Figuring machine rpm revisted.. Quick question

When you are trying to decide which pulley to put on a machine to give a certain rpm, do you use od or pitch diameter. I have had people tell me od and also others saying that is wrong pitch diameter should be used. Just wondering what you use.
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For the level of accuracy required for what you are doing it is not that important. A cpl of revs one way or the other will make no noticable difference. Personally I always calculate using o/d which is the sizes stamped on the pulleys. Unless you are using large differences in pulley diameters there is very little difference in pulley ratio between using o/d or pitch diameter which is what we are really concerned with. IE 1" to 2" pulley, 1 1/2" to 3" pulley, 2" to 4" pulley, 2 1/2" to 5" pulley. 3" to 6" pulley .... and the list goes on will all produce the same rpm at the head +/- 1%. If you really are so concerned with it get hold of a rev counter to attach to tha cutter and then have pulleys machined to the exact sizes you require. If you are going to that level of accuracy you might as well heve them dynamicaly balanced as well just to make sure there is no vibration.
Its not the size of the workshop or the size of the tool that matters. It is what is done with what you have that counts.
Dont get too technical, we are working with wood, not machining instrument components Stop playing with it ! Do some real work and enjoy it

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