3 Fundamentals of press design

466 Solid forming (Forging)
– small variations in diameter on the inside and outside surface of the
formed components should be avoided;
– material overflow must be provided for, due to fluctuations in billet
weight (e. g. during forward rod extrusion in the rod length, during
backward cup extrusion in the cup height);
– transition radii must be configured as large as possible; sharp edges
can only be pressed by using split dies;
– undercuts can be produced, but increase tooling costs;
– it is generally not possible to press plunge cuts, parallel key grooves,
transverse bores etc.;
– slip-on gearing can be produced by pressing, high-precision involute
gearing (running gears) can be produced with increased die and process
development effort.
The process engineering factors depend on the extrusion technique,
the selected processing steps, the workpiece material, the permissible
die loading and the respective tri-axial stress state. Process engineering
considerations may restrict the range of part geometries that can be
formed from billet (cf. Sect. 6.5).
Volume calculation
Following transformation of the end product (assembly ready part
geometry) into a formed part geometry, volume calculations are made
for the relevant geometry. In practice, the volume is calculated by
dividing the overall part geometry into simple volumetric elements,
such as cylinders, truncated cones or rings, whose calculated individual
volumes are added to obtain the total part volume. The calculation
must take into account punching and shearing scrap which allows for
material overflow during the forming process. Experience has shown
that the volume calculation of transition radii can be neglected, as
these volumes partially cancel each other out. A far more significant
influence is due to volumetric variations that result from the elastic
deflection and expansion of forging dies or material overflow which is
difficult to calculate. Complex rotationally symmetrical 2D geometries
are calculated easily using CAD and according to Guldin’s law.
Extremely complex 3D geometries are handled using the volumetric
calculation modules of 3D CAD systems.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998