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Selecting an RPM for Turning

The best speed for turning a particular piece depends upon many factors such as its diameter, the soundness of the wood, and whether the piece is balanced.  Your comfort level should be considered as well.  Therefore, it’s not possible to make a hard and fast rule such as, “All bowls 10 inches in diameter should be turned at 750 RPM.”  

However, turners generally agree on what speeds are suitable for pieces of a given diameter.  Some turn a bit faster and others a bit slower so what has emerged is a range of RPM that serves as an initial suggestion – before taking other characteristics of the wood into account.  Here’s the guideline; it involves a bit of arithmetic.

General guideline.  When you multiply the diameter of the piece in inches by the RPM of the lathe, you should get a number between 6,000 and 9,000.  

The diagram at right expresses this in graphical form.  It’s actually four graphs side by side with different vertical scales.  Locate the diameter of the piece at the bottom.  Then go straight up to the shaded region. Next, go horizontally to the left and read the RPM on the vertical axis.  

An example is illustrated by the red dots, for a piece 13” in diameter.  The minimum RPM is 460; the maximum is 690.  

A way to figure the RPM without using the chart is to simply divide 6,000 (or 9,000) by the diameter of the piece in inches.  The result is the minimum (or maximum) suggested RPM.  An easy example is a 10” bowl.  The minimum suggested RPM will be 600; the maximum, 900.  For a 6-inch bowl, the range is from 1,000 to 1,500 RPM.

Surface speed.  This is how fast the wood actually moves past the cutting edge of the tool.  At the lower end of the speed range suggested by the guideline, the surface speed is about 18 miles/hour (26.2 feet/second or 1,570 feet/minute).  

The values at the top of the range are about 27 miles/hour (39.3 feet/second or 2,356 feet/minute).  These speeds will exist only at the outer diameter of the piece;  the speed will be smaller as you move toward the center.

I’m not sure what the value is of knowing the surface speed, but the fact that it’s pretty high helps to explain why tools dull fairly quickly.  

Surface quality and high RPM.  To get a good surface on a part being turned on a metal lathe, a high RPM is used in conjunction with a slow feed for the cutting tool.  Only a thin line of metal is removed with each revolution of the part and the tool marks left behind are very fine and close together.

Experienced woodturners use the same principle in making finishing cuts.  The RPM is turned up, a very light cut is taken, and the tool is advanced rather slowly in the cut.  This combination gives the best odds for getting a smooth surface, but many other factors come into play.

Turning a lot of air.  The rim of a square-edge bowl is not continuous all the way around the piece.  Empty spaces exist between the corners and this complicates the simple act of advancing the tool in the cut.  There can be no bevel rubbing in the empty spaces so precise tool control is a must.  Turners refer to this as “turning air.”

A similar situation exists if a vessel has one or more large voids in its surface.  Contact with the wood is lost as the void passes underneath the tool.  Again, precise tool control is a necessity.

In such cases, the general recommendation is to increase the RPM so the void passes under the tool very quickly.  This keeps the actual distance the tool advances during the transit of the void to a minimum so that no sudden heavy cuts are encountered when the tool makes contact with the wood on the trailing side of the void. However, turning at a high RPM is not a substitute for precision in manipulating the tool.

Caution:  There are hazards associated with high RPM, especially with pieces having voids in the surface.  Indiscriminate use of high speed is dangerous, both to you and the workpiece.  More on this is given farther down in this article.

Turning degraded wood.  You must exercise some judgment in regard to using high RPM settings when turning either wood that has partially decayed or wood that has voids, bark inclusions, or ring shakes.  (See article on spalted wood, part 1.)  The centrifugal force associated with a high RPM can cause the wood to come apart.  

Some partially decayed wood exhibits spalting, but some does not. You cannot take the absence of spalting to be an indication that the wood is sound.

Truing a blank that is seriously out of balance.  Begin rotating the piece at the lowest RPM setting your lathe can achieve.  Then, very carefully, increase the RPM until the vibration becomes noticeable.  Back it off a little, and begin truing the blank. You can increase the RPM as you remove wood to bring the blank closer to a balance.  

Your comfort level.  If a chart or another turner suggests you should run your lathe at a speed higher than what you are comfortable with, don’t do it.  Nothing says you have to; it’s as simple as that.  

Just a Spattering of  Rotational Physics

Centrifugal force (CF) tends to make an object traveling in a circle fly away from the center of rotation.  When a bowl or vessel explodes off the lathe due to an overspeed, CF is the cause, together with whatever shortcomings the wood may have.

CF is directly proportional to the diameter of the circular path in which the object is traveling.  The CF exerted at points on the rim of a 10-inch bowl is twice that of a 5-inch bowl turning at the same RPM.

CF varies with the square of the RPM.  If you double the RPM, the CF increases to four times what it was.  Going in the other direction, if you cut the RPM in half, the CF is reduced to one-fourth of its original value.  A small change in RPM produces a much larger change in CF.

An increase in RPM of only 41% will double the CF.  As an example, if you speed up the lathe from 1,000 to 1,410 RPM, the CF acting on every point of the workpiece will be doubled.  On the other hand, a decrease in RPM of only 29% will cut the CF in half, such as reducing the speed from 1,000 down to 707 RPM.  

The kinetic energy of an object spinning on the lathe is a measure of its ability to tear stuff up or do bodily harm if it comes off the lathe.  And, naturally, the faster it spins, the more kinetic energy it will have.  

Kinetic energy varies with the square of the RPM in the same manner as centrifugal force.  Therefore, a small change in RPM gives a much greater change in the kinetic energy.  

Energy content of a spinning disk.  Here’s a way to visualize the energy contained in a disk turning at a certain RPM.  The disk is an approximation of a bowl or vessel of similar diameter.  

Suppose the disk is 10” in diameter and spinning at 800 RPM.  The energy it contains is enough to lift it from the floor to a height of 9.5 feet.  If the disk is dropped from this height and allowed to crash onto the floor, the energy dissipated in the crash will be the same as the energy it contains while rotating on the lathe.

Will it be a big crash?  That depends upon the weight of the disk. If the disk is made of balsa, which is very light, it won’t be much of a crash.  But if it’s wet sweet gum, for instance, it will be a much bigger deal.  

The following chart shows the effect of changes in the diameter and RPM.  The energy content varies as the square of both the diameter and the RPM, so a small change in either one will make a big difference.  

(Note:  The weight of the disk does not enter into the height calculation.  A heavy disk will contain more energy while it is spinning on the lathe, but a corresponding greater energy is required to lift it to the calculated height.  However, the weight of the disk has everything to do with the size of the crash if you drop it.)

So what does all this physics stuff mean?  OK, here it is. Suppose, heaven forbid, the disk comes off the lathe and strikes you in the face in such a manner that most of the energy it contains is expended during the impact.  The effect would be the same as if you were to lie down on the floor, facing upward, and have the disk dropped from the calculated height and hit you square in the face.  

This may be overstated just a bit.  If the piece comes off the lathe, the impact will probably not be quite as bad as the above implies, for a couple of reasons.  One is that it is not likely that all the contained energy will be imparted to your face.  And in a real hit, your face will recoil from the impact, which will lessen the damage somewhat.  If you were lying on the floor, the recoil would not be possible.

RPM and safety.  Turning at high RPM is more dangerous because (1) the greater centrifugal force is more likely to cause the workpiece to come apart, and (2) if the piece does come apart, the consequences are likely to be much more serious.

If you want to be safer while turning, keep the RPM down unless there is a good reason to crank it up.  And, always check the RPM setting before you start the lathe.

RPM and spindles.  The relatively small diameter of spindles reduces the contained energy dramatically in comparison to a disk. For example, the energy contained in a spindle 1.5” in diameter, 30” long, and spinning at 3,000 RPM is only about 10% of the energy contained in a disk 10” in diameter, 2” thick, and spinning at 800 RPM.

For this reason, the RPM used to turn spindles is much less of an issue as far as safety is concerned.

Selecting a Speed – A Summary

Here are several general statements and considerations that relate to selecting the RPM in various situations.  None are hard and fast rules.

1.  Use the guideline given at the beginning of this article to find the range of RPM for the piece you are turning.  

2.  Heavy, roughing cuts should be made at a lower RPM than what is suggested by the guideline.  

3.  Finishing cuts should be made toward the high end of the RPM range suggested by the guideline.  

4.  Turn degraded wood at an RPM somewhat lower than what you would use for sound wood.

5.  Objects such as natural-edge bowls, square bowls, and vessels with voids are typically turned at a higher speed in order to minimize the effects of “turning air.”  However, you must consider the integrity of a piece with voids before spinning it at high RPM.

6.  Avoid using high RPM settings unless there is a justification for doing so.  

7.  Always check the RPM setting before starting the lathe.  

8.  Do not push yourself to turn at an RPM higher than what your comfort zone allows.

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