True to Form: The Value of Metal Spinning and Hydroforming

Are your part quantities too low for a forming die? Do you only need a prototype? Here are two alternatives to stamping and deep draw stamping that are worthy of attention: metal spinning and hydroforming.

Parts shaped like top hats, mixing bowls and rocket nose cones are easy enough to make, but producing large quantities of them typically means investing in a forming die. This can add significant costs to a project that might need only a handful of parts—and weeks or months to the delivery lead time.

The good news is that there are options: Hydroforming and metal spinning are mature, predictable machining processes, though they are generally not well understood.

Both metalworking approaches are able to compete on lower-volume work that would otherwise require dedicated tooling and deliver parts that might otherwise be impractical to produce. Here’s what you need to know about metal spinning and hydroforming.

What Is Metal Spinning in Manufacturing?

For those machinists who quote on parts that resemble bowls and top hats and wonder how those parts are made, this is for you. Metal spinning works by gently forcing metal sheets over a mandrel machined to the same shape and size as the interior of the desired workpiece.

It is usually a “cold” process helping it to flow into tight areas and around corners or where work hardening is a concern. Take note: A blowtorch might be used to make the metal more malleable.

Any ductile metal is suitable for metal spinning, including aluminum, copper, brass, carbon and stainless steels, and superalloys, such as titanium and Inconel.

If you wish to make a stainless-steel bell, for example, you would first mount a bell-shaped mandrel onto a lathe spindle. The mandrel can be made of wood, hard plastic or metal for higher-volume work.

A disc slightly larger than the maximum length of the bell’s outer surface is needed and is fairly thin—roughly 1/8 of an inch, but it depends on the size of the bell. Some specialty shops suggest that materials up to 1 inch or thicker can be spun.

The disc is then positioned at the end of the mandrel and pinched against it with a rotating pressure pad similar to a tailstock-mounted live center. Once the lathe spindle is engaged, a tool appropriately called a “spoon” is used to apply pressure, gradually forcing the metal into the desired shape.

It is often a manual process. Craftspeople have been spinning metal parts for as long as there have been lathes and strong-arms. However, CNC spinning machines are also available that offer greater accuracy and productivity. In this instance, a roller-style tool would be used.  

There is very little waste with metal spinning.

In the bell example, the only scrap would be a narrow ring around the bell’s base that is trimmed away after forming. Uniform wall thicknesses with accuracies to +/-0.015” are possible with CNC, as are mirror finishes, assuming a burnishing-style spoon or roller tool is applied. If not, some tool marks may be visible.

 Do you need a technical question answered? Ask the MSC Metalworking Tech Team in the forum.

What drawbacks or advantages do you find in deep draw stamping? Have you evaluated spinning or hydroforming? Talk to your peers in the metalworking forum. [registration required]