How about stiffness.
When cutting once the tool/machine deflects more then the chip thickness the cutting becomes unstable and you get chatter as the machine/tool deflects and makes the chip thicker, and the load higher until it bounces back.
So once you have considered the force, you need to know how much the machine will deflect under load.
If it is too much you either must make the machine stiffer or reduce the cutting load.
A machine made from aluminum will deflect about 3 times as much as steel for a given structure and a wooden machine....??? Well lots more. eh.?
http://en.wikipedia.org/wiki/Young%27s_modulus
The above link should be of assistance.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.