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What is Metal Spinning Processes?

   25 May,2021

Metal Spinning Processes Classification:

Spinning technology means that deformed blank plate mount on the spindle end of the mandrel. and,  follows the rotation of the main spindle and the mandrel (it is also possible to rotate the rollers around it),  in which the rollers performs a feed motion relative to the mandrel. And, spinning the blank plate gradually by the spinning force which generated by the contact of the rollers with the blank plate.

The metal spinning machine can perform spinning, seaming, flange spinning, necking, shear forming, trimming, etc.

According to the wall thickness variation of the blank plate during the spinning process, the spinning technology divides into two categories: conventional spinning (the thickness is constant) and power spinning (the thickness is varying). So, the Conventional spinning is widely used in the deep drawing, necking and expanding of parts.

According to the type of spinning parts and the difference of metal deformation mechanism, the power spinning divides into cone-shape spinning (shearing spinning) and cylindrical-shape spinning (extrusion spinning).

Metal Spinning Processes

Metal Spinning Processes Steps:

step 1: The circular blank is properly centred and pushed by the tail stock against the front of the rotating chuck, usually made of wood. A wooden block is used from the tail-stock to support the blank.

step 2: The chuck is rotated by the head-stock of a spinning lathe machine. The spinning speed depends on the blank material, thickness and complexity of the part required. It is varies from a very slow to a high speed of about 3500 r.p.m.

step 3: A hard wood or roller type metallic pressing tool is pushed by the operator onto the external surface of the blank.

step 4: Then the tool is moved gradually on the blank so that is conforms to the shape of the form block or mandrel.

step 5: The blank slips under the pressing tool, which causes localized deformation. This deformation of the metal is proceeds by a mixture of bending and stretching.

step 6: To avoid wrinkles, a back-up support is applied to the opposite size of the pressing tool.

step 7: Finally, the blank takes the exact shape of the form mandrel.

Detailed Explanation of Metal Spinning Process:

Metal spinning has several iterations that suit different applications, and among them all, multipass spinning remains the most common. Performed either manually or on a CNC spinning lathe, a roller sweeps across the spinning disk multiple times, shaping the metal against the mandrel.

Another common iteration, shear forming, finishes a part in just one pass, with the shear forming roller pressing against the metal in a unique fashion. In multipass spinning, the unformed flange section of the disk bends forward and backward during the process depending on the type and direction of the spinning stroke. In shear forming, the roller keeps that spinning flange perfectly vertical during the process. A very skilled operator can perform shear forming on a manual machine, keeping just the right pressure to ensure that flange is perfectly vertical throughout the operation. Most shops, though, use a CNC machine for the process.

On the surface, multipass spinning and shear forming seem very similar, and they both can be performed on the same machines. But dig a little deeper and you will find significant differences. Consider what actually happens, metallurgically, when the spinning roller contacts the workpiece. The roller provides highly localized pressure that plastically deforms and cold-works the workpiece, compressing the grain structure and hardening the material. In shear forming, the cold working is especially pronounced, and it can make thin material just as strong as significantly thicker material, producing weight savings so sought after in industries like aerospace and automotive.

With this compression comes a reduction in wall thickness. Both multipass metal spinning and shear forming change the metal's wall thickness, but there's a difference: In shear forming, there's a deliberate reduction in wall thickness, while in multipass spinning the final wall thickness is determined through the number and direction of the roller's passes. When the roller moves forward, it thins the metal; when it moves backward, it displaces metal in the opposite direction. This ensures a consistent wall thickness and, on harder materials, overcomes issues like springback. In many CNC spinning machines, the operator can program in the final desired wall thickness, and the machine will adjust the number and direction of roller passes to suit.

Shear forming significantly displaces metal in the axial plane and, in so doing, forms the blank in only one forward pass. The amount of displacement affects the final wall thickness, which hinges on a specific formula called the sine law. The formula determines the gap, or space between the roller and mandrel, that will deform the workpiece enough in a single pass to overcome springback and produce the desired finished wall thickness. The formula involves the initial blank thickness; finished wall thickness; as well as the shear angle, or how many degrees the mandrel is off from horizontal.

The shear angle brings up shear forming's part geometry limitations. The simplest and most common shear forming operations involve conical shapes with just one shear angle from start to finish. The process also can work for parts with increasing shear angles, as long as they are between 12 and 80 degrees (though smaller angles to 8 degrees are possible with a second operation). Shear forming cannot produce cylindrical shapes, which have 0-degree shear angles. Compared to multipass spinning, the shear forming tool exerts greater axial force in one area, and if the shear angle drops too much, other stresses would emerge, causing distortion and, ultimately, total part failure. If the shear angle reduces to a cylinder shape (0-degree shear angle), the metal literally would start to wrinkle, fold in on itself, and fracture.

When performed correctly—that is, when the gap between the spinning roller and mandrel is programmed correctly—forming stresses are confined to where the roller hits the workpiece. The remainder of the workpiece remains virtually stress-free. And because the volumetric displacement occurs axially (up the contour of the mandrel), no radial stresses are induced, so the workpiece stays in the same annular position throughout the operation.

Besides multipass spinning and shear forming is another, less common metal spinning process called necking-in or reducing—sometimes referred to as "spinning on air." As the name implies, it usually requires no mandrel for internal support. It is commonly used for shapes such as gas bottles, which are necked-in out of a tube.

Any ductile material with more than 2 percent elongation can be formed with metal spinning. Put another way, any metal that can be formed or hot-formed on a press can be spun. The technology also can process a range of workpiece sizes, from very small up to 20 ft. in diameter.

Shops either purchase circular blanks or usually cut them out using a circle shear or laser. Blanks also can be preformed if necessary. For instance, the bullet-shape spun part used for some motorcycle light reflectors has a sharp point at its apex. The blank goes through a dimple-countersink operation first to form that bullet point, then onto a spinning machine.

Harder materials like stainless and titanium, because they work-harden quickly, require annealing. Shops either can stop midprocess and place the workpiece in an annealing oven, or use a gas torch or electrical induction to anneal the part in-process (see The Laser Meets Metal Spinning sidebar).

Tooling for metal spinning includes the mandrel and roller spinning tool. D2 tool steel is popular, but for small runs or prototypes, shops have been known to use wooden tooling. The radius of the roller should match the smallest radius of the part. If the part has a 1/4-in. radius, then the operation must use a roller with the same radius. Shear forming rollers require a sharper radius because usually the process involves only a straight wall (a single shear angle) and no radii.

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