BV30 Lathe

[Jonathan]

In the cross slide I have 40*30mm gap for a ballnut, has anyone got any suggestions as to a 10-16mm ballnut with pre-load that would fit?

Have you considered putting it on the end of the cross slide, not under? It will stick out at the back, but it means you're not limited by the size of the nut. That's what I think I'll end up doing on my lathe (Colchester student).

You say your spindle bore is MT5, but in the manual it says MT4 so is the manual wrong?

There's more to it than just getting a 'beefier' motor. Even just having a high/low gear with the ratio carefully selected (i.e. 2:1 is not likely to be optimal) will make a big difference. A reasonably approximation for a VFD is the motor will output roughly full power to half rated speed, but below that it drops proportionately to the speed. So with 2:1 you'd get full power all the way down to 25% speed, but much below that you're struggling.

For example lets say it's a 4-pole motor so about 1450rpm. If the ratio is greater, say 2.5:1, then you'll get full power from half way to full speed and full power from 1450/2.5 to half that. So that's rated power from 290 to 580rpm and 725 to 1450rpm. Between there the minimum power will be if you need just over 580rpm on the high gear, that's (725-362.5)/(580-362.5)=60% of rated power. So lets say the minimum power requirement is 500W, that means with 2.5:1 you'd need at least a 500/0.6=833W motor and the operating (>=500W) region would be 174rpm to 1450rpm compared with 580-1450 with direct drive.

Quick sketch:


So in short, plot a graph of the motor speed/power for 1:1 and the other ratio on the same graph and get them to overlap appropriately.
(Note I picked 2.5 pretty much at random, you'll need to work out how much power you need for each speed first.)