DESIGN STUDY OF PROGRESSIVE TOOL
ABSTRACT 1
Sheet metal is simply a metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Everyday countless objects are constructed of the material. Thicknesses can vary significantly, although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate. Design of sheet metal dies is a large division of tool engineering, used in varying degree in manufacturing industries like automobile, electronic, house hold wares and in furniture. In our project we will learn about different sheet metal dies, sheet metal operations and will study about the design aspects of progressive press tool.
INTRODUCTION TO SHEETMETAL
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Introduction Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material. Thicknesses can vary significantly, although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate.
Sheet metal processing The raw material for sheet metal manufacturing processes is the output of the rolling process. Typically, sheets of metal are sold as flat, rectangular sheets of standard size. If the sheets are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal process is to cut the correct shape and sized ‘blank’ from larger sheet.
Sheet metal forming processes Sheet metal processes can be broken down into two major classifications and one minor classification
Shearing processes -- processes which apply shearing forces to cut, fracture, or separate the material.
Forming processes -- processes which cause the metal to undergo desired shape changes without failure, excessive thinning, or cracking. This includes bending and stretching.
Finishing processes -- processes which are used to improve the final surface characteristics.
Shearing Process 1. Punching: shearing process using a die and punch where the interior portion of the sheared sheet is to be discarded. 3
2. Blanking:
shearing process using a die and punch where the exterior portion of the shearing
operation is to be discarded. 3. Perforating: punching a number of holes in a sheet
4. Parting: shearing the sheet into two or more pieces 5. Notching: removing pieces from the edges 6. Lancing: leaving a tab without removing any material
Fig.1: Shearing Operations: Punching, Blanking and Perforating
Forming Processes
Bending: forming process causes the sheet metal to undergo the desired shape change by bending without failure. Ref fig.2 & 2a
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Stretching: forming process causes the sheet metal to undergo the desired shape change by stretching without failure. Ref fig.3
Drawing: forming process causes the sheet metal to undergo the desired shape change by drawing without failure. Ref fig.4
Roll forming: Roll forming is a process by which a metal strip is progressively bent as it passes through a series of forming rolls. Ref fig.5
Fig.2 Common Die-Bending Operations
Various Bending Operations
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Schematic illustration of a stretch-forming process.
Schematic of the Drawing process.
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Eight-roll sequence for the roll forming of a box channel
Finishing processes Material properties, geometry of the starting material, and the geometry of the desired final product play important roles in determining the best process.
EQUIPMENTS Basic sheet forming operations involve a press, punch, or ram and a set of dies
Presses
Mechanical Press - The ram is actuated using a flywheel. Stroke motion is not uniform. Ref fig.6
Hydraulic Press - Longer strokes than mechanical presses, and develop full force throughout the stroke. Stroke motion is of uniform speed, especially adapted to deep drawing operations. Ref fig.7
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Fig.6 Mechanical Press
Fig.7 Hydraulic Press
Dies and Punches
Simple - single operation with a single stroke
Compound - two operations with a single stroke
Combination - two operations at two stations
Progressive - two or more operations at two or more stations with each press stroke, creates what is called a strip development
Fig 8 Progressive dies Punches 8
Tools and Accessories The various operations such as cutting, shearing, bending, folding etc. are performed by these tools.
Marking and measuring tools
Steel Rule - It is used to set out dimensions.
Try Square - Try square is used for making and testing angles of 90degree
Scriber – It used to scribe or mark lines on metal work pieces.
Divider - This is used for marking circles, arcs, laying out perpendicular lines, bisecting lines, etc
Marking and measuring tools
Cutting Tools
Straight snip - They have straight jaws and used for straight line cutting. Ref fig.10
Curved snip - They have curved blades for making circular cuts. Ref fig.10a
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Ref fig.10 Straight snip
Ref Fig.10a Curved Snip
Striking Tools Mallet - It is wooden-headed hammer of round or rectangular cross section. The striking face is made flat to the work. A mallet is used to give light blows to the Sheet metal in bending and finishing. Ref fig.11
Fig.11 Types of Mallets 10
Merits
High strength
Good dimensional accuracy and surface finish
Relatively low cost
Demerits
Wrinkling and tearing are typical limits to drawing operations
Different techniques can be used to overcome these limitations
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Draw beads
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Vertical projections and matching grooves in the die and blank holder
Trimming may be used to reach final dimensions
Applications
Roofings
Ductings
Vehicles body buildings like 3 wheelers, 4 wheelers, ships, aircrafts etc.
Furnitures, House hold articles and Railway equipment
MATERIALS Stainless steel The three most common stainless steel grades available in sheet metal are 304, 316, and 410. Grade 304 is the most common of the three grades. It offers good corrosion resistance while maintaining formability and weldability. Available finishes are #2B, #3, and #4. Note that grade 303 is not available in sheet form. Grade 316 offers more corrosion resistance and strength at elevated temperatures than 304. It is commonly used for pumps, valves, chemical equipment, and marine applications. Available finishes are #2B, #3, and #4. Grade 410 is a heat treatable stainless steel, but does not offer as good corrosion resistance. It is commonly used in cutlery. The only available finish is dull.
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Aluminium The four most common aluminium grades available as sheet metal are 1100-H14, 3003-H14, 5052-H32, and 6061-T6. Grade 1100-H14 is commercially pure aluminium, so it is highly chemical and weather resistant. It is ductile enough for deep drawing and weldable, but low strength. It is commonly used in chemical processing equipment, light reflectors, and jewelry. Grade 3003-H14 is stronger than 1100, while maintaining the same formability and low cost. It is corrosion resistant and weldable. It is often used in stampings, spun and drawn parts, mail boxes, cabinets, tanks, and fan blades. Grade 5052-H32 is much stronger than 3003 while still maintaining good formability. It maintains high corrosion resistance and weldability. Common applications include electronic chassis, tanks, and pressure vessels. Grade 6061-T6 is a common heat-treated structural aluminium alloy. It is weldable, corrosion resistant, and stronger than 5052, but not as formable. Note that it loses some of its strength when welded. It is used in modern aircraft structures, generally replacing the older 2024-T4 alloy.
GAUGE The sheet metal gauge (sometimes spelled gage) indicates the standard thickness of sheet metal for a specific material. For most materials, as the gauge number increases, the material thickness decreases. Sheet metal thickness gauges for steel are based on the weight of steel, allowing more efficient calculation of the cost of material used. The weight of steel per square foot per inch of thickness is 41.82lb (18.96kg), this is known as the Manufacturers' Standard Gage for Sheet Steel. For other materials, such as aluminium and brass, the thicknesses will be different.
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TOLERANCES
During the rolling process the rollers bow slightly, which results in the sheets being thinner on the edges.
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FORMING PROCESSES Bending The equation for estimating the maximum bending force is,
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, where k is a factor taking into account several parameters including friction. T is the ultimate tensile strength of the metal. L and t are Length and thickness of sheet metal respectively. The variable W is open width of a V-die or wiping die.
Curling Curling is a sheet metal forming process used to form the edges into a hollow ring. Curling can be performed to eliminate sharp edges and increase the moment of inertia near the curled end.[1] Other parts are curled to perform their primary function, such as door hinges
Decambering It is the metalworking process of removing camber, or horizontal bend, from strip shaped materials. The material may be finite length sections or continuous coils. Decambering resembles flattening or levelling processes, but deforms the material edge (left or right) instead of the face (up or down) of the strip.
Deep drawing
Example of deep drawn part Drawing is a forming process in which the metal is stretched over a form. In deep drawing the depth of the part being made is more than half its diameter. Deep drawing is used for making automotive fuel tanks, kitchen sinks, 2 piece aluminum cans, etc. Deep drawing is generally done in multiple steps called draw reductions. The greater the depth the more reductions are required. Deep drawing may also be accomplished with fewer reductions by heating the workpiece, for example in sink manufacture. In many cases, material is rolled at the mill in both directions to aid in deep drawing. This has a more uniform grain structure and is referred to as "draw quality" material which limits tearing .
Incremental sheet forming Incremental sheet forming (or ISF) is a sheet metal forming technique where a sheet is formed into the final workpiece by a series of small incremental deformations. However, studies have 15
shown that it can be applied to polymer and composite sheets too. Generally, the sheet is formed by a round tipped tool, typically 5 to 20mm in diameter. The tool, which can be attached to a CNC machine, a robot arm or similar, indents into the sheet by about 1 mm and follows a contour for the desired part. It then indents further and draws the next contour for the part into the sheet and continues to do this until the full part is formed. ISF can be divided into variants depending on the number of contact points between tool, sheet and die (in case there is any). The term Single Point Incremental Forming (SPIF) is used when the opposite side of the sheet is supported by a faceplate and Two Point Incremental Forming (TPIF) when a full or partial die is supports the sheet.
Ironing Ironing is a sheet metal forming process that uniformly thins the workpiece in a specific area. This is a very useful process when employed in combination with deep drawing to produce a uniform wall thickness part with greater height-to-diameter ratio. One example of ironing can be found in the manufacture of aluminum beverage cans, which are actually pressed from flat sheets of thicker material.
Laser cutting Cutting sheet metal can be done in various ways from hand tools called tin snips up to very large powered shears. With the advances in technology, sheet metal cutting has turned to computers for precise cutting. Many sheet metal cutting operations are based on computer numerically controlled (CNC) lasers cutting or multi-tool CNC punch press. CNC laser involves moving a lens assembly carrying a beam of laser light over the surface of the metal. Oxygen, nitrogen or air is fed through the same nozzle from which the laser beam exits. The metal is heated and burnt by the laser beam, cutting the metal sheet. The quality of the edge can be mirror smooth and a precision of around 0.1 mm (0.0039 in) can be obtained. Cutting speeds on thin 1.2 mm (0.047 in) sheet can be as high as 25 m (82 ft) a minute. Most of the laser cutting systems use a CO2 based laser source with a wavelength of around 10 um; some more recent systems use a YAG based laser with a wavelength of around 1 um.
Perforating Perforating is a cutting process that punches multiple small holes close together in a flat workpiece. Perforated sheet metal is used to make a wide variety of surface cutting tools, such as the surform.
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Press brake forming
Forming metal on a pressbrake This is a form of bending, used for long and thin sheet metal parts. The machine that bends the metal is called a press brake. The lower part of the press contains a V shaped groove. This is called the die. The upper part of the press contains a punch that will press the sheet metal down into the v shaped die, causing it to bend. There are several techniques used here, but the most common modern method is "air bending". Here, the die has a sharper angle than the required bend (typically 85 degrees for a 90 degree bend) and the upper tool is precisely controlled in its stroke to push the metal down the required amount to bend it through 90 degrees. Typically, a general purpose machine has a bending force available of around 25 tonnes per metre of length. The opening width of the lower die is typically 8 to 10 times the thickness of the metal to be bent (for example, 5mm material could be bent in a 40mm die) the inner radius of the bend formed in the metal is determined not by the radius of the upper tool, but by the lower die width. Typically, the inner radius is equal to 1/6 of the V width used in the forming process. The press usually has some sort of back gauge to position depth of the bend along the workpiece. The backgauge can be computer controlled to allow the operator to make a series of bends in a component to a high degree of accuracy. Simple machines control only the backstop, more advanced machines control the position and angle of the stop, its height and the position of the two reference pegs used to locate the material. The machine can also record the exact position and pressure required for each bending operation to allow the operator to achieve a perfect 90 degree bend across a variety of operations on the part.
Roll forming A continuous bending operation for producing open profiles or welded tubes with long lengths or in large quantities.Roll forming, also spelled rollforming, is a continuous bending operation in which a long strip of sheet metal (typically coiled steel) is passed through sets of rolls mounted on consecutive stands, each 17
set performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing constant-profile parts with long lengths and in large quantities.
Punching Punching is performed by placing the sheet of metal stock between a punch and a die mounted in a press. The punch and die are made of hardened steel and are the same shape. The punch just barely fits into the die. The press pushes the punch against and into the die with enough force to cut a hole in the stock. In some cases the punch and die "nest" together to create a depression in the stock. In progressive stamping a coil of stock is feed into a long die/punch set with many stages. Multiple simple shaped holes may be produced in one stage but complex holes are created in multiple stages. The final stage the part is punched free from the "web". A typical CNC punch has a choice of up to 60 tools in a "turret" that can be rotated to bring any tool to the punching position. A simple shape (e.g. a square, circle, or hexagon) is cut directly from the sheet. A complex shape can be cut out by making many square or rounded cuts around the perimeter. A punch is less flexible than a laser for cutting compound shapes, but faster for repetitive shapes (for example, the grille of an air-conditioning unit). A CNC punch can take 600 strokes per minute. A typical component (such as the side of a computer case) can be cut to high precision from a blank sheet in under 15 seconds by either a press or a laser CNC machine.
Rolling
Bending sheet metal with rollers In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the 18
temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes
Spinning Spinning is used to make tubular(axis-symmetric) parts by fixing a piece of sheet stock to a rotating form (mandrel). Rollers or rigid tools press the stock against the form, stretching it, until the stock takes the shape of the form. Spinning is used to make rocket motor casings, missile nose cones, satellite dishes and metal kitchen funnels.
Stamping Includes a variety of operations, such as punching, blanking, embossing, bending, flanging, and coining; simple or complex shapes formed at high production rates; tooling and equipment costs can be high, but labor costs are low. Alternatively, the related techniques repoussé and chasing have low tooling and equipment costs, but high labor costs.
Water jet cutting A water jet cutter, also known as a waterjet, is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance.
Wheeling Wheeling machine, is a metalworking tool that enables a craftsman to form compound (double curvature) curves from flat sheets of metal such as aluminium or steel. The process of using an English wheel is known as Wheeling. Panels produced this way are expensive, due to the highly skilled and labour intensive production method, but it has the key advantage that it can flexibly produce different panels using the same machine. It is a forming machine that works by surface stretching and is related in action to panel beating processes. It is used wherever low volumes of compound curved panels are required; typically in coachbuilding, car restoration, spaceframe chassis racing cars that meet regulations that require sheetmetal panels resembling mass production vehicles (Nascar),[1][2] car prototypes and aircraft skin components.
Fasteners Fasteners that are commonly used on sheet metal include:
Clekos
Rivets
Sheet metal screws
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DESIGN OF SHEET METAL DIES INTRODUCTION Design of sheet metal dies is a large division of tool engineering, used in varying degree in manufacturing industries like automobile, electronic, house hold wares and in furniture. There is no doubt that accuracy achieved by the new ideas in design and construction applied by the press tool designer, coupled latest development made in related fields made more productive, durable and economical.These are The variety in press specification gives the liberty to the designer to think innovative. The Safety Provisions has reduced the accidents and the productivity has been increased. “Simulation Software’s” give the designer freedom from taking risky decisions. The use and availability of Standard Elements has reduced the design and development period The concept of “Flexible Blank Holder” has given the scope to control the flow of the material in a better way. Hardened and toughened new martial & heat treatment process made the design easy. The latest machining process made the complex designs made easy, like wire cut, EDM, Profile Grinding.
Four factors are essential contributions to first class presswork are
Good operation planning Excellent tool design Accurate tool design Knowledge press setting
Design of any Press Tool involves the following Steps 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Determination of force (Press Tonnage) required for the operation Selection of Press for requisite force, work piece size and shape Determination of shut height of the tool Computing die thickness, and margins (minimum cross-section) Drawing Strip Layouts and comparing Material utilization Design of locating Elements Selection of Locating Elements Selection of Hardware Drawing die plan and selection of pillar die set Deciding punch length and mounting Finding Centre of Pressure and Checking scrap Disposal Drawing Details
DIE DESIGN 20
Die design is the preliminary stage in stamping die manufacturing once the product design is completed. Die design stage is very critical. A good die design can produce accurate components which can run for long time with less maintenance. Die design involves deciding the required stamping operations, basic layout, machining processes, type of stamping presses to be used etc. A die designer should have thorough knowledge about these elements to create a good die design. Computer aided design technology has developed really well during last decade to help die desingners. Below we will discuss about the various stages involved in precision metal stamping die design.
DIE DESIGN STAGES Die design is a step by step process which includes careful evaluation of the product characteristics and operations involved. Die designer then proceed with a strip layout which involves identifying different stamping operations which are required to produce the final stamped component. Let's discuss the stages involved in die design in detail.
Evaluate the Part design
At this stage of die design the designer does a through study of the blue print of the stamping component to understand its characteristics. The die designer has to study the product drawing carefully to understand the material composition of the stamped product, thickness of the material and complexity of the product design. This is important in identifying the material behavior against various stamping operations.
Strip Layout
This is probably the most important stage of die design. A good strip layout decides the types of stamping operations involved, size of the die, tonnage required and complexity of the stamping die design. While designing a strip layout the die designer has to think of least number of operations without making it complicated for manufacturing and maintenance of the die. Number of factors affects the strip layout design of die including but not limited to, the accuracy of the final product, operations involved, available stamping presses, easiness in manufacturing and maintenance and cost involved in manufacturing the die. Die Sets and Die Size Choosing the right die sets depends a lot upon the strip layout. The die design should make use of standard die sets readily available in the market to reduce the die set manufacturing cost. Most of die sets manufacturers supply catalogues with standard die set design which specifies the thickness of top and bottom bolsters, size of pillar and bush, effective working area and shut height.
DIES AND ITS TYPES 21
DIE The die may be defined as the female part of a complete tool for producing work in a press. It is also referred to a complete tool consists of a pair of mating members for producing work in a press.
TYPES OF DIES The dies may be classified according to the type of press operation and according to the method of operation.
(A): ACCORDING TO TYPE OF PRESS OPERATION According to this criterion, the dies may be classified as cutting dies and forming dies.
1: Cutting Dies: These dies are used to cut the metal. They utilize the cutting or shearing action. The common cutting dies are: blanking dies, perforating dies, notching dies, trimming, shaving and nibbling dies.
2: Forming Dies: These dies change the appearance of the blank without removing any stock. Theses dies include bending, drawing and squeezing dies etc.
(B) ACCORDING TO THE METHOD OF OPERATION According to this criterion, the dies may be classified as: single operation or simple dies, compound dies, combination dies, progressive dies, transfer dies and multiple dies. 1: Simple Dies : Simple dies or single action dies perform single operation for each stroke of the press slide. The operation may be one of the operations listed under cutting or forming dies. 2: Compound Dies: In these dies, two or more operations may be performed at one station. Such dies are considered as cutting tools since, only cutting operations are carried out. Figure shows a simple compound die in which a washer is made by one stroke of the press. The washer is produced by simulation blanking and piercing operations. Compound dies are more accurate and economical in production as compared to single operation dies.
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3: Combination Dies: In this die also , more than one operation may be performed at one station. It is difficult from compound die in that in this die, a cutting operation is combined with a bending or drawing operation, due to that it is called combination die.
4: Progressive Dies : A progressive or follow on die has a series of operations. At each station, an operation is performed on a work piece during a stroke of the press. Between stroke the piece in the metal strip is transferred to the next station. A finished work piece is made at each stroke of the press. While the piercing punch cuts a hole in the stroke, the blanking punch blanks out a portion of the metal in which a hole had been pierced at a previous station. Thus after the first stroke, when only a hole will be punched, each stroke of the press produces a finished washer.
5: Transfer Dies : Unlike the progressive dies where the stroke is fed progressively from one station to another. In transfer dies the already cut blanks are fed mechanically from one station to other station.
6: Multiple Dies:Multiple or gang dies produce two or more work piece at each stroke of the press. A gang or number of simple dies and punches are ganged together to produced two or more parts at each stoke of the press.
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BASIC DIE OPERATIONS
Most of the operations for shaping sheet metal in to finished products can be described as Drawing, Shearing, and Forming or Bending operation. These operations are generally referred to as stamping, press working or punch press working of sheet metal.
P R E S S W O R K IN G O P E R A T IO N S (S H E E T M E T A L ) DRAW
SHEAR
FORM
DRAW
B L A N K IN G
F L A N G IN G
REDRAW
P IE R C IN G
E M B O S S IN G
N O T C H IN G
JO G G LE
C R O P P IN G
R E S T R IK IN G
S H A V IN G
H E M M IN G
T R IM M IN G
B U R R IN G
S L IT T IN G
C U R L IN G
P A R T IN G -O F F
C O IN IN G
L A N C IN G
C O R R U G A T IN G
S L U G G IN G
Basic die operations
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BEND B E N D IN G
TYPES OF PRESS TOOLS 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 speciopified as blanking tool, piercing tool, bending tool etc.
CLASSIFICATION OF PRESS TOOLS Press tools are classified into some stage tools as follows :
Blanking tool When a component is produced with one single punch and die were the entire profile is cut in single stoke is called Blanking tool.
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.
Cut off tool Cut off operations are those in which 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 operation using this tool.
Parting off tool Parting off is an operation involve 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 double line. This is done for components with two straight surfaces and two profile surfaces
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.
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. 25
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 pres tools.
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. 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 sheet. This type of Press tools are used to perform only one particular operation .
Progressive tool Progressive tool differs from the stage tool by the following aspect, In progressive tool the final component is obtained by progressing the sheet metal or strip in many stages. In each and every stages the component will get its shape stage by stage the full shape will be obtained at the final stage.
Compound tool The compound tool differs from progressive and stage tool by the arrangement of punch and die. It is a inverted tool were blanking and piercing takes place in a single stage and also blanking punch will act as piercing die.
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.
PROGRESSIVE TOOL DESIGN
A die containing a series of stations that perform one press operation after another in series. A progressive die gradually forms a part as it moves through the die, and the last operation separates the part. Progressive dies provide an effective way to convert raw coil stock into a finished product with minimal handling. As material feeds from station to station in the die, it progressively works into a completed part.
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Progressive dies usually run from right to left. The part material feeds one progression for each press cycle. Early stations typically perforate holes that serve as pilots to locate the stock strip in later stations.
There are many variations of progressive die designs. The design shown here illustrates some common operations and terminology associated with progressive dies.
Progressive stamping is a metalworking method that can encompass punching, coining, bending and several other ways of modifying metal raw material, combined with an automatic feeding system. The feeding system pushes a strip of metal (as it unrolls from a coil) through all of the stations of a progressive stamping die. Each station performs one or more operations until a finished part is made. The final station is a cutoff operation, which separates the finished part from the carrying web. The carrying web, along with metal that is punched away in previous operations, is treated as scrap metal. The progressive stamping die is placed into a reciprocating stamping press. As the press moves up, the top die moves with it, which allows the material to feed. When the press moves down, the die closes and performs the stamping operation. With each stroke of the press, a completed part is removed from the die. Since additional work is done in each "station" of the die, it is important that the strip be advanced very precisely so that it aligns within a few thousandths of an inch as it moves from station to station. Bullet shaped or conical "pilots" enter previously pierced round holes in the strip to assure this alignment since the feeding mechanism usually cannot provide the necessary precision in feed length. 27
Progressive stamping can also be produced on transfer presses. These are presses that transfer the components from one station to the next with the use of mechanical "fingers". For mass productions of stamped part which do require complicated in press operations, it is always advisable to use a progressive press. One of the advantages of this type of press is the production cycle time. Depending upon the part, productions can easily run well over 800 parts/minute. One of the disadvantages of this type of press is that it is not suitable for high precision deep drawing which is when the depth of the stamping exceeds the diameter of the part. When necessary, this process is performed upon a transfer press, which run at slower speeds, and rely on the mechanical fingers to hold the component in place during the entire forming cycle. In the case of the progressive press, only part of the forming cycle can be guided by spring loaded sleeves or similar, which result in concentricity and ovality issues and non uniform material thickness. Other disadvantages of progressive presses compared to transfer presses are: increased raw material input required to transfer parts, tools are much more expensive because they are made in blocks (see fig. 1) with very little independent regulation per station; impossibility to perform processes in the press that require the part leave the strip (example beading, necking, flange curling, thread rolling, rotary stamping ect). The dies are usually made of tool steel to withstand the high shock loading involved, retain the necessary sharp cutting edge, and resist the abrasive forces involved. The cost is determined by the number of features, which determine what tooling will need to be used. It is advised to keep the features as simple as possible to keep the cost of tooling to a minimum. Features that are close together produce a problem because it may not provide enough clearance for the punch, which could result in another station. It can also be problematic to have narrow cuts and protrusions.
Applications An excellent example of the product of a progressive die is the lid of a beverage can. The pull tab is made in one progressive stamping process and the lid & assembly is made in another, the pull tab simultaneously feeding at a right angle into the lid & assembly process.
DESIGNING PROGRESSIVE DIES The decision to produce a part progressively is usually determined by two factors: the volume of production and the complexity of the part. These two factors are instrumental in the design and construction of the tooling. It is important to address all factors that will contribute to the desired level of part quality, tool maintenance, and tooling life. Trade-offs will be necessary to reach most decisions, and all will affect tooling costs. 28
PART ORIENTATION The process begins with determining how the part will be run through the die. This is governed by the features of the part and the locations of the datums and critical tolerances. Then, the trade-offs begin. Optimizing material usage may require rotating the part in the strip, which changes the grain direction of the steel in the part and thus can affect the strength of any forms in the part. Forming with the grain can cause cracking and fatiguing of the metal and make holding consistent form angles more difficult. Therefore, the form will be far more susceptible to problems associated with the chemical makeup of each coil that is run.
For example, Figure 1shows a part for the computer industry that was rotated in the strip to guard against inconsistent form angles that could be caused by differences between coils. The part contained critical dimensions with 0.025-millimeter tolerances dependent on the forms. Rotating the strip to ensure more consistent forms was not the most efficient use of material. In this case, however, part tolerances won out over optimizing material usage. Figure 1: This part was rotated in the strip to maintain critical tolerances better. Part configuration could provide a second motivation for rotating a part in the strip. If cam forming or piercing is required to make the part progressively, rotating the part may be the best, and sometimes only, option because the cam and driver can take up a significant amount of room. The part typically is rotated so that the cams' functions are perpendicular to the coil. This provides the easiest and most accessible condition for the cams. Often, a compromise between rotating a part to optimize material usage and angling the cams to keep them outside of the coil is the final result. This could increase piece part and tooling costs. To produce the part progressively, however, such a compromise may be necessary. A third consideration that may require rotating the part in the strip is the amount of lift that is needed to carry the strip through the die. Lift can sometimes be reduced significantly or eliminated by properly rotating a part. If all forms in a part are in the same direction, lift can be eliminated by forming upward. This usually adds to the cost of the die. When the part has forms in opposite directions, compromises must be made among excessive lift, poor material use, and the complexity and cost of the tooling. One such compromise is shown in Figure 2. The part is carried through with a ladder-style carrier, which adds material to the coil width because only two small areas are available for carrying the part. 29
Also, because of the shape and length of the forms, a significant amount of lift is needed. External stock lifters carrying the ladder strip work well in high-lift situations. Figure 2: Compromises among excessive lift, material use,and tooling cost and complexity were necessary to form this part. One final consideration for part orientation within the strip is that a part should be rotated so that the feed is as short as possible. This is especially true for heavier materials and narrow coils. The slitting process can cause camber in coils that can make feeding difficult. A shorter progression feed runs faster and has less chance to cause feed problems. When a substantial difference between the length and width of the part exists, it is usually more cost-effective to build the tooling with the shorter lead.
CARRYING THE PART How parts are carried in the strip affects how well the die feeds, the ability to lift the strip for feeding, and the ability to produce consistent-quality parts. Three basic options are available for carrying a part, although many variations of each also can be used. In the most straightforward approach, parts are carried by the scrap between them. Excess material equal to one to two material thicknesses per side is required for trimming. This method typically produces minimal scrap. Certain part configurations are needed to use this method. When rotated and laid out end to end, the parts must have enough usable area on both the leading and trailing edges of the progression (see Figure 3).
Figure 3: One side of the strip is used to carry this part through a progressive die
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The second basic strip option, in which a part is carried on one side of the strip, is shown in Figure 4. This style is suitable for parts that require a great deal of forming on as many as three sides. It also improves accessibility if cam piercing or forming is required. Lifting the strip through the die can become more difficult when this carrier option is used. A stock lifter on the edge of the strip is not sufficient—lifters are needed in the center of the strip for balancing, or feeding the strip through the die can become a problem. If large or numerous flanges are to be formed down, achieving the proper lift can be difficult. This type of carrier can cause another feeding problem. Trimming a large quantity of material from one side of the coil can cause camber in the strip as stresses are released from the steel. The more progressions in a die, the greater is the risk of feed and pilot alignment problems caused by camber. Part configuration, stock material thickness, and how narrow the carrier must be are all factors that influence whether camber becomes a problem. To prevent camber, the coil width should be increased so that the carrier side of the coil also can be trimmed. The additional trim releases stresses from the opposite side of the coil and balances the strip. Even with the additional trim, carrying the part on one side of the strip can be the most effective method to run a part from a material usage standpoint. The third carrier option is the ladder style. Some of the advantages of the ladder carrier were discussed earlier. These carriers work well with complex parts and with those requiring significant amounts of lift. Because this method allows a strip to feed easily, it also is often used in applications in which higher feed rates are needed. The ladder carrier uses more material per part. Often, however, a part cannot be produced progressively any other way. If production volumes are borderline to begin with in terms of justifying progressive tooling, the added costs of the more complex progressive die and additional material waste may make producing the part through multiple operations a better option. 31
Figure 4: The parts shown here are carried by the scrap between them, which also serves as stretch webs for the center draw
PILOTING Decisions on part rotation and carrier type must be made concurrently with a third consideration, piloting. The type, locations, and number of pilots all affect the progression, coil, and carrier type. Choosing pilots begins with examining the part configuration and tolerance requirements. Is piloting off of holes within the part possible or even acceptable? If a part contains holes, they must be large enough if they are to be used as pilots. Holes should be spaced as far apart as possible to help increase accuracy, and they must be in the proper locations if they are used to stabilize a strip and help with the forming taking place in the die. The tolerances of the proposed pilot holes in the part should be considered. If the hole diameter tolerance is very tight, even slight elongations caused during forming may produce scrap parts. Elongation could be caused by something as simple as an old feeder or one that is slightly out of adjustment. If a pilot is located in the scrap or the carrier, slight elongation is acceptable, as long as the piece part dimensions remain within tolerance. At times, two different sets of pilots may be required. In these applications, both sets of pilots should be pierced at the same time to provide an accurate transition from the first set of pilots to the second. When a significant amount of stripper travel is involved, problems can occur. The pilots will contact the material as they line up the strip. If considerable stripper travel is involved, the pilots will rub on the pilot hole for the complete distance, which can cause a burr on the hole and lead to galling of the pilots. The best solution is to guide the stripper and place the pilots in the stripper.
EXITING THE DIE Determining how a part exits from a die is often overlooked until the end of the design. It is at times, however, the pivotal factor in determining how a die is designed. Removing the part from the die may require rotating the part, using a different type of carrier, or changing the sequence of operations within the die. The locations of the forms in the part and their relationship to where the part is carried directly bear on how or whether the part comes out of the die. A ladder strip provides the easiest method for removing a part from the die. Usually, a part can be cut and blanked through the die. When a die is designed so that parts are cut and allowed to fall off the end, several factors must be considered. For instance, the part weight must be sufficiently off-balance to allow it to fall off the die block. A shedder pin can be added to the top stripper to ensure that the part exits the die.
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If form tabs or flanges are formed down on the part, clearances must be added. If that is not possible, it may be necessary to redesign the die to ensure that the part exits. If flanges are formed up, the advancement of the strip sometimes will kick the part out of the die.
DETERMINING THE NUMBER OF PROGRESSIONS Once the basic design is determined, the exact number of stations needed can be assessed. It is important to keep die construction in mind when finalizing strip layout. Often, empty stations should be included to prevent weakening a die if further modifications become necessary. In addition, the ease of maintaining the tool should be kept in mind. As the complexity of a tool increases, the degree of confidence in the design also plays a role in deciding how many stations should be included. If questions arise as to whether the part will draw properly or the form will come out as desired, one or more empty stations should be added—the more uncertainties, the higher the number of empties that should be added. If a die is built without empty stations and additional operations must be added later, options are few. In almost all such cases, the integrity of the die must be compromised to accommodate the modification. Oftentimes, very undesirable maintenance conditions must be built into the die. Either situation could result in producing a die that breaks repeatedly and is costly to maintain.
CONCLUSION Getting the fundamentals right is the key to producing a quality, cost-effective die and piece part. The more complex the die, the more important are the decisions on the fundamentals. With proper evaluation and the proper compromises, the best option can be determined. This will give a strong, good-feeding die that is easily maintainable. The die will produce consistent, quality parts to print. The proper decision should provide the best value for a company's tooling dollars.
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CONCLUSION
Sheet metal is simply a metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, which can be cut and bent into a variety of different shapes. Everyday Countless objects are constructed in the thin sizes of different materials with various thicknesses . these thicknesses can vary significantly. Extremely thin thicknesses are considered as foil or leaf, else pieces thicker than 6 mm (0.25 in) are considered as plate. Design of sheet metal dies has a large division of tool engineering, used in varying degree in manufacturing industries like automobile, electronic, house hold wares and in furniture. In our project we have learnt about different sheet metal dies, sheet metal operations and studied about different design aspects of a progressive press tool.
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