3-Axis CNC Mill Project

[Hobgoblin]

See now i just feel really thick. I had in mind 5mm pitch, 15mm belt. Ive already been having a lift with this from jazz and it has helped me to be in the right direction but i still dont fully understand this aspect of the machine im building. Not that im ignoring his advice i just thought id throw the Q on here and see what comes of it. Your last sentence went straight over my head jonathan, sorry .

[Jonathan]

If you use a 30T pulley on the ballscrew then you can put a 30T pulley on the motor to get 1:1, or use a 15T pulley on the motor (which is unlikely to be needed much if at all, but it's good to have the option) to get 1:2 ratio. It's best not to use smaller than a 15T pulley since the belt will wear quicker and you may not have enough teetth engaged with the pulley if it is too small.

The moment of inertia of a cylinder (which is essentially what the pulley is) is proportional to its squared, so a small increase in the pulley diameter will cause a relatively big increase of the inertia. The torque required from the motor is proportional to the inertia, so you want to minimise it if possible - hence use the smaller pulley, 30T instead of 40T.