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I was wondering if anyone knew of a formula or chart that would tell you what speed the outside of your bowl blank is turning in relation to the spindle speed. For example .... if you are turning a 12" blank at 500 rpm..what speed is the 12" part of the blank actually running at? I've searched online but have had no luck in finding anything....
Barry, Alan Lacer has an article on his website that contains a chart with rpm/speed for a given diameter. The chart is at the end of the article.
The basic formula to find velocity is distance divided by time. The distance traveled by a point on the perimeter of the bowl in one minute is PI x the diameter of the bowl x rpm (3.14 x 12" x 500 rpm = 18,840 " per minute). In one hour, the bowl will go 60 times that far or approximately 18 miles. So the rim speed of the bowl is about 18 mph at 500 rpm.
Thanks Steve and Tom for helping me understand this....
These guys have probably turned a couple orders of magnitude more bowls than I.
However I find more importany determinants of appropriate speed is the balance, thickness, mass, and chuck jaw size.
I turn up the speed until it starts to vibrate and then back off. I also find there appear to be harmonic points on a balanced blank whereby it may vibrate at one speed and then nonintuitively balance out at an even higher speed.
I personally believe that it's best to turn at the highest possible, balanced speed. This allows me to be gentler with the chisel which gives me best results for many reasons.