Metal spinning, or spin forming, is a metalworking process that transforms a flat circular blank or disc shaped workpiece into axially symmetrical round shapes. It is accomplished by the application of lateral force by a roller positioned against the surface of the disc blank and a mandrel that has the shape of the final part. Spinning along the axis of the mandrel at high speed transforms the metal workpiece into the desired shape. The mandrel supports the workpiece and gives it its final shape.
The process of metal spinning deforms a circular metal sheet such that it achieves various shapes without stripping away material. During spinning, the disc blank undergoes tensile and compressive forces as it is wrapped over the mandrel. Depending on the type of metal spinning used, the thickness of the original metal sheet may be reduced, a characteristic that is precision controlled to ensure the worked metal reaches the desired specifications.
Metal spinning is an ancient metalworking process that is a combination of art and science. It has been used for thousands of years dating back to the time of the Egyptians and has progressively improved and advanced over the years from a manual process to a technical manufacturing method involving the use of computers and computer numeric controlled (CNC) manufacturing.
Metal spinning became significantly important during the Middle Ages. Vases, tea kettles, and trophies were produced using the process, which were made on a wood spinning lathe using a rotational drive with a large flywheel that was manually powered by an assistant of the craftsman.
Although there have been significant upgrades to metal spinning, manual metal spinning is still one of the methods used to produce high tolerance and good finish quality products. The eye hand coordination and skills required to manually operate a metal spinning machine takes a long period of apprenticeship and a unique set of abilities.
Metal spinning machines, often referred to as spinning lathes or flow forming machines, are used to form and shape metal blanks into round, hollow cylindrical parts, cones, and complex contours. A rotating lathe and hardened roller apply localized pressure to accurately and efficiently shape blanks made of aluminum, stainless steel, copper, brass, and titanium.
The process of metal spinning uses advanced technology to deliver high repeatability and surface quality, while retaining flexibility for custom metal forming. The lathe is the primary machinery in the process. It rapidly spins the workpiece during the application of various tools. The other functions of the lathe include precision cutting, CNC drilling, and sanding. Custom mandrels are used by metal spinning to form the inner profile of a workpiece, which is coordinated with other components to form intricate shapes with exceptional tolerances.
The mandrel is a forming die that gives volume or shape to the metal disc and has the interior shape of the finished part. It supports the workpiece as it is rapidly spun. Continuous rotation and spinning of the workpiece ensures that it is deformed evenly without wrinkling or warble.
Prior to being placed on the mandrel, the metal disc is lubricated with grease or wax to make it easier to remove after shaping. Lubrication improves and enhances the surface finish of the final product.
Mandrels are machined from steel, aluminum, plastic, or wood. Less expensive materials are chosen for mandrels used for prototyping and short production runs. In all cases, the selected metal must be rigid enough to endure the force and stress applied by the rollers.
Rollers are a rigid tool that applies localized force over the mandrel to the workpiece for controlled plastic deformation. The force causes the workpiece to flow over the mandrel in order to transform it to the desired shape. The ball bearings of the rollers provide smooth operation, limited heat, and have minimal wear, which is critical for working hardened metals.
Adjustments to the rollers make it possible to balance the applied pressure and speed in order to produce uniform wall thicknesses and intricate contours. Rollers can be manually operated or CNC controlled, which has become more popular as a metal spinning process. Manual operation involves a tremendous amount of skill, training, and experience, skills that are necessary to compete with modern technological equipment.
The force from the rollers of metal spinning is much lower than that used by other metal working processes. High strength heavy gauge metals are formed to large diameters using the force of the roller with significant energy savings.
The work zone is a spiral path created by the rotation of the workpiece with the forward motion of the roller and the rotation of the workpiece. The result is seamless, high-strength spun parts with a twist or spiral that enhances their structural integrity.
The tailstock is located opposite the headstock. It holds the workpiece in place against the mandrel and supports it during its rotation. The tailstock is a precision adjustment mechanism that enables consistent pressure to securely clamp the workpiece, a factor that is crucial for ensuring accuracy during high-speed or heavy-gauge metal spinning.
The follower is a pressure pad attached to the end of the tailstock spindle to clamp the workpiece in place. Its size corresponds to the area of the base and matches the base of the spun part, evenly distributing holding force to prevent slippage. The follower ensures that the metal blank conforms to the mandrel during rotation. High-quality followers contribute to surface finish consistency and repeatability.
The spindle of the headstock is a shaft where the mandrel is mounted. It is responsible for rotating the mandrel and the workpiece during forming. A second rotating spindle is found on the tailstock where the follower is mounted. The main drive spindle serves as the axis for the mandrel and workpiece, providing rotational energy during the forming process. The tailstock spindle synchronized with the follower to ensure axial support and movement control. Direct-drive and belt-drive spindles are used to meet application speed and torque requirements.
The headstock is a frame that contains the mechanism responsible for transmitting the power required to drive the spindles and controls the spinning rpm. It contains the powertrain’s motors and gears for speed control, high torque, and spindle alignment. It supports and stabilizes the assembly during high-speed rotation, which are essential for safety and dimensional accuracy.
The lathe bed supports the headstock, tailstock, and the metal spinning machine components. It is the engineering foundation for the primary components with a rigid and vibration dampening. The lathe bed ensures that the headstock, tailstock, and tool rests are in alignment during high operational loads. An accurately machined and robust lathe bed ensures process consistency and equipment longevity.
CNC metal spinning involves the use of a lathe that controls and automates the metal spinning process. The initial steps of CNC metal spinning are similar to manual spinning. The operator places the workpiece on the CNC lathe drive where it is secured using a pressure pad.
Computer aided design (CAD) is used to create the form and shape of the final part. The parameters of a design are changed into G-codes and M-codes that are downloaded into the CNC metal spinning machine. They guide the components during the spinning process. When the CNC lathe is activated, it rotates and presses the workpiece against the mold. The rotational power of the CNC lathe, as with manual lathes, deforms the metal piece to the shape of the mold.
CNC metal spinning is low cost, provides superior tensile strength, excellent finishes, and refined grain structures. Dimensional stability is within 1000th of an inch or 100th of a millimeter. CNC metal spinning is used by industries that require low- to high-volume production and is used to produce aerospace components, lighting reflectors, gas cylinders, and custom engineered industrial products. The automation of the CNC process reduces operator error, cuts production costs, and is sufficiently flexible to allow quick switching between product runs.
With PNC metal spinning, a joystick is used to shape the workpiece manually. Once the designed shape has been achieved, the programmed changes are saved and used multiple times repeating the copied moves of the operator. Small changes and adjustments to the program help increase the accuracy of results, such as removing inefficient aspects of the program and replacing them with more efficient ones.
PNC metal spinning is effective for high volume production. In the playback mode, the lathe repeats the forming process as many times as necessary producing exact duplicates of the original part. An operator places the blank on the lathe and actuates the playback cycle.
Spinning is classified as compressive draw forming because of the radial, tensile, and compressive stresses that influence the metal flow in the localized work roller zone and adjacent areas. Tensile stresses are generated when the roller contacts the workpiece and rises with increasing axial roller feed to generate volumetric displacement in a plane radial to the mandrel.
Since the volume of the metal decreases closer to the center, compressive stresses develop between the volumes. These stresses are responsible for an increase in the thickness at the open end and the potential of buckling and wrinkling. This can be compensated by the direction of the feed.
In conventional spinning, the roller pushes the workpiece over the contour of the mandrel. The material thickness of the workpiece remains constant throughout the process. After forming, the depth of the workpiece increases while its diameter is reduced. It is considered the simplest type of metal spinning and requires simpler tooling and machinery.
In shear spinning, the rollers exert downward force to the workpiece as it moves over the contour of the mandrel. The material thickness of the finished part is less than that of its original blank form, but its diameter remains constant. The depth of the workpiece also increases. Since there are greater compressive forces acting on the workpiece, its mechanical properties (e.g., increased strength and hardness) are also enhanced.
Shear spinning requires a more robust tooling design and precise machining control because it all affects the dimensional accuracy and surface finish of the part. It imparts more friction on the workpiece and wear to the mandrel. Coolant is needed as this process generates a lot of heat.
The thickness of the resulting material in a shear spinning process is theoretically determined by the Law of Sines. According to the Law of Sines, the thickness of a part is equivalent to the original thickness of the flat metal sheet multiplied by the sine of the semi-apex angle of the cone. It is only applicable if the part has a conical profile formed by a single pass shear spinning. The unformed metal perpendicular to the spinning axis will retain its original thickness. The Law of Sines is a mathematical equation that relates the lengths of the sides of a triangle to the sines of its angles. It is used to compute the sides of a triangle when two of the triangle’s sides are known, a mathematical method known as triangulation.
In hot spinning, the temperature of the workpiece is brought to the forging temperature with the help of a heating torch. The heating torch is directed to the workpiece while it is being pressed over the mandrel. Hot spinning technique is used if the material has low ductility and malleability or if the metal sheet is too thick and therefore difficult to deform at room temperature. Spinning performed at room temperature (cold spinning) is suitable for any type of metal.
Since the workpiece is deformed in its plastic state, the hot spinning induces large amounts of deformation and the grain structure is refined as well, resulting in improved physical properties such as increased strength. However, the heated metal oxidizes rapidly. The overall process is difficult to control and is more expensive.
Tube spinning is a form of shear spinning used to elongate and reduce the wall thickness of hollow cylindrical tubes. The tube is first mounted and clamped in the mandrel. Drawing the tube over the length of the mandrel is accomplished by three or more rollers spaced equidistantly around the tube. The direction of the axial flow of material is similar to the direction of roller movement.
Tube spinning can be performed externally or internally on the mandrel. In external tube spinning, the tube moves over the outer surface of the mandrel. With internal tube spinning, the tube is spun inside a hollow mandrel.
Tube spinning can be used in fabricating tubes with multiple diameters if the wall thickness is not a concern. Like shear spinning, it requires a more intricate tooling design, and it also enhances the mechanical properties of the tube.
The thickness and finish of the workpiece may need to be adjusted by changing the RPMs, the shape and diameter of the roller, pressure, attack angle, and by changing feeds. A second pass may be made when it is necessary to reduce the outside diameter near the edge of the workpiece.
Adjusting the edge of the workpiece can be difficult since it can create an edge that needs to be trimmed or a razor sharp edge. Making adjustments has to be completed carefully. Every time the roller goes over the material, it becomes brittle and harder and springs back off the tool.
Necking and expanding are also possible with metal spinning. Necking refers to the gradual recession of the diameter in a particular section of the part. Expanding refers to the increase of the diameter in a particular section of the part.
The post-processes for metal spinning are planishing and trimming. Planishing is a finishing step in metal spinning performed to remove wrinkles or any marks left in the finished part by the tool. In this step, a planishing tool applies force to the workpiece while being spun at a slower rpm. Trimming is the cutting of any excess material present on the edge of the finished part.
Metal spinning can be performed on any type of metal. If a metal can be formed by metal stamping, it can be processed by metal spinning. In some cases, metals that cannot be formed by stamping are processed by metal spinning. Lathes for metal spinning operate at 15000 psi, which makes metal spinning capable of deforming any metal with the exceptions of carbon steel and tool steel.
Aluminum is an abundant metal that has a high strength-to-weight ratio, ductility, and malleability; these properties make it ideal for metal spinning. It is the most popular metal spun material. It has a high corrosion and chemical resistance and high thermal stability. Aluminum is a cost-effective and lightweight alternative to steel. Aluminum alloys best used for metal spinning are 1100, 2024, 3003, 5052, 6061, and 7075.
Examples of spun aluminum parts are cooking utensils, kitchenware, drums, funnels, decorative parts, components for electronic devices, appliances, and furniture.
Steel is an alloy of iron, carbon, and other elemental additives. The properties of steel and its workability during metal spinning depend on its composition and manufacturing process.
Stainless steel is a type of steel that contains a minimum of 10.5% chromium and other additives such as nickel, molybdenum, and manganese. It is known for its excellent chemical and corrosion resistance. It has high strength, toughness, and rigidity and all of these properties are maintained at high pressures and temperatures.
Carbon steel is a type of steel that mainly consists of iron and carbon. The carbon content is below 2%. The carbon content makes carbon steel stronger and more rigid, but it also makes it harder and brittle and less malleable and ductile. The increased carbon content also decreases its corrosion and chemical resistance. Despite this, carbon steel serves as a cost-effective alternative. nThe grades of carbon steel are low carbon or mild steel (less than 3% carbon), medium carbon steel (0.3 - 0.6% carbon), and high carbon steel (more than 0.6%).
The manufacturing process of the steel blank also affects its workability during metal spinning. Hot-rolled steel is milled at very high temperatures, temperatures higher than its recrystallization temperature. On the other hand, cold-rolled steel is rolled at room temperature. Hot-rolled steel is more ductile, malleable, and flexible than cold-rolled steel. However, hot rolling of steel is prone to oxidation, which causes a reduction of strength. Nevertheless, both types are suitable for metal spinning.
All types of steel are suitable for heavy-duty applications. Spun steel parts are commonly utilized in the automotive, construction, manufacturing, and aerospace industries. Steel is a popular material of construction for pressure vessels, tanks, and processing equipment.
Brass is a copper-zinc alloy. It has high thermal and electrical conductivity and good corrosion, microbial, and biofouling resistance, all of which are attributed to its copper content. It has an appealing appearance, with a dull yellowish to reddish color, making it suitable for decorative purposes.
Brass has good ductility and malleability. When formed by metal spinning, it exhibits excellent workability. Examples of spun brass parts are musical instruments, pipes and fittings, structural components, furniture, and electronic appliances.
Bronze is an alloy of copper, tin (its primary alloying element), manganese, and phosphorus. It is distinguished by its attractive yellowish brown appearance. It has higher strength and rigidity than brass. It weighs more than brass and stainless steel. It also possesses high thermal and electrical conductivity, and good corrosion, microbial, and biofouling resistance, all of which are attributed to copper. It also has good weldability.
Spun bronze parts are durable and have higher strength; examples include sculptures, musical instruments, trophies, engine parts, and structural components. However, it is harder and has lower malleability than brass.
Hastelloy is an alloy of nickel, chromium, and molybdenum. This special type of alloy is popular for its excellent chemical, oxidation, and corrosion resistance. Spun Hastelloy parts are widely used as tank heads for pressure vessels in oil and petroleum refineries, power plants, and chemical production sites and as components for automotive and aerospace equipment. Hastelloy has high strength and toughness and is maintained at high temperatures and pressures.
Titanium is a metal that has a high strength-to-weight ratio and chemical and corrosion resistance. It is lightweight, soft, and ductile, which makes it easy to deform using metal spinning. It is very expensive and is less frequently spun than other metals.
Metal spinning is used for low or medium volume production runs for concentric symmetric parts and where the cost of metal stamping dies is prohibitive. When the volume of metal spinning shapes rises above a certain level, metal stamping is used because the rate of production is higher with metal stamping and less expensive.
Some metal shapes are so unique that they cannot be produced using metal stamping and can best be manufactured using metal spinning.
Hemispheres or semi-spheres are formed from metal sheets where the height is equal to the radius and half of the diameter or half of a sphere. Semi-spheres are commonly used for tank ends or protective caps. They are a staple of metal spinning and are an essential part of the manufacture of seamless spheres.
The use of semi-spheres includes lighting fixtures, reflectors, mixing bowls, satellite dishes, tank heads, covers, caps, and domes and come in straight wall and without straight wall designs. During processing, the metal blank is pressure shaped around a semi-spherical mandrel. Hemispheres are made from cold rolled steel, stainless steels grades 304 and 316, aluminum, copper, and brass.
The process for producing spheres is an extension of hemisphere metal spinning. The steps for producing a sphere are the same as those used to produce a hemisphere. Unlike hemisphere metal spinning, spherical metal spinning includes an extra step where two hemispheres, after having been spun, are joined to form a sphere, which are welded together. The precision and accuracy of the process necessitates the use of highly skilled craftsmen who are trained in the metal spinning process. The forming of the hemispheres requires accuracy and precision to ensure easy joining of the halves.
The forming of Venturi tubes requires dimensional accuracy and precision due to the complexity of the shape of a Venturi tube. In many cases, due to the need for accuracy, Venturi tubes are made using CNC machining. The rigid requirements of the equipment and the high level of spinning technology makes CNC a necessity. As would be expected, the mandrel for Venturi metal spinning is in a Venturi shape. The use of metal spinning to produce Venturi tubes is due to the uniform stretching of the metal to produce a seamless Venturi tube.
Metal spinning is the ideal process for forming parabolic shapes since it is easy to deform metal along a parabolic curve. Parabolic shapes are essential to the communications industry for the transmission of accurate signals. Metal spinning is used to produce parabolic shapes due to the precision in the creation of parabolic geometries, which are essential to the demands of modern technology.
The spinning of cylindrical shapes presents a set of challenges that are somewhat similar to those associated with Venturi tubes. As with Venturi tubes, cylindrical metal spinning requires precision, strength, and the ability to transform metal blanks into axial symmetrical components. The main challenge of spinning cylinders is maintaining sufficient force to avoid uneven thicknesses, surface imperfections, and structural weaknesses. The success of shaping cylindrical forms is dependent on applying substantial power and force to the metal blank.
Conical metal spinning is similar to cylindrical metal spinning where a cone is produced instead of a cylinder. It is a shear spinning method that uses a conical shaped mandrel. As the roller stretches the metal, the compressive force, as the roller moves, ensures that the diameter of the finished workpiece.
Hopper lids and bases are hand spun for dispensing various types of materials. They are made from high quality aluminum that is spun and trimmed to customer specifications. In some cases, secondary processing is required such as the addition of holes and slots.
Metal spinning is one of the fabrication processes employed to create tank heads for pressure vessels, storage tanks, and other process equipment. Because vessels are often subject to high pressures and harsh environments, manufacturers want to produce seamless, durable, and stronger tank heads; this can be achieved by metal spinning. However, metal spinning of tank heads requires more precise control of the process as regulations are governing its dimensions, strength, and composition.
The common shapes of tank heads are:
Toroidal metal spinning involves shaping a flat metal disc into a doughnut shape, a hollow part that has a circular cross section. The mandrel for the formation of toroidal shapes are similar to those used for the manufacture of conical and other round shapes. The main use for toroidal shapes is as magnetic cores for electrical components. This particular function requires that a toroid have exceptionally accurate dimensional shape.
The advantages of the metal spinning process are the following:
The disadvantages and limitations of the metal spinning process are the following:
There are numerous machines available to perform metal spinning in the United States and Canada. These machines are essential in today's society because they enable the cost-effective and efficient production of various metal parts used in industries like aerospace, automotive, electronics, and more, contributing to technological advancement and economic growth. We examine many of these leading machines and their capabilities below.
The Leifeld PNC350-800 is a highly regarded CNC metal spinning machine. It's known for its advanced automation capabilities, precision, and reliability. This machine is designed to handle large workpieces, making it suitable for producing various metal parts used in aerospace, automotive, and other industries. The CNC control allows for programmable setups, making it efficient and versatile.
The MJC Engineering E-400 is another popular choice for metal spinning applications. It features a user-friendly interface and CNC control, making it easier to program and operate. The machine's rigidity and power enable it to handle heavy-duty metal spinning tasks with high accuracy and repeatability.
The Baileigh R-M10 is a manually operated metal spinning machine, ideal for smaller workshops and hobbyists. While not as automated as some CNC options, it remains popular due to its affordability, compact size, and ease of use. It's suitable for smaller-scale projects and prototype development.
The PNC-CNC series offers various models with CNC controls to cater to different metal spinning needs. These machines are known for their robust construction, user-friendly interfaces, and excellent performance. The PNC-CNC series is designed for both precision and productivity in metal forming applications.
The LNSpin LS-250 is a versatile automatic metal spinning machine, well-suited for high-volume production. It's equipped with automatic loading and unloading capabilities, reducing manual intervention and increasing production efficiency. The LS-250 is known for its reliability and fast cycle times, making it a preferred choice for many manufacturers.
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