Metal spinning is an age-old technique that has evolved to accommodate modern manufacturing demands. The metal spinning technique entails shaping sheet metal over a spinning mandrel using one or more stiff tools called rollers.
This process results in axisymmetric hollow pieces, typically with circular cross-sections. The procedure is akin to nett shape forming, which is known for its precision and efficiency.
When selecting the right rollers for metal spinning, factors such as material type, blank size, product radii, and desired surface smoothness should be carefully considered. Rollers come in various diameters, profiles, thicknesses, and materials, including plastic.
Let’s get straight to the point
Metal spinning is a technique for shaping sheet metal into hollow, axisymmetric forms using spinning mandrels and rollers.
The three main processes—conventional spinning, shear spinning, and flow forming—each serve different purposes: conventional spinning maintains wall thickness, shear spinning reduces it for complex shapes, and flow forming creates precise, high-strength components.
These methods are widely used in industries like aerospace and defence due to their precision, versatility, and cost-effectiveness.
Despite being an ancient practice, metal spinning continues to evolve with technological advancements, expanding its applications in modern manufacturing.
Brief History of Metal Spinning
The history of metal spinning is deeply rooted in traditional metalworking practices.
Metal spinning, which has been around for centuries, involves pressing flat pieces of sheet metal onto forms that spin on a lathe to create various products. Early artisans used this technique to make household items like saucers, trays, bowls, and vases.
As we entered the space era, metal spinning became vital in producing components for missiles, helium balloons, and nuclear power stations. This shows that while the technique is old, its applications have only grown with technological advancements.
Different Types of Metal Spinning Processes
Metal spinning processes can be categorised based on the wall thickness of the resulting product and the specific techniques used to achieve the desired shape.
The three main metal spinning processes are conventional spinning, shear spinning, and flow forming.
Conventional Spinning
Conventional spinning is one of the most common forms of metal spinning. During this process, the wall thickness of the final product remains almost unchanged from the original blank.
The spinning process involves applying localised pressure to a moving sheet metal blank using a roller tool, which gradually shapes the metal into the desired mandrel form.
In multipass conventional spinning, the final wall thickness is influenced by the number of roller passes and their direction.
This process may involve planishing, intermediate, and finishing passes to enhance accuracy and surface polish. Planishing, in particular, smooths out the waves caused by multiple winding phases, ensuring even wall thickness and reducing springback.
This method is especially useful for creating complex geometries combining various forms, such as domes, cylinders, and cones.
Additional Variants of Conventional Spinning
Conventional spinning can be further classified based on the specific spinning techniques employed:
- With or without mandrels: Some spinning processes require mandrels to shape the metal, while others do not.
- Opposing rollers: Spinning can involve one or more rollers working in opposition to shape the metal.
- Heat and cold spinning: Depending on the material, spinning can be done with or without applying heat.
- Computer Numerical Control (CNC) spinning: CNC technology allows for precise and automated control of the spinning process, ensuring high accuracy and repeatability.
Shear Spinning
Shear spinning, also known as shear forming, is a technique where the wall thickness of the blank is intentionally reduced.
This process typically involves shaping sheet metal over a conical mandrel, and the amount of thickness reduction is determined by the angle between the part’s wall and the axis of rotation.
Unlike conventional spinning, shear spinning involves compressing the metal blank as it is shaped, resulting in a product with the same external diameter as the original blank.
The shearing angle, which must be less than 80 degrees, plays a critical role in determining the outcome. This process is particularly useful for creating complex shapes like cones, nozzles, and flanges.
Advantages of Shear Spinning
Shear spinning offers several advantages:
- No need for heat: Shearing does not require heating the metal, making it suitable for cold working.
- Reduced waste: The process generates minimal waste, making it cost-effective.
- Versatility: Shear spinning can be applied to various metals, including iron, bronze, and aluminium.
Disadvantages of Shear Spinning
However, there are some limitations:
- Difficulty with hard metals: Shearing is not suitable for very hard metals like tungsten.
- Risk of distortion: Stainless steel sheets can be distorted if the process is not executed with precision.
Flow Forming
Flow forming, also known as tube spinning, is a process where the wall thickness of a hollow component is intentionally reduced to achieve a longer workpiece.
Unlike conventional spinning, flow forming requires specialised equipment designed to clamp a hollow component between a moving mandrel and a tailstock, with deformation occurring through the application of three rollers.
Flow forming is ideal for materials with low flow stress, high tensile elongation, and high tensile strength-to-yield strength ratio; however, even though materials can be successfully flow-formed, this process is highly versatile.
Flow Forming Process
The flow-forming process is somewhat analogous to the art of ceramics, where a soft material (in this case, metal) is shaped into a precise and durable product using mechanical rollers.
These rollers compress the workpiece, resulting in a more accurate and stronger product than traditional metal spinning methods.
Flow Forming Benefits
Flow forming offers numerous benefits:
- High accuracy: The process enhances the strength and durability of the metal by stretching its grain structure, allowing for extremely complex geometries with precise dimensions.
- Versatility: Flow forming can be applied to a wide range of materials, including stainless steels, aluminium alloys, and nickel-based superalloys like Inconel and Hastelloy.
- Cost efficiency: Flow forming is cost-effective due to its ability to produce entire workpieces in a single operation with minimal tooling requirements.
Wide Variety of Applications
Flow forming is widely used in industries like aerospace, nuclear, and defence, where components require precise dimensions, smooth surfaces, and enhanced metallurgical properties.
The process is particularly beneficial for creating components with large length-to-diameter ratios, such as missile casings and high-performance automotive parts.
Conclusion
Metal spinning is a versatile and efficient method for transforming flat sheets of metal into three-dimensional forms. By using rollers and mandrels, metal spinning can create complex, axisymmetric shapes with excellent dimensional accuracy and surface finish.
The three main types of metal spinning processes—conventional spinning, shear spinning, and flow forming—offer unique benefits and applications, making metal spinning a valuable technique in various industries.
While each spinning process has its own set of advantages and limitations, all three are integral to modern manufacturing.
Conventional spinning is ideal for maintaining consistent wall thickness, shear spinning is useful for creating complex shapes with reduced wall thickness, and flow forming is unmatched in its ability to produce high-strength, precision components.
As technology continues to advance, the role of metal spinning in manufacturing will undoubtedly expand, offering new possibilities for innovation and efficiency.
