IDEMI

5.1    WIRE METHOD OF LOCATING THE CENTER OF PRESSURE:

The center of pressure of a blank contour may be located mathematically, but it is a tedious computation.  Location of the center of pressure within 12 mm of true mathematical location is normally sufficient.A simple procedure accurate within such limits is to bend a soft wire to the blank contour.  By balancing this frame across a pencil, in two coordinates, the intersection of the two axes of balance will locate the desired point.  As an example of the marked influence this factor may have on tool design, a rather unusual blank is shown in Fig. 5-1.  Here, the center of pressure is near one end of the blank, and will require the indicated imbalance in the press tool design.

Die clearance is the space between the matting members of a die set. Proper clearance between cutting edges enables the fractures to meet and the fractured portion of the sheared edge has a clean appearance. For optimum finish of a cut edge, proper clearance is necessary and is a function of the kind, thickness and temper of the work material.

At Fig. 5-2, which shows clearance C for blanks of a given size, make die to size and punch smaller by total clearance 2C. At B, which shows clearance for holes of given size, make punch to size and die larger by the amount of the total clearance 2C

The application of the clearance for holes of irregular shape is diagrammed in Fig. 5-13; at B the hole will be of punch size, while at A the blank will be of the same dimensions as the die.

5.3    DIE BLOCK GENERAL DESIGN:

Overall dimensions of the die block will be determined by the minimum die wall thickness required for strength and by the space needed for mounting screws and dowels and for mounting the stripper plate.

Wall thickness requirements for strength will depend on the thickness of the stock to be cut.  Sharp corners in the contour may lead to cracking in heat treatment, and so require greater wall thickness at such points.

Two, and only two, dowels should be provided in each block or element that requires accurate and permanent positioning.  They should be located as far apart as possible for maximum locating effect, usually near diagonally opposite corners.  Two or more screws will be used, depending on the size of the element mounted.  Screws and dowels are preferably located about 1 ½ times their diameters from the outer edges or the blanking contour.

Die block thickness (see Table 5-1) is governed by the strength necessary to resist the cutting forces, and will depend on the type and thickness of the material being cut.  On very thin materials 13 mm. thickness should be sufficient but, except for temporary tools, finished thickness is seldom less than 22 mm., which allows for blind screw holes and also builds up the tool to a narrower range of shut height for press room convenience.

5.4    DIE BLOCK CALCULATIONS:

Method 1 (“Rule of Thumb”). Assuming a die block of tool steel its thickness should be 20 mm. minimums for a blanking perimeter of 75 mm. or less 25 mm. thick for perimeters between 75 mm. and 250 mm. and 32 mm. thick for larger perimeters. There should be a minimum of 32 mm. margins around the opening in the die block.

The die opening should be straight for a maximum of 3 mm; the opening should then angle out at ¼° to 1 ½° to the side (draft).  The straight sides provide for sharpenings of the die; the tapered portion enables the blanks to drop through without jamming.

To secure the die to the die plate or die shoe, the following rules provide sound construction:

1)    On die blocks up to 175 mm square, use two M10 cap screws and two dowels of dia. 10 mm.

2)    On sections up to 200 mm. square, use three cap screws and two dowels.

3)    For blanking heavy stock, use cap screws and dowels of dia 12-mm. diameter.  Counterbore the cap screws 3.2 mm. deeper than usual, to compensate for die sharpening.