microstepping and speed questions

Hi Guys, i now have my self a 6090 cnc from china and its working amazingly for the most part exeppt when it comes to speed, as for right now i am cutting at 500mm/m when i stepped it up to 750mm/m i lost steps over then course of the job (i am cutting aluminium with a 4mm endmill at 2mm doc) the microstepping is set to 1/8 and its running nema 34s at 70v. i could step it down to 1/5 micro stepping but that may be pushing it to low. so my question is what's best to do? i could go lower in the steps, i could buy new more powerful motors or maybe i am just stuck

thanks in advance Oli

Test your microsteps - it's a zero-cost/New Years Day-friendly approach to answer your own question.

Don't mistake microsteps for positional accuracy. As a technology it offers a smoother rotation - helps to reduce resonance, and notionally higher resolution, but resolution isn't the same as accuracy. Test with 5:1 or 4:1, see if that brings you joy.

Note, I'm not convinced that it will - but it's a zero-cost check. The problem is that I don't think it's micro-steps that's causing your issue (essentially you're in the electrical domain with micro-steps - the interface between the computer, the stepper driver and the stepper) - the numbers you're talking about don't feel too near any obvious limits (750mm/min = 12.5mm/sec = 2.5rps x 200 x 8 = 4000 steps/sec = 250us/pulse = should be fine/easy... you shouldn't be losing steps).

One thing to look at is your power supply. Is it's current rating adequate?

Even with small nema 34's and 70v you shouldn't be stalling at those feeds.

However this does depend on the pitch of the what ever makes it move ie: ballscrew
For instance if you have a 5mm pitch ballscrew then a nema 34 with 70v will start to lose torque at around 900rpm, so if direct drive you should easily reach 4500mm/min rapid speeds if the machine isn't binding.
But if it uses for instance some crappy lead screw like threaded rod with a pitch of 1.5mm then this changes things very much. Now you go down to 1350mm/min at the very best, then throw in the inefficency of the screw and your easily down to 750mm/min stalling speed.?

However, I'd take a guess and say it's using cheap Analog drives and you have a resonance issue.
Resonance can cripple a motor, changing the micro stepping can some times shift the resonance to a frequency that doesn't affect the drives or it can handle but not always and it's a fudge at best.

Digital drives are massively better than Analog and by far the easist way to get around resonance.

My only reservation on it being resonance is that it's stalling at such a low RPM, resonance usually only affects the motor as it try's to spin faster.

I'm going to assume a 5mm pitch with it being a Chinese 6090. So if it's stalling at 750mm/min the motor is only spinning at around 150RPM which a nema 34 on a 5mm pitch at this speed it should have enough power to dump the machine on it's head. So you could have some issue with serious friction causing the axis bind,

Give us some more details on the machine and electronics. What controller and method of communication ie: Parallel port, usb etc.