3 Fundamentals of press design

Fine blanking
s
h S1
tearing E
fracture
b A
h S2 h E
b E
achievable smooth cut ratio
h S1/sin%
100 100 90 75 50
h S2/sin%
90 75
Fig. 4.7.16 Terms defining the fine cut surface:
h S1 min. smooth cut section in % of the sheet metal thickness s in case of fracture;
h S2 min. smooth cut section in % of the sheet metal thickness s in case of shellshaped
fracture; b A shell-shaped fracture width (sum of all b A can be defined as
required by the user); h E die-roll height; b E die-roll width; E admissible tear in
accordance with VDI Guidline 3345/2906 part 5 relative to size no. 1, 2, 3 or 4
In the part drawing, the designer is required to define the necessary
smooth cut ratios, surface roughness characteristics, admissible tearing,
die-roll heights and widths and also blanking burr formation. The specifications
are made either on the basis of the cut surface standard
according to VDI 3345
R a
h h
s
RZDIN E
1 2
h
S1
E
h
S2
S S
s
or as per VDI 2906 page 5: whereby E represents the admissible
degree of tearing according to VDI 3345 in accordance with no. 1, 2, 3
or 4. In order to avoid the need for repeated complex specifications, the
blanked surfaces are marked with simple symbols z y ( , ,…)
, starting
with the last letter of the alphabet (z). The length of the cut surface is
indicated by a dotted line (Fig. 4.7.17).
Dimensional and form tolerances
The achievable tolerance levels depend on the material, the workpiece
thickness and geometrical shape of the part. The blanking press, the die
and the lubricant used are also significant in determining the achievable
part quality. The guideline values for achievable tolerances are
indicated in Table 4.7.2.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
347