

3.5.2 Piercing Tool

Piercing involves cutting of clean holes with resulting scrape slug. The operation is often called piercing, in general the term piercing is used to describe die cut holes regardless of size and shape. Piecing is performed in a press with the die. The piercing tool is used to pierce the holes as secondary tool such as after bending of component etc.

3.5.3 Cut off tool

Cut off operations are those in which a strip of suitable width is cut to lengthen single. Cut-off tools can produce many parts. The required length of strip can be cut off for bending and forming operations using this tool.

3.5.4 Parting off tool

Parting off is an operation that involves two cut off operations to produce blank from the strip. During parting some scrape is produced. Therefore parting is the next best method for cutting blanks. It is used when blanks will not rest perfectly. It is similar to cut off operation except the cut is in

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double line. This is done for components with two straight surfaces and two profile surfaces.

3.5.5 Trimming tool

When cups and shells are drawn from flat sheet metal the edge is left wavy and irregular, due to uneven flow of metal. This irregular edge is trimmed in a trimming die. Shown is flanged shell, as well as the trimmed ring removed from around the edge. While a small amount of Material is removed from the side of a component in trimming tool.

3.5.6 Shaving tool

Shaving removes a small amount of material around the edges of a previously blanked stampings or piercing. A straight, smooth edge is provided and therefore shaving is frequently performed on instrument parts, watch and clock parts and the like. Shaving is accomplished in shaving tools especially designed for the purpose.

3.5.7 Bending tool

Bending tools apply simple bends to stampings. A simple bend is done in which the line of bend is straight. One or more bends may be involved, and bending tools are a large important class of press tools.

3.5.8 Forming tool

Forming tools apply more complex forms to work pieces. The line of bend is curved instead of straight and the metal is subjected to plastic flow or deformation.

3.5.9 Drawing tool

Drawing tools transform flat sheets of metal into cups, shells or other drawn shapes by subjecting the material to severe plastic deformation. Shown in fig is a rather deep shell that has been drawn from a flat

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sheet.This type of Press tools are used to perform only one particular operation.

3.5.10 Progressive tool

Progressive tool differs from the stage tool by the following aspect, inprogressive tool the final component is obtained by progressing the sheet metal or strip in many stages. In each and every stage the component will get its shape stage by stage the full shape will be obtained at the final stage.

3.5.11 Compound tool

The compound tool differs from progressive and stage tool by the arrangement of punch and die. It is an inverted tool were blanking and piercing takes place in a single stage and also blanking punch will act as piercing die.

3.5.12 Combination tool

In combination tool two or more operations will be performed simultaneously such as bending and trimming takes place in a single stage. IN combination tool two or more operations such as forming, drawing, extruding, embossing may be combined on the component with various cutting operations like blanking, piercing, broaching and cut off takes place.

3.5.13 Notching

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

3.5.14 Perforating

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

3.5.15 Slitting

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

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3.5.16 Lancing

This is a cutting 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 actually removed, there will be no scrap.

3.5.17 Nibbling

The nibbling operation which is used for only for small quantities of component is designed for cutting out flat parts from sheet metal. The flat parts range from simple to complex contours. This operation is generally substituted for blanking. The part is usually moved and guided by hand as the continuously operating punch cuts away at the edge of the desired contour.

3.5.18 Squeezing

Under the operations, the metal is caused to flow to all portions of a die cavity under the action of compressive force.

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CHAPTER FOUR

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THEORY

4.1 Metal-Cutting Process

Metal cutting is a process used for separating a piece of material of predetermined shape and size from the remaining portion of a strip or sheet of metal. It is one of the most extensively used processes throughout die and sheet-metal work. It consists of several different material-parting operations, such a piercing, perforating, shearing, notching, cutoff, and blanking.

In blanking, the piece is cut off from the sheet, and it becomes a finished part. In piercing,the cutout portion is scrap which gets disposed off while the product part travels on through the remainder of the die. The

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terminology is different here, though both processes are basically the same and therefore belong to the same category, which is the process of metal cutting (Fig. 4-1).

The actual task of cutting is subject to many concerns. The quality of surface of the cut, condition of the remaining part, straightness of the edge, amount of burr, dimensional stability—all these are quite complex areas of interest, well known to those involved in sheet-metal work.

Most of these concerns are based upon the condition of the tooling and its geometry,material thickness per metal-cutting clearance, material composition, amount of press force, accurate locating under proper tooling, and a host of additional minor criteria. These all may affect the production of thousands and thousands of metal-stamped parts. With correct clearances between the punch and die, almost perfect edge surface may be obtained. This, however, will drastically change when the clearance amount increases, and a production run of rough-edged parts with excessive burrs will emerge from the die.

Highly ductile materials, or those with greater strength and lower ductility, lesser thicknesses or greater thicknesses—these all were found similarly susceptible to the detrimental effect of greater than necessary clearances.

The literature recommends different tolerance amounts for cutting tools. Some claim 0.06t (t = material thickness) to be sufficient for almost all applications. Others promote a 0.08t range, with 0.1t topping it off.

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(Fig. 4-1).

Naturally, a different type of separation must occur with a softer material than with its harder counterpart. The carbon content certainly has an influence on this process as well.

Therefore, the tolerance range must have a provision to change not only with the stock thickness but with its composition as well.

As already mentioned, good condition of tooling is absolutely essential to the cutting process. We may have the most proper tolerance range between the punch and die, and yet the cut will suffer from imperfections if worn-out tools are used.

4.2 Principle of Metal Cutting

The metal is brought to the plastic stage by pressing the sheet between two shearing blades so that fracture is initiated with the movement of the upper shear, finally result in the separation of the slug from the parent strip. The metal under the upper shear is subjected to both compressive and tensile stresses. In an ideal shearing operation,the upper shear pushes the metal to a depth equal to about the third of its thickness. Because of pushing the material into the lower shear the area of cross-section of the metal between the cutting edge of the shear decreases and causes the initiation of the fracture. The portion of the metal which is forced into the lower shear is lightly burnished and would appear as a bright band around the blank lower portion. The fractures which are initiated at both the cutting points would progress further with the movement of the upper shear and if the clearance is sufficient, would meet, thus completing the shearing action.The two shearing elements of the press tool are the hardened punch and the die plate having sharp edges and a certain shearing clearance. Both the shapes of the punch and the die opening conform to the required shape of the component. The punch is connected to the ram of the power press and while descending contacts the stock, exerts pressure over the stock around the cutting edges and shears it through. Exactly the same phenomenon that takes place where in blanking (or) in piercing (or) in any other shearing operation. In the process of shearing four important stages are usually distinguished according to the observation.

1. STAGE I: Plastic Deformation The stock material has been placed on the die and the punch is driven towards the die. The punch contacts the stock material and exerts pressure upon it. When the elastic limit of the stock material is exceeded, plastic deformation takes place.

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2. STAGE II: Penetration As the driving force of the ram continues, the punch is forced to penetrate the stock material and the blank or slug is displaced into the die opening a corresponding amount. This is true shearing part in of the cutting cycle, from which the term “shearing action” is derived.

3. STAGE III: Fracture Further continuation of the punching pressure that causes fractures to start at the cutting edges of the punch and the die. Under proper cutting conditions, the fractures extended toward each other and meet. When this occurs, the fracture is complete and the blank or slug is separated from the original stock material. The punch then enters the die opening, pushing the blank or slug slightly below the die cutting edge.

4. STAGE IV: As the punch completes the down stroke up to the lower point, the component of slug is pushed through the die opening. Strictly speaking this action is a consequence of the dynamic fracture at the stage III and only in certain case the push through takes place where the punch takes place where the punch travels beyond the land of the die. This is the simplest approach on the shearing action. Before dealing with the details of the phenomenon, the attention is drawn on the same other allied factors which calls for deeper deliberations on the shearing process.

4.3 Forces Involved In the Metal-Cutting Process

Aside from the press force acting upon the ram and applying vertical pressure to the die and subsequently to the steel-metal material, additional forces are involved in the metal-cutting process. As the punch enters the material, it pushes the bulk of it down through the opening in the die.

However, a small portion of metal is forced sideways, as seen inFig. 4-2.This flow, directed away from the cutting tool, is guided by the action of tensile and compressiveforces which develops within the cutmetal,andis thusGrain dependent: A different pattern of flow is seen along the grain than against it.

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(Fig. 4-2)

Such movement of material affects the structure of the sheet, especially in the immediate vicinity of the cutting station. Forced aside, the material becomes too crowded by such expansion in its content and it resorts to bulging through the only available outlet, through the surface of the sheet, which it deforms. In areas where piercing is more congested, the deformation progress is so widespread that the whole sheet becomes distorted, displaying either an excessive camber or waviness or any other

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Variation from straightness. The expanding material pushes also against the body of a punch, applying a side oriented or thrust force toward it. The punch is suddenly restricted in its movements by the squeeze of bulging material, which is accompanied by changes in friction, as well as increased heat. The stability of the punch is often threatened and slim and fragile tools can often break under such a load. The deformation of the cutoff portion of metal is often not so pronounced, which is probably due to its usually smaller size. It is obvious that the flow of tensile and compressive forces resulting in the development of side-oriented and expansive shifts within the material is also a great contributor to the emergence of wear of the working surfaces. According to some, a side-oriented force generated by the cut material may amount to 2 to 20 percent of the total blanking force, with its marked dependency on the material thickness, its composition, and the amount of clearance between the cutting surfaces.

Additionally, forces within the cut material further influence the size of a sheared opening. On the complete retrieval of the punch, the bulging material slightly flattens out, its movement being oriented toward the empty space, which subsequently gets reduced in size. Cutting clearances of up to 0.05t have been found to produce openings smaller than the size of the cutting punch.

As already mentioned, the punch on its way out of the cut material is restricted in movement by the emergence of frictional forces originating within the structurally altered material. For the punch to progress, a considerable force is needed to overcome this influence. This force, called a stripping force, may be calculated with regard to the material composition, its strength and thickness, the size of tooling, and its clearance. Naturally,

with increasing clearance between the punch and die arrangement, the amount of stripping force decreases. But the quality of the cut decreases along with it.

4.4 Alignment Of Cutting Tools

Punches entering the material must be absolutely concentric with the die opening below.But sometimes a shift from the mutual axis may be due to the assembly procedures; sometimesa minute movement in the frame of a press may cause a slight offset of the two centerlines, which ideally should

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match each other. Even with perfect positioning, a long, unsupported, and unguided punch may be swayed aside by the movement of metal during the cutting operation or by its own off-center punching, or by an action of some other demanding operation within the die. To alleviate this problem, punches should be guided in their movement unless their bulk is so great that they actually constitute the major portion of the die. The guidance can be provided through inserts in the stripper plate, which are appropriately called guide bushings. Slim punches should be further protected by punch sleeves, or wraps, and similar arrangements. Punches that have irregular shapes or those having their face area ground to an angle often utilize heels, which guide their progress during the cutting operation (Fig. 4-3).

Multipart retainers are an additional punch-guiding provision to a die. They resemble small, self-contained punch plates, and they come in various sizes and shapes and with different tool-retaining openings (Fig. 4-4). The whole unit, along with the punch or punches it holds, is secured to the holding plate with dowels and locked in this position by screws. The punch, equipped with a ball-retaining groove, is precision-located by a pressure of the spring-loaded ball.

Another help with tool guidance is that in which die shoes are aligned with precision guide pins. Four-pillar die sets were found to be the most accurately aligned instruments, surpassed only by subpress dies, which are actually considered small, self-contained, and self-aligned press units.Guide pin and pillar die sets are described in Sec. 3-1-2 and 3-1-3. Guide pins areprecision-ground and fit into bushings of equal quality. Their tolerance ranges are 0.0002 to 0.0004 in. (0.005 to 0.010 mm), and their smooth function is aided by the lubricants retained in the grooves in the bushing. Self-lubricating bushings are made of high-strength bronze material, where a lubricant is embedded throughout its structure. Such a lubricating arrangement usually lasts the entire life of the bushing.

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(Fig. 4-3 )

However, the absolute of the die alignments is the ball-bearing bushing (Figs. 6-8 and 6-9), which runs so tight that the effect of the side-oriented force on the tooling is almost eliminated.

4.5 Cutting Clearances

The amount of cutting clearance between the punch and the die is of great importance in allnsheet-metal work. It is usually given as a percentage of the thickness of cut material, as shown in Table 6-3.

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(Fig. 4- 4)

The cutting clearance is always added to the die bushing of the particular cutting station. The punch, as stated previously, has the exact size of the hole to be cut and a tolerance of

+0.0002 in. (+0.005 mm)

–0.0000 in. (−0.000 mm)

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The die opening, which is to contain the punch, will use its size, add the amount of cutting clearance to it, and attach a tolerance range, often similar to that of the punch, or

+0.0002 in. (+0.005 mm)

–0.0000 in. (−0.000 mm)

As an example, with the punch size of in metric, this becomes the die opening at 8 percent cutting clearance for fabrication of 1/16 in. (1.6 mm) thick material will be and in metric, Even though the correct cutting clearance is recommended to be somewhere between 0.08t and 0.10t per side, some manufacturers use clearances much broader, with up to 0.16t per side. Such a gap may often be excessive and the cuts it will produce are frequently rough and uneven. Yet, with larger-size punches and with thicker material, greater cutting clearances can be chosen with no detrimental effect on the outcome.

At the same token, manufacturers of tooling for automatic NC machinery (so-called turret presses) sometimes use extremely small clearances with impressive results. The trick is in the total guidance of the punch, which is restricted from any deviation by its precisionmade sleeve, and ultimately aligned with the die, both components being firmly retained within the heavy ring of a turret. This type of tooling is built as separate little dies with small spring-loaded strippers included in every assembly.

4.6 Stripping Pressure

A stripping pressure calculation helps to determine the correct amount of the spring pressure a spring-loaded stripper must produce. It usually varies between 3 and 20 percent of the blanking pressure and can be figured out using Eq.

Ps = 3.5 Lt

where all values are the same as with the blanking pressure. The amount of delivered stripping pressure depends mainly on the proper design and proper function of springs, which are supporting the stripper’s mass. The second influential factor is the thickness of processed material, which governs the demand for stripping pressure approximately as shown in Table 6-5.

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The calculation above is but an approximation of the actual pressure needed to strip the part. The precise amount is very difficult to establish, since it is influenced by too manyvariables. The condition of the tooling, cutting clearance, type of material, and lubrication of tooling are just several out of many factors influencing the amount of stripping pressure needed. Sheared punches may reduce the blanking pressure, but they have no effect on the stripping pressure requirements. However, staged punching, where the height of cutting tools is offset, will produce a decrease in demand for stripping pressure. Two levels of punches would halve the amount of stripping pressure otherwise needed. Three levels of punches will use up one-third of the pressure, and so on.

4.7 Major Operation by Compound Tool

4.7.1 Blanking

When a component is produced with one single punch and die where the entire outer profile is cut in single stoke is called Blanking.Blanking is the operation of cutting flat shapes from sheet metal.The hole and metal remained after blanking operation is discarded as a waste.Size of blank or

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(Blanking operation)

product is the size of the die & clearance is given on punch.It is a metal cutting operation.It is fast process and generally used for medium and mass production volumes.It is cheapest process in manufacturing.It is used to produce blanks of desired contour and size by cutting them out of the stock strip. Blank is the desired ‘piece part’ made by blanking die. The material remaining after blanking is called as scrap.

Critical stages of shearing action in blanking:-

First stage: Plastic deformation

Second stage: Penetration

Third stage: fracture

4.7.2 Piercing

Piercing involves cutting of clean holes with resulting scrape slug. The operation is called piercing, in general the term piercing is used to describedie cut holes regardless of size and shape. Piecing is performed in a press with the die. The piercing tool is used to pierce the holes as secondary tool such as after bending of component.This operation consists of simple hole punching. It differs from blanking in that the punching (or material cut from stock) is the scrap and the strip is

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the work piece. Piercing is nearly always accompanied by a blanking operation before, after or at the same time.

Critical stages of shearing action in piercing:

First stage: Plastic deformation

Second stage: Penetration

Third stage: fracture

4.7.3 CLEARANCE

It is defined as the intentional space between the punch cutting edge and die cutting edge. Theoretically clearance is necessary to allow the fractures to meet when break occurs. The amount of clearance depends upon the kind, thickness and hardness of the work material.In piercing the work material is placed between the die and punch, where the punch should be exact and the die requires a clearance for the action to be performed.

The die opening must be sufficiently larger than the punch to permit a clean fracture of the metal. This dimension between the mating members of a die set is called clearance.

In the blanking operation, where the slug or blank is the desired part and gas to be held to size, the die opening size equals the blank size and punch size is obtained by subtracting the clearance from the die opening size.

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‘C’ is the amount of clearance per side of the die opening. The clearance is a function of the kind, thickness and temper of the work material, harder materials requiring larger clearance then the soft material, the exception being aluminum. The usual clearance per side of the die, for various metals, is given below in terms of the stock thickness, t:

For brass and soft steel, c= 5% of t

For medium steel, c= 6% of t

For hard steel, c= 7% of t

For aluminum c=10% of t

The clearance may be determined also with the help of the following relations:

c = 0.0032 t √{shear strength of the material (in N/mm2) }

In blanking operation, the die size is taken as the blank size and the punch is made smaller giving the necessary clearance between the die and the punch.

Die size        = blank size

Punch size = blank size – 2 x clearance

Clearance   = k .t . 

Where t is the shear strength of material, t is the thickness of sheet metal stock, and k is a constant whose value may be taken as 0.003.

In a piercing operation, the following equations hold.

Punch size = blank size

Die size      = blank size + 2 x clearance

Clearance = k. t. 

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CHAPTER FIVE

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ACTUAL/PRACTICAL COMPOUND DIE DESIGN

5.1 BASIC APPROACH TO DIE DESIGN

With every new part produced, a complete evaluation of the stamping method and parameters must be performed. Based on the part’s flat layout, the sequence of tooling must be designed, which in turn dictates the size of the die. The economies of the strip must be assessed before the rest of the design is finalized. Seemingly small details such as the availability of strip material, the predetermined width, and its thickness and tolerance ranges may turn out to be of tremendous importance when it comes to production. For selection of the proper press, tonnage requirements must be calculated. Further, the amount of stroke, shut height, mounting arrangement, and other press- and productonrelated data must be compared to the capacities of the selected press equipment. Only then may the actual design be started, which always begins with the strip layout and its projection into the cross section of a die. Such a sequence of work process is intentional, as the cross-sectional view provides control of the placement of punches within the assembly. Where punch bodies or heads may be too large to fit the predetermined sequence of operations, or where an additional station may need to be added later on, one of the stations must be skipped with subsequent enlargement of the die. This can be readily assessed by comparing the cross-sectional view with the layout of the strip, whereas by looking only at the strip this may pass undetected. Both strip layout and cross-sectional view should be drawn to size or scaled. With accurately drawn punches and dies, the need for further detailing may often be eliminated. In questionable areas, some dimensions may be added instead of separate sketching or verbal explanations.

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5.2 Types of blanking dies:

5.2.1 There are two general types of blanking dies:

(a) Drop-through die- In this die, The die block assembly is mounted on thebolster plate or the press bed and the punch assembly on the press slide. The blank drops of its weight through the die opening and the clearance provided in the bolster plate and press bed. This design economical to build and maintain and is fast in working. However, this design is not suitable under the following condition:

1. When the blank is too thin and fragile to be dropped very far. 2. When the blank is too heavy to be dropped for any appreciable

distance.3. When the blank is too awkward to be removed from below

press. When the blank is larger than the press bed opening.

(b)Inverted type die: in this design, the punch becomes the lower stationary part and the die is mounted on the ram. This type is somewhat more complicated, more costly and slower in operation. The scrap disposal is much easier but removal of blank freom the die opening is used wherethe blank is large.

5.2.2 Die- block

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The die block is female half of two mated tools which carry the cutting edges. It is subjected to extreme pressure and conditions. Hence the die block is made of a superior quality of tool steel. A simple layout of die block is shown in the fig 5-1. The die block may be of solid or sectional construction, depending upon the size and contour of the die opening. If the die opening is small and its contour is simple, asolid die block is the choice. Sectional dies are made up of accurately ground matching components which may be assembled together easily.

(Fig. 5-1)

5.2.3 Die block thickness

The minimum thickness of the die block depends upon the strength required to resist the cutting forces, and it will depend upon the type and thickness of the material being cut. The determination of the die block thickness is usually based on the experience and thumb rule,

According to thumb rule, the die thickness may be obtained as follows:

Dia thickness= 19 mm, for blank perimeter>=75mm

Die thickness= 25mm, for blank perimeter= 75mm to 250mm

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Die thickness= 31mm, for blank perimeter > 250mm.

5.2.4 Die opening

The side walls of the die block opening should be provided with sufficient relief or taper so that the blank drops clear through. The taper can either start from the top surface itself or softer a straight land from the surface of the die, where the filling and grinding of the die is done by the machine, the fullytapered cavity is quicker to produce and is thereforecheaper. But if the die has to be finished on bench, the land is easier to file and sandstone.Soft metals such as cupper brass and aluminum tend to swell more rapidly softer being cut. This may be due to a slide spring back return of the material along the lines in which it which it which it has been stressed in compression. So, for such metals, the die cavity should be fully tapered. But for the steel, the taper or relief should start afterthe straight land from the die surface. The chipsadvantages of the straight land are that the original dimensions of the die are retained softenreportedregrinding. With fully tapered die cavity design, the die opening size increases after each regrinding. This increase, however, is very negligible to cause any appreciable effect on the blank dimensions.

5.2.5 Punch

The punch must be a perfect mate to the die block opening the size of the working surface of punch is obtained by subtracting the total clearance from the desire size of the blank. As shear is provided on the surface for blanking operation. The punch is provided with awide flange or shoulder to facilitate mounting and prevent its deflection under load. The minimum length of punch should be such that it extends far enough into the die block opening to ensure complete shearing of the blank. The punch length must also provide for the anticipated number of regrinds.

5.2.6 Back up Plate

For small punches, back up plates or pressure plates are often provided

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between the punch plate and punch holder. The punch plate or punch retainer fits closely over the body of the punch and holds it in proper relative position. It is attached to the punch holder. The back up plate is provided to take the cutting force of the punch head, provide a base and insurance against punch deflection when the punch does not have a flange or shoulder and prevent the hardened punch from being pushed into the softer punch holder, thus become loose.5.2.7 Methods of Holding Punches

The mounting of a blanking punch in the punch in the punch holder does not present any problems. Being relatively bigger, they are made with flanges that are dowelsinto position and directly fastened to the punch holder by screws without the use of punch plate and sometimes without even a back up plate. When used, the thickness of punch plate should be 1.5 times the punch diameter.

5.2.8 Centre of pressure

When the shape of blank to be cut is irregular. The summation of shear forces about the Centre line of the press ram may not be symmetrical. Due to this, bending moments will be introduced in the press ram, producing misalignment and undesirable deflections. To avoid thisCentre of pressure of the shearing action of the die must be found and while laying out the punch position on the punch holder, it should be ensured that the Centre line of press ram passes exactly through the Centre pressure of the blank. It should be noted that it is not the centroid of the area of the blank.

5.2.9 Strippers

After a blank has been cut by the punch on its downward stroke the scrap strip has a tendency to expand. On the return stroke of the punch, the scrap strip has the tendency to adhere to the punch and be lifted by it. This action interferes with the feeding of thestock through the die and some device must be used to strip the scrap material from the punch as it clears up the die block. Such a device is called stripper or stripper plate.It is two types,

1. Fixed stripper

Stripper is attached at a fixed height over the die block. The height should be sufficient to permit the sheet metal to be fed freely between the upper die surface and the under surface of the stripper plate. The stripper plate is

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usually of the same width and length as the die block. in simple dies, it is fastened with the same screws and dowels which are used for die block. In the complex dies, the strippers fastening willbe independent ofrefastening. The thickness of the stripper plate should be sufficient to withstand the forces needed to strip the scrap strip from the punch. The usual value 9.5 mm to 16mm.

2. Sprig loaded stripper

This type is used on large blanking operations and also on very thin and highly ductile materials where to utilize the pad pressure to hold the surrounding stock during the blanking operation. In this design the stripper plate is mounted over the compression springs and suspended by bolted from the punch holder, with the lower surface of the striper below the cutting end of the punch. As the punch travels downward from the blanking operation, the stripper plate contacts thestock strip first and hold s it until the clears the strip on its return stroke. As the punch rises, spring pressure holds the strip, stripping it forces may vary from 2.5 to 20% of cutting force. However, the more common values for most of the applications are 5 to 10%. 5.2.10 Stock Stop

The strip of sheet metal is fed guide, or through a slot in the stripper plate. After each blanking, the strip has to be advanced a correct distance. The

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device used to achieve this is called stock stop. The simplest arrangement may be dowel pin or a small block, against which an edge of the previously blanked hole is pushed after each stroke of the press. On its upward stroke, the punch carries the stock strip as so far as the underside of the stripper plate. Due to this, the stock strip gets released from the stop. With constant pressure exerted pushing the stock strip to the left, the stock will move as it is lifted clear, then drop with the next hole over the stop as the scrap strip is stripped from the punch.

5.2.11 Strip Feeding

Stock strip may be fed into the die either manully or mechanically. Manual feeding is suitable only for low production or with presses operating at low values of stroke per minute. Modern presses operate 200 to 300 strokes per minute. For such cases, manual feeding is not feasible and automatic feeding is the only answer. For this, the strip is prepared in large coils. The first step in the feeding of strip is the unwinding of coil. Two methods are in use for the purpose:

1) Reel 2) Coil cradle

The reel is considered to be better as it does not damage the strip in any way. The reel may be or may not be powerful driven. In the case of power driven reels, a roller at a end of a long loop arm, rides on the uncoiling strip. When the sufficient coil has been unwound, the loop arm raised, the power supply is switched on. In the case of unpowered reels, the coil is unwound by an external power source, which may be feeding mechanism or straightening rolls. When enough coilshave been unwound the reel is stopped on from uncoiling by a manual or automatic brake.After uncoiling and straighten, the final step is to feed the strip into the die. The main two types of feeding system are:

1) Roll Feed2) Slide And Hitch Feed

5.2.12 knockouts

The function of the knockout is to shed or eject a work piece from within the die cavity as the work piece may get jammed in the die cavity due to friction. A knockout may be actuated by springs or by a positive acting knockout pin and bar arrangement. The knockout pin usually leads through

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the shank. It may be single pin or a double pin fastened to a pad or collar above the shank. The working of a positive knockout pin arrangement is shown in the fig 2.3 for an inverted OBI press. On the return stroke of the press, the knockout pin strikes the knockout bar. This makes the knockout pin to accurate the knockout plate which forces the blank out of the die opening. The function of the knockout plate is to support and guide fragile punches.

5.2.13 the die shoe

The punch holder together with two or more guide posts constitutes a die. The die shoe and the punch holder are made of CI, CS and rolled steel. For smaller dies, CI is used, whereas for larger and special die sets, CS and rolled tool are used. Bushing is assembled to the upper shoe by press fitting and guide posts are press fitted in to lower shoe. The bushing and posts are sized to provide a slip fit. For average range of die sets, the diameter of guide posts varies from 2.5 cm to 7.5 cm. larger pins may be used if extreme alignment is required. When die is fully closed, the upper end of the guide posts should not project beyond the top surface of the upper shoe.

5.2.14Bolster Plate

When many dies are to run in the same press at different times, the ware occurring on the press bed is high. The bolster plate is incorporated to take the wear, plate is made from boiler plate or tbp=1.75(for steel) to 2.00 (for CI)* T. it is attached to the press bed and the die shore is then attached to it. It is machined so that its surfaces are flat and parallel. Bolster plates are relatively cheap and easy to replace. The other functions of a bolster plate are:

1) To provide attachment holes for the dies rather than drilling these holes in the press bed,

2) To support the die shoe when it is located over a large hole in the press bed.

3) To take up space in the press when the press shut height is too great for the die shut height.

4) To provide chutes for ejecting parts or scrap out the side of the press.

5.2.15 Stops

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Strip material, when first being guided into the die, must stop somewhere for the sequence of die operations to begin successfully. It is obvious that the strip should not go as far as the forming tool, which may need some pre-blanking work performed at the beginning. Advancing the strip too far may lead to greater than usual wear and tear of the tooling and its subsequent misalignment and breakage.For that purpose, stops are introduced in the die work. The first stop, which the strip meets on its way, is usually the first pierce and blank locator, which navigates the strip in such a way that all cutting is included prior to its arrival at forming and other stations.The automatic stop is a device which slides up and down along with the movement of the ram and either:

1) Forces the nose of the stop lever up, to release its engagement of the strip for the latter’sProgression (during the downward movement of the ram)

2) Releases its pressure on the lever, thus allowing its nose to come down, pushed by a forceof a spring. In such a position, the lever is ready for registration and retainment of theadvancing strip (during the upward movement of the ram).

5.2.16 Ejecting of Parts

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Spring-loaded stock lifters, pressure pins, or pressure pads may all be used as ejectors of finished parts, wherever these are not cut off in the last operation..

CHAPTER SIX

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STRIP LAYOUT

6.1 Strip layout: It plays an important role especially in the case of the design of the press tool. Strip decides the economic utilization of the work piece and helps in the decrease of cost of the job and reduction in the production time by increasing the number of components or layout the position of the work pieces in the strip and their orientation with respect to another. This is called ‘strip layout’. The factors which will influence the stock layout are:

1) Economy of material, 2) Direction of material grain or fibre,3) Strip or coiled stock,4) Press used,5) Production required,6) Die cost.

6.1.1 Economy of material

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In the above figure, the different ways of arranging to blank the given work piece are shown. The arrangement of Fig.(a), the strip would either have to be fed twice,once for each row or double blanking will have to be employedThe percent of material utilization may increase somewhat by the arrangement of Fig.(b), that is , by having two rows of blanks. Fig.(c) shown in a single row, double pass strip. This is called “stock nesting”. Here, the strip will have to be passed through the dies once, turned over and passed through dies a second time. Nesting considerably reduces the scrap. However the strip layout with maximum material saving may not be the best strip layout, as the die construction may become more complex which will offset the savings due to material economy unless a large number of parts are to be produced.Another important consideration in strip layout is the distance between the nearest points of blanks and between blanks and the edges of the strip. To prevent the scrap from twisting and wedging between blanks and the die, the distance must increase with material thickness. A general rule of thumb is to keep this distance, called web, at least 1.5 times the material thickness. However other factors such as strip thickness, hardness of the material, type of operation, shape of blank etc. may allow the web to be thinner. The various terms connected with strip layout are shown in below figure.

The distance between the blank and edge of strip, known as back scrap may be determined by the equation,

a= t + 0.015h

The distance between successive blanks and also the scrap bridge, b, is given in below table.

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Material thickness in mm b in mm0.8 0.80.8 to 3.2 tOver 3.2 3.2

In general softer material s requires larger spacing and thinner materials require larger spacing.

The feed or advance or the length of one piece of stock needed to produce oneblank is, S = w + b

The number of blanks which can be produced from one length of stock can be found out as, N = (L – b)/s

The scrap remaining at the end of one length of strip may be calculated from,

Y=L (Ns + b)

Measure of material utilization:

ηm=area of blank to be cut /area of material available

=B/A*100

% of scrap¿(A−B )A

∗100

Now area of material available per blank¿feed or advance * stock width

6.1.2 Direction of material grain or fibre

This factor is to be considered if the cut blanks have to undergo any subsequent operation, such as, bending or deep drawing. When the sheet metal strip is rolled in the mill, a fibre is produced in the direction of strip length. During subsequent bending operation on the blank, to obtain maximum strength from bend parts, the bend should be made across the strip or at an angle of 90° to the fibre. Therefore some part prints specify that the fibre is to run in the direction of an arrow shown on the print.

In such cases, the blanks may cannot be tripped or rotated to just any position desired.

6.1.3 Strip or coiled stock

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Another important consideration in the strip layout is whether the stock used will be in the form of a strip or coil. Whereas, the stock strip may be passed through the die more than once, the coiled stock is usually passed through the die only once.

When coiled stock is used, recoiling and recoiling of the stock is expensive. Thus coiled stocked is used when:

1) Production is high2) Thinner metal sheets are employed.3) The stock needs to be passed through the die only once.4) Strip stock is used when:5) Production is low6) Thicker sheet metals are used7) The stock needs to be passed through the die more than once.

6.1.4 Direction of burr

When sheet metal is cut in a die, a burr is produced on the die side of the scrap strip and on the punch side of the blank. If the burr has to be on the hidden side, then the expensive operation of removing the burr need not be done. For this, a note is often placed on the part drawing which reads “burr down”. To control the position of the burr may limit the stock-layout arrangement. Scrap may not be reduced to a minimum.

6.1.5 Press used

During production planning, a press has been assigned to the operation and the die. Therefore, the stock layout has to be such that it allows the die to be designed within the press capacity. Shear may have provided on punch or die, to limit the maximum cutting force within the press capacity.

Another factor is the bed area of the press. The relation of the press bed area to the blank area is a definite factor controlling the stock layout. The third factor is to have the cutting forces of the die evenly balanced around the center line of the press ram.

6.1.6 Production requirement

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The following guide lines may be followed when the production is the main consideration:

a) Low production –thin material:I. Strip stock and a single-pass layout.

II. Cutting of one or more blanks at a time.

b) Low production of thin material:I. Strip stock and a single or double-pass layout.II. Cutting one blank at a time.

c) High production –thin material:I. Coiled stock and a single pass layout.II. Cutting of one or more blanks at a time.

d) High production – thick material:I. Strip stock, and a single or double-pass layout.II. Cutting of more than one blank at a time.

6.1.7 Die-cost

A. Higher productions.B. Cutting more than one time, particularly when cutting extremely

complicated blank shapes, or when cutting extremely accurate blank sizes.

However, for simple round or square-edeged blanks, multiple cutting at one time is often practical. Also, double-pass dies are less expensive than cutting two at a time.

So, the designer has to decide while making the stock layout, as to which is preferred: more operator time per blank or more machine time per blank.

The first step in strip layout is defining the strip. This process involves naming the strip assembly and the strip part and defining the width and height of a station, the project shortcut, the number of stages, and an offset before and after the strip. You can specify a prefix for the name that is generated for parts placed inside the strip assembly. Parts include instances of the article and stamp reference parts.

Note: Terms used in a strip layout1) Scrap bridge

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a) this is the portion of the material remaining after blanks operation between one edge of the strip and the cutout portion.b) Thee portion of material remaining between the two adjacent openings after blanking is also called as the scrap bridge.2) FrontScrap: This is the scrap bridge on that edge of the strip which is towards the operator

In the design of blanking part from strip material, the first step is to prepare blanking layout, that is, to layout the position of the work pieces in strip and their orientation with respect to one another.While doing so, the major consideration is the Economy of material. Another important consideration in strip layout isthe distance between the blanks and the strip edge anddistance between blank to blank. To prevent the scrapfrom twisting and wedging between the punch and thedie. The distance must increase with material thickness.A general rule of thumb is to keep this distance equalto from 1 to 1.5times the material thickness.

6.1.8 STRIP ARRANGEMENT

Press tool design types may be categorized by layout as well as by motif or style of pattern. The term layout refers to the arrangement of motifs in the framework of the design plane. Unlike a painting or drawing, which isdesigned in relation to its boundaries or edges, the elements in a textiledesign are designed in relation only to each other. There are no boundaries,

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When the pattern is printed it will continue over yards and yards of cloth. For a textile design to be reproduced on fabric, it must eventually be developed into one standard unit containing a specific arrangement of the desired motifs. This one unit, called a repeat, will be repeated across the width and length of the fabric in a continuous manner.

6.2 CUTTING FORCES

In the cutting operation, as the punch in its downward movement enters the material, it need not penetrate the thickness of the stock in order to affect complete rupture of the part. The distance which the punch enters into the work material to cause rapture to take place called ‘penetration’ and is usually given as the percentage of the stock thickness.

The percentage of penetration depends on the material being cut and also on the stock thickness. When a hard and strong material is being cut, Very little penetration of the punch is necessary to caused fracture. With softer materials, the penetration will greater. For example, for soft aluminum, it is 60% of ‘t’; for 0.15% carbon steel annealed, it is 38% of ‘t’; and only 24% of thickness, being smaller for thicker sheets and greater for thinner sheets, as shown in the below table i.e., it is inversely proportional to stock thickness.

PENETRATION

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Stock thickness ‘t’ in mm

25

20

15

12.5

10

8 6 5 3 2.5

1.6

Below 1.6

Penetration % of ‘t’

25

31

34

37 44

47

50

56

62

67 70 80

Cutting force is the force which has to act on the stock material in order to cut out the blank or slug. This determines the capacity of the press to be used for the particular tool.Calculation of cutting force,Cutting force = l*s*T (max.)

where,

l=length of periphery to be cut in mms=sheet thickness in mmT max = shear strength of stock material in N/mm

Relationship between shearing action and cutting force the three critical stages of shearing action are related to cutting force. Resistance begins when the punch contacts the stock material. The load builds up rapidly during the plastic deformation stage. It continues to increase while penetration takes place. The accumulated load is suddenly released when fracture occurs. The curve levels off near the bottom.

FORMULATION

F= A x TßF = Cutting forceS = Material thicknessL = Total cutting lengthA = L x S = Shear area

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Tß = Shear strengthNote : Shear strength = 80% of tensile strength

6.3 PRESS CAPACITY

The specified amount of force that a press is capable of exerting near the bottom of its stroke in order to carry out a stamping operation.

First get the Dia of material in inches = A Get the thickness of material in inches = B Now, Multiplier for M.s is 80 & for soft material like copper, brass, aluminum is 6 0. now put the A X B X MULTIPLIER = TONNAGE Example, 1 inch dia 5mm thickness Material is M.S. SO TONNAGE is : 1 x 0.1968 x 80 = 15.75 is tonnage required

CHAPTER SEVEN53 | P a g e

ADVANTAGES AND DISADVANTAGES:

7.1 Advantages

1) Requires minimal space in the press.2) Leaves all burrs in one direction.3) Superior accuracy between holes and trim edges.4) More economical to build than a progressive die.

7.2 disadvantages

A disadvantage of building a compound blank die is the limited space which ends to leave die components thin and weak. This concentrates the load and shock on the punches and matrixes resulting in tooling failures.

7.3 Applications

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It is used in aerospace industries, house hold equipment’s, automobile parts, ship parts, electronic appliances. The best examples are daily used saving blade, drawing clips, cold rink bottle cap & cap opener, cane clip, etc.

CHAPTER EIGHT

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8.1 Conclusion

In today’s scenario of latest new product development trends, where the time to introduce a new product is under pressure, forming simulation for each sheet metal component has become essential. Sheet metal simulation help the tool designer to understand metal flow in a better way for complex shapes, which in turn increases the component quality and reduce the design cycle time and cost. It can be effectively used for optimizing the die design in order to improve quality, optimizing process parameters without any physical tool build.

8.2 References

1) Metal Cutting and Tool Design by B.J.RANGANATH.

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2) Mechanical Engineering Design by JOSEPH SHIGLEY &CHARLES M

ISCHKE.

3) Engineering Design by GEORGE DIETER AND LINDA SCHMIDT.

4) Fundamentals Of Tool Design By David Spitler, JeffLantrip, John N

ee and David A Smit

5) Production Engieering by Dr.P.C.ShramaS

chandpublication/chapter2

6) Hand book of die design second edition by Ivanasuchy.

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