Minster IOI, Service Manual

Page: 77 / 92

PUNCH SPEEDS
Velocity of the slide at the time the die strikes the work-
piece has a very definite affect upon quality of the part,
scrap loss, die life and economics of the operation.
Certain types of press work, such as blanking and
punching, are not restricted to maximum slide veloci-
ties. In fact, the only limitations seem to be those pre-
sented by feeding the stock, removing the blanks, and
speed of the press itself. On the other hand, speed of
the press used for deep drawing operations must be
limited and will depend upon the type of material being
processed.
Numerous charts and books are available which indi-
cate maximum drawing speeds for various metals. For
example, maximum drawing speed for steel will aver-
age about 55 feet per minute. Stainless steel is nor-
mally drawn at 35 feet per minute, while certain soft
deep drawing steels may be drawn at speed up to 80
feet per minute. Comparing this with other metals, alu-
minum may be drawn at 175 feet per minute and brass
at 200 feet per minute. In each case, a press should be
selected which has a slide velocity, near midstroke,
appropriate for the material being processed.
Note that drawing speeds are indicated in feet per
minute, whereas press speeds are specified as num-
ber of strokes per minute. The relationship becomes
further complicated because stroke lengths will vary
from one press to another. In order that the press user
may quickly determine slide velocity from these known
values, various charts and calculators are available.
However, if these aids are not handy, the following
equation may be used to determine approximate slide
velocity. The answer will be an approximation of the
slide speed encountered from 25% up on the stroke to
midstroke.
ENERGY CAPACITY
Energy capacity of a press is a measurement in engi-
neering units of the motor/flywheel capabilities. This
combination of parts is required to produce power,
store it, and then deliver it when needed to stroke the
press and complete the work therein. Usually energy
capacity is expressed in units of inch tons per stroke
available for work. Energy is supplied by the main drive
motor and stored as flywheel energy by rotating the fly-
wheel mass through a predetermined speed range.
It should be pointed out that not all of the energy stored
in the flywheel can be used in normal press operation.
Actually, just a portion of the stored energy is used to
do the work of forcing the slide and its associated dies
through a distance. By multiplying the amount of force
required times the distance through which it is applied,
the product will equal the energy load on the press. In
order to release any part of its energy for the work at
hand, however, the flywheel will be slowed down. It
then becomes necessary for the motor to restore the
spent energy by bringing the flywheel back to its origi-
nal speed.
Ideally then, the flywheel only releases a small portion
of its total energy through a short burst of power during
each stroke of the press. But, if the flywheel is slowed
more than a recommended amount, depending on size
of flywheel and type of motor used, the motor may be
overworked. Whenever the flywheel is slowed down,
the motor is also slowed. As a result, the motor will pull
a heavier current in an effort to regain speed. If repeat-
ed overload occurs, the motor temperature will rise and
open the thermal overload relays, if it is so equipped.
Actual amount of energy released by the flywheel
through one stroke of the press can be calculated if the
rim weight, initial and final speeds are known. Initial
and final speeds can be found by using a tachometer;
rim weight can be calculated from the flywheel mea-
surements. Substitute known values into the formula:
E1 =
W (V1
2
- V2
2
)
64.32
E1
= Energy in foot-pounds which the flywheel will
expend when speed is reduced from V1 to V2.
W
= Weight of flywheel rim, in pounds.
V1 2
= Square of velocity in feet per second at mean
radius of flywheel, before energy is given out.
V2 2 =
Square of velocity in feet per second at mean
radius of flywheel, after energy is given out.
64.32 =
Constant which is two times acceleration due to
gravity.
H - 8
Speed (ft. per min.) =
Stroke x S.P.M.
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Example: Find the approximate slide velocity at midstroke for
a press with a 10” stroke, operating at a continuous speed of
40 stroke per minute.
Speed =
10 x 40
= 100 feet per minute
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