Shearing of Sheet, Strip, and Plate
Shearing of Sheet, Strip, and Plate
Shearing is a method for cutting a metal piece into smaller pieces using a shear blade to force the metal past an opposite shear blade in a progression form. Shearing is widely used to divide large, flat metal such as sheet, strip, and plate. Shearing is a fast and inexpensive method to cut the flat metal. This method has been used since the beginning of the industrialization.
The different forms of shearing techniques are (i) simple shearing, (ii) punching, (iii) slitting, (iv) blanking, (v) notching, (vi) cut-off, (vii) nibbling, (viii) shaving, (ix) trimming, (x) dinking, (xi) lancing, and (xii) fine blanking. Shearing is the process when two blades cut a piece, but this process is called blanking, piercing, notching, or trimming when the blades are at an angle, while both the processes are referred to as shearing operations in terms of tool design and material behaviour. The concept of shearing is related to the way force is applied and the mechanical deformation which takes place. However, it is also considered as a manufacturing process.
Shearing is the mechanical cutting of sheet, strip, and plate of metals, frequently used to prepare content for subsequent operations, without the forming of chips or the use of burning or melting, and its performance helps to ensure the accuracy and consistency of the final product. Shearing is also referred to as sheet metal cutting.
When a shearing force is applied such that, the shearing stress exceeds the ultimate shear strength of the material, then the work metal fails and separates at the location of application. The shearing process makes use of two tools (upper blade and lower blade) which are located above and below the work-piece. The distance between the lower blade and upper blade (clearance) range between 4 % and 10 % of the thickness of the work-piece and is dependent on the material of the work-piece. Mechanical properties such as shearing strength of the work metal, the main cause of this clearance is to better cut a work-piece without ruining the material and make the work metal to deform plastically. Shearing is a sheet metal cutting operation along a straight line between two cutting edges by means of a power shear.
Shearing of sheet, strip, and plate is broadly classified according to the type of blade (knife or cutter) used as either straight or rotary. Straight-blade shearing is used for squaring and cutting flat work metal stock to the needed shape and size. It is normally used for producing square and rectangular shapes, although triangles and other straight-sided shapes are also sheared with straight knives. Rotary shearing (which is not to be confused with slitting) is used for producing circular or other contoured shapes.
Within the sheet metal industry, different shear cutting technologies are normally used in several processing steps, e.g., in cut to length lines, slitting lines, and end cropping etc. Shearing has speed and cost advantages over competing methods like laser cutting and plasma cutting, but involves large forces on equipment and large strains in the work metal material. Depending on material grade and work metal thickness, the shear process is normally adjusted in order to moderate the needed force and to control the final sheared edge geometry. Industrial shears can include some force measurement possibilities, but the force is most likely influenced by frictional losses between shear tool and point of measurement, and hence does not show the actual force applied to the work metal.
Well defined shears and accurate measurements of force and shear blade position are important for understanding the influence of shear parameters. Accurate experimental data is also wanted for validation of numerical shear models. With reliable numerical shear models, the appropriate shear parameters for new work metal grades can be found without the need for time consuming and expensive live tests in the production.
With the purpose to identify appropriate shear parameters, effective shearing and quality of the sheared edges are to be defined. Here, sheared edges are characterized from geometry, but accumulated plastic strain, stress history, and micro cracks in the edge are likely important for the sheared edge properties.
Shearing is a process for mechanical straight cutting of work metal without chip formation between two, against each other, moving tools. After the tools are in contact with the work metal, penetration starts and the work metal material experiences high shear stresses and ultimately failure. After a plastic penetration into the work metal, cracks start and propagate from both tools. Depending on work metal material properties and shear settings, the quantity of plasticity and penetration before crack initiation vary.
Shearing is normally used to produce rectangular shapes and the sheared pieces of the work metal normally have high tolerances and are used without further machining. Material waste and energy consumption in shearing are low compared to chip forming cutting and melting cutting. The work needed for shearing is normally applied either by manual, mechanical, hydraulic, or pneumatic means. The manual and pneumatic types are used for lighter shearing. High production rate is featured by the mechanical power driven shears but the hydraulic shears are more flexible and adjustable.
In straight-blade shearing, the flat work-piece is placed between a stationary lower blade and a movable upper blade. As the upper blade is forced downward, it cuts the metal into two parts. Straight-blade shearing is the most economical method of cutting straight-sided blanks from flat sheet, strip, and plate with thickness no more than 50 millimetres (mm). The process is also widely used for cutting sheet into blanks which are subsequently to be formed or drawn. Since shear gauging can be set within + 0.13 mm, the shearing process is normally limited to + 0.4 mm tolerances in 1.6 mm thick material.
The tolerance range increases with thickness. Straight-blade shearing is seldom used for shearing metal harder than around 40 HRC (Rockwell hardness, ‘C’ scale). As the hardness of the work metal increases, blade life decreases for shearing a given thickness of metal. In general, it is practical to shear flat work metal up to 38 mm thick in a squaring shear. Squaring shears up to 9 metres (m) long are available (even longer shears have been built), and some types are equipped with a gap which permits shearing of metal longer than the shear blade. When extremely soft, ductile metal (especially thin sheet) is sheared, the edges of the metal roll, and large burrs result.
Principle of shearing – The principle of shearing is simple. When the upper blade is forced to move downward, both the upper and lower blades gradually come together and contact the work metal being sheared. The blades penetrate to a certain portion of the metal thickness until its shear strength is reached, at which point the unpenetrated portion of the metal fractures, and the work metal separates (Fig 1). At the final stage of shearing, the upper blade continuously moves downward and results in freeing the sheared pieces from the original work-piece.
Fig 1 Schematics of straight blade shearing
The upper blade wall rubs against the metal edge to cause the cutting-edge area to burnish, while the lower blade wall rubs against the sheared piece edge to cause a second burnish area. A burr occurs on both the sheared piece and the original work-piece. The quantity of penetration depends largely on the shear strength and thickness of the work metal. The blade penetrates 30 % to 60 % of the metal thickness for low-carbon steel, depending on thickness. The penetration is higher for a more ductile metal such as copper. Conversely, the penetration is less for metals which are harder than low-carbon steel.
Shearing of sheet metal is accomplished by a shearing action between two sharp cutting edges. The shearing action is depicted in the four stop-action sketches of Fig 2, in which the upper cutting edge (the upper shear blade) sweeps down past a stationary lower cutting edge (the lower shear blade). As the upper shear blade begins to push into the work-piece, plastic deformation occurs in the surfaces of the work metal. As the blade moves downward, penetration occurs in which the blade compresses the work-piece and cuts into the metal. This penetration zone is normally around one-third of the thickness of the work-piece. As the blade continues to travel into the work-piece, fracture is initiated in the work-piece at the two cutting edges.
Fig 2 Shearing of sheet metal between two cutting edges
Shear geometry – Fundamental shear geometry is represented in Fig 3a and Fig 3c, where the work-piece metal is sheared between two shear blades and held in place by hold downs (clamps). Here, the geometric properties defined are work-piece thickness ‘h’, clearance ‘c’ and shear blade radius ‘r’. Shearing of wide strips normally includes the rake angle, created by rotation of one shear blade around the X-axis in Fig 3a. Use of rake angles limits contact length and forces but induces additional strains in the sheared material. With more 3D (three dimensional) strain state, distortion of the sheared piece can occur. For minimizing the strains, thorough clamping of the sheet, as close as possible to the shear blades, is important.
Fig 3 Shearing of steel sheet and representation of the shear geometry and boundary conditions
In general, existing shear settings affect the sheared edge geometry, which in turn is important for aesthetics, sharpness and the tendency to initiate cracks. Sheared edges have four characteristic zones, i.e., roll over, shear, fracture, and burr zones (Fig 3b). Roll over and shear zones arise from the plastic deformation of the sheet when penetrated by the shear blades, while the appearance of fracture and burr zones are determined by the crack propagation characteristics during fracture. Specific for shearing of high strength steel, are a small shear zone and large fracture zone. Burrs and rough fracture zones complicate the processing through inadequate tolerances which can imply additional machining and sharp edges, which can damage equipment or even cause injuries. Moreover, edge defects serve as initiation spots for cracks in later processing steps.
Within majority of the shear processes, the clearance between shear blades is normally adjusted along with changed work-piece metal grade or thickness, and is normally specified relative the work-piece thickness. Clearance and clamping have large impact on the sheared edge geometry. Ideally, cracks start from each blade radius and meet inside the work-piece. Depending on size of the fracture zone and the materials desired crack propagation angle, clearance is adjusted for the cracks to meet without overlap.
Experimental studies on blanking suggest 10 % clearance to minimize the needed force and the blade wear. Observations in sheet metal trimming show that both roll over and burr increases with higher clearance, especially when approaching 20 %.
A sheared edge is characterized by the smoothness of the penetrated portion and the relative roughness of the fractured portion. Sheared edges do not have the quality of machined edges. However, when blades are kept sharp and in proper adjustment, it is possible to get sheared edges acceptable for a wide range of applications. The quality of sheared edges normally improves as the thickness of work-piece decreases.
Shearing machines – Shearing machines are multi-functional machines being used to cut flat materials of carbon, alloy, and stainless steels as well as non-ferrous metals and their alloys. Smaller shearing machines use scissors as cutters or angular shearing movement to form sheet or strips of metals, larger machines use a straight shear operation with the blade set at an angle as opposed to the angular movement. The thickness of the work metals to be cut is a function of the rake (angular configuration of the blade) and the clearance. The normal shear consists of a fixed bed to which a blade is mounted, a vertically advancing cross-head and a set of pins or feet holding the material in place while the cutting takes place.
Machines for straight-blade shearing – Punch presses and press brakes can be used for shearing a few pieces or are used temporarily when more efficient equipments are not available. Production shearing, however, is normally done in machines which are designed for this operation. Squaring shears are normally used for trimming and cutting sheet, strip, or plate to specific size. These shears (also called re-squaring or guillotine shears) are available in a wide range of sizes and designs. Some types of lines also permit slitting when the work-piece moves for shearing.
The sheet, strip, or plate is held firmly by hold down devices while the upper blade moves down past the lower blade. Majority of sheets, strips, or plates is sheared by setting the upper blade at an angle (Fig 1a). The position of one of the blades can be adjusted to maintain optimal clearance between the blades. Squaring shears can be actuated mechanically, hydraulically, or pneumatically.
Mechanical shears – The power train of a mechanical shear consists of a motor, the fly-wheel, a worm shaft which is gear driven by a fly-wheel, a clutch which connects the worm gear drive to the driven shaft, and a ram actuated by the driven shaft through eccentrics and connecting links. Under the majority of the operating conditions, a mechanical shear can deliver more strokes per minute (spm) than a hydraulic shear. Some mechanical shears cycle as fast as 100 spm.
Another advantage of the mechanical shear is that, because of the energy stored in the fly-wheel, a smaller motor can be used for intermittent shearing. As an example, a mechanical shear with a no-cutting or free-running speed of 65 spm can make around six full shearing strokes (i.e., shearing maximum thickness and length of cut) per minute with a standard motor. However, when the same shear is cutting at full capacity in a rapid shear mode, a much larger motor is needed, since in rapid cutting, there is not enough time between cuts for the smaller motor to restore the speed of the fly-wheel. An additional advantage of the mechanical shear is that its moving blade travels faster than the moving blade of a hydraulic shear. In some cases, higher blade speed can decrease work metal twist, bow, and camber.
Majority of the mechanical shears are provided with enough power to build up the fly-wheel speed after each cut but not enough to allow the operator to run full-capacity cuts in a high-speed mode. It is to be noted that some of the mechanical shears are not of the fly-wheel type, rather, their motor directly drives the shear blade beam to move down to make a cut. Mechanical shears are rated in strokes per minute, not cuts per minute. Majority of the shearing applications do not need high-speed cutting.
Hydraulic shears – These shears are actuated by a motor driven pump which forces oil into a cylinder against a piston. The movement of the piston energizes the ram holding the upper blade. A hydraulic shear can make longer strokes than a mechanical shear. In general, long shears and shears with low-carbon steel capacities above 13 mm are almost all hydraulic. Hydraulic shears are designed with a fixed load capacity. This prevents the operator from shearing material which exceeds capacity and, hence, saves costly damage to the machine structure. This is a basic advantage of hydraulic shears.
The total load during shearing is related to the shear blade rake angle, sharpness of the blades, blade clearance and type, and the mechanical properties and thickness of the material being sheared. It is possible to stop the machine during shearing within a rated capacity if the clearance is incorrect, the blade is dull, or the back piece is excessively deep. In this way, the hydraulic shear is protected from damage caused by overloading. A mechanical shear is not constrained by an overload prevention system and continues to cut under nearly all conditions.
Pneumatic shears – These shears are used almost exclusively for shearing thin metal (rarely thicker than 1.5 mm) in relatively short pieces (rarely longer than 1.5 m). Activation of air cylinders makes the shear blade beam move to make a cut. Plant compressed air or a free-standing air compressor is used to provide power to air cylinders.
Alligator shears – These shears have a shearing action similar to that of a pair of scissors. The lower blade is stationary, and the upper blade, held securely in an arm, moves in an arc around a fulcrum pin. This type of machine is most widely used for shearing bars and bar sections and for preparing scrap. Alligator shears are available in different sizes, including those which can shear plate up to 32 mm thick by 760 mm long and plate up to 50 mm thick in shorter lengths. The lighter machines can be made portable. The heavier machines, however, are to be firmly anchored in concrete, especially if they are to be used in conjunction with roller conveyor tables in the shearing of plate.
Accessory equipment for straight-blade shearing
Some accessories have been incorporated into the majority of the shear designs and are needed for efficient and accurate straight-blade shearing.
Hold down – Hold downs (Fig 1a) are mechanical or hydraulic devices which hold the work metal firmly in position to prevent movement during shearing. The hold down pressure is to be higher than the forces generated in shearing the work material. These forces depend on the blade clearance, rake angle, and depth of material back piece. The most efficient hold down system is a series of independent units which securely clamps the work metal of varying thickness automatically and without adjustment.
The force on each hold down foot is to be substantial, ranging from several hundred kilograms on a machine for shearing sheet to several tons for shearing plate. Hold downs are to be timed automatically with the ram stroke, so that they clamp the work metal securely before the blade makes contact and release their hold instantly after shearing is completed.
Back gauges – They are adjustable stops which permit reproducibility of dimensions of sheared work-pieces in a production run. Majority of the gauges are controlled electrically. Push-button control provides a selection of high traverse speeds and slow locating movements for accurate final positioning. The addition of a computer numerical control (CNC) system permits dimensional accuracy and repeatability, increased productivity, and hands-off safe operations. For thin sheet, magnetic overhead rollers eliminate sag and support the sheet for accurate gauging. For rapid and accurate shearing, back gauges are equipped with electronic sensors which automatically trip the shear only when the sheet is accurately positioned.
Pneumatic supports are used to support thin sheet. Support arms are designed to raise into a horizontal position which is flush with the shear table, permitting material to be supported in the correct position against the back-gauge stop. Blank inaccuracies because of unsupported and poorly positioned work-pieces are almost eliminated.
Back gauges are also equipped with retractable stops for shearing mill plate. With the stops out of the way, mill plate of almost any length can be fed into the shear and cut to the desired length. When stops are not used, the work-piece can be notched or scribed to indicate the cut-off position.
Front gauges – When gauging from the front of the machine, the operator locates the work metal by means of stops secured in the table or in the front support arms. Power operation of the front support arms allows the blank dimensions to be entered digitally using a computer control. Front gauging is frequently done by means of a squaring arm.
Squaring arms – Squaring arms (Fig 4) are extensions attached to the entrance side of a shearing machine which are used to locate long sections of work metal in the proper position for shearing. Each arm is provided with a linear scale and with stops for accurate, consistent positioning of the work metal. Squaring arms are reversible to allow use of the shear at either end and to more evenly distribute the wear on the shear blades. Power operation can be added to the squaring arm for increasing blank accuracy and reducing the set-up time. This type of squaring arm prevents the arm from moving from side to side.
Fig 4 Squaring arm attachment
Blades for straight shear
Material used for shear blades depends on specific shearing operations. Majority of the shear blades are made in one piece from tool steel. Some are made of carbon or alloy steel. The composition, hardness, thickness, and quantity of metal being sheared are the most important factors in the selection of blade material. A D2 tool steel as per AISI (American Iron and Steel Institute) standard is frequently desired for cold shearing metals up to 6 mm thick sheets and plates as shown in Tab 1.
|Tab 1 Recommended material for straight shear blades for cold shearing of flat metals|
|Metal to be sheared||Thickness, 6 mm maximum||Thickness, 6 mm to 12.5 mm||Thickness 12.5 mm higher than 12.5 mm|
|Low production||High production||Low production||High production|
|Carbon and low alloy steels (up to 0.35 % C)||Modified A8, H13, and L6||D2||Modified A8, H13, and L6||A2||S5 (a)|
|Carbon and low alloy steels (0.35 % C)||Modified A8, H13, and L6||D2||Modified A8, H13, and L6||S5||S5 (a)|
|Stainless steels and heat resistant alloys||Modified A8, H13, and L6||D2||S5||S5||S5 (a)|
|Silicon electrical steels||D2||D2, Carbide||S5||S5||(b)|
|Copper and alloys, Aluminum and alloys||Modified A8, H13, and L6||A2, D2||Modified A8, H13, and L6||A2||S5 (a)|
|Titanium and titanium alloys||D2||D2|
|Grades are as per AISI standard, (a) S5 is preferred for plate thickness higher than 19 mm, (b) Plates higher than 12.5 mm rarely sheared.|
Blades made of modified A8, H13, or S5 tool steels are desired for low-volume production or for occasional shearing of metals up to 6 mm thick (except the more highly abrasive metals such as silicon steel). Blades made of A2 tool steel have been satisfactory for high-production cold shearing of soft non-ferrous metals. However, D2 blades are normally more economical because of better wear resistance, but they are not normally desired for cold shearing of work metals more than 6 mm thick since they are likely to break under impact loads.
However, depending mainly on blade design and length of cut, blades made of D2 tool steel have been successfully used for cold shearing aluminum alloys up to 32 mm thick. Modified A8 or H13 tool steels are suitable for some cold-shearing applications in which the work metal is more than 6 mm thick, as given in Tab 1. However, the shock-resistant grades S2 and S5 are normally desired for shearing heavy sections of all metals.
The length and design of the blade can influence the selection of blade material. Although, water-hardened tool steels such as W1 and W2 are suitable for several cold-shearing applications, the rapid cooling during heat treatment causes higher distortion than that in blades made from the oil-hardened or air-hardened steels such as D2. As an example, a bar 4 m long is needed to make a shear blade 4 m long from D2 tool steel. The same blade made from W2 steel needs a bar 4 m long. Both steels elongate when heat treated, but W2 steel bows more readily than D2 steel. Since straightening is difficult, the W2 steel blade hence is required to have more grinding stock. The additional grinding decreases the depth of the hardened shell and shortens the useful life of the blade.
Hardness – The rate at which a blade wears in cold shearing depends mainly on its carbon content, alloy content, and hardness. Insufficient hardness in a blade used for cold shearing shortens its service life. In one of the applications, a blade made of S5 tool steel with a hardness of 44 HRC has worn three times as fast as one with a hardness of 54 HRC used under the same conditions. In spite of the desirability of having shear blades as hard as possible to minimize wear, it is frequently necessary to sacrifice some hardness to prevent blade breakage as the hardness or thickness of the metal being sheared increases.
Hardness alone does not always guarantee blade performance and life, since different wear resistance, ductility, or toughness can be achieved at the same hardness. Improper quenching and tempering can result in a not-suitable micro-structure and hence insufficient wear resistance, ductility, or toughness for a specific shearing application even though the steel can have the same hardness as achieved in the same steel from proper processes.
Recommendations for the hardness of blades for cold shearing cannot always be made without knowledge of the details of the shearing operations. Such details include type, mechanical properties, and thickness range of the material to be sheared as well as shearing speed and blade dimensions. As an example, blades made of D2 tool steel have frequently been successfully used at 58 HRC to 60 HRC for shearing low-carbon steel up to 6 mm thick. However, the hardness of a D2 blade is to be kept below 58 HRC to prevent blade breakage if the work metal to be sheared is high-strength low-alloy (HSLA) steel. The shock-resistant S-grade tool steels are used in the hardness range between 50 HRC and 58 HRC.
The hardness of the blades is to be decreased from the higher end of the range to the lower end when the hardness or thickness of the work metals to be sheared increases. The higher end of this range is applicable to shearing of steels 6 mm to 12.5 mm thick and to non-ferrous metals. As shock loading increases with the shearing of harder or thicker work metals, blade hardness is decreased toward the low end of the afore-mentioned hardness range.
Operating parameters in work-metal cutting are clearance between the two blades, work metal thickness, type of metal and its strength, and length of the cut. These parameters have some of relationships among them.
Capacity – Majority of the shearing machines are rated as per the section size of low-carbon steel they can cut. The tensile strength of low-carbon steel sheet and plate is normally below 520 MPa and the yield strength below 350 MPa. Shears are frequently rated in terms of their ability to cut low-carbon steel with a tensile strength of 415 MPa and yield strength of 275 MPa. An allowance for normal over-tolerance work metal thickness is included in the capacity rating of the machine. The use of a machine for shearing other metals is mainly based on the relationship of the tensile strength and ductility of low-carbon steel to that of the particular metal to be sheared.
Metals with a tensile strength higher than that of low-carbon steel almost always reduce the capacity of the machine. For example, the machine capacity for shearing HSLA steels is reduced to around two-thirds to three-quarters of the rated capacity for low-carbon steel. On the other hand, for shearing aluminum alloys, machine capacity can range from 1.25 times to 1.5 times the rated capacity for low-carbon steel. As an example, a specific force is needed to shear 6 mm thick low-carbon steel. The same force can shear only a 4.8 mm thickness of type 302 stainless steel but can shear a 6 mm thick aluminum.
Ductility also can affect machine capacity. As an example, annealed copper, because of its high elongation, needs as much shearing effort as low-carbon steel, even though copper has considerably lower tensile strength. Similarly, carbon steel with very low carbon (less than 0.1 % C) and higher-than-normal elongation needs a higher-capacity machine.
Power requirements – The energy consumed during shearing is a function of the average stress, the cross-sectional area to be sheared, and the depth of maximum blade penetration at the time of final fracture of the work metal. For any metal, the quantity of energy consumed is hence proportional to the area under the shearing stress-strain curve up to the point of fracture. Fig 5 shows typical shearing stress-strain curves for hot-rolled steel and cold-rolled steel.
Fig 5 Typical curves for shear stress and shear strain
The distance through which the force acts (blade penetration) is around 35 % of the work metal thickness for hot-rolled steel and 18.5 % for cold-rolled steel. As an example, in the curve for hot-rolled steel, the average stress under the curve is 73.5 % of the maximum shearing stress Smax, and the distance through which the force acts is 35 % of the work metal thickness. Hence, the energy ‘E’ used in shearing hot-rolled steel is given by the equation 1 which is ‘E=0:735 Smax x Wt x 0.35t = 0:257 Smax x W(t-square)’, where ‘W’ is work metal width, and ‘t’ is its thickness. Applying equation 1 to the curve for cold-rolled steel in Fig 5 yields an energy consumption of 0.136 Smax x W(t-square).
The maximum instantaneous power Pmax needed for cutting work metal in a shear is determined by the equation 2 which is ‘Pmax = (Wt x Smax x V)/33,000’, where ‘V’ is the speed of the shear blade, and the other variables are as defined earlier. The average power requirement Pavg for a shear making ‘n’ cuts per minute in hot-rolled steel is given by the equation 3 which is ‘Pavg = [W(t-square) x n x Smax]/1,540,000’.
Equation 2 and equation 3 determine the net power needed for actual shearing of the work-piece. The quantity of power needed to operate the hold down system and to overcome friction is to be added to the net power.
Friction depends on the design of the shearing machine and the blade, type of bearings, alignment, lubrication, temperature of operation, and size of the machine in relation to the area of the section to be sheared. When shearing metal of nearly the maximum size for which a shear is designed, the loss of power by friction for well-designed machines rarely exceeds 25 % of the gross power.
Shearing force – It is to be noted that the shearing force ‘F’ needed to cut a piece of metal follows a proportional relationship, as given by the equation 4 which is, ‘F is proportional to [s x p x t-square (1 – p/2)]/R, where ‘s’ is the shear strength of the work metal, ‘t’ is the thickness of the work metal, ‘p’ is the percent of penetration of the blade into the work-metal, and ‘R’ is the rake of the shear blade.
It is not possible, hence, to calculate the needed shearing force for different work metal thicknesses based solely on change of rake. Even for low-carbon steel, the quantity of blade penetration prior to fracture can be as high as 60 % of the work metal thickness for 3.5 mm thick sheet and as little as 30 % for 19 mm thick plate by the same set of a shear.
Rake R – It is the tangent of the angle formed between the lower (fixed) shear blade and the upper (movable) shear blade (Fig 1b). It is normally expressed as the upper blade rise per unit length of the lower blade. As an example, a rake of 21 mm/m means that the upper blade rises 21 mm for each metre of linear distance along the blades. Rakes below 21 mm/m are rarely used. A rake of 42 mm/m or higher is typical of several plate shears.
A certain degree of rake is used to permit progressive shearing of the work metal along the length of the blade. As the rake decreases, the quantity of upper blade engagement increases as it travels through the material. This results in a higher needed shearing force. A higher rake reduces the shearing force and allows the use of a smaller machine to shear the material than is necessary if the cutting edges of the blades are parallel. However, a high rake increases the distortion in the sheared materials. A high rake also can cause slippage and hence heeds high hold down forces. A rake is to be as low as possible to reduce the quantity of distortion in the sheared materials.
Blade clearance – It is another important parameter. It is the gap between the upper blade and the lower blade as they pass each other. Very tight clearance results in secondary fracture after the first fracture, and the cut edge displays a characteristic ragged shape. On the other hand, too much clearance results in a tear and high burr rather than a clean cut on the cut edge. A more serious consequence of excessive clearance is that it can cause the work-piece to be pulled between the blades. This, in turn, causes overloading of the machine and can result in failure of machine components or shear blades.
The major effects of blade clearance are the appearance of the sheared edge and the squareness of the cut. As an example, for annealed mild steel, when sheared edge condition and appearance are critical, the blade clearance is between 4 % and 10 % of the material thickness. In contrast, it is 9 % to 15 % when the edge condition and appearance are not critical. The blade clearance also affects the degree of twist of the drop sheet and the shearing force needed.
Because of blade deflection during shearing in some shears, the clearance at the centre of the blade is normally set less than that at the ends. Blade clearance (except on machines using a fixed clearance) is normally increased as the work metal thickness is increased.
When soft metals are sheared, insufficient clearance causes double (secondary) shearing, which appears as a burnished area at the top and bottom of a sheared edge with a rough area between the burnished edges. Blade clearance normally is to be increased as the hardness of the metal being sheared decreases. Some mechanical shears are constructed to operate with a fixed clearance, and no adjustments are made for variations in work metal composition or thickness. The blade clearance is set for the thinnest material to be sheared. If the range of thicknesses sheared is not very large, then double shear can be avoided.
Ram speed – Ram speed in straight-blade shearing (and, in turn, the blade speed) has some effect on results in the shearing of flat sheet, strip, and plate. Low linear speed produces a rough sheared edge. As speed is increased, a cleaner sheared edge is achieved. In general, speeds in the range of of 21 metre per minute (m/min) to 24 m/min can be used without difficulty when shearing annealed metals. Regardless of the speed used, adequate hold down force is needed.
Shearing of electrical steel – Both the non-oriented electrical steel used to make motor cores and the oriented electrical steel used to make transformer cores need to undergo a shearing processing to get the desired shape. The electrical steel after shearing needs a low iron loss, no damage to magnetic properties, and a high lamination coefficient. As per a large number of studies and manufacturing techniques, the shearing process can generate plastic deformation and residual stress inside the material, resulting in micro-structural flaws. These micro-structural defects affect the structures and the wall movement of magnetic domains during the magnetization process, and hence cause adverse effects on the magnetic flux density and iron loss. Besides, studies have found that shearing processing increases the iron loss by 9 % to 20 %.
Magnetic materials with high permeability and low loss are the foundation for high-efficiency manufacturing in electric motors. Hence, investigating new ways to improve the sectional quality and reduce the damage to shearing edges has an important engineering significance and application prospects. Generally, the key factors affecting the quality of shear section of a metal sheet include shear clearance, thickness of sheet, lubrication and contact conditions, sharpness of the shearing edge of the blades etc. The shear clearance is a main factor affecting the quality of a cut section.
The impact of shear factors on the electro-magnetic characteristics of non-oriented electrical steel sheet has been studied. The results have indicated that the shearing process induces residual stresses inside the sheet and results in larger iron loss. Additionally, as the blade wears, the quality of shear section deteriorates and the depth of work-hardened layer increases, hence indicating worse magnetic properties. However, the shearing speed has a little effect on the shearing process.
In one of the studies on the effects of grain size and blanking clearance on the deterioration of magnetic characteristics of non-oriented electrical steel, it has been indicated that the ideal clearance is determined by grain size. In another study, it has been found that when shearing electrical steel sheets, a little distance between the upper blade and the lower blade results in superior magnetic characteristics than a big gap.
Shearing attributes and defects
Dimensional accuracy – Dimensional accuracy achieved in straight blade shearing is influenced by the capacity and condition of the machine, condition of the blades, blade clearance, and work metal thickness and condition. As an example, a total tolerance of 0.25 mm normally can be achieved in sheets with a thickness of 3.5 mm maximum when they are sheared to a size up to 3.6 m long. This tolerance applies to sheets which are essentially free from residual stress and flat within commercial limits. Sheets which are not flat or have residual stress, or both, cannot be sheared with the same accuracy.
Higher tolerances are needed in shearing of plate. A total tolerance of 0.5 mm to 1 mm can be maintained when plate is sheared in squaring shears. Dimensions can be held to a tolerance of around + 1.6 mm when shearing in alligator shears. In short, good sheared edge quality needs proper blade sharpness, blade clearance, and sufficiently high ram speed.
Bow, or cross bow, is one of three major shape defects introduced in short sheared pieces during shearing (Fig 6a). Bow is nearly proportional to the rake angle of the upper blade when a short piece is being sheared. A high rake causes the materials to bend during shearing. Reducing the rake angle minimizes a bow in a short sheet being cut when the incoming material has a good shape. The bow becomes negligible when the sheet being sheared is more than 100 mm long.
Fig 6 Three major shape defects introduced by shearing of short sheared pieces
Twist (Fig 6b) in a short-sheared piece is normally proportional to the rake of the upper blade. Thick plates twist more than thin sheets, if both of them have the same good quality of incoming materials in shape. Soft materials tend to have more twist than the materials. Wide incoming material has a higher tendency to twist in a short-sheared piece than narrow incoming material. Short sheared pieces appear more twisted than long sheared pieces. When a 6 mm thick plate is sheared, there is some twist in a 25 mm long sheared piece (drop). If the length of the sheared piece increases to 100 mm, no twist occurs visually. When 25 mm thick plate is sheared, the length of the sheared piece is to be higher than 125 mm for no measurable twist.
Camber (Fig 6c) cannot be eliminated in short sheared pieces (less than 100 mm) but sometimes can be reduced by lowering the rake angle of the upper blade. Sufficient hold-down pressure close to the shear can help to reduce camber.
Rotary shearing, or circle shearing (not to be confused with slitting), is a process for cutting sheet and plate in a straight line or in contours by means of two revolving, tapered circular cutters. Tab 3 gives recommended materials for rotary blades.
|Tab 2 Recommended blade materials for rotary shearing of flat metals|
|Metal to be sheared||Thickness to be sheared|
|4.8 mm or less||4.8 mm to 6.4 mm||More than 6.4 mm|
|Carbon, alloy, and stainless steels||D2 (a)||A2 (b)||S4, S5|
|Silicon electrical steels||M2 (c), D2 (d)||D2|
|Copper and aluminum alloys||A2, D2||A2, D2||A2 (e)|
|Titanium and titanium alloys||D2 (f), A2 (g)|
|Grades are as per AISI standard. (a) L6 is also recommended for shearing carbon and alloy steel sheet containing more than 0.35 % C. (b) D2 is also recommended for low C and low alloy steel sheet. (c) For sheet more than 0.8 mm thick. (d) For sheet more than 0.8 mm thick. (e) S5 is recommended for sheet more than 12.7 mm thick. (f) For sheet less than 3.2 mm thick. (g) For sheet more than 3.2 mm thick.|
For conventional cutting to produce a perpendicular edge, the cutters approach each other and line up vertically at one point (Fig 7a). The point of cutting is also a pivot point for the work-piece. Because of the round shape of the blades, they offer no obstruction to movement of the work-piece to the right or left. This feature permits the cutting of circles and irregular shapes which have small radii, as well as cutting along straight lines.
Fig 7 Rotary shearing with its set-up and tooling
Overlapping of the cutters to the position shown in Fig 7b permits the shearing of smooth bevelled edges in straight lines or circular shapes. With the cutters positioned as shown in Fig 7b, a bevel can be cut across the entire thickness of the work-piece, resulting in a sharp edge on the bottom of the work-piece, or (by varying the overlap of the cutters) only a corner of the work-piece can be sheared off, leaving a vertical edge (or land) for around half the work-piece thickness. The shearing of work-pieces into circular blanks needs the use of a holding fixture which permits rotation of the work-piece to generate the desired circle. For straight-line cutting in a rotary shear, a straight-edge fixture is used, mounted in the throat of the machine behind the cutter heads.
Applicability – Any metal composition or hardness which can be sheared with straight blades can be sheared with rotary cutters. In general, rotary shearing in commercially available machines is limited to work metal of thickness 25 mm or less. There is no minimum thickness. As an example, wire cloth made from 0.025 mm diameter wire can be successfully sheared by the rotary method. Circles up to 3 m in diameter or larger can be produced by using special clamping equipment. Minimum diameters depend on the thickness of the work metal and the size of the rotary cutters. Typically, with material up to 3.2 mm thick, the minimum circle which can normally be cut is 150 mm in diameter. For 6 mm thick work-piece, the minimum diameter is 230 mm, and for 25 mm thick work-piece, the minimum diameter is 600 mm.
Rotary shearing is limited to cutting one work-piece at a time. As in straight-blade shearing, multiple layers cannot be sheared, since each layer prevents the necessary break-through of the preceding work-piece. Rotary shearing, plasma cutting, laser cutting, water-jet cutting, gas cutting, and electric arc cutting are competitive for some operations. Each can produce straight or bevelled edges of comparable accuracy. The selection of one of these processes depends largely on the thickness of the work metal.
In general, rotary shearing is used for sheet and plate less than 13 mm thick, laser cutting is used for sheet and plate less than 25 mm thick, and gas cutting is used for thicknesses of 13 mm or more. Gas cutting is less suitable for cutting a single thickness of sheet or thin plate since the heat causes excessive distortion, but it is frequently feasible to minimize this issue by stack cutting (cutting several thicknesses at a time). Gas cutting is more versatile than rotary shearing, since it can produce smaller circles than rotary shearing and can produce rings in one operation. Gas cutting, however, produces a relatively large heat-affected zone (HAZ) on the work-piece.
Laser cutting produces an extremely narrow kerf (cut) which provides unmatched precision for cutting small holes, narrow slots, and closely spaced patterns. Complex openings, contours, and patterns, which are impossible to cut with conventional tools, are routinely cut using a laser and need little if any additional processing. A smaller HAZ than traditional thermal cutting processes minimizes distortion and improves part quality.
Circle generation – For cutting circles, the work-piece is placed in a special fixture consisting of a C-shaped, deep-throated frame having a rotating pin or clamp point at its outer extremity. The maximum circle which can be sheared is governed by the depth of the clamp throat and by the quantity of clearance necessary to permit the rotating work-piece to clear the deep part of the C-frame on the machine. Hence, when using square blanks, removal of the corners allows a larger circle to be cut.
There are two methods of holding the centre point of the work metal during circular shearing. In one method, the work metal is clamped by a screw-type hand-wheel or by an air cylinder, each of which incorporates two pivoting pressure disks, one above and one below the work-piece. The disks permit the work-piece to rotate in a horizontal plane. The other method is by centre pinning, in which a hole is drilled or punched in the work metal for locating and rotating it on a pin in the centre clamping attachment. The hole is at the pre-determined centre of the circle to be produced.
Of the two methods, centre pinning provides higher rigidity since the work metal cannot slip off the centre during shearing. The circle generated when the work metal is held by the clamping method is not going to be perfectly round if the clamping fixture has not been properly located or if it has shifted because of pressure on the cutters. The disadvantage of centre pinning is that a hole is to be made in the work and is to be closed by plug welding if it is not wanted in the finished product.
Adjustment of rotary cutters – The upper cutter head and drive of a rotary shear is raised and lowered by power, a clutch mechanism limits the upward and downward travel. Power movement of the upper cutter is necessary (especially when cutting plate material) since the shearing edges of the cutters are to be moved toward each other in proper alignment in order to create the initial shearing action. In setting up, the work metal is frequently rotated in the clamp attachment, with the cutter exerting light pressure, to determine whether a true circle is being generated. Additional pressure is then applied by the vertical screw-down of the upper cutter to cause sheari
Only the upper cutter is rotated by the power-drive mechanism. The pinching and rotating action of the upper cutter causes the work metal to rotate between the cutters, and the metal causes the bottom cutter to rotate. The position of the upper cutter in relation to the lower cutter is important. Fig 7a shows the setting for shearing a straight edge. Clearance between the cutters is as important as it is with straight blades. Overlap of the cutters, as shown in Fig 7b, produces a bevel cut. The degree of bevel up to a maximum of 30-degree can be adjusted by changing the quantity of overlap of the cutters.
Accuracy – Accuracy of the sheared circle depends on the rigidity of the centre clamping device, sharpness of the cutters, maintenance of optimal clearance between the cutters, thickness of the work metal, and cutting speed. For work metal up to around 3.2 mm thick, dimensional accuracy within + 0.8 mm can be achieved when generating a 750 mm diameter circle. With proper set-up of equipment, the sheared edge shows only a slight indication of the initial penetration.
Speeds – Speeds of 2.5 m/min to 6.5 m/min are normally used for the rotary shearing of metal up to 6 mm thick. Speeds of 1.5 m/min to 3 m/min are used for rotary shearing metal which is 6 mm to 25 mm thick.
Flanging and joggling – With cutters replaced by forming tools, the rotary shear can be used to form flanges and joggles on-flat material. The maximum joggle which can be produced is normally limited to the thickness of the work metal. Since the work metal is made to flow into a different shape during flanging or joggling, the quantity of energy needed reduces the capacity of the machine to 75 % of the rated capacity for shearing. Fig 7c shows a typical setup for forming a joggle.
Safety – Shearing machines are to be equipped with devices for protecting personnel from the hazards of shear blades, fly-wheels, gears, and other moving parts. The guards and safety devices used are to be sufficiently rigid enough to withstand damage from operating personnel moving heavy material into position. The squaring shears used for sheet metal is required to have guards on all moving parts, including fly-wheels, hold downs, and blades. The control lever, whether mechanical or electrical, is required to have a lock for supervisory control. Blade and hold down guard openings are to be large enough to provide visibility but small enough to keep the fingers of the operators out of the danger area. Proper opening dimensions are outlined in standards.
The shears used for shearing plate are more difficult to safeguard because of the higher clearances needed under the hold downs and upper blade to permit entry of the plate (especially when it is bowed or buckled). Guards on shears for plate are to be of the type which raises only when the plate is inserted and then rests on the surface of the plate. When there is no work-piece in the machine, the guards rest within 6 mm of the surface of the table.
Shears are to comply with the construction requirements of the safety standards. Safety regulations also cover the maximum noise level permitted from a shearing peration to prevent permanent impairment of hearing.