ac servo choice/advice

[iamspuddy]

hi i have a denford cyclone lathe it has 2 x 8nm nema 34 steppers on it 2:1 reduction 5mm pitch lead screw .
i wish to change them out for 2x ac servo motors.

can anyone recommend someone who sells them, and also what size would be ideal. 750w/1kw

ive seen these on eBay around £300 1kw 4nm
and
90ST-M02430 Matched Servo Driver 2.4N.M 750w servo motor

im new to servos ,but im after something with an encoder for accuracy and be compatible with my centroid cnc12 lathe control i just fitted

any help appreciated

[Doddy]

Theres a Czech-based supplier on eBay that I bought a 2kW servo (lathe) from - no drama and quick delivery, but the Chinese ebay vendors (750w - mill) are equally competent, just slower shipping, though very good at responding to queries (in my case a PDF of the manual prior to delivery). I run both in step/dir positional mode like a stepper, but both as spindles (one on a ML7 lathe and use the encoder for position feedback)

[Hood]

Not 100% sure but I seem to recall the Denford Cyclone to be a small lathe. If that is correct then I think 1Kw motors will likely be overkill, I would think 400W may be more suited.

[iamspuddy]

its not that big but they did come with up to a 5hp motor depending on original option taken.around 150 x z around 200

things are a lot better now i have centroid and an encoder on the spindle compared to mach3 before . But i think servos would be a better option and give a bit of a comfort blanket for accuracy if I'm not mistaken .
So if 750w looks more than enough from peoples experience that's fine i know what ball park to be in .
if anyone has links from any vendor they have used and trust that would be great

cheers

[m_c]

I put a 400W on the X-axis, and a 750W on the Z-axis of my Cyclone, although I think I changed the gearing on mine to give a greater ratio so the servos could run faster.

If I was doing one again, I'd probably just use two 400W servos with suitable gearing.

[iamspuddy]

thanks
is there any disadvantage to 750w or 1kw on both, as for gearing was it 1:1

please correct me if im wrong but im under the illusion that with the servos and driver kit if it "loses steps" ie out of position it will alarm or something to stop inaccurate turning

[m_c]

thanks
is there any disadvantage to 750w or 1kw on both, as for gearing was it 1:1

How much breaks if something goes wrong :-)
Although too big a servo on too light an axis can cause tuning/stability problems.

IIRC the amount of space available to put a bigger motor on the X-axis is limited, so it's something you have to consider.

I think I went to somewhere around 2:1 ratio, possibly a bit higher.
Steppers aren't fast, and it's unlikely you'd ever run one above 1000rpm (500rpm is probably more feasible for reliable operation), whereas most servos around a similar size will quite happily spin at 3000rpm.
If you direct couple a servo at 5mm pitch, at full speed you'll theoretically get 15m/min, but you're never likely to reach that speed due to the axes being so short, there simply won't be enough distance to accelerate/decelerate to that speed. So it's far better to gear to get better acceleration, and make better use of the servos potential rather than limit how fast it spins.

please correct me if im wrong but im under the illusion that with the servos and driver kit if it "loses steps" ie out of position it will alarm or something to stop inaccurate turning

It all depends on how you program them, but they should do. There will be a following error parameter, that can trigger a fault when a set number of counts out of position is exceeded.

[iamspuddy]

i asked a couple of vendors on eBay questions and unfortunately no replies

so if anyone can recommend a seller of reasonably priced servos and driver kit i would be grateful

cheers