BBBBflt] «BlJIUrIrlr - Clpdigital.org

216 Forging- Stamping - Heat Treating
harden excessively, resulting in considerable waste m
the product.
In the construction of dies, especially those which
are to be used very considerably, it is advisable to use
only the best of materials. The design should be such
that repairs and adjustments can be easily made and
the replacing of cutting parts does not involve completely
rebuilding the die. It is possible in some cases.
where parts to be produced are large and vary in
size and the production is small, to make dies which
are adjustable or which have portions that can be removed
and replaced with others. Such construction
has been followed on casket, jacket and furniture
dies.
It is extremely important to select a press which
is heavy enough to perform the work required of it
without overstraining. It is false economy in the purchase
of a press if its capacity is only equal to the
work it is to perform. The life of dies, especially
cutting dies, appears to be directly dependent upon
FIG. 3—Double crank press in a motor body plant working
on the back section of a closed car body.
the rigidity of the machine in which they are used,
and dies are the most expensive item in any press
business. One instance might be mentioned in which
a large electrical concern was using very expensive
dies in presses which appeared to be doing the work
all right. They were advised to use a press three sizes
heavier with the result that the number of punchings
obtained per grind on the dies was increased from
about 15,000 to 250.000 and the saving on die charges
exceeded the value of the press each month.
( )\ erloaded presses are subject to frequent breakdowns
especially in clutch parts and the resultant
delavs are usually the source of considerable loss to
production shops. Gap frame type presses are usually
June, 1925
favored for small blanking work especially where the
stock conies in strip form. This type of frame, however,
is more subject to spring than the straight sided
tvpe and consequently such presses should have a
greater excess capacity when used for blanking and
similar work.
The bulk of the presses used are single action machines,
that is, having just one slide and one action.
The most generally used are the crank presses, single
crank machines being used for small work and double
crank machines where considerable width is required
between the housings. Both single and double crank
machines are built with either the gap or cut back
type of frame or the straight sided type, which requires
less iron and is less subject to strain, especially
in the case of the built up construction used for the
medium and larger sizes. Built-up frames are made
up of four members, the bed, crown and two uprights,
which are tied together by steel tie rods shrunk in
so that they exert a pressure equal to the maximum
rated capacity of the machine. The smaller sizes of
gap frame presses are built with inclinable frames so
that the whole machine can be tipped back to permit
gravity discharge of the work from the dies.
The bulk of the energy required to perform each
operation is obtained from slowing down of the flywheel,
the maximum slow down under normal conditions
is about 10 per cent, running in extreme cases up
to 20 per cent. Fast presses for work at the bottom
of the stroke only are built with flywheels mounted
directly on their crankshafts.
Gearing of presses permits running the back shaft
and the flywheel at proportionately high speeds and
makes the press capable of working through a longer
stroke due to the greater energy available from the flywheel.
Some small gap frame presses are built with an eccentric
type shaft running front to back and use a
short stroke and a solid connection from the shaft to
the slide. These presses are especally suited to heavy
blanking and forming work, especially in hardware
plants.
In the larger sizes an eccentric type of shaft is
used with bearings on each side similar to the crank
presses and is capable of taking pressures about twice
as great as crankshafts of similar bearing sizes. This
construction is used with proportionately heavier
drives for short stroke presses for forging and stamping
work.
For extremely high pressure work such as coining
and embossing the presses are built with hardened
steel knuckles or toggles which are strengthened out
to exert the working pressure by a crankshaft at the
back of the machine. These presses require continuously
forced lubrication and create bearing pressures
as high as 15,000 lbs. per square inch.
Double action presses are those built with two
slides, one inside of the other, their function being to
hold with one slide while performing work, whether it
be drawing or forming, with the other slide. Generally
the dwell or holding period of the outer slide occurs
while the inner slide, which is crank driven, is moving
through the lower half of the down stroke.
Cam double action presses are built in most cases
with a pair of constant diameter cams mounted on the
slabs of the crankshafts and arranged with rollers
carried in yokes against their top and bottom surfaces