Any type of die that performs a drawing operation is referred to as a drawing die, regardless of its construction. Fig. 25 shows a drawing punch and die in its simplest form, producing only shallow cups, as in Fig. 26, from blanks made from another die. The chief difference between this punch and die and the plain blanking type is that the punch, instead of fitting the die, is made smaller than the die by an amount equal to twice the thickness of the stock to be drawn. Also the sharp corners of the die are removed to allow the stock to slide freely from the flat state to cup shape, and to prevent scratching the blank.
Fig. 27 shows types of cups that are formed by the drawing die in Fig. 28. When using this design of die, however, the blanks must be punched out with another die. This die should not be designed for use except when a small quantity of cups are required.
For producing the blanks seen in Fig. 29, the type of drawing die shown in its simplest form in Fig. 30 is employed. This die also requires blanks previously punched from sheet stock. As is the case with all the foregoing drawing dies, this type should not be designed where rapid production or quality is required.
Fig. 25. Simple Form of Drawing Punch and Die.
Fig. 26. Typical Cups Formed by Drawing Die.
We now come to that class of tools known as blanking-and-drawing dies. The designer is limited somewhat in his selection of a type of die that is best suited for the work, as he must design the dies that will be operative in the presses at hand. The die shown in Fig. 31 is designed for use in what is termed a single-action press, such as that in Fig. 32. In this case the press has only one crank on the drive shaft, and to that crank is attached the driving rod which actuates the ram, so that the ram has but one action. The die, Fig. 31, produces any of the cups shown in Figs. 26, 27, and 29, and has the advantages over any drawing die thus far shown of punching out its own blank and of drawing the blank to a cup at each stroke of the press. It will be noted that the die is somewhat complicated and that the blanking punch is also the drawing die. This type of die should be designed for rapid production only when compelled to use a single-action punch press.
Fig. 27. Types of Cups Formed by Drawing Die. Fig. 28.
Fig. 28. Drawing Die with Spring Stripper.
Fig. 29. Typical Blanks Produced by Drawing Die Fig. 30.
In operation, the blanking punch d cuts out the blank and the blank is pinched between the angular faces of the lower stripper c and the blanking punch. As the punch continues to descend, the lower stripper descends also and the forming punch e forces the blank up into the drawing die, which is the recess f in d. The stroke of the ram is so adjusted that at the extreme point of the downward stroke the forming punch e presses the cup firmly against the face of the upper stripper a, while at the same time the back face of a is pressing firmly against the bottom of recess f..
Fig. 30. Another Form of Drawing Die.
As the ram ascends, the lower stripper accompanies the blanking punch, that is, the stripper c, being actuated by heavy springs h, ascends at the same speed as that of the blanking punch and strips the cup from the forming punch e. The lower stripper e, therefore, performs the double function of stripping the cup from the forming punch and of pressing the stock against the angular face of the blanking punch to prevent wrinkling the stock. It is obvious that if the flat blank were not held under pressure, the blank would wrinkle when it changes from a flat blank, say 6 inches in diameter, to a cup shape 3 inches in diameter. The designer must employ powerful springs to prevent wrinkling. Near the end of the upward stroke of the ram, the stripper shank i comes in contact with a cross-rod in the press called the knock-out rod, which pushes the stripper a down, leaving it in the position shown in Fig. 31. This causes the finished cup to drop from the drawing die to the face of the blanking die.
Fig. 31. Blanking-and-Drawing Dies Designed far Single-Aclion Press.
The operator must remove the cup from face of the die before blanking and drawing the next one - a somewhat slower operation than when this same die is used in a press of the inclinable type such as shown in Fig. 33; the press being tilted on a decided angle, the cups cannot remain on the face of the die, thus enabling the operator to greatly increase the quantity of production. Therefore, the designer must learn which types of press he may have at his command, and when designing dies of this character he must select the die best adapted and have it fitted to the press that will insure the most rapid production. The designer should also see that the plans from which the tool-makers work contain notes calling for polished rounded corners on a drawing die, and for the sides of the drawing die and drawing punch to be highly polished.
Fig. 34 shows in section a drawing punch and die designed for a double-action press, Fig. 35, and to produce the cups shown in Fig. 27. The shape of the end of the drawing punch a, Fig. 34, governs the shape of the bottom of the cup. The term double action means that there really are two strokes to the press. The drive shaft has three cranks or eccentrics, as shown enlarged in Fig. 36. The two end cranks are connected to the large ram, Fig. 35, to which is fastened the blanking punch b, Fig. 34, while the central crank is connected to the smaller ram, Fig. 35, which slides inside the large ram. The position of the cranks is such that in operation the ram containing the blanking punch descends, and, before the blanking punch touches the stock to be punched, the inner ram containing the drawing punch a, Fig. 34, starts to descend. The rams are so adjusted that just as the blanking punch reaches its lowest point, which should be when the blank is firmly pressed against the face of the drawing die c, the drawing punch continues downward and pushes the blank down through the die.
Fig. 36. Crank Shift of Double-Action Press.
For producing the cups shown in Fig. 27, the die in Fig. 34 is the ideal one, due to its simplicity and speed of operation, for the blanks are pushed clear through the die. The double-action type of die, however, is not any better suited for those cups in Fig. 29 than the die for ft single-action press shown at Fig. 31, for in using either type the cups would have to.be removed from the top of the die unless the press were inclinable.