Speed Zone

By Alan Lacer

What speed do I turn at? A number of years ago in a class at Arrowmont, someone asked the instructor that question. After some thought he responded,"well I guess it should go around." What an insight! On one level you might think he was being a wise guy, but on another he was close to the answer. You really can turn at a wide range of speeds-and do excellent work. However, there are a number of factors that one balances in choosing a speed, and this is why I have never been a fan of the "speed selection charts" that often come with your lathe.

Working with miniatures Now we are into a red zone: large diameter (17"), heavy, out-of -round/balance, blank. If the lathe can handle such a piece, I progress from a point just below vibration to a modest speed as it becomes more balanced. However, with a rim speed of 51mph at 1000rpm I never find it necessary to crank up much speed.
When working small diameter miniatures you really can raise the speed to fairly high levels. However, I was getting clean cuts on this piece at speeds easily below 1000 rpm. Working with large diameter pieces

Diameters and Rim Speed

The rpm of the spindle is sometimes the least important number for me-and add the fact that even my most expensive lathe does not give a digital readout of rpm. No, the speed of the outside edge or surface may be far more telling in determining speed. (Comparison: the outer edge of a 10" table saw blade at 4000rpm is travelling at 119mph, while the 1/2" router bit at 25,000rpm is only travelling at 37mph). Simply look at the chart of rim speeds at different diameters, and see the dramatic differences. A miniature running at 1200 rpm may look like it is hardly moving, while a large bowl may overpower you and your lathe-which may place you into a danger zone.

Mass of the Object

The real impact of an object on the lathe is its velocity times its mass. So, a pen blank won't have a lot of force at 1800rpm-even if it came off the lathe-while the 14 lb wet bowl blank at 1800rpm can be lethal. The higher the speed the greater the forces-and at some very high rpm even the pen blank has real force.

Balance of the Object

Look at what a few ounces of lead in the wrong spot on a front wheel of your car can do: causes a two ton monster to shake and rattle at certain speeds. We have the same problem in woodturning: out of round, inconsistent densities of the material, or pieces with voids, can all lead to excessive vibration at certain speeds. And in reality, we may have some pieces that never balance-forcing us to work at slower speeds than we may desire.

Stability of the Lathe

This is related to everything I have already mentioned: some lathes simply start shaking with almost anything on them! And vibration is a curse to the machinist and the woodturner: we will have a rough ride, quality usually suffers, and it raises safety issues if we don't have some degree of stability of the lathe itself. Also, some lathes have awful stands/legs, flimsy headstock spindles, headstock bearings that are too few, underbuilt or just too close together-all of these factors impact lathe stability and therefore the speed you can turn at.

And one more factor: the low end speed of some lathes are simply not slow enough to do much bowl turning-they simply run too fast and are too underbuilt. Serious considerations in choosing a lathe if your interests are with bowls and vessels.

The Skill of the Turner

With NASCAR racing and woodturning, a true professional can often work at higher speeds. As your skill and control improve you can work at greater speeds. However, unless you are a production turner working on a piece rate schedule, high speeds are not really the answer-so be careful here, as even some production turners have had serious accidents related to speed. In most cases folks don't really care how quickly you made something, only how well it turned out.

The Material

I often hear it said that you get a better cut at higher speeds. Sometimes, but in reality there are still other factors related to the material that will effect the quality of the cutting action. The moisture content is one (generally the wetter the wood the cleaner the cutting action), orientation of the grain as well as consistency in grain direction (cutting against the grain or grain that is wild and erratic causes problems), species (compare the cutting qualities of fir against pear-don't even seem like the same material, i.e. "wood"). Sometimes I do get a cleaner cut by raising the speed (you are getting more cuts per inch of travel)-but sometimes I do better by not raising the speed and only slowing my feed rate (I move slower, and thereby get more cuts per inch of travel). And add to this the question of tool sharpness-working at higher speed becomes a smaller component of the equation. Finally, too much speed can contribute to the problem of ribbing or chatter-when due to the material flexing or distorting due to rate of speed.

Recommendations for Choosing a Speed:

Speed of Lathe - Turned Objects at Different rpm

RIM SIZE
(outside dia.)
  250 rpm 450 rpm 600 rpm 1,200 rpm 1800 rpm 3,000 rpm
1/2" dia. .4 mph .7 mph .9 mph 1.8 mph 2.7 mph 4.5 mph
6" dia 4.5 mph 8.0 mph 10.7 mph 21.4 mph 32.1 mph 54.0 mph
12" dia 8.9 mph 16.0 mph 21.4 mph 42.8 mph 64.2 mph 108.0 mph
14" dia 10.4 mph 18.7 mph 25.0 mph 49.9 mph 74.9 mph 125.0 mph

Alan Lacer is a contributing editor to American Woodturner living near River Falls, WI.

This article originaly appeared in the
American Woodturner Issue Spring 2004 Volume 19, No. 1