Die Design Fundamentals. Vukota Boljanovic
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