3 Fundamentals of press design

476 Solid forming (Forging)
Die angles range between 10 and 45° and generate tapers with shoulders
between � = 5 and 22.5° on the pressed part (cf. Fig. 6.1.2). The
punch load is of minor importance, and depends on the billet material.
It should not upset the billet plastically, and cause buckling stresses in
slender billets. Neither of these stress values should be exceeded. The
process is highly sensitive to fluctuations in the strength of the starting
materials and to the quality of lubrication, i.e. phosphating and soaping.
6.5.5 Ironing
Ironing is a process whereby a tubular part with a bottom (e.g. backward
extruded or drawn cup) is drawn to reduce the wall thickness (cf.
Fig. 6.1.2). This is a process in which the force is introduced mainly
through the bottom of the workpiece and partially by the mandrel friction
force. As a result, in addition to compressive stress at the die shoulder
inlet, tensile stress also occurs after exit from the die in the wall of the
workpiece (drawing process). This is in direct contrast to reducing, in
which the force and thus also the compressive stress occurs in the workpiece
cross section before the die shoulder inlet (press through process).
The part shape is generated by the drawing mandrel and the ironing (die)
ring. It is also possible to arrange several ironing rings one after the other.
The maximum degree of true strain achievable when ironing steel is
around � = 0.5 (� A = 40%) depending on the material characteristics.
For difficult to form materials this value drops to � = 0.35 (� A = 30%).
Die opening angles may range up to around 45°, which results in tapers
up to 22.5° in the finished part (cf. Fig. 6.1.2) Customary die opening
angles for ironing range between 5 and 36°. A minimum punch force is
achieved at an ironing angle of around 12°, when the probability of
wall fracture is minimum. Ironing is a process in which an extremely
good surface quality and excellent tolerances are achieved, and it is,
therefore, used frequently as a sizing operation.
6.5.6 Upsetting
Upsetting is defined as “free forming”, a process in which the part
height is reduced, generally between flat parallel die surfaces (upsetting
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998