Are they the same for a 1 or 2 or is it based on diamiter. How does it know which I have.
Signed Clueless
Nothing knows, it's just a fact of mathematics.
RPM is constant. No matter what is spinning on the lathe, any point on the workpiece will revolve around the center of the shaft the same number of times per minute.
However, what's important when you're touching a tool to whatever is spinning is how fast the surface of the material is running against the cutting edge of the tool. That depends on how far away the tool is from the center of the shaft. It's called the "Cutting Speed" and is usually in feet per minute.
Points on the workpiece (or grinding wheel or whatever is spinning) have a longer distance to travel to make one rotation the further out they are from the center of rotation. Just like in order to drive around a two mile racetrack in the same time someone else is driving around a one mile racetrack, you'd have to go faster along the path you're traveling.
For penturning, because our workpieces (pen barrels, usually) are
relatively the same size, we don't often talk about surface speed, we just refer to the RPM. Bowl turners on the other hand are very concerned about surface speed because that varies greatly from a small bowl to a large one, and because the cutting speed on the outside of a big bowl has to be controlled for safety and machinability reasons.
Hope that helps, but reading it over, it's as clear as mud :biggrin:
