What speed to use on the lathe?In general, what speed to use on the lathe is something you feel. Usually, I don't think of the lathe speed, I just increase or decrease it when I feel it is necessary. But that requires some experience, and before you get that, you may like to have some guidelines.
In general: start at a low speed. Then you may try to increase the speed in small steps afterwards. Be ready to switch off the lathe when you start it if the speed turns out to be too high.
The larger the diameter of the wood, the slower the speed. But the higher the speed, the smother the cut wil be - until a certain limit where the speed becomes too high and the wood or the lathe becomes unstable and starts vibrating. If the wood is vibrating, the tool will cut a spiralling pattern into the surface. If spiral shapes appear on the wood, either reduce the speed or try taking lighter and slower cuts.
If the blank is out of balance, e.g. a rough part of a trunk with bark on it, you have to use a slower speed than normally for the size of the wood to avoid vibrations.
For spindle turning of not too thick blanks - 5 cm or less - 2500-3000 RPM or even more is good, but less is always possible. If the blank is very long or slim, you may need a slower speed to avoid vibrations. If the blank is thicker, the speed is reduced correspondingly. The same speeds are used for small blanks mounted without support from a centre in the tailstock.
Face turning - bowls etc.: If the start blank is round, octagonal or square, the speed can bee about 500 RPM for a medium sized blank and higher (1500 RPM) for a small blank. When the blank has been turned round and it is balanced, the speed may be increased - maybe to 2000 RPM for small blanks.
A large piece of wood that is out of balance, e.g. a section of a trunk, will require a much lower speed - maybe 100 RPM.