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1. De Cambering
Steel Sheets are delivered from the steel mill in roll form. For fabrication purposes, the roll has to be split into sheet form. If the sheet is slit from the coil without de cambering, it tends to form camber or warping. This will not only cause a considerable amount of inconvenience during fabrication but also affects the quality of the components. Decambering involves flattening or leveling the sheet to make it crisp straight before being made available to the fabrication industry.
Adapted in low quantity manufacturing, sometimes even without electricity, there are hand-operated and foot-operated machines available for the purpose. Metal shearing can be performed on sheet, strip, bar, plate, and even angles, bars can be cut to length. Many geometrical patterns can be produced by shearing sheets and plates.
Hand and foot-operated shears usually result in visible deformed or warped edges along the line of cut and will require a post-operation finish. Power-operated production machines have provisions to minimize warping.
Shearing machines are normally called guillotine shears. Electric-powered machines are fast with a higher production rate than hydraulic shears. On the other hand, Hydraulic powered machines though a bit slower offer a better cut. The quality of shear ultimately is decided upon the sharpness of the blade, gap between upper and lower shearing blades. Power shears are equipped with “back Gauges’ which are essential stoppers to stop the sheet when it is pushed ahead between the blades. Depending upon the quality of these stoppers, a sheet can be sheared to an accuracy of +/_ 0.1 mm of the desired cut size.
Shearing operation of a sheet is normally carried out between a lower fixed blade and an upper moving blade. During the shearing operation, the sheet first undergoes through plastic deformation then a fracture occurs and ultimately shear happens.
The next most used of all operations in sheet metal is Punching. Punching is done by way of using Presses either hand-operated or mainly power-operated presses in semi-precision to high precisions operations. Punching is done to create holes, hole patterns, cut outs, embossing, shearing, plunging to mention a few.
A punching operation requires a die and a punch. The sheet is inserted between the die and the punch and force are applied. The sheet then takes the shape of the clearance between the die and punch. The size and complexity of the punched piece depend upon the quality and precision of tools and tonnage of the presses. Punch removes material from the Blank with the support of a Die.
Droops and Burrs
When the sheet is punched, few things happen to the sheet. A droop forms above the shear cross-section, fracture happens below the shear cross-section and burr formation happens below. They can be controlled with the right type of tooling with respect to sheet thickness. Burrs are removed in post-processing by way of the sander and grinding.
All most of all sheared and punched parts will have edge burrs which require removal using grinding and sanding tools, abrasive sheets, or abrasive blasting. The quality of tools used in punching influence the quantum of post-processing. Small-time fabricators instead of using good-conditioned tools opt for more post-processing work resulting in more man-hours of work. In high production, rate works automated methods are used. It is advisable to minimize post-processing works.
A good quality punched component depends mainly upon four components – The Die, the Punch, the Stripper, and the clearance between die and punch. Less the clearance between die and punch, better is the sharpness of punched component. Improper tooling results in more post-processing such as grinding and sanding of the edges of components adding to costs.
Nibbling is a process by which material is removed to form a cut by a series of overlapping strokes of the punch. It is a low-cost machine that can perform versatile operations. The nibbling machine operates the punch at a rapid rate of around 500 to 600 strokes per minute. Irregular contours can be cut conveniently. This process is often adapted to make prototypes and to produce a low quantity of parts. The limitations are the depth of the arbour which limits the length of the sheet to be placed on the machine and the size of the tools. Large size punches and dies are generally not used and the thickness of the sheet is limited to 6mm mild steel.
Computer Numerically Controlled (CNC) machines have virtually replaced traditional power presses for punching sheet metal. They are now central and foremost to the sheet metal fabrication industry. A CNC turret punch press contains a rotating turret that can hold a number of punches, some as many as 36 punches in the upper arbour of the machine. A die cavity is incorporated in the bed of the machine where the desired punch in the turret aligns with the die as per the program. There are varieties of machines with a varying number of tools in the turret, speed, and force. The CAD software optimises the layout of the sheet for maximum utilization. CNC punching process is a cheaper alternative to laser cutting in comparison with price and quality.
CNC Punching can be done on all sheet thicknesses available in the market. It is however advisable to keep the thickness under 6 mm.
Another important feature of the CNC punch press is that it can generate 3D profiles such as dimples, knockouts which are not possible in other high-tech cutting machines like water jet or laser cut machines. Cutting machines are plain cutting machines while punch press is also a pressing machine.
5. CNC Laser Cutting
CNC laser cutting is like CNC press punching except that the machine uses a laser beam to cut the sheet instead of punches and dies. The programming, nesting is all like in a punching machine, except that the process of cutting is different. Instead of a cutting tool, a high-energy laser beam generates heat sufficient enough to burn through the sheet. Depending upon the programme the laser beam moves along the determined path and cuts the material to the desired shape. Laser cutting is suitable for a wide range of materials, including metal, plastic, wood, glass, and paper. Complex and intricate parts can be produced without any mechanical tooling.
One main advantage of laser cutting over punching is that curved lines are better defined and cleanly finished because there are no serrations formed as in the case of punch and die-cutting. The cutting width of the laser is as small as 0.02 mm and the dimensional accuracy of cutting is in the range of +/_ 0.01 mm.
On the other hand, laser cutting demands high power consumption and the beam is delicate to handle. There is a limitation in cutting thick materials and materials like reflective materials Also, good care is required when metals like copper, cobalt, manganese are made to cut.
Water jet cutting machines use a high-powered water jet usually mixed with coarse powdered abrasive materials capable of cutting a wide variety of materials. A beam of water is ejected from the machine nozzle cutting through the material with a jet of a speed of over 700 m/s. Abrasive materials such as aluminium oxide are mixed with water at the end of the discharge nozzle. The mixture discharged at high speeds cuts through the materials.
Water jet cutting is the preferred method when the materials being cut are sensitive to the high temperatures generated by other methods. The width of the cut can be as small as 0.50 mm and the dimensional accuracy is about 0.15 mm. Water jet cutting is more effective as compared to laser cutting of punching in cutting thicker materials. There are 3 axis and 5 axis machines also available for intricate 3-dimensional profile cutting. Laser cutting and water jet cutting are both high-tech, highly accurate forms of sheet metal fabrication. However, laser cutting costs less than waterjet cutting.
CNC routers conduct milling operations. There are both benchtop and floor-standing industrial versions available. Almost all materials metal to plastic to leather can be routed using CNC routers. CNC routers. However, they are mainly used in the sign industry to route graphic patterns on aluminium and aluminium composite panel sheets. Modern routers can accommodate up to 2000 mm wide sheets with the length being not a constraint.
|Comparison of CNC Punching, Laser, Water Jet and Routing processes
|Accuracy mm||+/_ 0.1||+/_ 0.025||+/_ 0.05||+/_ 0.5 – 1.0|
|75.00||M.S < 6.00||M.S up to 3.00 Al up to 6.00||up to 25.00 for wood Up to 3.00 for Al|
|Heat Effected Zone||No||Yes||No||No|
|V Grooves possible|
Blanking is a process during which a metal workpiece is removed from the primary metal strip or sheet when it is punched. The material that is removed is the blank.
9. Fine Blanking
Fine blanking is where the component under blanking is subjected to an equal amount of pressure from both the die and punch. As compared to the normal blanking process, fine blanking provides excellent definition, flatness, and excellent edge quality. A remarkable feature of fine blanking is that shearing happens with practically no fracture zone. Under ideal conditions, very close tolerances between +/- 0.01–0.05 mm are possible, in fine blanking depending on the base material thickness, tensile strength, and the quality of the tooling.
Chemical blanking process is adapted to produce machined features on thin sheets and foils. The material is actually etched out by a chemical process. In some sense, it is chemical milling. Certain micro components like very thin gears, levers are done by chemical blanking.
Chemical etching or milling is also done to produce shallow cavities with complex profiles on the surface of metal sheets and plates. The depth of material removed can be as deep as 10 mm.
Lancing is a process wherein the material from a sheet is sheared and expanded further to form a profile in the same stroke. There is no loss of material but only a modification carried out in the sheet with the parent material still attached to the slit material. Lancing process is used to make ventilating louvers, tabs, bridges, flanges, and the like. Lancing is recommended to be done along the grain lines of the sheet.
Embossing is used to create raised or sunken reliefs in ductile sheet metal. This can be done either by pressing the sheet between a die and punch or by passing through rollers with the etched patterns. Roller embossing gives a high rate of production. Embossing is sometimes combined with foil stamping to create decorative 3D effects. Embossing is usually limited to a maximum of 4 to 5 mm in depth.
Coining as the name implies is used to produce coins, buttons, badges, etc., It is a kind of precision cold forging process wherein the material is tightly compressed between two dies using a good amount of force. The surface of the material is subjected to such high stress that it induces plastic flow on the surface to produce finer definitions. The surface of the sheet is subjected to work hark hardening. Coining ensures very close tolerances, smoother surfaces, and practically no tapered edges.
Bending is the most widely used operation in sheet metal fabrication. The machine that carries out sheet bending operation is called a “Press Brake”. For small size manual bending “Fly Press” is used. Press brake also comes with a manual or CNC-controlled back gauge that allows positioning the bend line accurately.
The sheet is placed on a “Die” with a ‘V” groove and the “Punch” applies downward pressure on the sheet above the die groove causing the sheet to bend. The bending process can be vertical or horizontal. The sharpness of the bend depends upon the groove width, sheet thickness, and the force applied. However, one hundred percent sharp bends are practically impossible due to the thickness of the sheet and in turn, their bending factors involved.
Metal sheets have grain directions. These are formed at the time of manufacturing the sheet when the sheet is rolled and the grains elongate due to pressure in the direction of rolling. During the bending process the sheet contracts inside due to compressive forces and expands on the outside due to tensile forces. The bend line should preferably be perpendicular to the line of grain flow. While it may not always be possible, at least important bend lines should follow this guideline. The type of grain has also some influence on the bending force to be applied.
When a sheet is bent with a stroke of the punch and when the punch is withdrawn, invariably the sheet springs back a little. On the shop floor, the press brake operator applies some allowance known as “spring back allowance” to compensate for the deviation. If bending is carried out little more than theoretical normal, the sheet springs back to the desired dimension. This practice is more of a trial and error method. The spring-back effect may vary from batch to batch of materials as well cold rolled or hot rolled sheets.
When a part is folded to form a three folded structure, as shown above, a corner relief hole is important for a sharp and undistorted corner joint. Without this hole, there will be interference of material from the three sides and tend to form a bulge. Also, the sheet tends to develop some stresses and tear even when two adjacent bends meet each other. In general, a minimum bend relief is equal to the material thickness plus the inside bend radius.
In a simple tray-type bending the vertical gap between two adjacent folds in the corner always is an irritant. If it is filled with weld seam, the subsequent griding makes an appearance not very appealing, if not filled the gap may not be uniform. Unsupported vertical edges tend to show some buckle. To overcome this problem, flange material is made to firmly sit on the adjacent sheet edge as shown in fig B or corner bends are incorporated as shown in fig C.
An example of bending a sheet metal part without a bending machine. Slots can be cut at the bending line to selectively weaken the material. The tabs can then be bent manually using a plier or even folded by hand. Since the slots weaken the material, such practices are not suitable for load-bearing, but suitable for small prototype enclosures.
Forming is essentially causing deformation to the sheet metal by applying pressure to derive curvy or other intricate shapes. Almost all forming operations are cold working, which means they are not heat softened but worked on using the ductility properties of the sheet.
Bending and Forming are two different operations where the sheet metal is bent along an axis. When the axis of bent is a straight line then it is called bending and when the axis of bend is curved it is known as forming. The common feature between these two operations is that there is plastic deformation only near the axis of a bend.
16. Deep Drawing
Deep drawing is one of the most widely used processes in sheet metal forming. It is widely used in the automobile industry to produce large auto body parts, in domestic products to produce kitchen sinks vessels with large depths, one side closed-end-tubes, etc. Deep drawing preferably requires softer and stretchable materials such as aluminium, copper, brass, while stainless of particular grades are also used with higher forces. Deep drawing occurs under both tensile and compressive conditions and is generally carried out in stages.
During deep drawing, work hardening takes place on the material. Therefore deep drawn products are considerably stronger.
Progressive Die Stamping.
Stamping is adapted in the high rate production of small-sized components such as lugs, terminal strips. Stamping may involve a single punching method or a combination of punching processes such as press punching, blanking, bending, coining, embossing, flanging, etc.,
18. Spin Forming
The spinning process is a process by which a disc or tube of metal is rotated at high speed against a mandrel of the desired shape and formed into an axially symmetric part. Spinning process does not involve material removal but essentially involves press forming over an existing shape. Commercial applications include nose cones, gas cylinder parts, kitchen utensils, etc.,
Explosive forming is a technique in which explosive charges are used instead of a punch to form the component against a cavity. Explosive forming offers interesting and unique possibilities where conventional methods cannot be applied such as one-off-large-sized components, or when very high pressures are required in a very short span of time.
Sheet sizes of more than 10 sq. mt, various types of metal sheets of varying thicknesses some upto 100 mm have been successfully formed by explosive forming.
In the stretch forming method, the sheet is stretched and bent simultaneously over a block to form large contoured components.
The sheet is firmly gripped at the ends by gripping jaws and placed over the stretch block. The block which has the desired curved contour is moved upwards to deform the long sheet. Any ductile sheet can be formed by this method, but normally aluminium alloy sheets are used to form large parts like aircraft fuselage cladding. Stretch forming as can be understood is limited to one side countered curvature only but not dual axis forming like bowls.
Rolling is the process of inducing curvature in sheets. In rolling the sheets are rolled between a set of rollers into cylindrical or any curved shapes. Rolling of sheets with varying radii is also possible. The number of rollers typically varies from three sets to seven sets depending upon the accuracy required. There are simple manually operated machines to CNC controlled power machines are available. In these machines, not only sheets but profiles such as flats rods can also be rolled. The rollers in such cases will have grooves to accommodate the profile of the stock to be rolled.
Rolling when done with proper engineering allows precision joinery to the closest possible tolerances. There can be no other way to form radii in sheets of high thickness and large sizes as possible with the rolling process.
Roll forming is a continuous process that converts a sheet by passing through a series of rollers into complex profiles. It is economical than press brake operations to produce components in large quantities. Since the raw material stock is normally a coil, there is no limitation on the length of the part other than handling issues. Almost all ferrous and non-ferrous materials such as steel, stainless steel, aluminium, and special alloys can be roll-formed. Since it is a continuous line process, secondary operations such as painting, powder coating, galvanizing even mechanical operations like holes, hemming, knockouts can be integrated within the production line making it a very cost-effective process. The number of rollers sets in roll forming typically varies from three to nine. The higher the sets, the more intricate are the profile of the derived component.
Screws require threading to lock themselves in. Plunging operation allows part of the parent material to squeeze out and with enough wall thickness to permit threading by way of tapping.
Plunge and Tap accommodate screws without any nut. The basic material of the sheet is made to Stretch down without tearing to make allowance for tapping. This process is generally limited up to M5 size screws.