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
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.
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.
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.
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.
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.
*This is one article in a series of How to Buy Metalworking Equipment. Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.