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.
118
Blanking and Punching Dies
Section
A-A
d
Methods for assembling punches.
1 -punch
2-punch plate 3-backing plate
4-screw 5-ball
6-spring a-hole
c Fastening with a screw. Screw fasteners are used for simple dies, and the punch is fastened direct- ly
on the ram of the press. The domain of application is for punching holes d
=
20 to 60 mm.
d Fastening with a specially shaped screw head. When the screw is turned 90 degrees, the punch is freed
Sometimes, it is more economical to use a machine steel spacer and a tool steel plate or ring for a punch than it is
to
make a large blanking punch out
of
one piece
of
tool steel. At other times, it may be easier and more economical to make punches out of sections and fit them
into the desired pattern than it is to attempt to make the punch from one piece. Such a design may be desir- able when the punches need to be large or irregularly shaped. Another advantage is that a section may be
removed and replaced if it becomes worn or broken.
9.3.3 Punch Calculations
By using the ratio between the length and the cross-section area of the punch, the dimensions of the punch may be calculated in two ways:
Compression stress. For short punches may be calculated by the formula:
F A
where: F
=
punching force
A
=
punch cross section area
=
permissible compression stress. For hardened tool steels, steels,
=
980
Blanking and Punching Dies
119
b Buckling calculation. If the punch is fixed at one end as shown in Fig the critical force
should be calculated initially using the Euler formula:
41
=
where: Imin.
=
minimal second moment of area, 1
=
free length of punch, E
=
modulus of elasticity. If critical force
equals punch force then the maximum length of the punch may be calculated by the
following formula:
where: L
=
length of cut, T
=
thickness of material. For a punch fixed at one end and guided at the other end, as shown in Fig.
the critical force may be calculated by the formula:
=
The critical force exerted by a guided punch is 8 times greater than that exerted by a free-end punch. Consequently, the maximal length of a guided punch is
=
2.8
times larger than that of a free-end punch. Special attention needs to be paid to the design of the die for punching small-diameter holes in thick
material because greater than allowable compression stress in the punch may occur. For punching any material where the shearing stress is
the punch diameter must be greater than the thickness of the workpiece.
cr
Fig.
Different end conditions: a one end is fixed, and the other end is free.
b
one end is fixed, and the other end is guided.
120
Blanking and Punching Dies
MATERIAL Types
of work processes
THICKNESS Simple punching or
Compound punching or Punching and
at blanking
g same
time
9.4 STRIPPER PLATES
When a punch shears its way through a work piece, the material contracts around the punch to the degree that it takes a substantial force to withdraw the punch from the material.
Efficient removal of the workpiece and scrap from the die increases productivity, quality, and work-
place safety.
,
9.4.1 Stripper Force
The force required to strip the material from the punch, as shown in Fig. 9.1 1, may be calculated by the
following equation: 9.10
where:
=
stripping constant F
=
punch force. Values for
are given in Table 9.3.
to
I I
I
1.0 to
5.0 0.06 to 0.08
12 0.12 to0.15
Over 5.0
0.08 to 0.
0.15 to0.20
1 2
1
-punch plate
3-punch plate 4-stripper spring
5-workpiece 6-stripper
7-die
Fig 9.1
Schematic illustration showing positioning
of
the stripper.
Blanking and Punching
Dies 121
9.4.2 Stripper Design
Generally, there are two types of strippers: the solid stripper and the elastic stripper.
a Solid stripper. The cheapest and simplest design for a stripper is shown in Fig. 9.12. Two additional
methods for mounting solid stripper plates are shown in Fig 9.13. In Fig. a spacer is provided to
raise the stripper so that there is clearance between the work strip and the stripper plate. In Fig. the
stripper plate has had a channel milled into its lower surface. If a die uses a guide system with a guide plate, as shown in Fig. 9.24, the guide plate has the function
of a stripper plate. In all types of solid strippers for stripping scrap or workpieces, the force of the press is used for the stripping operation.
1
-die ring
3 -stripper
4-ram motion
5-frame of press
Fig. 9.12
Schematic illustration of mounting a simple solid stripper. 1
I
1 -punch
2-die 3-stripper
4-work strip
Fig. 9.13
Schematic illustration of mounting a solid stripper: a stripper plate fixed on one side; b stripper plate fixed on two sides.
b Elastic
stripper.
Sometimes it is desirable to hold the scrap strip in a flat position before the punch makes contact with the workpiece. This arrangement is advisable when it is necessary to be very accurate,
when punching very thin material, or when thin punches are used. These types of strippers use a com- pression spring or rubber pad to produce the stripping force. The stripper is generally made from a plate
that provides the desired configuration and is suspended from the punch holder with stripper bolts and compression springs. An example of such a stripper plate is shown in Fig. 9.14.
There are many ways of retaining springs. In Fig. 9.14, both the punch holder and the stripper plate are counterbored to provide retainers. However, a counterbore in the stripper plate is often not possible.
