Nema23 425oz/in steppers for 1720 XMTC?

I've had my 1720 XMTC since new - 1998 - and always intended to CNC it but lacked the funds. I have the original CNC hardware kit with the pulley driven Z axis via the fine feed handle. Looking into upgrading it now the kids/leeches are leaving the nest and things are a lot cheaper these days. The machine also has the original single acme nuts on XY, so backlash is going to need addressed.

I have ordered a 3 axis DRO and glass scales (on sale from China) thinking that would be a Good Thing, and digging around on the current state of the technology I'm seeing that stepper motors have come a long way in recent years in terms of strength (rare earth magnets?) and price. Specifically it is really hard to find Nema34 steppers in the size recommended for this machine any more - they seem to be all 1000oz/in and up, which is overkill and with the drivers and power supplies still pretty pricey. On the other hand I have seen 425oz/in(3Nm) Nema23 motors now at a much more affordable price point (set of 3 with drivers, PSU and optoisolated DB25 breakout for $180 on eBay right now). Obviously I'd need to adapt the mounts and sleeve/replace the cogs for the (rather skinny) 8mm shaft on these.

Anybody gone the Nema23 route? Good/bad experience? Pitfalls?

I'm thinking I'll drive this with LinuxCNC and experiment with position feedback from the DRO scales before saving up for ball screws.

Alastair

The trouble with some of the china motors and drivers is that the instructions and support may not be very good. I suggest taking a look at eDealers Direct on ebay and look at his nema 23 packages. He sells the good Gecko drivers and good quality motors. You will pay a little more but get better products and better service. He (Vince) also has a youtube channel under corvetterguy50 you can reach him at. He is also on here as storm2313

Hi,
most first time buyers look only at the holding torque. The more torque the better right?? Well, not always...Stepper motors, all stepper motors
lose torque the faster they go. Its not uncommon for a stepper to have as little as 5% of its holding torque at 1000 rpm.

The principle determinant of how badly the torque degrades with speed is inductance. For 23 size motors look for inductance of 1-2mH, closer to
1mH preferred. For a 34 size motor look for 2-4mh, 2mh preferred. Such motors when matched with the highest voltage power supply and
driver you can find will have 25-50% of its rated holding torque at 1000 rpm.

Low inductance motors tend to have high current ratings for a given torque having fewer turns of larger diameter wire and commensurately low
inductance.

Get the highest voltage power supply you can find drivers for. The best drivers readily available are 80V. Gecko are the gold standard but can be up to
$150 each. Leadshine AM822's are about $75, 80V and 8.2A capable.

Most people get switchmode power supplies, they are cheaper at higher power outputs but toroidal transformer types are much more forgiving and a much
more graceful overload capacity.

Craig

Thanks for the science. The cheap-on-ebay set I am looking at are 6.8mH - so those are not so good as "1-2mH, closer to 1mH preferred".

They are, however, so cheap I'm tempted to get them anyway and replace later as funds come available.

With a 2:1 step down on the belt and a 10 tpi lead screw 1ips = 1200 rpm so that's fast enough to be a problem

Hi,

Quote:

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They are, however, so cheap I'm tempted to get them anyway and replace later as funds come available

---

You'll be disappointed......they are made and sold to people who don't know any better. I would guess that those units will struggle
to move your machine at 200 rpm.

Craig

using the math here: https://duet3d.dozuki.com/Wiki/Choos...stepper_motors and the advertised specs and 36V included supply gives 337rpm - and "In practice the torque will drop off sooner than this because of the back emf".
So 200rpm sounds about right. Which is 10 inches per minute. Ouch.

Hi,
I would disregard that stuff, certainly the voltage calculation. The rule is use the highest voltage possible, the practical limit
based on available drivers is 80V.

How many hundreds of thousands of cheap CNC machines are out there not being used because the moment someone tries to operate
the machine above glacial speed I stalls due to high inductance steppers? That represents many millions of dollars....all wasted.

If you buy high inductance steppers you will add to that total.

Craig

I recently picked up a similar vintage 1720 and have been also looking for info on a conversion to CNC, so I'll be following this thread with interest!!

Closing out this thread - what I ended up with, after issues with in-stock and deliverables, are stepperonline 34HE31-6004S Nema34 units. These are 4.8Nm =679oz/in holding torque. They claim to be

• Inductance: 3.5 mH ± 20%(1KHz)

As recommended on this thread "For a 34 size motor look for 2-4mh, 2mh preferred" they are in the range but not great.

I have my max velocity on the axes set to 0.5in/sec = 30ipm = 600rpm of the motor. It did twice that in testing but those flying handwheels are too scary for comfort. This speed roughly matches what I could rapid by hand.

running at 60V through DM860I drivers with microstepping set at 8.

I'm using encoder feedback for position, so lost steps on the rapids would be no big deal anyway.

Seems to work nicely.

Thanks for the tips.