What is Metal Spinning?
by Fred Holder
[This article was published in the January 1997 issue of More Woodturning as a part of the issue theme of Metal Spinning. Other articles on this subject were included in that issue.]
Metal spinning is the process by which a piece of flat sheet metal is formed over a pre-turned, three-dimensional pattern while it is spinning on a lathe. This forming is done by applying pressure to the metal as it spins to press it against the wooden form and cause the metal to take on the form of the pattern. The male (wood form) pattern must be turned from a good hard dense wood such as maple or a material of equal hardness. Normally, such spinning is for open forms; i.e., the opening is larger diameter than the rest of the form, thus allowing the spun metal to be easily removed from the pattern. When closed forms are required, it is normal to form the object as several pieces and then solder them together to form the final closed form. However, it is possible to fabricate the male patterns such that they can be disassembled after the spinning is completed; thus, removing the pattern from the spun work through disassembly.
Normal metals suitable for spinning include copper, brass, bronze, pewter, aluminum, silver and mild steel in gauges from 22 (.025") to 14 (.064"). It is important that the metals be annealed prior to beginning the spinning process. If the process it too severe, it may even be necessary to anneal the metal a second or third time before the spinning can be completed. Metal that is too hard to begin with may readily crack rather than form to the mold.
To begin the spinning, the metal disk is centered between the male pattern (mounted on the headstock) and a wood follower mounted on the tailstock (a live center is desirable). Clamping pressure to hold the metal in position is applied by cranking the tailstock tightly against it. This pressure is the only thing holding the metal in place when you first begin, so be careful, that spinning disk would be like a knife if it came loose from the lathe. However, if you center the disk carefully and don't run the lathe too fast, there should be very little danger.
The outside of the metal disk (tailstock side) must be lubricated to prevent galling. Tallow, beeswax, grease or soap will all work. Bill Moore gave a formula recommended by Dave Hout that consists of:
- 3 parts beeswax
- 1 part paraffin
- 1 part toilet bowl seal wax.
I presume you melt these all together, stir them thoroughly and then allow to cool before using. Once your metal is spinning, you apply pressure to the metal with a blunt, smooth tool that has no sharp edges. I've read that a hardwood tool can be used for this purpose, but I suspect that a polished steel rod with a blunt, rounded end such as Bill Moore uses would be most suitable. When Jim Hume needed tools for his tail light can project (described briefly in our lead story), he visited a local blacksmith/toolmaker (Savage Forge in Clearlake, Washington) to have his tools special made for the task. This would likely be a good move unless you are a blacksmith or toolmaker yourself.
You do need a special tool rest with holes in it to take a pin that can be used as a fulcrum when levering the tool against the form to shape the metal. It would be best if the tool rest has several holes so that the fulcrum pin could be moved as the work progresses toward the outside or top of the formed vessel (assuming it is a vessel).
Bill Moore states that the stroking should be from the smaller diameter (the foot or the tailstock point) toward the larger diameter (the headstock side). The point of contact of the forming tool with the spinning metal should be below center line and the stroke should be from the center to the outside edge. The repetition of the pressure draws the metal down over the male form, causing it to take the shape of the pattern against which it is being spun.
If you're making a piece of art or production pieces, the process is the same. The pattern can be used again and again if one takes care of it and exerts reasonable care in the spinning process. It's amazing how quickly a piece can be formed, trimmed to size, and polished--ready to use in a short time.