In order to bend metals to various forms, dies are made for use in punching presses, drop hammers, and various other machines. A simple form of bending die is shown in Fig. 352. The shape of the upper and lower parts of the die is such that when the upper part is brought down on the blank B (shown by the dotted lines) that will be bent to the required shape. The shoulder A forms a locating stop, against which the blank rests before bending.
Fig. 348. Sheet-Metal Blank.
Fig. 349. Simple Stamping. Twelve Produced at a Stroke.
Fig. 350. Gang Punch and Die for Simple Punching.
Dies for extremely soft metals may be made of the exact shape of the model, or the shape the piece should be when finished; but if the piece is of stiff material which bends with difficulty, it will be necessary to make the die of a form that will give the article more bend than is required, as the piece will spring back somewhat as soon as released by the return of the upper part.
The bending of articles of certain shapes requires tools so designed that certain portions of the piece will bend before others. Any attempt to make the tools solid, and thus to do the bending of the various portions at once, would result in stretching the stock. As a rule it is not advisable to stretch stock, and dies are constructed to do away with this trouble.
Fig. 351. Punching! Too Close Together.
Under certain conditions a bending die which has a horizontal surface for the work to rest on and a vertical-sided punch does not work in a satisfactory manner-this is especially true when the stock is stiff. In that case, a die of the design shown in Fig. 353 works well, as the angle may be made other than 90 degrees to allow for the spring of the metal.
This design of die may be used for angles other than right angles. It is especially satisfactory for bending springs and other pieces made from a stiff stock that is liable to spring back somewhat after bending, as the lower block may be made with an angle greater than 90 degrees to allow for this factor.
For comparatively light work, the form of bending die shown in Fig. 354 is very satisfactory, and may be used for a variety of shapes and angles. The die block a is drilled and reamed to receive the shouldered portion b. The rectangular groove, to receive the pad, is milled or planed, and the pad is fitted and forced in. The proper angle or shape is then milled in the block a and pad b. The surfaces are carefully finished and the pad forced out and drawfiled until it slides nicely in the groove. The spring c forces the pad against the washer d. Gage plates are provided to locate accurately the pieces to be bent.
Fig. 362. Simple Bending Die.
Fig. 353. Bending Die for Right Angles.
Fig. 355 shows a die, the upper part of which has the portion a so constructed that it engages the stock first and forces it down into the impression in the lower portion. The resistance of the coil spring is then overcome, and a is forced up into the opening provided for it. The arms cc bend the ends of the piece. After bending, the article is of the form shown at b.
In Fig. 356 is shown a form of die used in bending bow spring and looped wire for armature connections or other looped wire-work. The work is placed in the die, and the punch, as it descends, bends the wire to the shape of the die. The spring just back of the punch is compressed; this allows the punch holder to descend and force the side benders BB toward the punch by means of the wedge pins AA, and thus forms the piece into a circle. Fig. 357 shows the punch when down. It is obvious that it is necessary to make the shape of the punch and die different. The lower die must have its bending surface a curve of a radius equal to the radius of the punch plus the thickness of the material.