Punching Holes Piercing

In addition to punching or piercing through holes, punch press tooling and computer numerical controlled punch press machinery are very effective in creating sheet metal parts with formed features. Some examples of features produced via forming operations are as follows ... 

Smooth wall hole piercing special punch profiles are used to produce crack-free holes. 

Loch, n. hole, opening cavity eye pore prison, -blende, /. perforated screen or diaphragm. -eisen, n. puncher, pimch. locben, v.t. perforate, punch, pierce, locherig, a. full of holes, perforated, porous. Locherstelle,/. open place, gap, hole. Loch-frass, m. pitting (as of metals), -karte,/. punch (ed) card, -kultur,/. (Bact.) stab culture. -scheibe, /. perforated disk, apertured disk. 

The tools used in presses are punches and dies. The punch is attached to the press slide and is moved into the die, which is fixed to the press bedplate. In blanking and piercing, the punch and die are the shape of the required blank and hole and the metal is sheared by passing the punch through the die. In bending, the punch and die are shaped to the required form of the bend and no cutting takes place. In each case, the punch and die must be in perfect alignment. The complete assembly of punch and die is known as a press tool. 

The principle Is the same as for simple blanking but the punch plate has two punches fitted - one for blanking and one for piercing - and there are two holes of the required shape in the die. Fig. 16.16. The work material Is again in the form of strip, fed by hand. 

Greater accuracy of the inner and outer profiles of a workpiece can be obtained by fitting a pilot in the blanking punch (Fig. 16.16). In this case the fixed stop is used for approximate positioning and is arranged so that the work material is drawn slightly away from it as the pilot engages in the pierced hole. 

In blanking and piercing operations, the punch establishes the size of the hole and the die establishes the size of the blank. Therefore in piercing, where an accurate size of hole is required, the punch is made to the required hole size and the clearance is made on the die. 

Fig. 17.17(a). The punches descend and the hole for the first workpiece is pierced. The punches are then raised. The first sliding stop is then retracted, the second sliding stop is pushed in, and the work material is pushed against it. 

Fig. 17.17(c), again positioning the pierced hole under the blanking punch. The operation is then repeated, a completed workpiece being produced at each stroke of the press. 

The life of a punch, pierce, or blank die should be evaluated with reference to the various copper-clad materials that may be used. One way to evaluate die wear caused by various materials is to weigh the perforators, or punches, very accurately, punch 5,000 pieces, and then reweigh the punches. Approximately 5,000 hits are necessary for evaluation, because the initial break-in period of the die will show a higher rate of wear. Also, of course, the quality of the holes at the beginning and end of each test must be evaluated. Greatly enlarged microphotos of the perforator can be used for visual evaluation of changes in the die. 

Flash-free, molded holes can be produced during stamping. When the hole is vertical or parallel with the press ram motion, fixed pierce punches are used. The punches must be located on the side from which the part is to be stripped. As a rule, the part will bind on the protruding punches, eliminating the need for additional undercuts. The part will nearly always stay on the mold half with the punches. 

Be designed with 0.001 in. total clearance with hole-in-mold button. The holes for the mold button and pierce punches must be line-bored to obtain the close tolerances needed. 

A hydraulically driven hollow pierce punch can be used to mold a hole on a sidewall. The following design parameters are recommended (see Fig. 7-23)" ... 

The hollow pierce punch should start moving within about 2-5 deg before the press reaches bdc. A longer delay will prevent formation of molded walls of the hole. 

The hollow pierce punch should remain in its most forward position for at least 5 s. This time is needed to cool the slug/hole walls sufficiently to prevent resin flow and subsequent welding. 

Tubular rivets. This is a standard solid rivet with a hole drilled along the length of its shank to a depth of over 112 % of the shank s diameter. The idea of a hole is to allow for a recess that will accept punched out material as the rivet was originally designed as a self-piercing fastener. 

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