Metal spinning, or spin forming, is a metal working process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part using tools. Metal spinning is often performed by hand to produce decorative items, or using machine tools, such as CNC lathe, when tight tolerances are required. Metal may be formed into a die to shape the outside diameter or onto a mandrel to size the inner diameter.
Metal spinning ranges from an artisan’s speciality to the most advantageous way to form round metal parts for commercial applications. Artisans use the process to produce architectural detail, speciality lighting, decorative household goods and urns. Commercial applications include rocket nose cones, cookware, gas cylinders, brass instrument bells, and public waste receptacles. Virtually any ductile metal may be formed, from aluminium or stainless steel, to high-strength, high-temperature alloys. The diameter and depth of formed parts are limited only by the size of the equipment available.
Metal Spinning by hand
Metal Spinning is a process by which circles of metal are shaped over mandrels (also called forms) while mounted on a spinning lathe by the application of levered force with various tools. It is performed rotating at high speeds on a manual spinning lathe. The flat metal disc is spun against the mandrel and a series of sweeping motions then evenly transforms the disc around the mandrel into the desired shape. It takes a very skilled workman to correctly shape and finish a hand spun piece.
When spinning metal by hand, care must be taken to not touch the spinning metal with one’s hands until the metal edge has been “turned over” (rolled to a rounded edge so that the bare edge of the metal stock is protected). This is mentioned specifically because wood turners are accustomed to touching the spinning wood in the lathes (once it reaches relative smoothness) to monitor their progress. This practice is very dangerous in metal spinning. Lexan/Clear plastic lathe shields and guards are recommended.
Metal spinning tools
The basic hand metal spinning tool is called a Spoon, though many other tools (be they commercially produced, adhoc, or improvised) can be used to effect varied results. Spinning tools can be made of hardened steel for using with aluminium or solid brass for spinning stainless steel/mild steel.
The mandrel/chuck can be made from wood, steel alloys, or synthetic materials. The choice of material is dictated by the hardness of the material to be spun and by how many times the tool is expected to be used.
Cutting of the metal is done by hand held cutters, often foot long hollow bars with tool steel shaped/sharpened files attached. This is dangerous and should only be done by skilled tradesmen.
In CNC applications, traditional carbide or tool steel cut-off tools are used.
Some metal spinning tools are allowed to spin on bearings during the forming process. This reduces friction and heating of the tool, extending tool life and improving surface finish. Rotating tools may also be coated with thin film of ceramic to prolong tool life. Rotating tools are commonly used during CNC metal spinning operations.
Commercially, rollers mounted on the end of levers are generally used to form the material down to the mandrel in both hand spinning and CNC metal spinning. Rollers vary in diameter and thickness depending the intended use. The wider the roller the smoother the surface of the spinning; the thinner rollers can be used to form smaller radii.
Woodworking lathes are often used, although a Wilson lathe is the most common manual spinning lathe for spinning metal by hand. The mandrel having been formed from wood on the lathe or steel chuck machined on a CNC lathe previous to mounting on the metal stock. All stock sizing is done prior to the spinning.
Comparison to other forming techniques
Other methods of forming round metal parts include hydro-forming, stamping and forging or casting. Hydro-forming and stamping generally have a higher fixed cost, but a lower variable cost than metal spinning. Forging or casting have a comparable fixed cost, but generally a higher variable cost. As machinery for commercial applications has improved, parts are being spun with thicker materials in excess of 1″ thick steel. Conventional spinning also wastes a considerably smaller amount of material than other methods.
Advantages of Metal Spinning
These are several benefits of spinning and shear forming. Several operations can be performed in one set-up. Work pieces may have re-entrant profiles and the profile in relation to the centre line virtually unrestricted.
Forming parameters and part geometry can be altered quickly, at less cost than traditional metal forming techniques. Tooling and production costs are also comparatively low. Spin forming is easily automated and an effective production method for prototypes as well as high production runs.