Page 17 - MetalForming August 2019

Therefore, a large ± range in the for- mula ensures that the selected roller diameter is less than the component diameter. Large roller nose radii will produce more uniform thickness dis- tribution and low surface roughness.
Mandrels are made from a range of materials such as cast iron, mild steel, tool steel, aluminum or magnesium. Larger parts produced from light-gauge, formable materials can be made with aluminum, wood or, potentially, plastic. Larger parts produced from more dif- ficult materials can be constructed via tubing and steel or custom casting. Mandrels also function as supporting and rotating members in the metal spinning setup, meaning that they must be statically balanced. High speed calls for the dynamic balancing of large man- drels. In these cases, cored casting of steel or iron is preferred, in order to reduce the weight of the mandrel.
A blank-support attachment may be required in the early stages of spinning when high feed rates or thin materials
are used. The blank, positioned between the support and the work roller to help control buckling and wrinkling, is sim- ilar to a blankholder in deep drawing.
Process Parameters
Key process parameters for spinning include roller feed rate, spindle speed, feed ratio, temperature and lubricants.
Roller feed rate—the distance the roller travels into or along the work- piece with respect to time and meas- ured in mm/sec. or mm/min.—affects formability and forming quality. A high feed rate causes the workpiece to con- form more closely to the mandrel at the expense of surface finish. Too high of a feed rate causes buckling or wrin- kling to occur. Reducing the feed rate improves surface finish, but at the expense of cycle time, thickness uni- formity and dimensional accuracy.
Typically performed at room tem- perature, spinning processes may require heating above room tempera- ture, especially when it comes to thick
parts. At elevated temperatures the flow stress—the stress level required to cause or continue yielding—drops, making the blank easier to form. Increased formability proves useful when spinning machine capacity is insufficient to cold form the compo- nent or the alloy ductility is too low at room temperature.
Lubricants, normally required dur- ing spinning, provide lubricity and cooling. Restricting the amount and location of the lubricant contributes to part quality and extended tool life. For example, an increase in blank tem- perature occurs during forming, reduc- ing the flow stress in the blank. If wall- thickness control is important, this lessens part quality. On the other hand, an increase in ductility at elevated tem- peratures may be desirable when spin- ning materials with low ductility. In either case, lubricant decisions play a key role in achieving desired results.
Next month: advancements in metal spinning technology and tooling. MF
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Tooling by Design