Blanking and Punching
39
4.5
SHAVING
Sometimes it is necessary to achieve a very clean hole. In order to do this, a process called shaving is used. This is the process of the removal of a thin layer of material with a sharp punch. It can be used with a
blanked and punched workpiece.
4.5.1 Shaving a Punched Workpiece
The edges of punched pieces are generally unsquare, rough, and uneven. This process is particularly use-
ful
in: improving the surface finish of a punched hole, and
improving the dimensional accuracy of punched parts and distances between holes.
Shaving may be done as a separate operation or it may be incorporated into one station along with punch- ing in a progressive die. A thin ringed layer of metal is removed by shaving Fig. 4.12.
Scrap
\
Fig. 4.12
Phases of shaving punched hole
It is necessary, of course, to provide a small amount of stock on the punched work piece for subsequent shaving. This amount is:
4.1 1 The value of is 0.15 to 0.20 mm for a previously punched hole and 0.10 to 0.15 mm for a previously
The diameter of punch in Fig. 4.13, can be calculated from the formula drilled hole.
4.12 where:
=
diameter of hole after shaving,
=
production tolerance of the hole,
=
amount of compensation for tightening of the hole area after shaving.
40
Blanking and Punching This amount is:
i
=
0.007 to 0.017 mm for brass,
i
=
0.005 to 0.010 mm for aluminum,
i
=
0.008 to 0.015 mm for low-carbon steel.
The diameter of the die needs to be 20 to 30 bigger than the diameter of the punch; that is:
=
1.2
to
1.3
4.13 When shaving, the following should be considered:
Always maintain the recommended close shave clearance for the shave operation, One of the natural problems with shaving is keeping the scrap out of the die. For removing this
scrap, Arnold recommends the patented Bazooka Bushing, which creates a vacuum force to pull the scrap out of the die button.
Another problem with shaving in automatic dies is progression control. French notching or trim- ming both sides of the strip in the die plus adequate piloting should be considered before beginning.
Workpiece Punch
Fig.
4.13
Schematic illustration
of
an addition
for
shaving, and production tolerance
of
the hole.
The shave force can be calculated from the formula:
where:
A
=
cross-section of ringed scrap in Fig. 4.13:
A
=
+
+
+
-
p
=
specific pressure Table 4.5 gives the value for specific pressure
Blanking and Punching
0.4 to 0.5 0.6
to
2.0 2.1
to
3.0
41
0.10 0.15
0.20 0.25
0.27 0.30
0.05 1961
1765 1560
1375 1175
1080 980
2100 1900
1670 1130
1225 1125
1030 2250
2010 1765
1520 1275
1080
Table 4.5 Value for specific pressure p
RINGED SCRAP CROSS-SECTION
Material
thickness
Punching and shaving operations can be done with the same punch at one stroke of the press. For such combinations, different shapes of punches may be used depending on the workpiece material. The gradual
punch shown in Fig. 4.14 is used for shaving brass, bronze, and aluminum alloys.
Workpiece Punch
Fig
4.14
Gradual punch for punching and shaving.
The first step of the punch is a smaller dimension, punching the work material, and the second level of punch is a larger dimension, shaving the hole. At the punching stage, the material is sheared by plane
AB, between the cutting edges of the die and the first punch level. If the punch is given the shape of a conic section, as shown in Fig
4.15, the process is completed in two phases. In the first, the punch penetrates into the material to a depth of
0.5 to but the materi-
al does not divide. In the second phase, the first crack appears and the scrap begins to slip. This method improves the surface finish of the punched hole. Another advantage of this type of punch is its ability to
be sharpened many times.
Punch
Fig.
4.15
Frustum cone punch for punching and shaving
