Introduction:
Products made of sheet metals are everywhere. They include beverage cans, cookware, file cabinets, metal desks, appliances, car bodies, trailers and aircraft fuselages. Sheet forming dates back to about 5000 B.C., when household utensils and jewelry were made by hammering and stamping gold, silver and copper. Compared to castings and forgings, sheet metal parts offer light weight and versatile shape. Numerous processes exist for making sheet-metal parts. The term pressworking or press forming is common in industry because these operations are typically performed on presses using dies. A sheet metal part produced in presses is called a stamping, after the word stamp meaning to force downward or to pound. This term parallels forging or casting. Low carbon steel is the most common sheet metal due to low cost and good strength and formability. Newer alloys like TRIP and TWIP steels are popular for automotive applications because of high strength, offering good crash protection in lightweight designs. Aluminum is the most common for beverage cans, packaging, kitchen utensils and corrosion resistant applications. Aircraft and aerospace materials are aluminum and titanium, but they are increasingly replaced by composites. Most sheet metal processes are at room temperature. Hot stamping increases formability and reduces loads, typical materials are titanium alloys and high strength steels. This article describes blank cutting from large sheets, subsequent forming methods, characteristic features of sheet metals, formability tests, forming‑limit diagrams and all major processes and equipment.Sheet metal forming is a cornerstone of manufacturing, offering versatility, light weight and cost effectiveness across countless products. From basic shearing to advanced techniques like fine blanking, FLDs, and specialized processes, the industry continuously improves precision and efficiency. Each process has unique advantages, limitations, and cost drivers. The comprehensive array of methods like bending, drawing, spinning, hydroforming, explosive, electromagnetic, peen, laser, micro and others ensures that virtually any shape can be produced economically when the right process is matched to the application.
Shearing:
Before forming, a blank is removed from a large sheet (often a coil) by shearing—cutting with shear stresses using a punch and die. Sheared edges are not smooth nor perpendicular. Cracks form on top and bottom edges and meet to separate the material. Rough fracture surfaces are from cracks; smooth burnished surfaces result from rubbing against punch and die walls.Key parameters:
It include punch/die shape, punching speed, lubrication and clearance (c) between punch and die. Clearance is major: larger clearance increases deformation zone and roughens edges, the sheet pulls into the clearance making edges rougher. Secondary operations may be needed, raising cost. Edge quality improves with higher punch speed, up to 10‑12 m/s. Sheared edges undergo severe cold working, reducing ductility and affecting subsequent bending/stretching. The burnished to rough area ratio increases with sheet ductility and decreases with thickness and clearance. Deformation zone extent depends on speed, higher speed confines heat, giving narrower, smoother sheared surfaces with less burr. Burr height increases with clearance and ductility, dull tool edges contribute greatly. Burr shape/size affect subsequent operations, deburring processes exist.Common Shearing Operations:
Punching (slug is scrap) and blanking (slug is the part) are the most common. These and others are often performed on CNC machines with quick change toolholders useful for prototypes.Die cutting includes:
Perforating: punching multiple holes.
Parting: shearing into two or more pieces.
Notching: removing pieces from edges.
Lancing: leaving a tab without removing material.
Perforated sheet metals with hole diameters from ~1 mm to 75 mm are used as filters, screens, ventilation, machinery guards, noise abatement and weight reduction. They are punched in crank presses at rates up to 300,000 holes/min with special dies.
Fine blanking produces very smooth, square edges with clearances about 1% of thickness (typically 0.5‑13 mm). A V‑shaped stinger locks the sheet to prevent distortion. Dimensional tolerances are ±0.05 mm, and edge perpendicularity can be within ±0.025 mm. Developed in the 1960s.
Slitting uses circular blades (like a can opener) along straight, circular, or curved paths. Slit edges have burrs that may be rolled flat. Improper slitting causes distortions.
Steel rule dies are thin hardened steel strips bent to shape (like a cookie cutter) on a wood/polymer base, used for soft metals, paper, leather and rubber.
Nibbling uses a small punch moving rapidly; the sheet is fed to make overlapping holes along any path. Intricate slots/notches can be made with standard punches; economical for small runs because no special dies are required.
Scrap can be as high as 30% on large stampings. Nesting efficient arrangement of shapes reduces scrap, CAD techniques help minimize it.
Formability Tests:
Sheet metal formability is defined as the ability to undergo shape change without necking, cracking or tearing. Two basic modes: stretching and drawing. Different parameters apply.Cupping tests (Erichsen test):
A sheet clamped between flat dies and a ball/punch forces into it until cracking; punch depth at failure measures formability. Easy but not reliable for complex parts.
Forming limit diagrams (FLDs):
A grid of circles (2.5‑5 mm, preferably as small as practical) is marked on the blank. The blank is stretched over a punch, and circle deformation is measured in failure regions (necking/tearing). FLDs predict formability more accurately.
Bending and Related Operations:
Press brake forming:
Bends sheets or strips up to 7 m long using long dies in mechanical/hydraulic presses, suitable for small runs. Tooling is simple, motions up/down, easily automated. Die materials range from hardwood (low strength, small runs) to carbides (abrasive materials); carbon‑steel or gray‑iron dies are common.
Four slide machine:
Bending handles short pieces; lateral die movements are synchronized with vertical movement to form shapes like seamed tubing, conduits, bushings, fasteners, and machinery components.
Roll bending uses three rolls; adjusting distances gives various curvatures. Flexible and used for boilers, pressure vessels, curved structural members. A compliant polyurethane roll conforms to the strip as the upper roll presses.
Beading bends the sheet periphery into a die cavity, increasing moment of inertia (stiffness), improving appearance, and eliminating sharp edges.
Flanging bends edges typically to 90°.
Shrink flanging subjects the flange to compressive hoop stresses; excessive stress causes wrinkling, increasing with smaller radius.
Stretch flanging subjects the periphery to tensile stresses; excessive stress causes cracking along the periphery.
Roll forming forms continuous lengths for large runs. Metal strip passes through consecutive rolls; formed strip is sheared to length. Products: panels, frames, channels, gutters, siding, pipes/tubing with lock seams. Length limited only by coil stock. Thickness typically 0.125‑20 mm. Speeds generally below 1.5 m/s. Roll design/sequencing requires experience; tolerances, springback, tearing, buckling must be considered. Rolls are carbon steel or gray iron often chromium plated for finish and wear. Lubricants reduce wear, improve finish, cool rolls and sheet.
Tube bending and forming requires special tooling to prevent buckling/folding. Old method: pack with sand, bend, then remove sand. Flexible internal mandrels serve the same purpose. Thick tubes with large bend radius can be bent without fillers.
Dimpling, piercing, flaring:
In dimpling, a hole is punched then expanded into a flange. Piercing with a shaped punch produces flanges. Tube ends can be flanged similarly when bend angle <90° (conical ends), it is called flaring. Edge condition is important; stretching creates tensile hoop stresses that can crack/tear. As flange to hole diameter ratio increases, strains increase. Rough edges promote cracking; shaving improves edge finish.Hemming folds the edge over itself, increasing stiffness, improving appearance, eliminating sharp edges.
Seaming joins two edges by hemming; double seams use shaped rollers for watertight/airtight joints (food/beverage containers).
Bulging places a tubular/conical/curvilinear part in a split female die and expands it, usually with a polyurethane plug. Retracting the punch allows the plug to recover, the part is removed by opening dies. Products: coffee/water pitchers, beer barrels, oil‑drum beads. Shaped plugs apply higher pressure at critical regions. Polyurethane plugs resist abrasion and do not damage surface finish.
Segmented dies are individual segments placed inside the part and expanded radially; they are retracted to remove the formed part. Inexpensive and usable for large runs.
Stretch forming clamps sheet edges and stretches over a male die (form block/punch) moving up/down/sideways. Used for aircraft wing‑skin panels, fuselages, boat hulls. For Boeing 767/757 aluminum skins, tensile force 9 MN; sheets are 12 m × 2.5 m × 6.4 mm. Generally low‑volume but versatile and economical for aerospace. Blank is rectangular, clamped on narrow edges, stretched lengthwise, allowing width shrinkage. Control stretching to prevent tearing. Cannot produce sharp contours or reentrant corners. Accessory equipment includes further forming with both male/female dies under tension. Dies: zinc alloys, steel, plastics, wood; little or no lubrication.
Deep Drawing:
Many cylindrical or box shaped parts pots, pans, containers, sinks, canisters, fuel tanks are made by forcing a punch into a die cavity. The process is called deep drawing though it also produces shallow parts. It is one of the most important metalworking processes. A round blank is placed over a die opening held by a blankholder and the punch forces it into the cavity to form a cup.Rubber Forming and Hydroforming:
Rubber forming (Guerin process) uses a flexible die typically polyurethane membrane, for one of the die halves. Polyurethanes offer abrasion resistance, fatigue life, and resistance to cutting/tearing. In bending/embossing, the female die is replaced by a rubber pad. The outer sheet surface is protected from scratches. Pressures ~10 MPa.Hydroform (fluid‑forming) controls pressure over the rubber membrane up to 100 MPa during the cycle, preventing wrinkling/tearing. Deeper draws are possible because pressure forces the cup against the punch, increasing friction and reducing tensile stresses, delaying fracture. Friction control and proper lubricants are critical.
Tube hydroforming forms metal tubing in a die with internal fluid pressure usually water. It can form simple tubes and intricate hollow shapes for automotive‑exhaust and structural components. Advantages: complex shapes, laminated sheets/coatings, flexibility, no surface damage, low die wear, low tooling cost.
Spinning:
Spinning forms axisymmetric parts over a mandrel using tools/rollers, like pottery on a wheel.
Conventional spinning:
a circular blank is held against a rotating mandrel while a rigid tool deforms it. The tool may be manual or computer‑controlled. Requires skill and multiple passes. Suitable for conical/curvilinear shapes that are difficult/uneconomical otherwise. Diameters up to 6 m. Most at room temperature; thick/high‑strength/low‑ductility metals require elevated temperatures.Shear spinning:
(power spinning, flow turning, hydrospinning, spin forging) produces conical/curvilinear shapes, reducing thickness while maintaining blank diameter. One or two rollers (two preferred for force balance). Parts: rocket motor casings, missile nose cones. Diameters up to 3 m. Little waste, short cycle (seconds). Tooling simple (tool steel). Spinnability is maximum thickness reduction without fracture, related to tensile reduction of area. If reduction of area ≥50%, thickness can be reduced 80% in one pass. Low‑ductility metals require elevated temperatures (heating and rapid transfer).
Tube spinning:
reduces thickness of hollow cylindrical blanks on a mandrel using rollers, lengthening the tube. Can be external or internal; various profiles from constant‑wall blanks. Forward/backward spinning analogous to extrusion. Maximum reduction per pass similarly related to tensile reduction of area. Uses: rocket, missile, jet‑engine parts, pressure vessels, automotive wheels.
Specialized Forming Processes:
Explosive forming uses explosives as an energy source. The blank is clamped over a die, lowered into a water tank, air evacuated, explosive charge detonated. Shapes possible if material is ductile at high deformation rates. Versatile, no size limit; suitable for low‑quantity large parts (aerospace). Steel plates 25 mm thick and 3.6 m diameter, tubes with 25 mm wall have been formed. Smaller scale uses canned explosive for thin‑walled tube bulging/expanding. Mechanical properties similar to conventional. Dies: aluminum alloys, steel, ductile iron, zinc alloys, reinforced concrete, wood, plastics, composites.
Electromagnetically assisted forming (magnetic‑pulse) discharges capacitor bank through a coil. A ring coil over a tube collapses it by magnetic forces. Eddy currents in the tube create opposing fields. Higher workpiece conductivity increases forces; magnetism not required. Advantages: increased formability, improved accuracy, reduced springback/wrinkling. Coil design critical. Flat coils for embossing/shallow drawing. Effective for aluminum alloys. Since the 1960s, used for collapsing tubes over rods/cables/plugs, compression‑crimp sealing oil filters, specialized sheet forming, bulging/flaring, swaging end fittings for Boeing 777 torque tubes.
Peen forming produces curvatures by shot peening one surface. Compressive stresses expand the surface layer; the unpeened substrate resists, causing curvature. Also induces compressive residual stresses improving fatigue. Cast‑iron/steel shot from wheel or air blast. Used for wing skins; shot 2.5 mm diameter at 60 m/s formed panels 25 m long; heavy sections use 6 mm shot. Also straightens twisted/bent parts and out‑of‑round rings.
Laser forming applies laser beams as heat sources, creating steep thermal gradients that cause localized plastic deformation permanent bending without dies. In laser assisted forming, the laser reduces strength locally, improving formability and flexibility. Applications: straightening, bending, embossing, complex tubular/flat components.
Microforming produces very small parts (submillimeter, milligram weights). Examples: wristwatch with camera and 1 GB storage, micromotor shafts, springs, screws, coldh aded, extruded, bent, embossed, coined, punched, deep drawn parts.
Electrohydraulic forming (underwater spark or electric discharge) uses a spark between electrodes connected by a thin wire; rapid capacitor discharge creates a shock wave. Energy lower than explosive (few kJ), pressure sufficient to form parts. Batch process for small parts.
Gas mixtures as energy source: ignite a gas mixture in a closed container; pressure forms parts. Similar to internal combustion engine, not often used.
Liquefied gases (e.g., liquid nitrogen) expand to gas at room temperature in a closed container, developing pressure to form shallow parts; not used in practice.
0 Comments