# Determining the tip speed of a rotating disk

 Forum topic by Chris posted 03-30-2010 04:21 PM 2073 views 0 times favorited 9 replies
 Chris 1867 posts in 2735 days 03-30-2010 04:21 PM I was having a discussion the other day and the question of how fast the outside edge of a bowl blank is actually moving came up I couldn’t remember the math… been to long. Do any of you lj’s or engineers remember the formula for that? Lets say that the lathe is running at 500rpm and you have a 6” diameter blank mounted; how fast is the edge actually going? -- "Everything that is great and inspiring is created by the individual who labors in freedom" -- Albert Einstein

## 9 replies so far

 PaulfromVictor220 posts in 2089 days #1 posted 03-30-2010 04:27 PM Here is my guess somebody is welcome to correct… Circumference = pi x diameter, so 3.14×6 = 18.84” circumference (distance around) If that is spinning 500 times in a minute , that = 9420 inches a minute. That equals 785 feet per minute. To have more fun… x 60 that equals 47,100 feet per hour…divided by 5280 = 8.92 miles per hour. CharlieM195815777 posts in 2962 days #2 posted 03-30-2010 04:27 PM How fast is the edge going? 500rpm. :-) But I guess you wanted miles per hour? Feet per second? -- Charlie M. "Woodworking - patience = firewood" PaulfromVictor220 posts in 2089 days #3 posted 03-30-2010 04:32 PM Now to have yet more fun… If a 6” spinning bowl blank leaves chicago at 10:00 a.m heading for new york at 500 rpm, when will a 9” bowl blank leaving Chicago at noon heading toward New York spinning at 550 rpm pass the 6” bowl blank? (my brain hurts just writing that question…) CharlieM195815777 posts in 2962 days #4 posted 03-30-2010 04:39 PM In Pittsburgh? -- Charlie M. "Woodworking - patience = firewood" Chris 1867 posts in 2735 days #5 posted 03-30-2010 05:16 PM I guess I could have worded that better…. The real question would have been regarding velocity…. Tip speed in Feet p/Sec i guess…. versus the rpm. You see the discussion was regarding the proper rpm vs the blank size which kind of morphed into this side question. -- "Everything that is great and inspiring is created by the individual who labors in freedom" -- Albert Einstein JJohnston1593 posts in 2035 days #6 posted 03-30-2010 05:39 PM Do you mean, “for a specified tip speed and diameter, what should your RPM be”? Then it’s just a matter of rearranging the equation (solving for RPM) and boiling down the constants. For tip speed in feet/sec and diameter in inches: RPM = 229*tip speed/diameter -- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger PaulfromVictor220 posts in 2089 days #7 posted 03-30-2010 05:41 PM 13.08 feet per sec. CharlieM195815777 posts in 2962 days #8 posted 03-30-2010 07:39 PM The next question this raises for me is: What is a good tip speed for shaping a bowl blank? I did a little research and found several references to this simple formula: The diameter of the blank in inches multiplied by the lathe rpm should fall between 6,000 and 9,000. So, for example, you would want to turn a 10” blank somewhere between 600 and 900 rpm. A 5” blank should be turned at between 1,200 and 1,800 rpm, and so on. This video explains it: http://www.youtube.com/watch?v=-3mqZjTgVc4 -- Charlie M. "Woodworking - patience = firewood" cosmicturner403 posts in 2139 days #9 posted 03-31-2010 01:49 AM I can answer the safest speed is 0.0hard to say it depends on the cut your making and the type wood no real rule more of a feel but less viberation is better unless you want texture…chatter timehappy turning -- Cosmicturner