PUNCHES, DIES, AND PRESS WORK

There are two methods of giving clearance to dies: In one method, the clearance extends to the top face of the die; and in the other, there is a space about 1⁄8 inch below the cutting edge that is left practically straight, or having a very small amount of clearance.
For very soft metal, such as soft, thin brass, the first method is employed, but for harder material, such as hard brass, steel, etc., it is better to have a very small clearance for a short distance below the cutting edge. When a die is made in this way, thousands of blanks can be cut with little variation in their size, as grinding the die face will not enlarge the hole to any appreciable extent.
Lubricants for Press Work.—Blanking dies used for carbon and low-alloy steels are
often run with only residual mill lubricant, but will last longer if lightly oiled. Higher alloy and stainless steels require thicker lubricants. Kerosene is usually used with aluminum.
Lubricant thickness needs to be about 0.0001 in. and can be obtained with about 1 pint of fluid to cover 500 sq. ft of material. During successive strokes, metal debris adheres to the punch and may accelerate wear, but damage may be reduced by application of the lubricant to the sheet or strip by means of rollers or spray. High-speed blanking may require heavier applications or a continuous airless spraying of oil. For sheet thicker than 1⁄8 in. and for stainless steel, high-pressure lubricants containing sulfurs and chlorines are often used.
Shallow drawing and forming of steel can be done with low-viscosity oils and soap solutions, but deeper draws require light- to medium-viscosity oils containing fats and such active elements as sulfur or phosphorus, and mineral fillers such as chalk or mica. Deep drawing often involves ironing or thinning of the walls by up to 35 per cent, and thick oils containing high proportions of chemically active compounds are used. Additives used in drawing compounds are selected for their ability to maintain a physical barrier between the tool surfaces and the metal being shaped. Dry soaps and polymer films are frequently used for these purposes. Aluminum can be shallow drawn with oils of low to medium viscosity, and for deep drawing, tallow may be added, also wax or soap suspensions for very large reductions.
Annealing Drawn Shells.—When drawing steel, iron, brass, or copper, annealing is necessary after two or three draws have been made, because the metal is hardened by the drawing process. For steel and brass, anneal between alternate reductions, at least. Tin plate or stock that cannot be annealed without spoiling the finish must ordinarily be drawn to size in one or two operations. Aluminum can be drawn deeper and with less annealing than the other commercial metals, provided the proper grade is used. If it is necessary to anneal aluminum, it should be heated in a muffle furnace, care being taken to see that the temperature does not exceed 700 degrees F.
Drawing Brass.—When drawing brass shells or cup-shaped articles, it is usually possible to make the depth of the first draw equal to the diameter of the shell. By heating brass to a temperature just below what would show a dull red in a dark room, it is possible to draw difficult shapes, otherwise almost impossible, and to produce shapes with square corners.
Drawing Rectangular Shapes.—When square or rectangular shapes are to be drawn, the
radius of the corners should be as large as possible, because defects usually occur in the corners when drawing. Moreover, the smaller the radius, the less the depth that can be obtained in the first draw.
The maximum depths that can be drawn with corners of a given radii are approximately
as follows: With a radius of 3⁄32 to 3⁄16 inch, depth of draw, 1 inch; radius3⁄16 to 3⁄8 inch, depth 11⁄2 inches; radius3⁄8 to 1⁄2 inch, depth 2 inches; and radius1⁄2 to 3⁄4 inch, depth 3 inches.
These figures are taken from actual practice and can doubtless be exceeded slightly when using metal prepared for the process. If the box needs to be quite deep and the radius is quite small, two or more drawing operations will be necessary.
Speeds and Pressures for Presses.—The speeds for presses equipped with cutting dies
depend largely upon the kind of material being worked, and its thickness. For punching and shearing ordinary metals not over 1⁄4 inch thick, the speeds usually range between 50 and 200 strokes per minute, 100 strokes per minute being a fair average. For punching metal over 1⁄4 inch thick, geared presses with speeds ranging from 25 to 75 strokes per minute are commonly employed.
The cutting pressures required depend upon the shearing strength of the material, and the actual area of the surface being severed. For round holes, the pressure required equals the circumference of the hole × the thickness of the stock × the shearing strength. To allow for some excess pressure, the tensile strength may be substituted for the shearing strength; the tensile strength for these calculations may be roughly assumed as follows:
Mild steel, 60,000; wrought iron, 50,000; bronze, 40,000; copper, 30,000; aluminum,
20,000; zinc, 10,000; and tin and lead, 5,000 pounds per square inch.
Pressure Required for Punching.—The formula for the force in tons required to punch a circular hole in sheet steel is πDST/2000, where S = the shearing strength of the material in lb/in.2, T = thickness of the steel in inches, and 2000 is the number of lb in 1 ton. An approximate formula is DT × 80, where D and T are the diameter of the hole and the thickness of the steel, respectively, both in inches, and 80 is a factor for steel. The result is the force in tons.