Blanking and Punching Dies
125
where:
=
strip width, c
=
clearance. The clearance for fixed guide rails is
=
0.25 to 0.75 mm. For the elastic-type guide rails see Fig. the clearance is c
=
2.5 to 4.0 mm. The tunnel height
H
in
Fig. 9.18 provides clearance in the vertical direction. This dimension depends
on the material thickness and method of work-strip feeds manual or automatic. Advisable values for H and h are given in Table 9.4.
Table 9.4 Values for
and h.
Automatic
feed
Sometimes, to provide smooth movement to the workpiece in the production of large and complicat- ed parts, four guide pins two on each side may be used instead of guide rails see Fig. 9.4.
1
-guide rail 2-stop
pin
3-punch 4-die
5-guide plate 6-work strip
A
Fig. 9.18
Schematic illustration
of fixed guide rails. In compound dies, the elastic type of guide rails are used. Fig. 9.19 shows
two types of design for elastic guide rails. These designs provide positive guidance of the work strip, and facilitate feeding through
the die.
126
Blanking and Punching Dies
I
1-elastic guide
3-work
Fig. 9.19
Elastic guide rails.
9.5.2 Die Stops and French Notch Punch
a Die stops are used to stop the material strip after each feed movement is completed. Solid pins, with or without heads, may be used as stops. The pin should be lightly press-fitted into the die shoe and
should extend above the die block face. The extension h of the die stops is a function of the thickness of the material. The value of h is given in Table
9.4. A clearance hole should be provided in the stripper plate
and under the pin in the die shoe for removal of the pin when necessary. b The French notch punch is used for trimming away a length of work strip that is equal to the pro-
gression of the die. This action provides a fixed stop feature for strip progression such as is shown in Fig. 9.20. French notching is one of the best ways to control the problems of strip width tolerance, strip
1 -French notch punch
2-punch 3 -punch
4-work strip
I
5-stop
Fig. 9.20
Die with French notch punch.
Blanking and Punching
Dies
127
Material
thickness
Up
to
1.5 Over
1.5
ber, and progression control. If French notching is used on only one side of the strip, a pilot punch must be used to allow the strip to back up 0.05 mm.
The value of the notch width
w
is a function of the thickness and kind of material. These values are given in Table 9.5.
Kind of
material Steel
Other
softer
materials
2 to 3T 2T
Sometimes the back gage is extended and used to support a stop. This arrangement is especially use- ful with long workpieces. Such a stop is shown in Fig 9.2
1. Finger stops are used to stop new strips in the proper location in a die. They are operated with the finger by pushing them into the stock channel until
they seat. The press is tripped when the stop is released to its “out” position and it is not used again until a new strip is started. One type of finger stop is shown in Fig. 9.2 1. The die shown is sometimes used for
punching holes and cutting workpieces.
Cutoff
punch
Fig. 9.2
Compound die for punching holes and blanking long parts with finger stop and adjustable stop.
9.5.3 Positioning the Individual Blank
There are many design options for positioning individual blanks or workpieces. The selection depends on the shape and dimension of the workpiece. Fig. 9.22 shows three methods of positioning the individual
workpiece by using:
a Three dowels, b A ring, or
c A combination of dowels and guide-rails.
128
Blanking and Punching
Dies
Fig. 9.22
Methods
of
positioning the individual blank
or
workpiece:
with
dowels, b
with
ring, c
with
rail.
9.5.4
Pilots
Pilots are used in progressive and compound dies to position the work strip so that the relationships
between stations or previously punched holes and the outside blanked contours of work pieces may be maintained. In Fig. 9.23 are shown various methods for mounting pilots in punches.
The hole that receives the pilot should be extended through the punch so that the pilot may be
removed if it breaks. The pilot should fit the work hole with a tolerance of from 0.02 to 0.15 mm. Pilots are generally made
from tool steel, hardened and polished. They may have a radiused end that terminates in the diameter of the pilot, or the end of the pilot may be blunt.
3
1 1 -punch
5-backing plate 9-spring
2-punch holder 6-die
1
0-screw 3-punch plate
7
-stripper
1
l-die shoe 4-pilot
8-bushing 12-work strip
Fig. 9.23
Various methods mounting
of
pilots
in
punches.
