Discovering a new use for a tool I already own always gives me a special sense of satisfaction. So I was excited when I realized I could form metal workpieces on a wood lathe. With the addition of just a few parts, I could fabricate one-off velocity stacks for my hot rod or build my own retro-style oil lamp — the possibilities seemed endless.
Metal spinning is nothing new; in fact, it’s one of the oldest forms of metal shaping, dating back thousands of years to early Chinese, Indian and Egyptian craftsmen. The process remains largely unchanged from its beginnings, although modern lathes, tools and materials have enabled parts to be spun with far greater quality and predictability.
Virtually any type of sheet metal can be formed by spinning, though metals with greater elasticity are the best choice for complex shapes and deep draws. Aluminum, brass, bronze, copper and stainless steel are typically the most elastic non-precious metals and are most commonly used by metal spinners. Purer annealed aluminum alloys (i.e. 1100-0) are best for forming; avoid using heat-treated or strain-hardened metals. During the forming process the part may become work-hardened and resist additional forming, so it will then need to be annealed to relieve internal stresses and improve its cold working properties. Sheet metals as thick as .100 in. can be formed with hand tools, though thicker and stiffer materials will require significantly more pressure to form properly.
Tools to Use
Professional metal spinners have a vast array of tools to use during the forming process, and they commonly build their own tools to meet specific needs. For the project shown in the photos (below), I used only three tools: a general-purpose combo tool, a beading tool and a trimming tool. The tools are used to form the metal over a mandrel or buck while the lathe spins at 900 to 1,200 rpm (1,800 rpm for finishing). The mandrel is commonly made from hardwood such as maple, but in industrial applications steel is used for large production runs.
Mandrels should be designed without undercuts to allow the newly formed part to be removed when complete. Mandrels with an undercut or waist must be made up of multiple parts so they can be removed without damaging the completed workpiece.
Experience pays off in the forming or spinning process. Tool speed, pressure and control all come into play as metal is drawn over the mandrel. As with developing any new skill, you can’t expect stellar results on your first attempt. Work in smooth, steady rowing strokes, and the material will form gentle hyperbolic curves at first. Once the material has moved about an inch, it should be seated over at least the first 1/2 in. of the mandrel. With the part seated, the remaining forming proceeds quite predictably.
1. Long handles on the forming tools provide the leverage to move material across the mandrel. The butt of the tool is placed in the right armpit, and body weight provides the force to move the material. The left hand, also known as the clamping hand, grasps the pin on the tool rest and ensures the forming tool remains in contact with the pin, which is used as a fulcrum when applying pressure. The right hand is then placed midway on the tool and used to guide the tool over the part.
2. The mandrel should be securely mounted to the headstock or headstock mounting plate. Avoid using jaw-type chucks on the headstock to hold the mandrel — they pose inherent risks, and their 1,000-rpm limit will adversely affect finishing.
3. Sheet metal should be mounted close to center between the mandrel and the follower. The metal disc is then trued by starting the lathe, loosening the tailstock slightly while touching a piece of scrap wood lightly to the disc, and then retightening the tailstock when the disc runs true. The size of the metal disc should equal 80 percent of the part’s final length plus its radius: D = .8 (length + radius)
4. Good lubrication is essential in metal spinning. Specially designed spinning waxes and beeswax are commonly used, although they can be a little messy as lubricant spins off of the workpiece. (Use tarps to protect nearby items from lubricant spatters.) Reapply lubricant as needed during the forming process.
5. After bringing the lathe up to speed (900 to 1,200 rpm), use the combo tool to apply pressure to the sheet-metal disc, working in slow rowing strokes and moving from the inside of the disc to the outside to form, then back to move metal back into the newly formed shape.
6. Once the sheet metal has been formed approximately 1 in. at the outside edge, it should be seated to the mandrel. Working from the inside out, push the metal over the mandrel for at least the first 1/2 in. As the metal contacts the mandrel, the tone will change to a low groan.
7. Using the same simple rowing strokes with the combo tool, move material over the remaining mandrel. The tool should always be moving to prevent galling the surface or spot-hardening. Remember to reapply lubricant as needed.
8. Before completing the top edge of the piece, use the carbide trimming tool to clean up the edge and correct any off-center issues from the forming process. Bring the trimming tool very slowly into contact with the edge of the part.
9. Finishing the edge is the last step of the forming process. There are several ways to complete the edge; we created a rolled edge using the beading tool. With approximately 3/8 in. of material remaining to be formed, apply the back stick to the edge to start it in the right direction; then use the beading tool to roll the edge over (below).
10. Once the forming is complete, it’s time for final finishing. Remove the tool rest before you begin finishing. For best results, increase the lathe’s speed to 1,800 rpm and use sandpaper or a Scotch-Brite pad to smooth the surface. Metal polish or buffing compounds may also be applied.
11. With a little practice, you’ll be able to complete parts in minutes and achieve high-quality results.
Spinning metal at high speeds while applying force against it poses inherent risks. Follow these safety precautions to ensure your project doesn’t take a wrong turn.
Wearing protective safety glasses and/or a face shield is a must. Wear heavy leather gloves (especially on your clamping hand) to protect your skin from the sharp edges of the moving part and to absorb vibrations from the spinning tools. While spinning parts, the fingers of your clamping hand should remain curled around the tool post, away from the workpiece.
It’s much better to directly mount the mandrel to the headstock or headstock plate instead of using three-jaw chucks. The direct-mount method ensures that the mandrel is centered on the headstock and eliminates the risks associated with tools coming into contact with chucks.
Tightly clamp all components (headstock bolts, tailstock, tool rest, etc.) when you spin parts, although you’ll need to remove the tool rest during finishing operations such as sanding, filing or polishing. During the spinning process it’s important to be aware of the material’s condition: Localized hardening, thin areas, possible shearing and wrinkling can be avoided if the signs are detected early.