3 Fundamentals of press design

Fine blanking
case hardening nitrited steel, heat-treatable steel, tool steel, rust-proof
ferritic steels, rust-proof austenitic steels and special steels (cf. Sect.4.2.2).
These steels can be processed in the form of hot or cold strip or flat bar
steel. Flat products must be free of scale (Table 4.7.1).
Material stress and properties
There is an underlying difference in the material stress which occurs
during fine blanking and standard blanking. This corresponds to a flow
shearing process which is only possible if certain specific material properties
are fulfilled. The grains of the metal microstructure are subjected
to a marked cold forming process by the blanking punch prior to the
point of separation. This grain deformation calls for a formable material
with a high formability, i.e. with a high degree of ultimate elongation
and fracture necking.
The flow processes taking place in the shearing zone and the work
hardening are illustrated in Fig. 4.7.9. The cold forming of the grains
increases from the die-roll to the burr side of the part, and decreases
from the cut surface towards the center of the workpiece. At a certain
distance from the surface, the grains which make up the microstructure
are no longer affected by the fine blanking process.
The fine blanking capability of steels is determined on the basis of
their chemical composition, their degree of purity, their microstructure,
their treatment and the prevailing mechanical and technological con-
sheet metal thickness [mm]
4
3
2
1
Fig. 4.7.9 Material flow (left) and work hardening (right) in the shearing zone
0.05mm
0.15mm
0.30mm
0.50mm
1.00mm
0 100 200 300 400
hardness[HV 0.2]
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
die-roll side
sheet metal
thickness
burr side
distance from cut surface
339