As I’ve only ever worked on a VS lathe is there a lot to learn regarding a GH lathe?
Don’t try to change gears with it running !
More seriously you will be unlikely to stop the spindle in the lower gears compared to a vari speed machine, where the drive may stall if you have a problem – something will give first. Discounting losses, 0.75kw at 40 rpm equates to 178Nm at the spindle.
I will always remember an incident in the apprentice training school (Harrison L5s or 140s IIRC, so similar size to a GH600) when we were screwcutting. My friend Andy was on the next machine to me and, hearing a groaning noise from his lathe, I looked across in time to see the whole toolpost & compound slide leaning in towards the workpiece, before a big bang & the groaning stopped. He had run the tool into the blind end of the thread on what was supposed to be the screw for a G clamp (no run-out groove) on a late pass. When the tool went against the full depth end of the thread the machine didn’t stop, he panicked and didn’t disengage the halfnuts or stop the spindle in time & the bang was the HSS tool letting go. He was lucky – the broken toolbit went past the side of his head & nicked the top of his ear. I can still visualise him just standing in front of the machine in a state of shock, with a trickle of blood running down the side of his head.
Otherwise you will always be compromising to some extent on speed, having only 12 steps to choose from & obviuosly adjusting speed on the fly can’t happen (which may be useful when facing large diameters to increase surface speed towards the center). On the upside, no expensive electronics to go bang at an inconvenient time, just a couple of switches & generic contactors. I like the saddle mounted spindle start lever, though that is mainly due to familiarity with this style of operation on the various Harrison lathes that were present where I worked – this can’t be knocked on accidently as the lever has to be moved sideways before being moved down (to start forwards) or up (to start reverse), but it just has to be moved back towards the centre position to stop. It falls readily to your right hand when operating the machine & just feels natural to use.
Good luck with your deliberations,
Nigel B.