Conventional Stamping

Simply put, stamping is a method of cold-working sheet metal to a prescribed size and shape by means of a die and a press. The die determines the size and shape of the completed workpiece. The press provides the force needed to effect the change.

Each die is specially constructed for the operation to be performed and is not suitable for other operations. The stamping die is in two halves, between which the sheet metal is placed. When the two halves of the die are brought together, the operation is performed. Normally, the upper half of the die is the punch (the smaller member), and the lower half is the die (the larger member). When the two die halves are brought together, the punch enters the stamping die.

The die, or matrix, has the desired opening cut into it by various methods. The punch has a shape that that corresponds to that of the die but is smaller by an amount determined by the required punch and die clearance, which in turn is determined by the type and thickness of the material and the operation to be performed.

The machine used to consummate these changes of shape has a stationary bed, or bolster, on which the die portion is clamped. A guided slide, or ram, which has the punch portion clamped to it, moves up and down perpendicularly to the bed. The motion and force of the ram are provided by a crankshaft, eccentric, or other mechanical means. Hydraulically actuated presses are also employed.

The two parts are mounted in a die set, or subpress, the die (in a simple blank through stamping die) being mounted on the base and the punch on the upper shoe. The use of a stamping die set ensures proper alignment of the punch and die regardless of the condition of the press. The simplest dies are those for punching holes in a blank.

The stamping of sheet metal involves cutting or shearing, bending or forming, and drawing or deep-drawing operations. Cutting around the entire periphery of a part is called blanking. Cutting holes in a workpiece is called punching or piercing. A description of each category follows:

Blanking or Piercing. Blanking or piercing to a contour progresses through three stages: (1) plastic deformation, (2) penetration, and (3) fracture. In stage 1, the punch makes contact with the material, and pressure begins to be exerted until the elastic limit of the stock is exceeded and plastic deformation commences. In stage 2, the continuing pressure causes the punch to penetrate the stock, thereby displacing the blank or slug into the die opening, the displacement equaling the amount of penetration. In stage 3, the fracturing occurs. At this point, the blank or slug is separated from the parent stock.

Generally, the straight or cut band of the material will average approximately one-third of the stock thickness. This, of course, depends on the material’s brittleness. The punch could penetrate anywhere from 15 to 40 percent of the thickness before fracture occurs.

Forming. In forming, the operation produces one or more plane surfaces that are at an angle to the original or flat plane of the blank. Any change in the shape of the blank, no matter how small, is classified as forming.

The material to be formed must have the proper ductility to ensure its retention of deformation in tension without rupture. A schematic of the bending action in a pad die

Drawing. When a part is designed so that no seams or other mechanical joints are permissible, it can be described as a “hollow” or “cup-shaped” body. The “body” is manufactured, by using a flat blank, with the drawing method. Drawing probably ranks second in importance to the cutting operations of press-metal working.

Generally speaking, drawing operations require the use of a triple-acting toggle joint or drawing press. Two or more draws (with necessary heat treatment between draws, depending on the material used) are required when the depth of the cup-shaped area is more than three-fourths of its diameter or width. In a simplified description, blanks are drawn by first confining them between the die and a pressure pad (this is where the triple-acting press comes in); then, as the press continues its cycle, the punch (which has the reverse shape of the die but is smaller by stock thickness) forces the material to flow inwardly from between the two confining surfaces.

The pressure applied between the die and the pressure pad during the entire press cycle must be such that the blank is kept from wrinkling. Wrinkles will prevent obtaining a smooth cup, and the possibility of tearing the cup increases. The pressure required is mainly the result of experience plus trial and error. For a schematic view of a typical drawing operation.

In addition to the three basic methods, conventional stamping includes shaving, trimming, embossing, coining, and swaging.