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92 Fbrging-Stamping - Heat Tiearing
turned up between the punch and the die and leaves
FIG. 13- -Gear shift control base for automobile
transmission case
an even and true edge. The tool steel die is subject
to considerable abuse due to the heavy blow of the
punch. The die is therefore built up of a number of
pieces, any one of which may be replaced at much
less expense than if it were made of one solid. The
labor in shaping and fitting a die of this sort runs
high so that the cost of replacement must be considered.
A more difficult form of drawn part is the conical
shaped stamping shown in Fig. 13, being the gear shift
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FIG. 14—Die for first draw on gear shift control base.
control base mounted on top of an automobile transmission
case. The upper portion of this part is symmetrical
but the lower portion blends into an irregularly
shaped rectangular form before flattening out
into a flange or base.
The determination of the blank for a shape of this
sort is most difficult, but it can be estimated quite
March, 1925
accurately. The area of the blank required corresponds
approximately to the surface area of the finished
stamping allowing for scrap. It is apparent
that the blank will be a modified rectangle, but inasmuch
as the exact shape cannot be found until the
drawing dies are finished, the estimate is based on a
circular blank.
A section is laid out corresponding to the conical
and to the diagonal of the
lower portion. The area of a part of this cross section
may be found by splitting it up into horizontal sections.
The upper portion being cone shaped its area
is readily determined. The portion below may be
considered a cylinder which area may be easily computed
as well as that of the flat flange. The summation
of these areas gives the area of the approximate
blank required. The check on this computation is
made when the drawing dies are completed and before
the blanking die is made. Some variation would be
found in a difficult part like this, although the more
FIG. IS—Fifth reducing operation for gear shift control base.
symmetrical a part may be, the easier it is to make
an accurate estimate of the blank size.
A tapered shape is among the more difficult to
draw and in this particular part it is complicated by
the irregularities of the base. A good set of tools
will produce a part of uniform thickness throughout
and with a surface free from draw rings, the mark of
successive drawing operations. The experience of the
tool designer plays a big part here for only the proper
selections of drawing operations will produce best results.
The blank size for a cylindrical shell may be figured
approximately from the formula
D = \/d2 + 4dh
ivhere D is the diameter of the blank, d the diameter
of the shell and h the height of the shell. This method
does not take into consideration the "stretch" of the
metal in drawing or the thickness or quality of the
metal. Variations in the details of the die construction
may vary the result, but for all practical purposes this
formula is sufficiently accurate.