Ok so am I thinking this wrong
FPM - is not just the longitude travel speed but also material removed on the circumference of the material?
I hope I can make this more clear instead of less. It's two things, that you are trying to make into one. Two totally independent things. Both related to the same cutting operation, and the outcome of the part, but two totally separate things.
Feet per minute is NOT the speed that the carriage moves at. On a lathe, It is the speed at which the work piece rotates past the tool. On a typical lathe operaton, it's measured in a vertical direction from high to low. Think of it like this- Put a piece of round bar in the chuck Any size, it doesn't matter. Draw a dot on the piece. Move the tool next to that dot, and turn the lathe on. No cuttting, no movement of the carriage, no turning cranks, just turn the lathe on and watch it run. If the lathe is running very slowly, the dot will travel past the tool bit very slowly. If the lathe is running very fast, the dot will travel past the tool bit very fast. The speed at which that dot you made passes by the tool bit, that is the surface feet per minute. It could be inches per minute, it could be kilometers per hour, it doesn't matter. Feet per minute is just the customary units that have been adopted over the years. That speed, and that speed alone is the is the FPM number that you must calculate. That number for any given size is calculated from the spindle speed in revolutions per minute, and the circumference of the work in the lathe. That's it. You're done. Nothing else goes with it.
The feed per revolution for power feeding is not calculated based on time. That is set up separately, with gears, so it is not tied to time at all, it's tied exactly to the number of rotations of the spindle. Every time the spindle turns one time, the carriage will have moved (usually to the left) by exactly that much. Another way of thinking of it is this- You use the cross slide (or compound slide) dial to move the tool bit in an exact amount, to get a specific depth of cut in one direction, that is the direction away from you, towards the lathe. That sets how wide your chip of removed material is. The feed per revolution is the second dimension, in most cases, it's how deep you cut to the left, which sets the thickness of your chip. So it's not a speed adjustment at all, but rather it's one of the two "depth of cut" adjustments.
Both of those settings will affect the cut, the finish, the horsepower requred, and all that, but both settings are determined separately and independantly, and serve two very different functions.
