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INTRODUCTION TO DIE DESIGN
Die design, a large division of tool engineering, is a complex, fascinating subject. It is one of the most exacting of all the areas of the general field of tool designing.
How then shall we enter into the study of die design? Obviously, we shall have to begin cautiously, learning each principle thoroughly before proceeding to the next one. Otherwise it is quite likely that we should soon become hopelessly involved in the complexities of the subject and in the bewildering number and variety of principles that must be understood. What, then, is a die?
The word “die” is a very general one and it may be well to define its meaning as it will be used in this text. It is used in two distinct ways. When employed in a general sense, it means an entire press tool with all components taken together. When used in a more limited manner, it refers to that component which is machined to receive the blank, as differentiated from the component called the “punch,” which is its opposite member. The distinction will become clear as we proceed with the study.
The die designer originates designs of dies used to stamp and form parts from sheet metal, assemble parts together, and perform a variety of other operations.
In this introduction you will learn basic meanings and the names of various die components; then, operations that are performed in dies will be listed and illustrated. In other sections of the book, the design of dies and die components will be explained in a far more thorough manner, so that your understanding will be complete in every respect.
1.1.1 Part Drawing
To begin our study of the various components that make up a complete die, let us consider the drawing of the link illustrated in Figure 1.1. This part is to be blanked from steel strip and a die is to be designed for producing it in quantity. The first step in designing any die is to make a careful study of the part print because the information given on it provides many clues for solving the design problem.
Figure 1.1 A typical part drawing.
Figure 1.2 A complete die drawing.
1.1.2 Die Drawing
Figure 1.2 is a complete die drawing ready to be printed in blueprint form. To the uninitiated it might appear to be just a confusing maze of lines. Actually, however, each line represents important information that the die makers must have to build the die successfully. In illustrations to follow, we will remove the individual parts from this assembly and see how they appear both as three- and as two-view drawings, and as pictorial views, to help you to visualize their shapes. As you study further, keep coming back to this illustration to see how each component fits in. When you are through, you should have a good idea of how the various parts go together to make up a complete die.
1.1.3 Blueprints
After a die has been designed on tracing paper using traditional techniques or AutoCAD, blueprints are produced for use in the die shop where the dies are actually built by die makers. This is how a blueprint of a die drawing appears. From such prints, die makers build the die exactly as the designer designed it. The drawing must be complete with all required views, dimensions, notes, and specifications. If the die maker is obliged to ask numerous questions, the drawing was poorly done. Figure 1.3 shows a typical blueprint.
Figure 1.3 A typical blueprint.
1.1.4 Bill of Material
The bill of material (Figure 1.4) is filled in last. This gives required information and specifications for ordering standard parts and for cutting steel to the correct dimensions. This material is cut and assembled in the stock room, then placed in a pan, along with a print of the die drawing. When filled, the pan must contain everything the die maker will require for building the die, including all fasteners and the die set.
Figure 1.4 A typical bill of material.
Figure 1.5 A pictorial view of an entire die.
1.1.5 Die Assembly
Figure 1.5 is a pictorial view of the entire die as shown in Figure 1.2. The die pierces two holes at the first station, and then the part is blanked out at the second station. The material from which the blanks are removed is a cold-rolled steel strip. Cold-rolled steel is a smooth, medium-hard steel, and it gets its name from the process by which it is produced. It is rolled, cold, between rollers under high pressure to provide a smooth surface. The strip A is shown entering the die at the right.
1.1.6 Scrap Strip
A scrap strip (Figure 1.6) is designed as a guide for laying out the views of the actual die. Figure 1.6a shows a typical scrap strip. This illustration shows the material strip as it will appear after holes have been pierced and the blank has been removed from it. We would first consider running the blank the wide way as shown at A. When blanks are positioned in this manner, the widest possible strip is employed and more blanks can be removed from each length of strip. In addition, the distance between blanks is short and little time is consumed in moving the strip from station to station. However, for this particular blank there is a serious disadvantage in this method of positioning. Because the grain in a metal strip runs along its length, the grain in each blank would run across the short width; the blanks would be weak and lacking in rigidity. This defect is important enough for the method to be discarded. Instead, the blanks should be positioned the long way in the strip as shown at B. The grain will then run along the length of each blank for maximum stiffness and strength.
Figure 1.6 Scrap strips: (a) Typical scrap strip layouts and (b) Three views of the scrap strip.
Three views of the material strip are shown in Figure 1.6b exactly as they appear in the die drawing in Figure 1.2. In addition, a pictorial view is supplied at the upper right corner to help in visualizing the strip. In other words, this is the way you would imagine the strip if you were to draw it in three views. The top or plan view shows the strip outline, as well as all openings. This would be made actual size on the drawing. The holes are represented by circles at the first station, and the blanked opening is shown at the second station. At the lower left, a side view of the strip is drawn. It is shown exactly as it would appear at the bottom of the press stroke, with the pierced slugs pushed out of the strip at the first station and the blank pushed out of the strip at the second station. The narrow end view at the lower right corner is shown as a section through the blanking station, and the blank is shown pushed out of the strip. The strip in many instances is often drawn shaded to differentiate it from the numerous lines that will represent die members. In the upper plan view, shading lines would appear on the surface of the metal. In the two lower views, the lines are shown in solid black to further differentiate the strip from the die members.
1.1.7 Stampings
Stampings are parts cut and formed from sheet material. Look around you! Wherever you may be, you will find stampings. Many are worn on your own person; the ring on your finger is probably a stamping. Most of the parts in old-fashioned wrist watches are stampings, including the wristband. Your belt buckle, the metal grommets through which your shoe laces pass, eyeglass frame, the clip on your ball point pen, and zipper—all these are stampings.
Look around the room, any room, and you will find products of the pressed-metal industry. Most of the parts in the lighting fixture are stampings; so are threaded portions of light bulbs, door knobs, and the radiator cover. The list is a long one indeed. In the home we find stampings by the score: pots and pans, knives, forks, and spoons, coffee pot, canister set, pie plates and muffin pans, cabinet handles, kettle, can opener, and more.
The refrigerator is almost entirely made of stampings. So are the stove, toaster, and other appliances. And each single part in all these requires an average of three to six dies to produce.
Every automobile contains hundreds of stampings. The largest are the roofs, hoods, quarter-panels, doors, etc. Even the wheels are stampings. There are hundreds of smaller parts, many of which are covered and seldom seen. For example, even the points require very complex dies with multiple stations each, costing thousands of dollars to build, in addition to assembly dies for joining the components.
Office machines and computers provide another big stamping field. So do adding machines, calculators, and dictating machines. We could go on and on; the list is almost endless. Radio and television components require thousands of dies. So do streamlined trains, aircraft, and missiles. All of these are improved from year to year, so an enormous number of new dies is constantly required.
The foregoing should give you some idea of the great size and importance of the pressed-metal industry.
Figure 1.7 A typical punch press.
1.1.8 Punch Press
Figure 1.7 is a photograph of a typical mechanical punch press in which dies are operated to produce stampings. The bolster plate A is a thick steel plate fastened to the press frame. The complete die is clamped securely on this bolster plate. The upper portion of the die is clamped in ram B, which is reciprocated up and down by a crank. As the material strip is run through the die, the upper punches, which are fastened to the moving ram B of the press, remove blanks from it.
Figure 1.8 is an exploded drawing of the die shown in Figure 1.5 with the names of various die components listed. These names should be memorized because we will refer to them many times in future work.
Figure 1.8 An exploded view of the die shown in Figure 1.5.
Figure 1.9 A typical die set.
1.2.1 Die Set
Figure 1.9 shows a die set, and all parts the die assembly comprises are built within it. Die sets are made by several manufacturers and they may be purchased in a great variety of shapes and sizes. The “center posts” A are called “punch shanks” in the die set manufacturers’ jargon. And, no, they cannot be used for clamping the punch holder, but they can be used for aligning the die in the press. Ram mounting holes must be provided in the punch holder for mounting. In operation, the upper part of the die set B, called the “punch holder,” moves up and down with the ram. Bushings C, pressed into the punch holder, slide on guide posts D to maintain precise alignment of cutting members of the die. The die holder E is clamped to the bolster plate of the press by bolts passing through slots F.
In Figure 1.10, the die set is drawn in four views. The lower left view shows a section through the entire die set. The side view, lower right, is a sectional view also, with a portion of the die set cut away to show internal holes more clearly. The upper left view is a plan view of the lower die holder with the punch holder removed from it.
The punch holder is shown at the upper right, and it is drawn inverted, or turned over, much like an opened book. In the complete die drawing, Figure 1.2, all punches are drawn with solid lines. If the punch holder were not inverted, most lines representing punches would be hidden and the drawing would contain a confusing maze of dotted lines.
Figure 1.10 A die set.
Another reason for inverting the punch holder is that this is actually the position assumed by the die holders and punch holders on the bench as the die makers assemble the die, and it is easier for die makers to read the drawing when the views have been drawn in the same position as the die on which they perform assembly motions.
1.2.2 The Die Block
Figure 1.11 shows the die block of the die shown in Figure 1.2. The die block is made of hardened tool steel into which holes have been machined, before hardening, at the piercing station and also at the blanking station. These are the same size and shape as the blank holes and contour. Other holes are tapped holes used to fasten the die block to the die holder, and reamed holes into which dowels are pressed to fix the block’s location relative to other die parts.
Figure 1.11 The die block.
The top view is a plan view of the die block. The lower left view is a section through the holes machined for piercing and blanking. Lines drawn at a 45-degree angle, called “section lines,” indicate that the die block has been cut through the center, the lines representing the cut portion. Similarly, the end view is a section cut through the die block at the blanking station. A tapped hole is shown at the left and a reamed hole at the right side. These are for the screws and dowels that hold the die block to other die components. Sectioning a die, that is, showing the die as if portions were cut away to reveal the inside contours of die openings, is a very common practice. In fact, practically all dies are sectioned in this manner. The die maker can then “read” the drawing far more easily than he could if outside views only were shown because these would contain many dotted or hidden lines.
Always remember that all drafting is, in a sense, a language. A die drawing is a sort of shorthand, which is used to convey a great deal of information to the die makers. Anything that can be done to make it easier for them to read the drawing will save considerable time in the shop.
Now refer back to Figure 1.2 and see how easily you can pick out the three views of the die block. That is exactly what the die maker has to do in order to make the die block.
1.2.3 The Blanking Punch
The blanking punch (Figure 1.12) removes the blank from the strip. The bottom, or cutting edge, is the shape and size of the part. A flange at the top provides metal for fastening the blanking punch to the punch holder of the die set with screws and dowels. Two holes are reamed all the way through the blanking punch for retaining the pilots, which locate the strip prior to the blanking operation. Locate the views of the blanking punch in the die drawing, Figure 1.2, to improve your ability to read a die drawing.
1.2.4 Piercing Punch
A piercing punch (Figure 1.13) pierces holes through the material strip or blank. It is usually round and provided with a shoulder to keep it in the punch plate. When a piercing punch penetrates the strip, the material clings very tightly around it. A way must be provided to strip or remove this material from around the punches. The means employed to remove such material is called a “stripper.”
Figure 1.12 The blanking punch.
Figure 1.13 A piercing punch.
1.2.5 Punch Plate
The punch plate (Figure 1.14) is a block of machine steel that retains punches by keeping the punch heads against the punch holder of the die set. The punches are held in counterbored holes into which they are pressed. Four screws and two dowels retain the punch plate to the punch holder of the die set. The screws prevent it from being pulled away from the punch holder. Dowels, which are accurately ground round pins, are pressed through both the punch plate and punch holder to prevent shifting. Locate the front view and plan view of the punch plate in the die drawing Figure 1.2.
Figure 1.14 A punch plate.
Figure 1.15 A pilot.
1.2.6 Pilot
Pilots (Figure 1.15) are provided with acorn-shaped heads, which enter previously pierced holes in the strip. The acorn shape causes the strip to shift to correct register before blanking occurs.
1.2.7 The Back Gage
The back gage (Figure 1.16) is a relatively thin steel member against which the material strip is held by the operator in its travel through the die. The front spacer is a shorter component of the same thickness. The strip is fed from right to left. It rests on the die block and is guided between the back gage and front spacer. The distance between the back gage and front spacer is greater than the strip width to allow for possible slight variations in width.
Figure 1.16 The back gage.
Figure 1.17 A finger stop.
1.2.8 The Finger Stops
The finger stop (Figure 1.17) locates the strip at the first station. In progressive dies having a number of stations, a finger stop may be applied at each station to register the strip before it contacts the automatic stop. Finger stops have slots machined in their lower surfaces to limit stop travel.
1.2.9 Automatic Stops
Automatic stops (Figure 1.18) locate the strip automatically while it is fed through the die. The operator simply keeps the strip pushed against the automatic stop toe, and the strip is stopped while the blank and pierced slugs are removed from it, then it is automatically allowed to move one station further and stopped again for the next cutting operation.
1.2.10 The Stripper Plate
The stripper plate (Figure 1.19) removes the material strip from around blanking and piercing punches. There are two types of stripper plates: spring-operated and solid. The one illustrated is solid. The stripper plate has a slot A machined into it in which the automatic stop operates. Another slot B at the right provides a shelf for easy insertion of a new strip when it is started through the die.
Figure 1.18 An automatic stop.
Figure 1.19 The stripper plate.
1.2.11 Fasteners
Fasteners hold the various components of the die together. Figure 1.20 shows the commonly used socket cap screw. These fasteners are available from various suppliers, and all have a threaded portion and a larger round head provided with an internal hexagon for wrenching. As you have been doing for previous illustrations, pick out the fasteners shown in the die drawing, Figure 1.2. Note that in section views, screws are shown on one side and dowels on the other.
Figure 1.20 Socket head cap screw for use as a fastener.
Let us now consider the steps taken in designing, building, and inspecting a representative die. At the same time, you will gain an insight into the operation of press shops, tool rooms, and manufacturing plants so that your understanding of tooling and manufacturing will be better than average.
1.3.1 The Product
First, we will consider the product to be manufactured. The product engineering department designs the product. In most plants, the work consists in improving the product from year to year to meet changing styles and changing requirements of customers.
After management has decided upon the final form of the new or improved product, a directive is sent to the process planning department to route the various parts through the appropriate manufacturing departments. The process or methods engineers then plan the order of manufacturing operations and decide what operations will be used. They request that the tool design department produce designs of all jigs, fixtures, cutting tools, and dies needed for efficient production of the parts.
After a product designer has prepared layouts and assembly drawings of the product to be manufactured, the engineering department prepares detail drawings of each component the shop has to produce. These drawings contain all required views, dimensions, and explanatory notes to represent all detail features of the objects.
The part which is to be machined, formed, pressed, or inspected is called by one of the following terms:
•Part
•Work
•Workpiece
Part is the preferred term, but workpiece or, simply, work are often employed as alternate names; all three terms will be used interchangeably throughout this book.
The print on which this part, work, or work-piece is represented is called a part print. In designing a die for producing a stamping, the die designer works from a part print.
1.3.2 Process Planning
Prints of detail drawings are sent to the process planning department. When stampings are required, it is the function of this department’s employees to determine how the stampings are to be made. They decide how many operations will be required and what presses will be employed to make them. This department thus assumes the responsibility of determining the sequence of manufacturing operations. The information is noted on a series of forms:
a) Route Sheet
The route sheet (Figure 1.21) is designed to suit the requirements of the individual plant and, therefore, the information route sheets contain will vary. However, the following elements are usually included:
1.The heading. This is located at the top of the sheet and contains information such as:
•Part name
•Part number
•Drawing number
•Number of parts required
•Name of product engineer
•Date
In addition, the product name and model number may be included.
2.The number of each operation required to make and inspect the part. Numbers are most frequently listed in increments of 5, such as 5, 10, 15, 20, etc., to provide numbers in sequence for additional operations which may be found necessary in manufacture or when changes are made in the design of the product.
3.The name of each operation.
4.The name and number of the machine on which the operation is to be performed.
5.Estimates of the number of parts that will be completed per hour for every operation. These estimates are altered after production rates have been measured accurately by the time study department. Route sheets are supplied to the following departments:
•Tool design department
•Production department
•Inspection department
Of course, any machine or product will contain many components, which have been standardized and which can be purchased from outside suppliers or vendors. Such items would include screws and dowels, bearings, clutches, motors, and many others. The purchasing department would be supplied with a bill of material, and purchase orders would be issued for all parts to be bought.
Figure 1.21 A typical route sheet.
Figure 1.22 A typical tool operation sheet.
b) Tool Operation Sheet
The tool operation sheet (Figure 1.22) is prepared from the route sheet and it usually lists the following:
•Number of each operation
•Name of each operation
•Machine data
•List of all standard and special tools required for the job
•Names and numbers of all special tools that are to be designed and built. These numbers are marked on tool drawings and later stamped or marked on the actual tools for identification.
Tool operations sheets are helpful in planning and developing a tooling program. Copies go to the tool designers and to the tool purchasing department. Before proceeding further, study carefully the tool operation sheet illustrated.
c) Design Order
The design order (Figure 1.23) authorizes work on an actual design. An order is prepared for each die or special tool required and the information is taken from the route sheet. In addition, the design order may give instructions regarding the type of die preferred. The following lists the information usually given on a design order:
Figure 1.23 A typical design order.
•Department name
•Tool name
•Date
•Tool number
•Part name
•Part number
•Operation
•Machine in which tool is to be used.
1.3.3 Designing the Die
Before designers begin to draw, they must seriously consider a number of things. It is now possible for them to list all the items that will be required so they can begin designing intelligently. These items are:
•The part print
•The tool operation sheet, or route sheet
•The design order
•A press data sheet.
In addition, designers may have either a reference drawing of a die similar to the one they are designing or a sketch of the proposed design prepared by the chief tool designer or group leader suggesting a possible approach to solving the problem. Let us consider further the information required:
Part print. The part drawing gives all necessary dimensions and notes. Any missing dimension must be obtained from the product design department before work can proceed.
Operation sheet. The operation sheet or route sheet must be studied to determine exactly what operations were previously performed upon the workpiece. This item is very important. When the views of the stamping are laid out, any cuts that were applied in a previous operation must be shown.
Design order. This item must be studied very carefully because it specifies the type of die to be designed. Consider particularly the operation to be performed, the press in which the die is to be installed, and the number of parts expected to be stamped by the die. The latter will establish the class of die to be designed.
Press (machine) data sheet. The die to be designed must fit into a particular press and it is important to know what space is available to receive it and what interferences may be present.
In time you will come to realize the importance of careful and repeated study of the part print, operation sheet, and design order because there can be no deviation from the specifications given.
a) Die Drawing
If the information on a drawing is complete, concise, and presented in the simplest possible manner, the die maker can work to best advantage. The first step in originating plans for a new die is to prepare a sketch or sketches of significant features of the proposed die. These are a guide for beginning the actual drawing of the full-size layout. However, it is a mistake to spend too much time in this phase of the work or to try to develop the entire design in sketch form because doing so can result in arbitrary and inflexible decisions.
Always keep your mind open to possible improvements as you develop the design in layout form. You will often find that the first or second idea sketched out can be considerably improved by alteration as work progresses. Often the first idea proves entirely impractical and another method of operation must be substituted.
Before beginning the sketch, gather before you the part print, operation sheet, and design order. The three must be studied together so that a complete and exact understanding of the problem will be realized. This study will form the basis for creating a mental picture of a tool suitable for performing the operations—one which will meet every requirement. The sketch you make may be a very simple one, for simple operations, or it may be more elaborate. In fact, a number of sketches may be required for more complex operations and intricate designs. In any event, the sketch will clarify your ideas before you attempt a formal layout. In addition, it will form the basis for a realistic estimate of the size of the finished die so that you may select the proper sheet size for the layout.
Layout. Laying out the die consists of drawing all views necessary for showing every component in its actual position. In the layout stage, no dimensions are applied and neither is the bill of material nor the record strip filled out. After the die has been laid out, the steps necessary for completing the set of working drawings are more or less routine.
Assembly drawing. A properly prepared assembly drawing contains six general features:
1.All views required for showing the contour of every component including the workpiece.
2.All assembly dimensions. These are dimensions that will be required for assembling the parts, as well as for machining operations to be performed after assembly.
3.All explanatory notes.
4.Finish marks and grind marks to indicate those surfaces to be machined after assembly.
5.A bill of material listing sizes, purchased components, materials, and number (quantity) required for all parts.
6.A title block and record strip with identifying information noted properly.
Detail drawing. After the assembly drawing of the die has been completed, detail drawings are prepared, unless all dimensions were previously placed on the assembly drawing (as is done for simple dies). Detail drawings are drawings of individual components. They contain all dimensions, notes, and supplementary information so that each part can be made without reference to the assembly drawing or to other detail drawings. Such information usually includes 10 distinct elements:
1.All views required for identifying every detail of the part must be drawn.
2.Every dimension needed for making the part must be given.
3.Suitable notes for furnishing the supplementary information that dimensions do not cover must be applied.
4.Finished surfaces must be identified.
5.The name of the part and its number must be given.
6.The material from which the part is to be made must be specified.
7.The number of each material required per assembly must be stated.
8.The scale to which the drawing has been laid out must be listed.
9.The draftsman’s name or initials must be signed.
10.The date must be specified.
Dimension and notes. With the die design completed, all dimensions and notes are applied to the drawing. Figure 1.24 shows the die set note, which tells the die maker exactly what die set to order and gives required information about punch shank diameter, type of guide bushings, and diameter and length of guide posts.
Figure 1.24 A typical die set note.
b) Checking the Die Drawings
After a set of drawings has been completed and the designer has reviewed them for possible omissions or errors, the set is turned over to the group leader, who will bring it to the checkers for further review. The design order, part print, and any notes or sketches that may have accompanied the drawings when the designer first received the job will now travel with the drawings. Checkers require all of these in order to do their work properly.
Checkers first study the operation of the die to make sure that it will function properly and that its cost will not be excessive for the work it is to perform. After they are satisfied that it has been designed properly, they will check every dimension, note, and specification for accuracy. They usually work from a check print. This is a blue and white print having blue lines and a white background. With a yellow crayon, they will cover every dimension they find to be accurate, and with a red crayon they will cover every dimension they find to be wrong. Above or to the side they will write the correct dimension in red.
The tracings, along with the check prints, are then returned to the designer for correction. Incorrect dimensions are carefully erased to remove all graphite from the paper. An erasing shield is ordinarily used to prevent smudging of other dimensions or lines. Correct dimensions are then lettered in place.
After the tracings have been corrected, they are returned to the checkers, who review the job again to make sure that no correction was overlooked. They then sign the drawing in the space provided and enter the date the drawing was checked.
After drawings have been completed and checked, they must be approved by the chief designer, chief tool engineer, and possibly the plant superintendent and others who are held responsible by the management for the cost and quality of dies used in the plant. Usually, these approvals are routine after drawings have been approved by the checker. However, it may sometimes happen that these personnel will refuse to sign because they believe that the die will not work as well as expected, will not deliver the number of parts required per day, will be too expensive to build, or for some other reason. If they convince others that their objections are valid, the drawings will have to be altered or a new design begun, depending upon the extent of the changes to be made.
c) Prints and Distribution
After drawings have been approved, blueprints are made from the tracings, or originals. A small print is taken of the bill of material only. This is sent to the stock cutting department where steel is stored and cut as required. The stock cutter goes over the list, selects bars of proper thickness and width, or diameter, and saws the bars to the lengths specified for each item listed. These cut blocks and plates are placed in a pan, along with screws, dowels, and other parts, which are kept in stock. When purchased components are delivered to the plant, they are also placed in the pan. Finally, the pan contains a set of die prints and a part print and it is delivered to the tool room, where the tool room foreman turns it over to the die maker, who will build the die.
One of the prints is sent to the purchasing department. There, orders are written to authorize purchase of all components needed to build the die. If the entire die is to be built by an outside tool shop, a purchase order is sent to it. If it is to be built within the plant, an order authorizing construction is sent to the tool room. In addition, requisitions are made out for the following:
•Standard parts or assemblies that are not kept in stock and which must be purchased.
•Castings, forgings, or weldments required for construction of the die.
•Steels of special analysis not carried in stock.
•Special sizes of steels or other materials not stocked.
The purchasing agent must plan for delivery of all these components before the date set for beginning construction of the die.
The files for the die drawings—whether electronic or card—are then made out by the die design department. The file contains the number of the drawings and the job by name and number. Each die has only one file.
1.3.4 Tool and Die Inspection Department
After a die has been designed, a set of prints is sent to the tool inspection department. Then after the die has been built, the department will inspect it to make certain that it was constructed to specifications given in the tool print.
When the die is built by an outside tool shop, it is inspected by the tool inspection department upon delivery. The same inspection procedures are followed to determine if the stampings it produces hold to tolerances specified on the part print.
1.3.5 Production
After the tool inspection department has approved its construction and accuracy, the die is delivered to the production department where it will be used. The set-up person for that department installs it in the press where it will be operated. A few sample parts are then produced under the same conditions in which the die will run in actual production. These parts are taken to the production inspection department. There, they are inspected to determine whether or not sizes hold to tolerances specified on the part print.
Once the production inspection department has determined that the samples are satisfactory, a form is issued and signed by the chief inspector authorizing production with the die. After receiving production orders from the production department, the production foreman will proceed to go into production of the stampings. Production orders specify how many parts are to be run, when they will be required, and where they are to be delivered.
After a new die has been in production for a few hours or so, and it is found to perform satisfactorily, the order that was issued to the tool room to build the die is closed. No more time may be charged against it. In this regard, it is worth noting that records are kept of all time devoted to designing, building, inspecting, and trying out the die in order to determine the actual tool cost, illustrating perfectly that “time is money.”
Just exactly what operations are performed in dies? This question is asked often and we have prepared the following illustrated list of the 20 types of operations.
1.4.1 Blanking
Stampings that have an irregular contour must be blanked from the coil or from the strip (Figure 1.25). Piercing, embossing, and various other operations may be performed on the strip prior to the blanking station.
Figure 1.25 A blank and the strip from which it is been cut.
1.4.2 Cut-off
Cut-off operations (Figure 1.26) are those in which strip of suitable width is cut to length. Preliminary operations before cutting off include piercing, notching, and embossing. Although they are relatively simple, many parts can be produced by cut-off dies.
1.4.3 Piercing
Piercing dies pierce holes in previously blanked formed, or drawn, parts (Figure 1.27). It is often impractical to pierce holes while forming or before forming because the holes would become distorted in the forming operation. In such cases they are pierced in a piercing die after forming.
1.4.4 Piercing and Blanking
Compound dies pierce and blank simultaneously at the same station (Figure 1.28). They are more expensive to build and they are used where considerable accuracy is required in the part.
Figure 1.26 Part separated from strip in cut-off operation.
Figure 1.27 Holes pierced in a previously drawn part
Figure 1.28 Part is blanked and pierced simultaneously in a compound die.
Figure 1.29 The result of trimming in a trimming die.
1.4.5 Trimming
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. Figure 1.29 shows a flanged shell, as well as the trimmed ring removed from around the edge.
1.4.6 Shaving
Shaving consists of removing a chip from around the edges of a previously blanked stamping (Figure 1.30). A straight, smooth edge is provided. Therefore, shaving is frequently performed on instrument parts, watch and clock parts, and the like. Shaving is accomplished in shaving dies especially designed for the purpose.
Figure 1.30 The result of shaving in a shaving die.
Figure 1.31 Serrations applied in a broaching die.
1.4.7 Broaching
Figure 1.31 shows serrations applied in the edges of a stamping. These would be broached in a broaching die. Broaching operations are similar to shaving operations. A series of teeth removes the metal instead of just one tooth, as in shaving. Broaching must be used when more material is to be removed than could effectively be done with one tooth.
1.4.8 Horning
Horn dies are provided with an arbor or horn over which the parts are placed for secondary operations such as seaming (Figure 1.32). Horn dies may also be used for piercing holes in the sides of shells.
1.4.9 Side Cam Operations
Piercing a number of holes simultaneously around a shell (Figure 1.33) is done in a side cam die. Side cams convert the up-and-down motion of the press ram into horizontal or angular motion when the nature of the work requires it.
Figure 1.32 The seam on this part is done as a secondary operation in a horn die.
Figure 1.33 The holes are pierced simultaneously in a side cam die.
1.4.10 Bending
Bending dies apply simple bends to stampings. A simple bend is one in which the line of bend is straight. One or more bends may be involved (Figure 1.34). Bending dies are a large and important class of press tool.
1.4.11 Forming
Forming dies apply more complex forms to workpieces. The line of bend is curved instead of straight and the metal is subjected to plastic flow or deformation (Figure 1.35).
Figure 1.34 Stamping bent in a bending die.
Figure 1.35 Stamping formed in a forming die.
Figure 1.36 Shell drawn from a flat sheet.
1.4.12 Drawing
Drawing dies transform flat sheets of metal into cups, shells, or other drawn shapes by subjecting the material to severe plastic deformation. Figure 1.36 shows a rather deep shell that has been drawn from a flat sheet.
1.4.13 Curling
Curling dies curl the edges of drawn shells to provide strength and rigidity (Figure 1.37). The curl may be applied over a wire ring for increased strength. You may have seen the tops of sheet metal pails curled in this manner. Flat parts may be curled also. A good example is a hinge in which both members are curled to provide a hole for the hinge pin.
Figure 1.37 Lip on this drawn shell produced in curling die.
Figure 1.38 Bulge in this drawn shell produced in bulging die.
1.4.14 Bulging
Bulging dies expand the bottom of previously drawn shells (Figure 1.38). For example, the bulged bottoms of some types of coffee pots are formed in bulging dies.
1.4.15 Swaging
In swaging operations, drawn shells or tubes are reduced in diameter for a portion of their lengths (Figure 1.39). The operation is also called “necking.”
1.4.16 Extruding
Extruding dies cause metal to be extruded or squeezed out, much as toothpaste is extruded from its tube when pressure is applied. Figure 1.40 shows a collapsible tube formed and extruded from a solid slug of metal.
Figure 1.39 Drawn shell that has been swaged.
Figure 1.40 Drawn shell that has been extruded.
1.4.17 Cold Coining
In cold forming operations, metal is subjected to high pressure and caused to flow into a predetermined form. In coining (Figure 1.41), the metal is caused to flow into the shape of the die cavity. Coins such as nickels, dimes, and quarters are produced in coining dies.
1.4.18 Progressive Operations
Progressive operations are those in which progressive dies perform work at a number of stations simultaneously. A complete part is cut off at the final station with each stroke of the press. Figure 1.42 shows part and strip produced in a progressive die.
1.4.19 Sub Press Operations
Sub press dies are used for producing tiny watch, clock, and instrument components, represented by the watch needles shown in Figure 1.43. Sub presses are special types of die sets used only for such precision work.
Figure 1.41 Cold-coining part in which metal flow is caused by high pressure.
Figure 1.42 Part and strip produced in a progressive die.
1.4.20 Assembly Operations
Assembly dies represent an assembly operation in which two studs are riveted at the ends of a link (Figure 1.44). Assembly dies assemble parts with great speed; they are being used more and more.
From the foregoing, you can perhaps appreciate what a wide field die design engineering really covers. You must have come to realize that it is indeed a pleasant and interesting occupation, one which will stimulate your mind in much the same manner as the working out of fascinating puzzles. In addition, you will come to find that is a very profitable one.
Figure 1.43 Typical precision parts produced in sub press dies.
Figure 1.44 Part produced in an assembly die.
As you study the chapters that follow, you will be introduced, step by step, to the fundamental die components. You will learn the methods by which die designers assemble these components when they design dies. When you have completed the book you will know the elements of die design quite thoroughly. Knowledge such as this is well-compensated professionally. You will have acquired the foundation of a career that can benefit you for the rest of your life.
