3 Fundamentals of press design

356 Sheet metal forming and blanking
pins 7/8 and positive press-off pins 4 are required. The coil guide not
only ensures coil guidance but also stripping of the scrap web from the
pilot pins 7/8. The initial blanking stop 3 ensures precise initial blanking
of the sheet metal at the start of a coil or strip. The pilot pins 7/8 are
responsible for precisely positioning the sheet metal in each die station,
i.e. ensuring that the prescribed feed step is adhered to precisely. The
task of the positive press-off pins 4 is to compensate for the counterpressure
so that it does not have to be absorbed fully by the individual
piercing punches 6 during initial blanking in the first die station.
Die design
The factors which determine the design layout of a fine blanking die are
the geometrical shape and size of the part and the type and thickness of
the sheet metal being processed. These parameters determine both the
type of die used and also the die system in general.
Further definition of the type of die design must be carried out
according to the following conditions, which are basically the same as
those also valid in standard blanking. In order to achieve optimum
blanking conditions, the line of application of the slide force must
coincide as far as possible with that of the blanking or forming force (cf.
Sect. 4.5). The more precisely this condition is complied with, the better
the blanking result and the lower the degree of die wear.
The part drawing must contain all the dimension and tolerance specifications,
and the cut surface properties of the part must be indicated
(Fig. 4.7.17). Depending on the type of die used, the strip layout must
indicate the position of the part in the sheet metal or the processing
sequence (Fig. 4.7.2 and cf. Sect. 4.1.1). The most important aspect of
the strip layout on the sheet metal is always the achievement of optimum
material utilization (cf. Fig. 4.5.2 to 4.5.7). The strip layout is also
used to define the strip width and feed step.
Calculation of press forces
The following forces are exerted during fine blanking: the blanking
force, vee-ring force, counterforce, stripping force and ejection force
(Fig. 4.7.3). The first three forces determine the configuration of the
press, adding up to create the total machine force:
FGes = FS + FR + FG[ N]
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998