High speed CNC router spindle question

[Arbutus]

I would like to use an air-cooled high speed spindle (2.2Kw) controlled by a 220V single phase VFD for a precision CNC grinder. The spindles typically run up to 24,000 RPM, but for a grinding application that will need to be much less.

The maximum wheel size I will be using is 4" so the permissible RPM (according to Norton Abrasives) is between 9,000 and 15,000 RPM.

When looking at the various spindles available, one manufacturer stated ~ since this is a high speed spindle it should not be used below 9000 RPM because the motor can overheat ~

Now I could understand if the fan was ineffective at that speed (?!) but why would a spindle overheat at 40% of its rated speed?

Is this a common problem with these integrated spindles? If you own one, what is the lowest practical RPM that you can use it at?

Don

 

[DavidR8]

I have a water cooled 24K max rpm spindle on my CNC router.
The problem with running them at low speed is at the bottom of their rpm range they have very little torque. I can run mine down to 8000 rpm but at that speed I can stall it with a gloved hand on the collet nut. Even at 15,000 rpm the torque is greatly reduced.
Honestly I'd be terrified to run any vitrified wheel at 18,000 rpm. The risk of an explosion is very real regardless of what Norton says.
You can buy low speed spindles so maybe that's a better option?

 

[Arbutus]

I have a water cooled 24K max rpm spindle on my CNC router.
The problem with running them at low speed is at the bottom of their rpm range they have very little torque. I can run mine down to 8000 rpm but at that speed I can stall it with a gloved hand on the collet nut. Even at 15,000 rpm the torque is greatly reduced.
Honestly I'd be terrified to run any vitrified wheel at 18,000 rpm. The risk of an explosion is very real regardless of what Norton says.
You can buy low speed spindles so maybe that's a better option?

I'm still digging for good info about these spindles. Any recommendations for low speed options?

 

[mbond]

I'm not sure exactly what your setup looks like, but since you mention VFD and overheating i can make a very general suggestion. More heat caused by higher current through the windings at lower frequency (and slower speed)

 

[DavidR8]

I'm still digging for good info about these spindles. Any recommendations for low speed options?

Zero experience with this:

https://www.aliexpress.com/item/1431431309.html

Or this:

https://www.amazon.ca/Brushless-Motor-Spindle-Compatibility-Factories/dp/B0D328LQTM

 

[Arbutus]

Zero experience with this:

https://www.aliexpress.com/item/1431431309.html

Or this:

https://www.amazon.ca/Brushless-Motor-Spindle-Compatibility-Factories/dp/B0D328LQTM

I like that one. ER20 as well.

 

[DavidR8]

Note that it's water cooled.
I'm considering swapping to a low-speed spindle on my router so that I can more easily machine metal.

 

[PeterT]

I've had my eye on the 110V 1.5KW version for toolpost-like grinding & similar 'high rpm' grinding projects. Compared to the CNC router duty they are intended for, short 15 min grind jobs should not be an issue, at least that is my hope. But I'm waiting on verdict from a buddy who bought (essentially same) from Amazon. He has experience with these & I trust his judgement. He thinks the fan might be ok at high rpm, but a but anemic at mid-low rpm for sustained periods. And I think that was even mentioned in his <cough> documentation. The fan is direct drive so low rpm = low CFM. It looks like a glorified computer fan. Anyway he installed a deeper fan body with more aggressive blade pitch but he powers it remotely so spin high regardless of spindle rpm. Supposedly I will hear a progress update soon. The other thing I heard is bearings can be anywhere from perfectly acceptable, even AC type with proper shaft heat compensation, to a cruel joke. I have seen more than one rebuild where they pulled seized roller bearings out & they had 2 chisel marks on them.... you know like typical AC bearings might have a front & back orientation? I guess the chisel marks are intended to ward off evil thermal ju-ju?

 

[Maker Mike]

I have an Ugra air cooled spindle with max rpm of 12000. They say don't run it less that 4000rpm. The torque curve is linear so as DavidR8 said the torque is pretty much non-existent at the really low speeds. I am currently switching to a belt driven spindle with a servo motor so I can run at low speed and do steel.

 

[Arbutus]

I am currently switching to a belt driven spindle with a servo motor

Yeah, thats where I'm going too. These integrated spindles with their built-in collets are efficient and obviously useful in typical routing applications where high speed is needed and the materials are relatively very much softer.

My application is for a small benchtop surface grinder using a rigid gantry design, with 4 inch wheels. I figure about 5000 RPM max. The motor will need to be around 250-350 watts (24V 10-15A). I already have an ER-20 on a 25mm shaft and I'm looking for a suitable motor. This is also a much less expensive option.

The ER-20 collet system on the integrated spindle was what attracted me at first. Thanks for the wisdom folks. Much appreciated!

 

[PeterT]

There are 500watt brushless motor/VFD packages like this in 3000 & 6000 (max) RPM flavors on Ali. Min rpm is 500. I think they start at 400W & up from there. But it wont have a multiple, hopefully low tolerance bearing package like the integrated spindle motors. It would be a better drive motor to a separate spindle. I'm considering one myself for an unrelated application but have not pulled the trigger.