A common question on the various woodturning forums is "what is the best turning speed?".
The answer depends mostly on the lathe operator's skill level, but on a few other things as well. A little finial or similar spindle turning is not likely to injure anyone if your speed is too high, but any bowl or hollow vessel bigger than say, 4 or 6" in diameter can hurt someone if it flys apart.
Bill Grumbine says "if the lathe isn't shaking, and the turner isn't shaking, then the speed is fine.". Makes a lot of sense, but some people need more specific detail than that.
Just don't get too hung up on numbers. Numbers don't mean a lot, as far as RPM goes. Don't get too excited about tachometers that give readouts to one-thousandth of a revolution per minute, or even to the nearest 50 RPMs.
What matters is the operator's comfort level, and to what extent he/she is prepared to deal with a situation that goes beyond that. If you have enough experience to know how to react instinctively in a situation, and have the proper personal safety gear (good face protection being the most important) then consider yourself well prepared.
When you are starting out with turning, a speed of 500 RPM might make you feel a bit queasy. That's generally the slowest speed available on the entry-level lathes commonly available on the market. That's fine! It's good to be extra cautious when you are starting out. Don't let anyone dare you into going faster than your comfort level. As you gain experience at the lathe, you can judge if you are ready to crank the speed up a little more.
There are guidelines for determining lathe speed, and they are rather loose. The main thing they depend on is the biggest diameter of the piece you are turning, or the diameter of the area you are currently turning which may be smaller than the largest diameter.
To be more specific, it's the
surface speed of the wood spinning past the tool which is the bottom line here. If too slow, you have to slow your tool travel speed (also referred to as tool FEED) way down to where it feels clumsy and uncomfortable. There's that C word again, COMFORT. With too slow of a speed, if you push just a little too hard and too fast, you will turn a spiral! Or just a ragged, ugly groove in your work.
Too fast of a speed, and the shavings FLY off the piece. This can be fun, but most likely, those shavings are really HOT. They can even burn your fingers if they are right near the cut. This will also dull your tool a lot faster and could take the temper out of a high-carbon turning tool. OK, not many turners use high-carbon steel anymore, but sometimes it's useful for custom tool shapes.
Here's the guideline/rule of thumb I have seen recommended by Mike Mahoney, Dale NIsh, and other top turners:
Diameter multiplied by RPM's should equal 6000-9000. Another way of putting it:
6000-9000 divided by Diameter gives recommended RPM. That range of 6000-9000 is a range of
constants. (A mathematical term) It's not numbers with any particular units, but a multiplier to get the desired results using the other numbers which do have units: RPM's, and Inches. The constant here is applicable only to inches, so if you measure in centimeters you have to convert the constants to apply. Give me a day or so, I'll try and do that.
Turners with less experience should go with the lover constant, and higher for the more experienced turners.