Press sliding punch

The pressworking process is carried out by placing the sheet metal between a punch and die mounted in a press. The punch is attached to the moving part - the slide or ram - which applies the necessary force at each stroke. The die, correcdy aligned with the punch, is attached to the fixed part or bedplate of the press. 

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. 

In order to ensure perfect alignment in the press, the punch plate and the die are secured in a die-set Standard die-sets are available in steel or cast iron, and a typical example is shown in Fig. 16.13. The top plate carries a spigot which is located and held in the press slide. The bolster contains slots for clamping to the press bedplate. The bolster has two guide pins on which the top plate slides up and down through ball bushes which reduce friction and ensure accurate location between the two parts. 

In rotary tablet presses, only the dies are fixed in the die table, whereas the punches are vertically mobile and are not fixed to a holder. They slide up and down in the turret bores, driven by cam tracks guiding the punch heads. The dwell time, i.e., the period during which the tablet is under full compression, is determined by the diameter of the flat part of the punch head. 

As discussed in Section 8.2 (Fig. 8.3), interparticle and wall friction, force dissipation from particle to particle, and sliding under shear cause differences in density distribution in a compact that is produced in a die with one-sided compression by one of the punches. To obtain a somewhat more uniform structure, compaction can be carried out by both punches. If both move at the same rate and for the same stroke length, thus exerting identical forces, and assuming uniform filling of the die, a mirror image of the density distributions that were shown in Fig. 8.3 (Section 8.2) develops along a neutral plane which, under those conditions, is located in the middle of the compact. Machines that operate in this manner may be identified as presses with double pressure (see also below). 

In operation, the vertical handle is grasped and the handle is partially rotated. This provides, through the multi-start thread, a vertical movement of the slide. A punch and die fitted in the slide and on the bed, and correctly aligned with each other, are used to carry out the required press-working operation. 

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