Basic knowledge can impact productivity, safety
July 26, 2001
Understanding shears is a matter of understanding shear features, including design and drive systems. This article offers information about evaluating shears and includes a list of 20 enhancements and explains each of them.
A guillotine shear has a moving blade that runs on straight slides. The moving blade is almost parallel to the fixed blade during the entire stroke.
Sheet metal and plate shearing machines are used in many fabricating and sheet metal operations. Before selecting a shearing machine, several factors must be evaluated, including the type of shear, required capacity, productivity enhancement options, and safety.
Shear type is determined by many factors, including the material length that it can process and the thickness and type of material that it can cut.
Shearing machines can be broken down into types by shear design and the drive systems that are used in the design. Two design types are common to power squaring shears: the guillotine (also know as the slider unit) and the swing beam.
The guillotine design (see Figure 1) uses a drive system to power the moving blade down and in a position almost parallel to the fixed blade during the entire stroke. Guillotine machines require a gibbing system to keep the blade beams in the proper position as they pass each other.
A shear with a swing beam design has a blade that pivots around a fixed point.
The swing beam design (see Figure 2) uses one of the drive systems to pivot the moving blade down on roller bearings. This eliminates the need for gibs or ways to keep the blades in proper position as they pass.
The drive system powers the moving blade through the material to make a cut. Drive systems can be categorized into five basic types: foot or manual, air, mechanical, hydromechanical, and hydraulic.
Foot Shear. A foot shear is engaged when the operator steps on a treadle to power the blade beam to move down to make a cut.Foot shears are commonly used in sheet metal applications ranging up to approximately 16-gauge capacity and with lengths up to 8 feet, although 8-foot machines are not as common as those with shorter capacities.
Air Shear. To use an air shear, an operator steps on a pedal that activates air cylinders to make a cut. Shop air or a freestanding air compressor is used to power an air shear.
Air shears are used in shops for cutting material up to about 14 gauge with lengths up to 12 feet. Air shears have a simple drive design, and they provide overload protection. Overload protection is designed for proper operation and generally for straight-down loads. For instance, even when cutting a material thickness that is within a machine's capacity, the equipment can be damaged if the material is cut without using a hold-down or if the blade gap is not properly adjusted. This applies to hydraulic machines as well.
Direct-Drive Mechanical Shear. This shear operates when the operator steps on a pedal to turn on the motor that brings the beam down to make a cut. The motor turns off at the end of the cycle, and the blade beam returns to the top of the stroke. This design is suitable for shears when they are not in constant use because the machine uses power only when it is activated.
Flywheel-Type Mechanical Shear. When using a flywheel-type mechanical shear, the operator steps on a pedal to activate a clutch that engages the flywheel to generate the power to move the blade beam down.
Mechanical machines are fast and have a better design for cutting certain types of material. Most mechanical machines being purchased today are used for materials up to 10-gauge thickness and in lengths up to 12 feet.
Hydromechanical Shear. This shear has a hydraulic cylinder or cylinders that power a mechanical device such as an arm to move the blade beam down to make a cut. Some feel that a smaller hydraulic system can be used with this type of shear because the mechanical device produces the power.
Hydraulic Shear. This shear is powered when the operator steps on a pedal to activate the hydraulic cylinders to power the blade beam.
One consideration used in evaluating shears is the capacity required for specific jobs. The machine specifications for almost all shears list capacities for mild steel and stainless steel. To compare a fabricator's requirements to those of the machine, the fabricator's material specifications must be checked against the machine's capacity.
Some shear capacities are rated on mild steel, which may have 60,000 pounds per square inch (PSI) tensile strength, while others are rated for A-36 steel or 80,000 PSI tensile strength. Capacities for stainless steel are almost always less than those for mild or A-36 steel. It may be surprising to some metal fabricators that certain grades of aluminum require as much power to shear as is required to cut steel. It is always best to check with the shear's manufacturer when there are concerns about capacity.
The problems that commonly influence cut quality are illustrated here. A shear's rake angle plays an important role in determining cut quality.
The rake angle of the blade (the angle of the moving blade as it passes the fixed blade) is important in determining the quality of the cut. Generally, the lower the rake angle, the better is the quality of the cut. Problems with cut quality, such as bow, twist, and camber (see Figure 3), are seen on shorter pieces (up to 4 inches long) that fall behind the shear after they are cut. Machines with lower rake angles require more power than those that have a higher rake.
Some guillotine-type machines have a variable rake, a rake angle that can be adjusted to suit the length of the part that is being cut. To evaluate whether this variable rake design is a better option for a fabricator, the type and thickness of the material being cut, the length to be cut, how much of it will fall behind the shear, and the rake angle available for the job must be determined.
For example, if a fixed rake angle has a 1-1/3-inch fixed rake and the adjustable rake machine has a range of 1 to 3 degrees using the 3-degree setting for the 1/4-inch thickness, the fixed rake will produce a better-quality cut on a 3-inch strip. The variable rake machine, on the other hand, may give a better-quality cut on a 1/2-inch strip of 24-gauge material.
Generally, one should not expect a good cut on a strip that is smaller than eight times the material thickness (example: 2-inch strip of 1/4-inch steel). Variable rake machines are generally found in shops with thicker capacity requirements such as 1/2 inch and higher. In the case of these heavier machines, changing the rake angle allows for better cuts on a wide range of thicknesses and types of materials.
Many shear users depend on important standard features and optional accessories that can increase their productivity. This increased productivity can come in many forms: labor savings, better material flow, increased accuracy, better cut quality that can prevent the need for secondary operations, and most important, improved safety. Items that can help to increase productivity include:
1. Squaring arms. A squaring arm is used for squaring the sheet for a trim cut. It can also be used, depending on its configuration, for front gauging and support of longer sheets.
2. Support arms. Support arms are used to support material in the front of the machines and, with the proper front gauging stops, can be used for measuring the cut part.
3. Stops. Different types of stops, such as swing stops anddisappearing stops, are used on the squaring and support arms. The stops are used for front gauging the material. Generally, the disappearing stops are the more widely used stops on front support arms. Disappearing stops allow the plate to be fed over the stop, which becomes recessed and flush with the top of the machine's table and then pops up when there is no more material on top of the stop.
4. Programmable backgauges. A programmable backgauge allows the backgauge dimension to be set and, because it is a programmable device, can possibly control other accessories.
5. Sheet support device. This device (some include the front support arms or front squaring arm as a sheet support device) is generally located behind the blades to hold up the material to the backgauge and support it to prevent sagging and assure an accurate cut. When a sheet support device is used to support the material, the sequence is as follows: the foot pedal is stepped on; the hold-downs come down and clamp the material; the front support device moves or falls down out of the way; the material is cut. Upon completion of the cycle, the process reverses to the neutral position so that the next cut can be made.
6. Manual or power front-operated backgauge. This backgauge is used to adjust the setting of the backgauge dimension to control the size of the pieces dropping behind the blades.
7. Different types of blade material to provide maximum blade life. Depending on the type of material that is being cut, different types of blades are available. The goal is to provide the best and most long-term blade life for the dollars invested.
8. Manual blade gap adjustment. Current blade gap adjustments allow the operator to set the machine at the proper blade gap setting for thickness, type, and dimension of the material that is being cut. It is preferable for this adjustment to be made from one side of the machine. On certain types of machines, the gap adjustment has to be made from each end frame, adding the possibility for an error if both adjustments are not the same.
9. Power blade gap adjustment. The power blade gap adjustment adjusts the blade gap with a motor. This adjustment can often be controlled by a computer control on the machine after the parameters for thickness, type of material, and size to be cut are entered into the controller.
10. Ball transfers. Ball transfers are the accessories mounted on the machine's table to facilitate movement of the plate. These are especially advantageous when shearing thicker material.
11. Conveyer/stacker/scrap separator unit. A conveyor/stacker unit is added to the back of the machine. The conveyor moves the material either down into a stacking unit or, in certain cases, will return the plate after it has been sheared back to the front of the machine for another cut. A scrap/separator attachment to a stacker/conveyor unit separates the trim cut or scrap material from the usable material.
12. Front return unit. This is a conveyor type of unit that feeds the material back to the front of the machine through the blade area so that subsequent cuts can be made from the piece.
13. Manual one-shot lube system. A manual lube system lubricates points on certain locations.
14. Automatic lube system. This system automatically provides the appropriate lubrication to the lubrication points on a systematic basis.
15. Gap in frame for slitting. A gap in the frame for slitting allows parts longer than the blade length to be cut.
16. Light beam/shearing line. This line is provided to facilitate cutting material on a scribed line. The operator marks the plate. The shadow line that is cast allows the operator to line up the scribed line with the edge of the blade for a more accurate cut.
17. Pads or similar device on bottom of hold-down. These devices can be added on the bottom of a hold-down to prevent marring the material.
18. Vibration isolation pads. Vibration isolation pads can be used to facilitate the installation of a shear, especially when it is not practical or feasible to put in a large pit when it is required.
19. Stroke adjustment. The stroke adjustment allows the operator to set the length of the cut.
20. High-speed devices. These devices increase the number of strokes per minute.
The safe and proper operation of all metal fabricating machinery by properly trained personnel is a must. A shear is no exception. A potentially dangerous machine, the shear must be operated in accordance with all federal, state, and local ordinances and the manufacturer's instructions.
Shears should come with safety enhancements such as point-of-operation guards and warning labels which must be left in place. Some operations require the use of additional safety enhancements such as rear guards and light curtains. Generally, the larger the opening under the guard for material thickness, the greater is the distance from the blade that must be protected.
In addition, using proper material handling techniques is critical for parts that are being fed into the machine as well as those being removed from the back of the machine.
Depending on a shear's size and design, a mirror is often mounted on the machine to allow an operator to see around it. Proper lockout/tagout procedures should be used on the shear during repairs or when the blade is changed. In addition, the work area should always be clean.
The basic movement of a shear will always remain the same—the upper blade crossing the lower blade to shear metal. However, maintaining a safe environment and operating shears with various productivity enhancements can help make them safer and more productive.