[Nikola010]

Last time I was doing something on CNC machines it was few years ago and when I done it in practice I was learning from oldschool mechanic guy. Milling machine had powerfull spindle motor I dont know exactly what was the horsepower but i know it had low rpm, about 1000 (with mechanical speeds). I always thought that machine could work a lot faster but I didnt have much free time with it and he sad 0.6mm in depth per pass and feed rate of about 50mm/min,dont change that and make me programs. I was ok how you wish.

Now I am making my own machine and I started to choose spindles and I am confused. I have always been taught that high speed could burn the milling cutter and now all spindles I find are 10000-20000rpm. I am looking at smaller spindle motor of 0.8kw. I am interested if someone can explain what is the cache, can most of the materials be milled with higher speeds even wood without burning it and does that mean that feedrate should also go up? Can I go to lower cutting depths without any problem? Is there some constrain with those spindles that I should be aware of?

[dazp1976]

You mentioned a mill but are you looking at building a mill or a router?
Type of motor can you find on a mill:
https://www.ebay.co.uk/itm/1-5KW-5NM...s/282813413367
Type of motor you can find on a router (with spindle fitted):
https://www.ebay.co.uk/itm/ER16-1-5K...MAAOSw-itXtn~c

I'd say that machine likely had one of these type:
https://www.ebay.co.uk/itm/Three-Pha...E4d4jZyiaeqAxw

[Nikola010]

I worked on mill, it was conventional mill made from casting iron, converted to cnc, had servo drives etc. Really powerfull. Now i am building cnc router.

But my question is what's the difference and advantages/disatvantages with higher spindle speed that I shoduld know and account for. Earlier when i worked everyone was like dont go too high spindle speed and it was like 2000rpm is too high, but they never worked with cnc, but hey you need to learn from somebody. And now i look everything is 10k+ rpm, that's why i am asking what is difference when you go with higher speeds.

Thank you for replays

[RCaffin]

Wood is soft: you can cut it at high speed.
But if you run the spindle fast, you MUST also move fast, otherwise the cutter spends most of its time rubbing rather than cutting. Rubbing makes heat and burns things. Manual machines cannot move the axes fast, so you have to keep the spindle speed down on those.

Cheers
Roger

[joeavaerage]

Hi,
high speed spindles are good for wood and engraving/light cutting of aluminum but absolutely the wrong choice for steel.

With a carbide tool the suface speed in steel should be 100m/min, any faster and you'll cook the tool in short order.
Lets say you have a tool 6mm in diameter:

6 (mm) x PI=6 x 3.141
=18.85mm circumference.
=0.01885m

100/0.01885=5305 rpm.

Most highspeed spindles (unless VERY powerful) are hopless at low speeds, they'll overheat and they just don't have the torque so the tool stalls.
If you want to cut steel, or worse stainless, then high speed spindles are just not torquey enough.

Craig

[Superman]

Generally..
the lower the RPM, the more torque that can be supplied by the spindle motor. And the larger diameter cutters that can be used, deeper cuts are possible
...now a high RPM spindle doesn't have low RPM torque, which means you can't use large diameter cutters ( may be possible with very small cut depths)
It depends on what materials and shapes you need to work with.

... plus what Roger has posted

[joeavaerage]

Hi,

the lower the RPM, the more torque that can be supplied by the spindle motor.

Lets imagine OPs spindle is 800W at 24000 rpm, very common in Chinese asynchronus spindles.

Power= torque x angular velocity

torque = power / angular velocity
angular velocity = (24000 /60) x 2 x PI
=2513 radians/sec

torque =800 /2513
=0.318Nm

Less than 1/3 Nm, thats not enough torque to run a 3mm tool in steel unless you take a miniscule depth of cut.

Torque does not increase as the rpms drop, torque is proportional to current, and the max current is fixed by the spindle heating.

If you tried to use a bigger tool say 6mm or maybe 8mm it would almost certainly stall the moment you offered it up to the material.
I have a 750W 24000 rpm spindle, and I use it alot for making circuit boards and it does a fair job in aluminum with 3mm and 6mm tools
but its hopless in steel with even 3mm tools....and I have tried.

What I have done is make another spindle based on a second hand 1.8kW Allen Bradley AC servo and drive. 3500 rpm rated, 6.2Nm cont,
18Nm overload. It works in steel and stainless really well.

Craig

[Nikola010]

Thank you all for great replays, now I know in which direction to look. One thing I am not sure is why there are only 10000+ rpm motors for spindle, even really powerful ones when you can't cut steel on that speed. I cant find any spindles in my country or on aliexpress that have lower speeds. Did I understand something bad or its just that I need to construct my own spindle with classic 1450 rpm asynchronous motor and some gearbox (talking about cheaper solution) and eventually frequent regulator if I want to cut steel?

[joeavaerage]

Hi,
if you look at the spec for VMCs out there you'll commonly see spindle powers of 10kW, 15kW at 12000 or 15000rpm.

lets take a spindle of 10kW at 12000 rpm:
Torque= 10000 / ((12000 /600 x 2 x 3.414)
=7.95Nm

Thus you could run this spindle very much slower, say 1000 rpm but still have about 8Nm torque. This is how VMC's can cut steel despite having
what seems an insanely fast spindle. The problem for you and I is that such spindles are huge, very expensive and require three phase power, not good
for hobbyists.

My approach to this problem is to have two spindles, one is 750W@24000 rpm, for engraving, aluminum etc and the other 1.8kW@3500rpm for steel.
Not a perfect solution but within my budget.

Craig

[abrackers]

Hi, I am looking into the same thing, and have found a cheap chinese 900W spindle that spins up to 6500RPM max, with an advertised 2NM of torque. Does anyone have any opinions or experience as to why I may not want to go with this for a cnc router / mill I'm building?
https://www.aliexpress.com/item/3300...18db2e0eX43uyN

I have emailed the seller and am awaiting a reply requesting the torque curves and minimum speed for this. I am primarially wanting to machine aluminium (normally with 1/4 or 1/8 inch bits), although in future I may want to cut out shapes from thin 1 - 2mm sheet stainless (basically doing the job of a waterjet / laser cutter).
According to cnc cook books feed rate calculator the spindle speeds seem within range (4000 rpm for 1/4 inch milling aluminium, 2000rpm for 1mm cutter on stainless steel, 8000rpm but can run at 6000rpm for 1/8 inch milling aluminium although all these speeds are based on HSS bits, not carbide). According to custompart.net's milling horsepower calculator, the motor should have enough power for all of these operations provided I keep below ~1/4 inch depth of cut in aluminium. Steel and stainless actually seem to require much less power as they have a much lower feed rate due to the lower SFM rating.

[RCaffin]

900 W in a body that small? Hum ... I have my doubts, at least for continuous operation. It might glow rather quickly.

What is not clear is whether the price is just for the motor, or whether it includes the electronic driver. Very vague.

It might handle sheet aluminium with a thin cutter and shallow DoC, but I have real reservations about it handling SS. The idea of it handling a 1/4" depth of cut with a 1/8" cutter is ... well, I would not try it myself.

Sorry
Roger

[abrackers]

900 W in a body that small? Hum ... I have my doubts, at least for continuous operation. It might glow rather quickly.

What is not clear is whether the price is just for the motor, or whether it includes the electronic driver. Very vague.

It might handle sheet aluminium with a thin cutter and shallow DoC, but I have real reservations about it handling SS. The idea of it handling a 1/4" depth of cut with a 1/8" cutter is ... well, I would not try it myself.

Sorry
Roger

Hi,
Thanks for your reply. Volumetrically the size felt in line with the power compared to the 65mm dia 1.5kw units. It has a volume of 453kmm^3 for 0.9kw, compared to 663kmm^3 for 1.5kw (so its actually slightly lower power per volume)
From another seller who sells the parts individually for ~£70 each, I'm fairly confident it comes with both motor and controller.
Are you saying you wouldn`t try 1/4" doc with 1/4" cutter with a 900w 2nm 6500rpm spindle, or that you doubt this one actually has those specs so you wouldn't risk it assuming it has much less power / torque?
Thanks

[RCaffin]

You can buy 1kW high-speed motors which are even smaller, but they are for model aircraft and they rely on a huge airspeed for cooling. Without that airflow, they glow real fast! What can this one do? No idea.

One part of the ad says milling; another part of the spec says electric bikes. Very Chinese. Frankly, I do NOT trust Alibaba/AliExpress vendors.

No, I would not try a 1/4" DoC with a 1/4" cutter myself. Um - not unless it was a really top-grade (ie very expensive) cutter mounted in a very rigid/solid machine and I had done a lot of tuning of the feed rate - and I was running some serious misting and air blast. And I was desperate for production speed and damn the cost of the cutters.

My 2c.
Cheers
Roger

[joeavaerage]

Hi,
I have some Destiny Tools Raptor brand endmills which I swear by, they are great in steel and stainless.

The 1/8th inch tool I will use in my highspeed low torque (24000rpm, 0.3Nm) in steel for very small jobs when I can't be bothered
to swap to my low speed high torque (3500rpm,6.2Nm) spindle.

I run the 1/8th inch Raptor at 9000rpm, about as low as I want to run my air cooled spindle. In mild steel I can cut 50% step over (1.5mm) but only
0.5mm DOC. Any more and it will stall the spindle, the classic 'torque limited' situation. At 9000 rpm I flood cool the tool with as much coolant as I can
direct at it. Without coolant you'll turn the tip of the tool cherry red in seconds and these Raptor tools cost $20-30USD a piece!!!!

If the job is bigger with significant material to turn into chips I'll fit my high torque spindle. Then I'll run a 1/4 inch Raptor at 25% step over (1.5mm) but 6mm
DOC. I'll run the feed as fast as I can with respect to tool bending. In short I give it hell! This is the classic 'tool strength limited' situation.

If I want to hog lots of material I use a 16mm coated carbide tool. The tool is strong and I can use all the torque and thrust of the machine but my
mini-mill starts flexing despite bing made out of cast iron. This is the classic 'rigidity limited' situation.

Perhaps you guys have seen some of my other posts and will be aware that I'm building a new mill but bigger and alot more rigid than my existing
mini-mill. I'm having the mill beds cast in grey iron, about 75kg each and the frame that holds it all together is 275kg of SG iron. This is to overcome
the rigidity limitaion of my existing mini-mill.

To OP, when it comes to cutting steel and stainless there is always a limitation. In my case the first limitation I encountered was lack of torque with
my original high speed spindle. So I made a low speed high torque spindle (pic attached in previous post) and the limitation is now tool strength with
small diameter tools or machine ridgity with far stronger large diameter(16mm) tools. Ergo my new build mill. Its a never ending quest.

May I suggest if you are of the opinion that the spindle you have linked to is adequate and affordable that you buy it and try it out. My experience
is that you will encouter limitations, be it the torque of the spindle, the strength or otherwise of the tools or the rigidity and thrust of the
machine. Whether you will be dissapointed or not when you encounter the limits is more than I can say. I recall being dissapointed when I found
that my otherwise perfect German made high speed spindle proved to be very poor in steel, but then I realized that I had learnt an exteremly
valuable lesson. Once I'd sucked up the lesson and built my second spindle I found new limits....and my learning curve continues.

Craig

[RCaffin]

What Craig wrote.
+1 on the learning curve too. But that is part of the fun!

Cheers
Roger

[machinehop5]

Wood is soft: you can cut it at high speed.
But if you run the spindle fast, you MUST also move fast, otherwise the cutter spends most of its time rubbing rather than cutting. Rubbing makes heat and burns things. Manual machines cannot move the axes fast, so you have to keep the spindle speed down on those.

Cheers
Roger

x2 and very simply put..x2

[dazp1976]

Hi,
if you look at the spec for VMCs out there you'll commonly see spindle powers of 10kW, 15kW at 12000 or 15000rpm.
Thus you could run this spindle very much slower, say 1000 rpm but still have about 8Nm torque. This is how VMC's can cut steel despite having
what seems an insanely fast spindle. The problem for you and I is that such spindles are huge, very expensive and require three phase power, not good
for hobbyists.
My approach to this problem is to have two spindles, one is 750W@24000 rpm, for engraving, aluminum etc and the other 1.8kW@3500rpm for steel.
Not a perfect solution but within my budget.

Craig

Tell me if I'm wrong but in my ideal world for my mill.
I've got 3000rpm max, 1.1kw, roughly 4nm.
I'd be really keen to up it to 5000rpm (at some point distant) so.....
I've got my eye on a 2.3kw motor, 7.7nm, 2500rpm rated.
The plan, to make a new motor mount, bigger pulleys, and overdrive it 1:2 or a slighter increase.
Should technically give me the same torque I have now for near double the rpm.
Correct?

That's the advantage of having a motor and a spindle as 2 seperate units.
Seems slim pickings overall if you need your spindle and motor as one unit.

[abrackers]

Thanks Roger and Craig for your very helpful replies.
No I probably wouldn't go near 1/4" doc. That was just what custompart.net's milling horsepower calculator suggested would be the max for 900w. I realise now I also hadn't taken into account tool strength or machine rigidity. I probably will get that spindle as for £90 off taobao, the worst case is I get a nice vfd heatsink (I need one about that size anyway for a 3d printer enclosure heater). Also my experience with taobao shows that the quality of stuff sold internally in China is much less questionable than the stuff sold externally on aliexpress
Thanks,
Andrew

[joeavaerage]

Hi,

Should technically give me the same torque I have now for near double the rpm.
Correct?

Yes, that is correct. Many industrial installations have a separate spindle motor and spindle.

You are correct there is limited choice when it comes to a spindle and integrated motor, at least cheaply.
Heavy duty integrated spindles are easy to find if you have $10,000!!

Craig