Turning Archive 2007

Subject:
Wood surface speeds

Troy German
>I just went though this exercise for myself and thought others would find the results interesting. I’m sure this is old hat for many of you.

To calculate the surface speed of your wood the following formulas should work for all practical porpoises [insert picture of practical porpoises here]:

radius (inches) x rpm / 2 = ft/min (That’s radius times half the lathe speed…easy!)

radius (inches) x rpm / 170 = mph (At 1700 rpm, that’s radius times 10.)

radius (cm) x rpm / 1000 = m/sec (That’s metric.)

For example, a 10” turning blank (5” radius or 12.7 cm) at 500 rpm has a surface speed of (5 x 500 / 2) 1250 ft/min, or (5 x 500 / 170) 15 mph, or (12.7 x 500 / 1000) 6.4 m/sec. So, if you run a 10” blank at 500 rpm the tip of your gouge cuts 1250 ft of wood every minute! (I know. The radius gets smaller as you cut which throws off the numbers, but accounting for that involves other variables…and calculus.)

As far as safety is concerned, I would imagine that momentum is more important than speed alone, so weight of the blank would have to come into play. Is this true? Do some of you have guidelines about safe surface speeds?