The earliest lathes were made of wood, and the use of wood as a bed material has survived to the present (I designed and turn on such a lathe ). More commonly today, though, lathes are made with all-metal parts. Yo u’re likely to encounter several lathe-construction materials: cast iron, fabricated steel, steel stampings, cast aluminum, cast zinc, and extruded aluminum. I have not mentioned plastic here because it is not yet used in major structural components; rather it is reserved for handles, knobs, and housings.
Please see “History of Lathe” for more details
Of great concern to turners is the amount of vibration inherent in a lathe. Any turning convention will find turners waxing lyrical about how quiet and vibration free their pet lathes are. Any machine has a natural frequency of vibration when given an impulse from a variety of sources such as the motor, work by the operator, or in the case of lathes the work itself. The natural frequency of vibration is the frequency at which the machine will continue to vibrate after an initial impulse, regardless of the source. This frequency is directly proportional to the stiffness of the building material and inversely proportional to mass. This means that the stiffer the material, the higher the frequency, but the more massive the machine, the lower the frequency. The old adage that “you can’t beat a good heavy machine” is undoubtedly true.
CAST I RON
Cast iron is a time-honored material for lathe construction that’s still hard to beat. The inherent mass of cast iron combined with a favorable modulus of vibration makes for a sweet-running machine. Most woodworking machinery is cast from grade-25 gray iron, which has a nice balance between strength, damping effect, and machinability. Castings require expensive tooling in the form of patterns, and the casting process itself is expensive-especially in small lots.
To p-quality lathes are made with a heavy cast- iron headstock, tailstock, and tool-rest assembly. In the past, the bed would also have been cast iron, but today makers are increasingly turning to fabricated structural steel as the bed material.
A lathe with cast-iron major components mounted on a wooden bed is known as a “High Wycombe lathe,” so named after the lathes that were popular with turners who turned furniture parts around High Wycombe, England, until the early part of the 20th century. Many of the photographs in this book are of a High Wycombe lathe that I code signed. The best economy lathes are often made with cast-iron parts, but the castings are light in keeping with the lathe. On more expensive lathes, the castings are “filled”-a substance not unlike auto body filler is squeegeed on the castings-before painting, resulting in a much better paint job. This difference is purely cosmetic but does reflect the attitude of the manufacturer, just as a better car has a better finish.
Fabricated lathes are made by welding together pieces of structural steel. Fabrication first became popular in the late 1950s as a less-expensive alternative to cast- iron castings. Its biggest advantage is that little or no tooling cost is required, which makes fa brication particularly well suited to small production runs where amortizing the costs of patterns for castings would be difficult. While traditional machine-design theory holds that steel has poor damping qualities compared with grade -2S cast iron, that’s only part of the story. It is true that the stiffness of steel makes for high-frequency vibration, but the welds tend to act as barriers to the transmission of vibration. Instead they act much like a cracked glass, stopping vibration. Fabrication is used in both the most expensive and the cheapest lathes, so use common sense when making buying decisions.
Steel stampings are made by placing sheet steel between male and female dies mounted in a press. The press closes the dies on the sheet steel and forms it into the desired shape. Complicated shapes often entail a progression of dies. The cost of the dies for stamping parts can be considerable, but the stamping process itself (unlike foundry work) is cheap. This makes traditional stampings great for high-production work but expensive for short-run work. The last decade has seen the introduction of computer numeric machines that can do short-run stamping on a cost-effective basis. These machines can “nibble” out the basic shape with small round or square dies, then form the material on various standard-forming dies. A stamping made from heavy-gauge metal can be very good, but the most charitable thing that can be said about stampings from light-gauge material is that they are vibration prone. Stampings are used extensively for machine stands, belt covers, and the like.
CAST ALUMINUM AND ZINC
Parts such as pulleys, knobs, and hand wheels are often made fr om cast aluminum or zinc. These are often called die castings because the low melting temperature of aluminum and zinc allows them to be cast in metal molds or dies. Die castings are almost perfect directly from the mold and require little machining. The low weight of aluminum makes it a good material for pulleys because balancing is less of a problem than with cast-iron pulleys. Die-cast knobs and levers are a much better option than plastic.
Occasionally lathes are made of extruded aluminum. Although extrusions can be tempered, the alloys used for the process are soft and gummy. This softness, as well as the tendency of raw aluminum to turn anything it touches black, is somewhat lessened by anodizing, a plating process that puts a thin, hard coat of aluminum oxide on the surface of the metal. However, dents that go through the anodized surface into the soft aluminum substrate are a potential problem.
There are more and more extrusions being used in woodworking machinery today-my table saw fence is an extrusion, for example. Although extrusions are acceptable for miniature lathes and parts of lathes, I’m not convinced that they are suitable for an entire full-sized machine. Before you buy a lathe made principally of extrusions, consider how much and what type of use you intend for the machine.
Most lathes available today are hybrid designs, incorporating two or more of the construction materials outlined above. A common design is a cast iron headstock, tailstock, and tool base mounted on a structural-steel bed or, on economy lathes, on solid- or hollow-steel tubing. Steel stampings, sheet metal, aluminum extrusions, and plastic are often used for stands, belt covers, and knobs.