SHEARS, ROTARY (Including Ring Shears & Circle Shears)
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Details to look for during new and used machinery inspection:Capacity in soft steel
Diameter of cutters
Depth of cutting head
Depth of circle arm
Will circle from square blanks
ACCESSORIES
Describe special attachments
How to Buy Shears
SHEARS: THE SHEARING OF METAL
The shearing process of metal is complex
and highly technical when it involves
factors such as clear structure, slippage
planes, brittle fracture and anisotrophy.
For the user, it's important to know
that both tensile stress and compressive
stress are involved. During a cut, a
shear goes through horizontal stress,
vertical stress, and torsional stress.
When the knife makes contact with the
metal being sheared, the top surface
is put under tension and with the support
of the lower knife, the bottom surface
is put under compression. As the elastic
tolerance of the metal is exceeded,
it is stressed in shear until its maximum
strength is exceeded, where the piece
then breaks away completely from the
parent metal. If the knives are sharp
and the clearance between the top and
bottom knife edges are correct, the
sheared edge will be clean and close
to the perpendicular. Thus, to insure
accurate shearing, the crosshead and
bed must be rigid enough to resist deflection.
There are three internal stresses present during shearing: twist, camber & bow. Twist is the tendency of the off cut to curl up and spiral. A result of excessive rake (the angle of the cutting tool in a plane perpendicular to the work surface) in the knife and can be kept to a minimum if the knives are sharp and the clearance and rake are properly set. Camber is the tendency of a sheared strip to have an arc while laying flat. Instead of the rake having effect, it's caused by internal stresses, inferior material, or improper clearance. Similar to twist, it's most severe on off cuts which are narrow. Bow is the tendency of a piece of sheared material to hump in the center. It is usually due to deficiencies in the material.
Like press brakes, shears are designed to deliver precise vertical blows. Therefore, their basic design is the same.
The housing or end frames are heavily
constructed to provide structural support
for the machine.
The bolster plate is secured to the
press bed. It positions and supports
the die assembly.
The bed is a stationary mounting surface
for the lower die or blade.
The ram carries the upper blade. It's
positioned on the front of the housing
and maneuvers vertically. It is designed
for rigidity.
The drive gives vertical motion to the
slide. It may be mechanical or hydraulic.
Gibs provide a sliding surface for the
ram.
The drive gives vertical motion to the
slide. It may be mechanical or hydraulic.
SELECTION CRITERIA
When selecting a shear, the user must
make a number of decisions based on
production requirements. If you're using
it for high volume work, a rugged piece
of equipment for whatever tonnage is
needed. If the shear is intended for
short-run or intermittent applications,
then the user will likely select a machine
more suited for light work.
For example, there are considerations of mechanical vs. hydraulic drive, overdrive or underdrive, conventional or high speed, and standard or CNC.
Shears are evaluated on the basis of their ability to shear mild steel of a given thickness and length. This capacity rating should never be surpassed. The load on a shear remains constant regardless of the length of a cut, and a fractional increase in metal thickness can cause overloads of 200% or more. However, a machine's mild steel rating is not detailed enough for the user to determine tonnage requirements when materials other than mild steel are to be sheared. Each metal has its own shear strength, the higher the strength in psi (pounds per square inch), the more tonnage is needed to shear it. Tonnage requirements also increase as metal thickness increases.
HYDRAULIC VS MECHANICAL SHEARS
The shear is a precision machine designed
to meet severe demands for accuracy
and performance. They are available
with both hydraulic and mechanical
drive systems. Hydraulic drives
offer advantages in control of
speed and striking force while
the mechanical shear cycles its
ram faster. For the hydraulic shear,
the blade is driven by two cylinders
mounted to the frame on each side
of the ram. The major advantage
here is in its provisions for rake
adjustment. By increasing the rake
angle of the blades, more thickness
of metal can be handled and the
capacity of the shear effectively
increased. The determination of
rake angle is the user's exercise
in compromise, producing a commercially
acceptable off cut at a minimum
practical loading force for the
frame, bed and crosshead. Shears
have also been developed to perform
specialized kinds of work. The
billet or structural shear cuts
flat and round bars and certain
types of structural shapes. The
nibbler, used primarily for thin-gauge
sheet metal work, duplicates the
action of a scissors by cutting
progressively along a straight
or curved line. Also, special shears
have been made for shearing masonite,
clad and exotic metals, wire mesh,
stone, rubber and even burlap.
OVERDRIVE VS UNDERDRIVE SHEARS
There are three basic approaches to
the structural design of shears:
They may be made of: 1. Rolled
steel plate. 2. Machined and assembled
with keyed and bolted construction.
3. Welded steel or cast iron. Most
shears use overdrive, or gap-frame
construction where the drive shaft,
gearing, flywheel and motor are
above a throat or gap provided
in the side frame members. Since
the stock passes through this gap,
overdrive construction permits
the slitting of stock longer than
the shear is wide. Most overdrive
mechanical shears have a provision
for raising the crosshead without
changing the rake angle. This vertical
adjustment capability makes slitting
easier because the knives can be
set so that they do not overlap
at the high end. Underdrive shears
are more compact and have a lower
silhouette than overdrive shears.
They offer improved visibility,
which is a good advantage in straight-through
work or when the shear is incorporated
into a fabrication line.
HIGH-SPEED SHEARS
There are two classes of high-speed
shears: One is a speeded-up version
of the conventional shear where
the ram speed is increased by 75%
to 100%. This type of equipment
is usually used for cutoff work
or in long-run production on coil
stock. The second class of high-speed
equipment is capable of shearing
12 gauge mild steel, 72 inches
wide at 300 strokes per minute,
found in high-speed automatic cutoff
lines. There is very little humping
of the stock and the cutting action
actually improves as the speed
of the knife increases. These shears
have been successful in blanking
silicon steels and a number of
synthetic materials.
INSPECTION
NON-POWER
Check the frame. Are there any cracks,
breaks or welds evident?
Check the condition of the blade. Look
for any cracks, breaks, etc., as an
indicator to general machine condition.
Uncover the gear boxes and check the
gears for broken teeth and other signs
of excessive wear.
UNDER POWER
Take a cut on a piece of work across
the length of the blade. The break
should be clean throughout the
cut.
Check the rear gauge for accuracy by
adjusting it to a specific length and
then measuring a cut piece to size.
Check the clutch for noise and slippage.
Listen to the gears mesh. Are there
any abnormal sounds?
Verify that all controls perform as
they should. Make sure inch-control
and foot pedal mechanisms work properly.
Verify that the hold-downs are functioning
properly. Whether mechanical or hydraulic,
the stock should be held firmly without
moving.
Check the hydraulic system for worn
hoses, leaking cylinders, etc.
If the shear is equipped with a shadowlight,
make sure that it works properly.
