Understanding Metal Spinning: Conventional vs. Shear Forming

Metal spinning is a common metallurgy process that turns a blank workpiece into an axially symmetrical part or component with a hollow inside. Machinists use lateral force to press rollers into their workpieces on a high-speed spinning mandrel. The result is a tailored shape that can be used in countless applications.

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That’s because spun metal doesn’t lose its material volume or density during production. Instead, the workpiece is subjected to both compressive and tensile forces to achieve the desired effect. And while certain types of metals may react differently to spinning or sustain a minimal decrease in thickness, the overall characteristics of the finished product are indicative of this less invasive machining method.

What is Metal Spinning Used For?

Also known as spin forming, metal spinning is usually performed by hand or with an advanced CNC lathe, and it’s one of the most beneficial methods when rounding metal components for practical and/or decoration purposes. However, in many large-scale metal fabrication projects, the lathe can become automated. In turn, that helps to accommodate precision, consistency, product complexity, and lead time specifications.

In fact, metal spinning is often the preferred machining technique for a wide range of manufactured products, from common household goods and durable cookware to specialty instruments and aerospace parts. Meanwhile, nearly any ductile metal can be spun by skilled hands, including high-strength alloys. That’s why you’ll find spun metals in modern architecture, innovative art, contemporary lighting, and countless public amenities.

DID YOU KNOW: Despite the frequently interchanged name, conventional metal spinning and shear forming are two entirely different CNC machining processes.

Metal Spinning vs. Shear Forming

Whether the blank is flat or a preformed metal sheet, it’s important to understand the differences between conventional metal spinning and shear forming. Metal spinning involves the use of a mandrel and hand or CNC lathe to shape a workpiece, while shear forming involves the use of two rotating disks that shear the workpiece into a specific shape—without the use of a mandrel.

Let’s dive a little deeper:

Conventional Metal Spinning

Conventional metal spinning is a process of shaping a flat metal disk or sheet by using a tool called a mandrel, which rotates at high speed while the metal is pressed against it with a roller or other tool, creating a hollow, symmetrical part.

The workpiece depth thereby increases as the diameter is reduced, making conventional metal spinning one of the simplest and most affordable tooling techniques available.

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Shear Spinning/Forming

Shear spinning, also known as shear forming, is a metal spinning technique that involves stretching a metal workpiece over a mandrel using high-pressure rollers. Unlike conventional metal spinning, shear forming reduces the thickness of the material while also reducing the diameter of the workpiece.

Shear forming requires more precise machining and tooling structures than conventional metal spinning, and it generates more heat during the process, requiring cooling to be implemented. Be sure to ask your machining team which coolants are best for the metals and processes you’re using.

Common Types of Spun Metal

Nearly any ductile metal can be spun, but some metals may be more challenging to spin due to their physical properties, such as hardness or ductility. In those cases, specialized techniques may be required to achieve the desired result. Additionally, the thickness of the metal being spun can also affect the ease of the spinning process, with thinner gauges generally being easier to spin than thicker gauges.

The most common metals, included some precious metals, that can be spun are:

• Titanium
• Aluminum
• Bronze
• Copper
• Steel
• Stainless Steel
• Brass
• Hastelloy
• Gold
• Silver

Because this machining technique is primarily used to fabricate hemispheres, parabolas, venturis, cylinders, and cones, it’s generally reserved for expert CNC machinists, skilled artisans, and experienced hobbyists with proper training and equipment who know how to manipulate metals according to their innate properties. In turn, a unique design emerges despite the obvious benefits and drawbacks of the process.

It should be noted that while nearly any ductile metal can be spun, there may be limitations based on the size and shape of the workpiece being spun, as well as the machinery and tools available. Additionally, some metals, such as those with low melting points, may be more difficult to spin due to the heat generated during the process.

The Pros and Cons of Metal Spinning

Not only is metal spinning an ideal machining method for prototype design and innovative product development, but it also involves the following advantages and disadvantages:

Pros

• The entire metal blank is used to create the finished product, resulting in minimal waste and lower material costs.
• Spun metal projects typically produce less waste than conventional metal forming methods.
• Overall tooling costs are relatively low compared to other machining methods.
• The technique is cost-effective and can help shorten lead times for small-to-medium production runs.
• Metal spinning is a simple and efficient manufacturing process that can be used for innovative product development.

Cons

• Surface irregularities or imperfections may occur during the spinning process, which could affect the quality of the finished product.
• Depending on the specifications and tolerances of the part, some parts may fail inspection due to imperfections.
• Consistent product quality can be difficult to achieve with this method, as it depends on the skill and expertise of the machinist.
• Machining conditions for each part must be carefully controlled in order to ensure the best possible outcome.

NOTE: If you’re controlling spun materials in a CNC machine shop, be sure to check the offsets to determine the best machining conditions for each part.

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Well I got into it but then got married and moved and had to spend almost a year getting the shop back up and running. So what I'm saying is it's almost like starting new. I will try to answer questions. There are two guys who do it a lot and I'll try to remember their names. Probably hit me about 2am when I wake up in the middle of the night. I took a class with David Marks and had a ball. I apparently had a good touch and took to it pretty fast. About half the class or more wrinkled up their first few vessels. by the end of the weekend we all were doing pretty well. I made my own tools. I don't really know where to buy them right now. Penn State Ind. was selling tools and materials for a while but they quit. Oneway sells a metal spinning tool rest. I built my own for that also. I will have to do some more research to find other metal spinning resources. There is an excellent DVD listed online. I"ve never purchased it but heard a lot of good things.
there are also some pretty good youtube videos although many of them are too professional for most of us. Mike, can you give us an idea of what you want to spin? Size, material, etc, etc.

In general, you'll want annealed material which is sometimes not so easy to find in small quantities. Possibly, a method of annealing if working with material that tends to work harden. Copper alloys will need a pickling solution to remove heat scale after annealing.

Tools,... you can make those yourself. The lathe will need a tool rest with pivot pin holes, again a DIY item.

I would say becoming proficient at metal spinning is a bit more challenging than wood turning. Doug,

One of the items I will be forming will be metal spheres in various sizes. 4" thru 12" diameters
Intend to use aluminum and copper metal mostly. I located some 4 foot steel pry bars that
can easily be worked into spinning tools. One end is tapered to a point the opposite end has a
curved flat duck bill. A little work with a grinder and should have a usable tool. I have some carbide
cutting tools for my metal lathe that I can braze or weld onto another steel bar for a trimming tool.

Doug,

One of the items I will be forming will be metal spheres in various sizes. 4" thru 12" diameters
Intend to use aluminum and copper metal mostly. I located some 4 foot steel pry bars that
can easily be worked into spinning tools. One end is tapered to a point the opposite end has a
curved flat duck bill. A little work with a grinder and should have a usable tool. I have some carbide
cutting tools for my metal lathe that I can braze or weld onto another steel bar for a trimming tool.

Mike, not to spoil all your fun, but.........

http://www.necopperworks.com/copperballs.html

A 4" ball is $29. Buying the material at onlinemetals.com a 12" x 12" piece of dead soft .040" copper is $24. So about $12 in materials per 4" ball, using two 6 x 6" pieces. Doug,

I have a supply of copper and aluminum sheet metal for this project already. The spheres need to be made to a specific diameter to fit the intended application. I am sure I could have someone make them but I would need to pay for the mandrel forms for each specific size which I intend to make myself. Once I get all of my mandrels made I might look at jobbing it out, but the market I work in has a limited volume of business. I have a few other items besides the spheres that I also want to spin. I also have a couple of Tesla coils that I would like to finish one of these years with a custom made toroid.

Paul,

Most of the retail tools sold for spinning are somewhat undersized for the abuse these tools take when spinning the bigger items and harder metals. I already have a metal spinning tool rest for my lathe I intend to use for my metal spinning.

John,

I have access to a full equipped metal fabrication and tool shop at my day job. I can heat treat and weld just about any kind of metal there. I can turn some metal rollers on my metal lathe for the more complex beading and roller tools and set them up on a tool shaft and handle. The tools I am making will be about 4 foot long with the handles when done.

Some spinners use copper sheeting from the lumber yards that are used for gutter and flashing applications.

You have to be very aware of thickness when you compare prices and buy. Online's price I mentioned for dead soft C110 copper was .040" thick (the thinnest they sell in 0 temper). Roofers copper from Online Metals in .043" thickness is even more expensive for the 12" square.

A friend who works exclusively in copper tells me Online's prices are about 3 times what he pays in volume purchases. Places like Online are the only place to buy small quantities though.

I believe typical roofers copper at lumberyards would be .021" thick, know as 16 oz. Probably half hard so might need annealing prior to spinning and mighty be too thin (depending on the spinner's experience of course).

Copper is expensive wherever you buy.

Anyone on the site using their wood lathe for spinning metal? I have several jobs coming up where I need to spin some metal parts. Looking for any resources that may be available tools and materials etc.

I’ve done a bit of metal spinning, just a few pieces, and made my own spinning tools and toolrest, used copper as it is easy to soften and I had/have some copper sheeting, so never had to buy any.

The sheets I used came from an pressure-less water heater that had started to leak at the seams, for the rim fold over I just two tools at the same times to hold and raise the edge and then roll back.

A skew works just fine for trimming the edge.

If you can turn the forms from hard wood the rest isn’t that hard to do, even if you have to anneal the metal more than once to get there.

These are nothing special, just some of what I made,
The professional spinner that i met one time used an Oak stick as a back up especially on larger platters. I set up a demo for club. He was turning the lower half of the Kerosene well on an Aladan's kerosene lamp. It literally took him about 30 seconds or less. After he finished he said does anyone want to see it again. We said yes because we didn't see it the first time. The scary part was he chained himself to the lathe and used his whole body to swing the tool. These were about 12" discs when he started and ended up being about an 8" bowl with a lip. He would turn two of these and put them together by spinning epoxy into the joint. That was the reservoir for the kerosene for those lamps.