Turning metal on a wood lathe is possible, even though the wood lathe is not designed for it. Lathes are designed specifically for either woodturning or engineering purposes, rarely both. Heavily-built engineering lathes work better than wood lathes. Their tool holder rigidly clamps the cutting tool and moves mechanically. But if you don’t have an engineering lathe and aren’t too ambitious, you can turn many small items in brass, aluminium or even steel freehand quite successfully on a wood lathe. The finial shown above was turned on my Graduate Shortbed lathe, mainly using a small gouge. It was threaded onto a bit of 12 mm studding held in a chuck, with tailstock support, and sanded to an acceptable finish. It is about 5 inches tall.
There are problems to overcome. Holding the tool so that it cuts steel is difficult. The hardness of the metal resists the cut, and the cut is very liable to ‘chatter’. This is vibration that leaves a rough or ridged surface on the work. Working freehand, accuracy is harder to achieve. Without a tool slide, accuracy comes from the turner’s skill. Making true cylinders or flat surfaces accurate to a thousandth of an inch freehand is not easy. But many items don’t need such precision.
Interrupted cuts, such as turning the corners off something, are particularly difficult freehand because it is hard to control the cutting tool. It is risky, too. Some wood lathes have a proper slide rest as an accessory.
Woodturners are familiar with the problem of chatter. When turning metal on a wood lathe it is hard to avoid. To prevent chatter, you need a strongly built lathe, with good bearings. It must hold the workpiece firmly, with minimum projection from the headstock. For example, modifying a drive centre while it is in the spindle taper is easy. A similar job held in a chuck is harder, because the metal can move away from the cutting tool.
Use tailstock support whenever possible. A Jacobs chuck will hold small items. Light cuts using a robust and sharp tool, with minimum projection over the toolrest, should then produce an acceptable result.
You can use a high speed steel woodturning scraper or a graver. A graver, which you can easily make yourself, was traditionally made from square section tool steel with a diagonal flat, leaving a long point at one corner. You could convert a triangular or square file, but a high speed steel tool bit would be better. A round high speed steel bar ground with a pyramid point would work. It would be similar to a woodturning point tool, but with a more obtuse point. Use a graver a bit like a skew chisel. Its edges (not the point) can plane off long, thin curly shavings from steel.
Brass likes tools with zero top rake, so responds well to scrapers. Tools leave a polished surface on brass. Aluminium turns with a graver or even a small short-beveled bowl gouge. Cutting speeds are lower than for wood, but because only small items are possible, the normal low-speed setting on the lathe is probably OK. Some metal alloys are more free-cutting and ‘turnable’ than others. A file will shape the item and remove chatter marks if necessary. Even on a lightweight lathe you can make simple shapes in steel using a file or a rotating grinding wheel held in a drill chuck.
Turning metal on a wood lathe is tiring, particularly with steel, because the tool must be held firmly up to the work. It helps to use a pivot pin in the toolrest to lever the tool into the work. This gives more control. If the workpiece is held rigidly, the pivot pin can help prevent chatter too. A heavy cut can increase it.
Safety when turning metal on a wood lathe
Turning metal freehand is hazardous, therefore precautions are necessary. It is essential that the workpiece is secure in the lathe. Chunks of metal flying out of the machine are even more likely to do you harm than are lumps of wood. Eye protection is a must. The swarf is sharp and hot – wood chips hitting your hand are annoying, but metal swarf can cut. Long strands could even catch your fingers and drag them in. Never clear away swarf while the lathe is running.
Turning with a scraper can make chips like little needles. They can get in your skin like splinters. But gloves are risky around moving machinery because they can catch and drag your hand in. Thin ‘rubber’ gloves that tear are safer. A bad dig-in could wrench the cutting tool badly enough to break it and perhaps cause injury. A file thrown back by the chuck jaws can injure you. They must always be used with a proper handle to stop the tang impaling your hand.