Blanking and Punching Dies
115
If the die block has a shape similar to that in Fig. the bending stress may be calculated by the formula:
This method of calculation is approximate, but for practical use, it is good enough. More precise calcula- tions are based on mechanical theory.
Example: For example, it may be required to calculate the dimensions of the rectangular die block of heat treat-
ed alloy tool steel. The die block is supported by two supports at a distance of 1
=
120 mm. The workpiece material has a thickness of T
=
3 mm and
=
372 The workpiece dimensions are a
=
100 mm and
=
120 mm. Solution:
c
= The height of the die block is:
The height of the die block can be rounded to
=
40 mm. The wall thickness
e
is: e
= The value of
e
may be rounded to e
=
50 mm. The die block dimensions are:
A
=
a
+
2e
=
100
+
=
200 The blanking force is:
F =
+
= The bending stress is:
B
-
b H
=
0.75
= =
300
These dimensions are acceptable because
=
=
116
Blanking and Punching Dies
9.3 PUNCHES
Standard punches are available for a wide variety of round, oblong, and square holes. The manufacturers furnish these punches in standard sizes as well as to special order. The main considerations when design-
ing punches are 1 they should be designed so that they do not buckle; 2 they should be strong enough to
withstand the stripping force, and 3 they should not be able to rotate as a result of the cutting action.
9.3.1 Punch Face Geometry
It is possible to control the area being sheared at any moment by making the punch and die surface at an angle beveled, as shown in Fig. 4.7. In Fig. 9.6, several types of punch face geometry are shown:
a Flat face surface, b Concave face surface,
c Bevel face surface, and d Double bevel face surface.
If the surface of the punch and die are flat, the punch force builds up rapidly during cutting because the entire cross-sectional area is being cut at one time. The punch face geometry in Fig. 9.6 is particular-
ly suitable, with an adequate shear angle on the die, for shearing thick blanks because it reduces the force required at the beginning of the stroke. The angle also reduces the noise
combinations are: a Flat punch
- double bevel die b Concave punch
- flat die c Bevel punch
- flat die d Flat punch
- concave die.
Fig. 9.6
Types of punch face geometry: a flat, b concave, c bevel, and d double bevel.
It is possible to use other combinations depending on the purpose of the die. When blanking soft and thin material, a tubular punch without a die block may be used.
9.3.2 Methods for Assembling Punches
There are many methods for assembling blanking and piercing punches on a punch holder. In Fig. 9.7 are shown several forms at one end of the punches for such assembly.
Piercing punches are smaller in cross-section and generally longer than blanking punches. They must be designed to withstand shock and buckling. Because of the high probability of damage, they must be
designed so that they can be easily removed and replaced. Deflection or buckling of punches may be avoid- ed by making the body diameter of the punch larger than the cutting diameter or by guiding the punch
Blanking and Punching
Dies
117 Bushing
Punch
Fig.
9.7
Forms at one end of piercing punches.
through a bushing as shown in Fig. and Fig.
Sometimes it is necessary to insert a hardened back- ing plate between the head of the punch and the punch holder. Whether or not it is necessary to use a back-
ing plate is dependent on the specific pressure between the head of the punch and the punch holder. If the following condition is satisfied,
F 4F
-
=
a backing plate is not necessary Fig. However, a backing plate is necessary under the conditions
illustrated in Fig. 6
5
- ..
-die 2-stripper
3 -punch 4-punch plate
5-backing plate 6-punch holder
7-work material Fig. 9.8
Assembling punch a without backing plate, b with backing plate.
Because of the high probability of damage to piercing punches, they must be designed so that they can be easily removed and replaced. In Fig. 9.9 are shown four methods of designing such punches, which
all allow for quick removal and replacement at the punch plate. a Fastening with ball and screw. This type of fastener is used with complex dies.
b Fastening with a ball under spring pressure. The punch can be released by pushing the ball through
hole a. This type of fastener is used on dies for punching holes of d
=
3 to 30 mm in material thickness up to 3 mm.
