Die Design Fundamentals. Vukota Boljanovic
surfaces having a uniform gray color. Oiling protects the surfaces against rust.
These sheets are readily stamped or welded. Long-lasting painting or enameling is possible because of the absence of scale. Pickled and oiled sheets are used for household appliances, automotive parts, toys, and the like.
b) Copper-Bearing Sheets
Copper-bearing sheets are hot-rolled sheets having 0.20 percent minimum copper content. They are used for parts designed for outdoor exposure, or for indoor use under corrosive conditions. These sheets have a service life from two to three times longer than can be expected from non-copper-bearing steels. They are used for roofing and siding, farm and industrial buildings, truck bodies, railroad cars, farm implements, signs, tanks, dryers, ventilators, washing machines, and other similar applications.
c) Medium-Carbon Sheets
Hot-rolled sheets having a 0.40 to 0.50 percent carbon content provide hardness, strength, and resistance to abrasion. They can be heat-treated to make the material even harder and stronger and are primarily used for scrapers, blades, hand tools, and the like.
3.2.2 Cold-Rolled Sheets
Cold-rolled sheets have a smooth, deoxidized satin finish, which provides an excellent base for paint, lacquer, and enamel coating. Thicknesses are held to a high degree of accuracy. Cold-rolled steel is produced by the cold rolling of hot-rolled sheets to improve size and finish. Refrigerators, ranges, panels, lockers, and electrical fixtures are among their many uses.
a) Possibility of Deformation
Six tempers of cold-rolled steel sheets and strips are available; it is important to know exactly what operations can be performed on each (Figure 3.1):
1. Hard. Hard sheets and strips will not bend in either direction of the grain without cracks or fracture. These tempers of steel are employed for flat blanks that require resistance to bending and wear. Direction of grain is shown along lines A in the illustration. Hardness is Rockwell B 90 to 100.
2. Three-quarter hard. This temper of steel will bend a total of 60 degrees from flat across the grain. This is shown as dimension B in the illustration. Hardness is Rockwell B 85 to 90.
3. One-half hard. This temper will bend to a sharp 90-degree angle across the grain, shown as dimension C. Hardness is between Rockwell B 70 and 85.
4. One-quarter hard. This commonly used temper of steel will bend over flat on itself across the grain and to a sharp right angle along the grain. Hardness is Rockwell B 60 to 70.
Figure 3.1 Various tempers of cold-rolled steel from hard (1) to dead soft (6) and kinds of deformation possible with each.
5. Soft. This temper will bend over flat upon itself both across the grain and along the grain. It is also used for moderate forming and drawing. Hardness is Rockwell B 50 to 60.
6. Dead soft. This temper of steel is used for deep drawing and for severe bending and forming operations. Hardness is Rockwell B 40 to 50.
b) Finish
Cold-rolled steel is available in three grades of finish:
1. Dull finish. This is a gray lusterless finish to which lacquer and paint bond well.
2. Regular bright finish. This is a moderately bright finish suitable for most work. It is not recommended for plating unless buffed first.
3. Best bright finish. This finish has a high lustre well suited for electroplating. It is the brightest finish obtainable.
c) Stretcher-Leveled Sheets
These are cold-rolled steel sheets that have been further processed by stretcher leveling and resquaring. They are used in the manufacture of metal furniture, table tops, truck body panels, partitions, and other equipment requiring perfectly flat material.
d) Deep-Drawing Sheets
Deep-drawing steel is prime quality cold-rolled steel having a low carbon content. Sheets are thoroughly annealed, highly finished to a deoxidized silver finish, and oiled. Deep-drawing sheets are used for difficult drawing, spinning, and stamping operations such as those which produce automobile bodies, fenders, electrical fixtures, and laboratory equipment.
e) Silicon Steel
Also called “electrical steel,” silicon steel is extensively used for motors and generators. Lighter gages are suitable for transformers, reactors, relays, and other magnetic circuits.
The shearing process involves the cutting of flat material forms, such as sheets and plates. The cutting may be done by different types of blades or cutters in special machines driven by mechanical, hydraulic, or pneumatic power.
Figure 3.2 shows the mechanics of shear in 8 steps:
1.This illustration shows the cutting edges of a die with clearance C applied. The amount of this clearance is important, as will be shown.
2.A material strip is introduced between the cutting edges and is represented by phantom lines. Cutting a material strip occurs when it is sheared between cutting edges until the material between the edges has been compressed beyond its ultimate strength and fracture takes place.
3.The upper die begins its downward travel and the cutting edge of the punch penetrates the material by the amount A. The following stresses occur: The material in the radii at B is in tension; that is, it is stretched. The material between cutting edges C is compressed, or squeezed together. Stretching continues beyond the elastic limit of the material, then plastic deformation occurs. Observe that the same penetration and stretching is applied to both sides of the strip.
4.Continued descent of the upper cutting edge causes cracks to form in the material. These cleavage planes occur adjacent to the corner of each cutting edge.
5.Continued descent of the upper die causes the cracks to elongate until they meet. Here then is the reason for the importance of correct clearance. If the cracks fail to meet, a bad edge will be produced in the blank.
6.Further descent of the upper die causes the blank to separate from the strip. Separation occurs when the punch has penetrated approximately 1/3 of the strip.
7.Continued descent of the upper die causes the blank to be pushed into the die hole where it clings tightly because of the compressive stresses introduced prior to separation of the blank from the strip. In other words, the material at C in step 3 was compressed and it acts like a compressed spring. The blank, confined in the die hole, tends to swell, but it is prevented from doing so by the confining walls of the die block. Conversely, the material around the punch tends to close in and, therefore, the strip clings tightly around the punch.
Figure 3.2 Mechanics of shear-enlarged views of clearance between cutting edges of a shearing die (step 1) and material undergoing shear (steps 2 to 8).
8.The punch has now penetrated entirely through the strip and the blank has been pushed entirely within the die hole. Observe that the edge of the blank and the edge of the strip have identical contours except that they are reversed. The strip will cling around the upper punch with approximately the same pressure as the blank clings within the die hole and a stripper will be required to remove it.
3.3.1 Sheared Edges
It now becomes necessary to understand exactly what occurs when sheet material is cut between the cutting edges of a punch and die.