Re: [CAD_CAM_EDM_DRO] Stepper performance

ccs@... wrote:

> I've temporarily mounted an older (but new-in-box) 4.4v 3.5A 300 ozin
> NEMA 34 stepper to the 8 tpi leadscrew on my 9" lathe with 4:1 belt
> reduction. Drive is a G210 running off 25 volts with the motor wired
> half-coil. It certainly works and will make the cuts I need, however
> rapids leave much to be desired at only around 8 ipm for reliable
> operation (ie, that's over two minutes to traverse the 18" working
> length of the lathe) This isn't the end of the world as the fact that
> I'm driving the leadscrew via the halfnuts means I can easily
> reposition the carriage manually, but it would be nice to run faster.
>
> The most obvious suspect is the low power supply voltage, which was
> set by having a transformer that could only be wired up in
> increments of 25 volts and an older cross slide drive that was limited
> to 40 something volts. I just aquired a torroid with 24 volt AC
> secondary which I believe will yield about 34 VDC. Switching both
> drives to this may help, or maybe I'll leave the gecko on the present
> supply, but rewired for 50 volts (75v on an unregulated supply strikes
> me as pushing it too close?)

You will only get 70 V from that arrangement. The factor is 1.414, minus
a bit for diode drop.

>
> Another possibility is the 4:1 reduction. I did that because the
> inefficiency of the acme leadscrew coupled with all the drag in the
> rack and pinion of the carriage was quite a hefty load. 8 IPM with
> 4*8tpi=32 revs per inch works out to 256 rpm. Would I do better to
> change the reduction so that the motor stays in its better torque
> range?

These things are hard to predict without the manufacturer's torque
curves. but, it is most likely your rapid feed rate would almost double.

> I was trying to decide if one could learn anything about the
> performance of a stepper/drive system by looking at the waveform with
> a scope. In full stepping mode, it would seem that chopping duty
> cycle would approach 100% as the reactance approaches the limit of the
> supply voltage.

Yup, this is correct. Just be careful not to ground the scope probe
to the wrong terminal.

> To ease the friction loads I'm probably also going to add
> needle-roller thrust bearings to the leadscrew mount, and hanging a
> cheap ballscrew on the back of the bed is within possibility -
> especially as it would save wear on the leadscrew.

The roller bearings are most likely insignificant compared to the
friction of an Acme screw. Anyway, it is DEFINITELY insignificant
compared to the cutting loads, unless your leadscrew is REALLY
stiff!

Jon