A dull or broken cutting edge produces smearing. Instead of a cleanly sheared surface, the finish looks torn up. The balling-up of material on the surface and jaggedness of the cut are the visual results.
Modern industrial lathes using the latest hi-tech carbide tooling routinely produce parts with a mirror-like finish. Even parts made from the toughest alloy steels come “off tool” with surfaces that require little, if any, additional work (such as grinding or polishing) to refine them to all but the highest standards. Unfortunately, most home shop lathes lack the horsepower, heft, and rigidity to achieve a superfine off-tool finish. That doesn’t mean you can’t get an acceptable finish, but given the inherent limitations of small machines in general, getting a good finish can sometimes be a challenge.
The key to a good finish starts with understanding what can cause a bad finish. The number one enemy of a good finish is chatter. Chatter is often the result of turning a part that is too thin, too long, and/or unsupported (see “Be Steady,” January 2017). But chatter can also be caused by tool flex, tool height, tool profile, the depth of cut being too light or too heavy, or any combination thereof.
Dull or chipped tools are another common culprit. High speed steel (HSS) tools can be resharpened on a home shop grinder (see “Daily Grind,” September 2017), but there’s not much you can do when carbide-tipped or -insert tools wear out and become unusable. Yes, it’s true: Tough as they are, carbide tools eventually go bad. Unlike HSS tools, where the wear is more or less proportional to use, carbide tools will stay sharp for a long time, but at some point the cutting edge will chip or fracture and it’s a goner. Parts machined with a damaged edge will have irregular grooves and/or small beads of material balled up on the surface. This is sometimes referred to as “smearing,” which describes what’s happening to the material as it encounters the dull tip of the cutter.
Available power, the rigidity of the tool, and heft of the lathe all play into the equation when turning. A 0.250-inch depth-of-cut on 440 stainless steel is no problem for this 2-hp lathe.
Another potential cause of smearing, especially when turning aluminum, is friction welding. This occurs when small globules of material adhere to the tip of the turning tool. This effectively blunts the tool and can result in any number of surface issues, from roughness and globular deposits, to a sudden change in the expected dimension. Usually the offending globule can be knocked off and the tool is good as new. If you encounter this type of smearing (friction welding), it can usually be avoided by adjusting the depth of the cut or the feed rate. If neither of those work, change the tool for one with a different geometry. Aluminum in particular responds to tools with higher rake angles (for more on insert tooling, see “Cool Tools for Lathe Work,” August 2015).
If your tools are sharp and in good condition, you should be able to get a smooth finish on any material. Maybe not mirrorlike, but respectable. Experimenting with different depths-of-cut and feed-rates will tell you what’s possible with your particular lathe.
This close-up shows the difference feed-rate can have on surface finish. Using the same tool, the same depth of cut (0.015 inches), and rpm (600), the off-tool finish on this 440 stainless bar improved dramatically using a finer feed-rate.
Hand- Versus Power-Feeding
Engaging the carriage feed usually assures the most consistent finish, but there are times when hand feeding is preferred to power feeding. On some materials, hand feeding provides an element of tactile feedback that you can’t get with power feed.
On certain machines, particularly small benchtop lathes, your choice of feed rates may be too limited to achieve a desired finish.
All metal lathes come with a chart to show which gear combinations to use for cutting different threads (in threads per inch) and power feeding (in carriage distance traveled per revolution of the spindle). See “Chasing the Elusive Thread,” in the May 2017 issue for more details.
Unlike larger lathes, most small benchtop lathes don’t have a dedicated carriage-feed mechanism. By design, these “mini lathes” utilize the thread-cutting mechanism for power feeding. This limits power feeding to one or, at most, two options. For example, all variants of the basic 7×12-inch mini lathe come with gearing that provides the user with 0.0039 inch per revolution power feed. On lathes with traditional carriage-feed systems, this would be in the “midrange” of available feed rates. (A “fine” feed rate would be 0.001 inch or less per revolution.)
Fancy tooling is not needed to get respectable results. A good, sharp, HSS tool can match the results of carbide tooling.
Another drawback to power feeding with a mini lathe is that engaging the feed mechanism often adds a significant amount of drag in the form of gear train friction. This extra friction can make power feeding impractical for all but the lightest depths of cut (less than 0.010 inch).
While it’s impossible to hand feed with the same precision as power feeding, with some practice and by going at rates slower than the machine’s slowest feed rate, you can get a pretty good, and consistent, finish.
In an upcoming column I’ll cover power polishing on the lathe to show how you can refine any lathe-turned machined surface to a mirror finish. In the meantime, get out in the shop and make some chips!