Spindle motor upgrade choice? AC Servo/BLDC, Induction?

Hi guys, I have just received my Harbor Freight knee mill, entry level obviously. I like the machine, but I am looking to sell its monstrous AC motor in the back and do an upgrade. I am leaning towards buying an AC Servo (possibly 240V one) instead of that motor. Googled for quite some time but was not able to find any good comparison of different motor types for this application. All I was able to find is extremely dated and isn't true anymore.

I am looking to keep it on current NEMA14-50 socket, so high voltage isn't an issue, I have that already. what would be your suggestions for 2021? :)

The machine has 9 speeds, and a max RPM of 2800, but realistically max RPM is 2300 as the belt starts rubbing on a gear on that setting, not really usable. And such RPM is really limiting obviously, I would like to have the ability to mill at at least 10000 RPM, as I will be doing a CNC conversion for it shortly also. That was the whole point of buying it :)

Thanks in advance!

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Originally Posted by z05

Hi guys, I have just received my Harbor Freight knee mill, entry level obviously. I like the machine, but I am looking to sell its monstrous AC motor in the back and do an upgrade. I am leaning towards buying an AC Servo (possibly 240V one) instead of that motor. Googled for quite some time but was not able to find any good comparison of different motor types for this application. All I was able to find is extremely dated and isn't true anymore.

I am looking to keep it on current NEMA14-50 socket, so high voltage isn't an issue, I have that already. what would be your suggestions for 2021? :)

The machine has 9 speeds, and a max RPM of 2800, but realistically max RPM is 2300 as the belt starts rubbing on a gear on that setting, not really usable. And such RPM is really limiting obviously, I would like to have the ability to mill at at least 10000 RPM, as I will be doing a CNC conversion for it shortly also. That was the whole point of buying it :)

Thanks in advance!

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If you will be doing a CNC conversion for it shortly, then a full CNC package with AC servo spindle motor and driver would be better, for the best performance and best compatibility.

I am already doing a CNC conversion actually. Since you are in China, are you selling such motors by any chance? haha. I would be interested. Also the real question, if I get an AC servo with controller, what kind of speed I will be limited to. And what kind of gear reduction should be done for that kind of motor vs spindle axis.

Hi,
AC servos are very torque/power dense, probably the best output for a given size. Downside is that they are probably the costliest of your options.

A significant fraction of the cost comes from the absolutely superb control options that you have with a servo, more really than is required for a spindle,
but if you want an indexing spindle or using it as a Caxis for rigid tapping, all the required control is there.

Typically large servos, the sort that you might consider useful for a spindle, are rated to about 3000rpm, often with a max speed (torque reduced) of 5000 rpm say.

I have a second hand 1.8kW 3500 rpm Allen Bradley AC servo as a spindle motor for my mini-mill. I use it only when I want lower rpms with good torque authority, ie milling steel
or stainless. For that purpose it works great.

Craig

Hi Craig, I am really not looking for an overkill premium solution at higher price, I just need speed control for it and faster RPM, and if motor will be more silent, its a plus. I could throw probably $500ish into a new motor setup, but probably not more. After all the whole mill costs $1850 lol. And I could probably sell its existing motor for a couple hundred to offset some of the costs of this upgrade. Going to need ballscrews probably, and this machine turned out to have a super beefy 20mm ballscrews on it, cant even find a correct size couplers for its 16mm shaft now.

Found these options online, can get 3.8kW actually, not sure if its needed tho. What do you guys think? Thinking of eitehr getting 2.6kW or 3.8kW model.


https://www.cnczone.com/forums/attac...d=460904&stc=1

Hi,
the specs you have posted are not uncommon and are typical of cheap Chinese servos.

To be fair to them they have packed a lot in and sell it at VERY attractive prices.....but....it's not all plain sailing. These servos seldom have any
PC based set-up and tuning software...you have to program them by pushing buttons on the drive like a microwave. There are a lot of parameters that need to be
programmed and button pushing is very tedious and error prone. Also the documentation is Chinglish at best, often really confusing.
Backup and support is questionable.

If you have had experience in programming AC servos then these drawbacks will not trouble you. If however this is your first AC servo you are in for a hell of a
learning curve.

I use Delta AC servos, a Taiwanese brand manufactured in China. They have been in the market for years with mature product offerings. The documentation is good,
near the best I've seen from Taiwan/China, they have good PC based set-up and tuning software and good support. They are more expensive than the el-cheapos you have
linked to but still very much less than Japanese/US/European brands.

Even Delta prices, or indeed the ones you have linked to are still going to blow your $500 budget. For instance a 1hp Delta (B2 series, 160,000 cpr encoder) is $438USD
and your el-cheapo ones are likely to be $350 USD, and thats for only 750W (1hp). Larger, say 1.5kW and 2kW are going to be double or nearly so.

I also question whether you can run a 2.3 or 2.6kW servo let alone a 3.8kW servo on a single phase supply. A 2.2kW servo is about the practical maximum power you
can get from a single phase domestic 230VAC supply. You'd need special wiring, fusing and plug sockets to get to 3.8kW. At a guess you'd need a 50A socket
or a 32A at the very least.

The bottom line here is that AC servos are very good but are not a cheap source of motive power. Even the cheapest Chinese brands are still more than your budget
and I have outlined some shortcomings of the very cheapest servos....I personally wouldn't touch them.....especially when for about another 25% you can have Delta or DMM.

Another alternative, very widely used by hobbyist and professionals alike is a VFD and induction motor.

I will draw a distinction between ordinary industrial induction motors, 60Hz or 50 Hz models used for fans, pumps, machines....you name it and the highspeed, 24000 rpm typ,
spindle motors abounding on EBay.

You could easily get a 1.5kW or 2.2kW watercooled spindle motor and VFD for $500USD. Most people have a good run with these units provided you don't let them overheat
or mis-program the VFD. These spindles are great for engraving and softer metals like brass and aluminum but are no good in steel. Steel requires low rpms, and these spindles
Achillies Heel is at low rpms, and good torque authority, there again just where these spindles are weakest.

A I posted earlier I have an AC servo based spindle for low rpm/high torque when I want to mill steel, but 90% of the time I use my 800W German made 24000 rpm spindle. I've been
thrashing it for six years and its still going. The one thing I absolutely avoid is the temptation to run it below 9000 rpm.......as I risk overheating it....and its just too damned good and useful to
risk it!

You need to decide whether you need to mill steel. If you do......then a high speed spindle like the ones we are discussing are not the right choice. If you don't care
to machine steel then these spindles are just the ticket and competitive Chinese market has produced some great units at unbelievable prices.

If you need to do steel then an industrial induction motor may be the answer. Officially an induction motor that was intended that it be driven by a VFD is a special design
called 'inverter ready'. In reality an inverter ready motor is nearly identical to an ordinary motor but with much more care given to insulation of the windings, often with
vacuum impregnated windings. Many ordinary industrial motors of quality design and construction (US/Japanese/English/European) will run on a VFD without problems.
Cheap Chinese and Indian made motors are likely to fry up though.

A well made 60Hz two pole (one pole pair) induction motor has a synchronous speed of 3600 rpm. With a VFD you can of course make it go faster, you will get away with 5000 rpm probably,
and if you are a praying man, a prayer might get you to 6000 rpm without the rotor or bearings bursting!

Industrial induction motors are not as torque/power dense as an AC servo....but not to far off. Thus you could pick up a second hand motor, old but good quality, throw in some new bearings,
add an external fan if you want to run at low rpm for extended times AND a VFD and still have change from your $500.

I note that a lot of people are using treadmill motors as motive power for spindles. There are very few examples of them in New Zealand, so I've seen very few of them......and none
of them were as spindle motors. Maybe some of the US based members might chime in with what can be expected from these units.

Craig

Hey, thanks a lot for taking so much of your time to reply to me in such great detail. Really appreciate that!

Being in the industry and working with China for many years, I can definitely add my 2 cents to "el cheapo". El cheapo only applies in china when you are looking for bottom line products and even then, you gotta be looking on retail network like aliexpress to be able to get the real junk. I only search on wholesale B2B networks therefore I usually get superior products for a much lower price compared to what I could get in USA for example, which in most of cases are just rebranded motors with unreasonable profit on top of that. Such buy makes no sense to me, as those middle men dont really add anything to the product, except of a lot of margin.

I dont think that my motor is VFD compatible at all, as it is single phase. Buying a used induction motor also doesnt make much sense to me, as I am gonna again end up with something inferior, used, and $500 is not really a big money compared to how much time I will need to spend with the machine installing any motor. So if I get into that, it's better be worth it. I am not a typical mill user in a way, that I personally think noise level of the motor is actually important.

AFA as chinese motors go, I personally have never had a single motor of any type fail on me, ever, including some super cheap ones from public chinese retail networks.

Watercooled spindles are for toy routers which is not the machine that I have now. I did have a watercooled spindle on my chinese 6040 router back in a days, 2.2kW one and it was a joke, it would never be able to handle my current 700lb machine lol., but anyways RPM is way too much and at low RPM it totally lacked the torque. If you can link any watercooled spindle that can be reasonable to use on a knee mill, I would love to see that. So far all I saw is a toy size things. They are super silent though.

Linking the machine, to make sure we are on the same page: https://www.harborfreight.com/vertic...ine-40939.html

I think there is quite a bit of miscommunication here honestly, because it really seems to me that you think I am having a toy router or something, while I am having a serious toy mill :) lol Which is using an R8 chuck etc, I may even make an ATC for it in future. Collets are ER32, and the ones I had on my toy 2.2kW spindle were ER11 if I recall that correctly.

So coming back to that screenshot from Motor factory, I can actually pickup both of those models for under $300 with a driver and everything. On B2B network of course, and just throw it to my weekly DHL shipment box with DHL. They suggested 2.6kW actually due to high amp load on 3.8kW@220V. Those are more meant for 380V applications.

RPM is 2500 only which is what bothers me a bit, because I machine aluminum in 80% of cases, maybe a bit of titanium alloys, rarely steel. However I do not want to limit myself as well. Because I may grow on steel and start using it more when I actually get a capable machine. I will almost certainly need to use some sort of RPM increase setup, in which case who knows if 10Nm will cut it for me or not, as for example with 1:4 reduction/increase, I will get an output of 10000RPM @ 2.5Nm only... But is it really "only"? as I think my 2.2kW spindle from toy router actually had way less, maybe like 0.5Nm, if we dont count the own inertia of the motor at that speed of course. In any case thats defintiely not the best driver in the hood, no canbus etc, but I dont really need that I think, in order to control it with linuxCNC. Driver does have pulse input and all other standard I/O, and it does have RS485 on it, so that should probably help. In any way I dont know what I am doing anyways, and it looks like a fun project to do for a very little investment, as I can sell the existing motor in USA for 200ish, it's new after all. :)

Hi,

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I dont think that my motor is VFD compatible at all, as it is single phase.

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You are correct, single phase motors are totally unsuitable for use with VFDs.

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Buying a used induction motor also doesnt make much sense to me, as I am gonna again end up with something inferior,

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I don't agree. At a previous place of work we had a lathe spindle motor running on a VFD. The motor blew up so the boss got an Indian made motor (new),
and it lasted a fortnight. No to be put off he bought another Indian made motor (new), different and supposedly better brand, and it lasted a week. He then bought
what looked to be a well specified and made Chinese motor (new), and it lasted three months.

I got an old three phase motor from the local butchers shop when they were upgrading their refrigeration plant. Didn't cost a cent. It is an English built motor
and must have been thirty years old when I got it. I fitted it to the lathe....and to my knowledge its still there twelve years later. Don't turn your nose up at old
but high quality motors....they will outlast the bloody lot of us!!

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Watercooled spindles are for toy routers which is not the machine that I have now.

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I disagree here too....it horses for courses. My little 800W 24000 rpm spindle is certainly a 'toy' by comparison to a serious mill spindle, but I use the little spindle
all day every day and it pays for itself two or more times a month. The 'serious' mill spindle wont do a quarter of the work it does, the mill spindle does not spin fast enough.
If your work involves engraving tools or endmills down to 0.5mm, and even smaller drills as my work does a quality highspeed spindle of very low runout is a must.

For my new build mill I really want another highspeed spindle. On the basis of outstanding service I would not hesitate to buy (again) from Mechatron Gmbh. The model I want
is 2.2kW (S100) and 42,000 rpm with HSK25 toolchange. At 5,700 Euro however I may have to wait....forever!!!

https://www.mechatron-gmbh.de/en/pro...sional-series/

When I want to do steel I use my 1.8kW Allen Bradley AC servo based spindle. Sure it only does 3500 rpm but has 6Nm cont torque, and does an excellent job within
its power limits.

I have another servo, a remanufactured 2.8kW Vickers servo from the late 90's. Its a superb piece of work, 12 Nm cont and 48Nm overload at 4000 rpm.
I bought on Ebay for $110USD. It cost more ($130) in freight to get it to New Zealand than I paid for it.

Only downside is that it has a resolver rather than an encoder and so I'm building my own kick-arse servo drive, electronics is my thing. It will require a 32A
single phase connection. It will have a rectifier/boost-buck regulator setup to get near unity power factor and make the most of what I've got. My new mill will
handle the extra weight and power a breeze.

I am not averse to buying good quality second hand....the vast majority of my purchases of second hand gear have proven to be extremely good and certainly better than
I could have afforded to buy new.

Craig