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214 Forging - Stamping - Heat Treating
ming of forgings and of drawn shells, where there is
just a flange with an irregular edge to be trimmed
to shape by pushing through the die, the construction
followed i^ similar to that in the case of blanking
dies. For trimming shells with straight sides or with
hinged lugs or notches, especially where a square edge
is required, dies are sued which fit the inside and the
outside of the shell. They are held in proper relation
for cutting and given motion in four directions,
to give a smooth edge. The machines in which such
work is done arc known as flat edge trimming machines.
Blanks cut bv ordinary blanking dies have an edge
which is partly cut and partly broken due to the shearing
action after the punch has penetrated only a part
of the distance. Therefore for greatest strength or
where accuracy or square finish are required it is
necessarj to trim or shave the blank. Shaving dies
are built verv much like blanking dies but practically
FIG. 1—Single action double crank press equipped with dies
and a spring pressure drawing attachment.
without clearance, and arc designed to lake off say
.005-in. the first time, and for extreme accuracy .003in.
in a second cut and ,002-in. in a final cut. The
figures mentioned would apply to a blank of say T,s-in.
thickness. An even higher degree of accuracy and finish
is obtainable by pushing the shaved blank through
a burnishing die with round edges and highly polished
surface and made about .001 to .0015 tight according
to the hardness of the material.
It is best on all shearing and blanking work to use
a press with just as short a stroke as is possible. The
velocity with which the punch strikes the material has
a decided bearing on the life of the tools and for this
reason a press with a long stroke and a proportionately
high linear velocity of crankpin cannot be run as
fast on blanking work as a short stroke press. Of
June. 1925
course, where mechanical feeds are used the stroke
must be long enough to give a sufficient time for feeding.
Work which stresses the material chiefly in tension
includes also in most cases a certain amount of internal
compression of the material also. This is especially
true of drawing work. In drawing a shell
from a flat blank, pressure is applied by the punch to
that section of the blank which will be the bottom of
the shell. The rest of the material is held flat by a
blank-holder and as the punch descends drawing the
surplus down into the shape of the shell there is a
tensile stress in the side wall, and a combined tensile
and compression or crowding stress under the blankholder.
The crowding stress is very high and will
form wrinkles which cannot be ironed out if it is not
held flat. This pressure on the blank results in considerable
pressure on the edge of the drawing die over
which the material is being pulled. Therefore this
radius must be finished very smooth, and should not
be so sharp as to flex the metal unduly. The material
used for the drawing die is of importance on account
of the tendency of some metals to pick up when
being pulled over the drawing edge, thus, cast iron
and especially a chrome nickel mixture makes better
drawing dies for sheet steel than steel itself, but is not
as suitable as steel for drawing aluminum or copper.
Drawing work is done cold and as the metal is
naturally deformed a great deal in dragging it from
the flat shape to a cup or shell there is considerable
strain hardening similar to that in cold rolling. This
and the relation between the tensile strength of the
material and the resistance of the flat blank to the
crow fling action of drawing, limit the amount of
work that can be done in a single operation. For
round work it is generally taken that the depth of a
first operation shell cannot exceed its diameter for
economical results. For square and rectangular shells
a general rule is that the length of the shell cannot
exceed six times the corner radius. In rectangular
work the greatest crowding action occurs at the corners
and consequently the tendency to wrinkle is
greater so that more care must be taken to squeeze
the metal there and to hold it out. In redrawing or
reducing operations in which a shell drawn from a flat
blank is drawn down to a smaller diameter, a blankholder
may be used fitting the inside of the shell or
the stiffness of the shell itself may be counted on to
hold out wrinkles. Common practice permits 25 per
cent reduction in diameter for the first redrawing
and a little less in each case for subsequent redraws
when a blankholder is used. Without it 15 per cent
is about the maximum for the first case with proportionate
decreases for the following redraws. There is
a tendency in all drawing work for the metal to thicken
at the outer edge or at the top of the shell. Where
this must be prevented, that is, where the shells must
have parallel walls as in the production of races for
ball bearings, a so-called ironing operation must be
performed. This merely requires construction of the
drawing die with a clearance between punch and die
equal to the final metal thickness required and therefore
less than a normal thickness of the shell if not
ironed out. Where wall thickness is not important
but the shell must be held accurately to size the final
operation is called sizing and while it may amount to
ironing in some cases, it may also require striking
home at the bottom to bring the corners up.