3 Fundamentals of press design

Materials, billet production and surface treatment
ranges from 5 to 40 mm in diameter. Wire diameters of up to 50 mm are
processed in individual cases, although the handling capability of the
wire bundle becomes increasingly difficult as the coils increase in weight.
Low-cost rolled wire in accordance with DIN 59 110 and 59 115 has
a wide diameter tolerance and a rough surface finish. It therefore
requires preliminary drawing on a wire breakdown machine. The
reduction in cross section should be between 5 and 8%. The wire must
be annealed, de-scaled and phosphated.
Drawn wire after DIN 668 K (K = cold drawn) is used to produce parts
with a lower true strain (small reduction in area), as the material has
already undergone a certain degree of preliminary strain hardening
during the drawing process. This material also has to be descaled, phosphated
and possibly also lubricated (stearate drawn). When a higher
degree of true strain is involved (e.g. cup backward extrusion), after preliminary
drawing, annealing (G) or spheroidized annealing (GKZ) must
be performed in order to restore the original material formability. The
form of delivery is specified with the designations K+G or K+GKZ. Light
oiling helps prevent rust formation on the wire. In the case of highalloy
steel qualities, a further finish drawing process takes place after
annealing with cross section reductions of less than 6%. The form of
delivery is then specified as K+G+K or K+GKZ+K.
In practice, bar stock can also be used economically in the larger
diameter ranges. With diameters of up to 70 mm, bars of between 6 and
12 m in length are produced. Hot-rolled round steel to DIN 1013 and
DIN 59130 is the most favorable selection in terms of cost.
The allowable diameter tolerances cannot be reduced by drawing,
leading to volume variations. These must be fully compensated for by
excess material during the forming process. For a component weighing
around 1.3 kg with a diameter of 46 mm, a weight tolerance of 88 g
(average deviation) or 45 g (precision deviation) must be integrated into
the die if the admissible tolerances are fully utilized. The transformation
clearance in the first stage has to take account both, the diameter
tolerance and also the ovality of the billet caused by the shearing
process. For a material with a diameter of 46 mm, the first transformation
clearance will be approx. 2.5 mm.
Where round steel is being cold forged, the descaling, phosphatizing
and lubricating operations take place prior to forging. In the case of
warm forging, the billets are fed directly to the heating line.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
455