3 Fundamentals of press design

Materials, billet production and surface treatment
When using bar and flat material, the billets are generally sheared by
the forger and subjected to heat and surface treatment. Strain-hardening
effects which occur partially at the flat surfaces of the billet due to
the shearing process are thus eliminated prior to forging.
Billet separation
The most commonly used billet separation process is shearing. Slicing
on the trimming lathe is used only rarely, for example for test purposes.
Blanking and fine blanking are explained in Sects. 4.5 and 4.7.
Shearing is characterized by practically loss-free separation at an
extremely high level of output, in terms of quantity. The shear blade
plastically deforms the material until its deformation limit in the shearing
zone has been exhausted, shearing cracks appear and fracture
occurs. With all four shearing principles – without bar and cut-off holder,
with bar holder, with bar and cut-off holder and with axial pressure
application – plastic flow lateral to the shearing direction is increasingly
prevented, while compressive stress increases during the shearing
operation. Both tendencies exercise a positive influence over the geometry
(ovality, tolerance) of the sheared surface. The most accurate billets
are produced using the shearing principle with bar and cut-off holder.
For cold forging, the sheared billets should have the greatest possible
rectangularity, volume control and little plastic deformation. The
sheared surfaces should be free of shearing defects and exhibit only a
moderate amount of strain-hardening. The appearance of the sheared
surfaces is the result of interactions between workpiece characteristics,
tool, machine and friction. The shearing clearance exercises a major
influence here. The greater the strength of the steel, the smaller is
the shearing clearance (cf. Fig. 4.5.12). However, aluminium and lead
always require a small blanking clearance. The following values may be
taken as a guideline for the shearing clearance of steel:
soft steel types 5 – 10 %
(of the starting material diameter in mm)
hard steel types 3 – 5 %
brittle steel types 1 – 3 %
Rough fractured surfaces, tears and seams indicate an excessively wide
shearing tool clearance. Cross fractured surfaces and material tongues
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
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