High Speed Spindle for Taig - Brushless Motor?

[GammaWill]

Ok, so I broke down and bought a high speed spindle from Wolfgang for my soon-to-arrive Taig. Since he just released a heavy-duty Taig mount for his spindles, the combined purchase was too hard to resist.

I'll be using the spindle to finish mill 6061-T6 aluminum using 1/32" cutters. The spindle includes a 540 can motor (brushed and inefficient) running flatout at 20K RPM, but I'd like to use a brushless motor instead.

The spindle is rated at 100K RPM, but I'd like to target around 40K RPM.

The brushless hobby motors used in RC airplanes look perfect, but I'm not sure they can handle the continuous operation (up to 12 hours continuous). I would suspect that the bearings would be the part that would fail over time, and I'd like to run it at 20K RPM using a 2:1 belt between it and the spindle, giving me 40K RPM at the cutter. Load would be light, since the 1/32" cutter would be eating at only about 15 IPM max.

However, I don't want to waste my money buying something that might be unsuitable. Has anyone else used a brushless motor like this with their CNC mill? Are any "warning bells" going off in your heads as you read this?

Thanks,
---Will

[gun_mike]

I havent used brushless motors on a machine yet, but i have been using them on model airplanes for about a year now and they run great.
It sounds like a good idea to put them on a cnc mill for hs spindle. the first brusless i bought was a year ago and i still use it about 2-3 times a week at the local park. after a 15-17min flght, the motors do become pretty warm. I'm only using 300 sized motors and they run at full power for most of the flight. Ive seen 600 size brushless motor used in large helicopters, these are the ones I would recomend. One concern would be the motor case, i know for a fact that some of the cases are made of carbon fiber, and if the motor is run for say 12 hours im sure the heat for defiinitly burn the epoxy in it. another concern is the controler, there motors have three leads, so you would need to build/buy a controller (maybe varialble speed). you wont be able to use an RC brushless controller they are ment to be hooked up to a reciever. hope this helps

Mike G

[in2steam]

Look into proxon, they have a brushless dc dremel like tool that many have used with succes. I don't think you will get the desired torque out of a can motor and the bearings won't handle the load.

chris

[GammaWill]

Mike, the brushless motors I'm looking at have metal cases. That should help with the heat dissipation I hope. As for driving them, the controllers expect a .5ms to 1.5ms width signal to vary their speed. 10 years ago, I happened to have built servo driver using a 555 timer to generate the pulsewidths. It drove a servo just fine, and should drive the hobby-type brushless ESCs. What would be really cool is to get MACH3 to output a variable voltage that I could use to drive the ESC. A simple PIC (even a puny 16F84) could handle the voltage-to-PWM conversion for me. I'll look into it and report back when I have some results.

Originally Posted by in2steam Look into proxon, they have a brushless dc dremel like tool that many have used with succes. I don't think you will get the desired torque out of a can motor and the bearings won't handle the load. chris

Chris, thanks for the tip. I looked into the Proxon, first, and realized it was overkill for my needs. I'll be using the high-speed Wolfgang spindle to cut very small cavities in aluminum. No cavity is over 1/4" deep or over 1.5" in length. With an ultra-small stepover and stepdown of .00625", it should be hardly any stress on the cutter. I don't think a 1/32" cutter could handle much stress any way. I've heard those little babies snap off and go flying across the room at the slightest abuse. I'll definitely have an enclosure around my mill when it is whirring away at 40K RPM with a tiny cutter in the collet!

As for load on the bearings, the motor drives the spindle, so I'm not expecting any undo stress on the motor bearings. If the brushless motor shaft were the spindle, then I'd be in real trouble, wouldn't I!

Regarding torque, I wonder how I can calculate that? Does anyone know how I might determine the torque on a 1/32" 2-flute end mill running at 40K RPM, cutting .00625" into 6061-T6 aluminum at 10 IPM? I suspect it is negligible at that rate.

[in2steam]

Originally Posted by GammaWill Mike, the brushless motors I'm looking at have metal cases. That should help with the heat dissipation I hope. As for driving them, the controllers expect a .5ms to 1.5ms width signal to vary their speed. 10 years ago, I happened to have built servo driver using a 555 timer to generate the pulsewidths. It drove a servo just fine, and should drive the hobby-type brushless ESCs. What would be really cool is to get MACH3 to output a variable voltage that I could use to drive the ESC. A simple PIC (even a puny 16F84) could handle the voltage-to-PWM conversion for me. I'll look into it and report back when I have some results. Chris, thanks for the tip. I looked into the Proxon, first, and realized it was overkill for my needs. I'll be using the high-speed Wolfgang spindle to cut very small cavities in aluminum. No cavity is over 1/4" deep or over 1.5" in length. With an ultra-small stepover and stepdown of .00625", it should be hardly any stress on the cutter. I don't think a 1/32" cutter could handle much stress any way. I've heard those little babies snap off and go flying across the room at the slightest abuse. I'll definitely have an enclosure around my mill when it is whirring away at 40K RPM with a tiny cutter in the collet! As for load on the bearings, the motor drives the spindle, so I'm not expecting any undo stress on the motor bearings. If the brushless motor shaft were the spindle, then I'd be in real trouble, wouldn't I! Regarding torque, I wonder how I can calculate that? Does anyone know how I might determine the torque on a 1/32" 2-flute end mill running at 40K RPM, cutting .00625" into 6061-T6 aluminum at 10 IPM? I suspect it is negligible at that rate.

Thats not torque thats side load, torque is what the motor produces. Its most often referenced to horsepower but can also be used to refer to holding power of a servo/stepper. The rest is your feed rate, which really has nothing to do with torque. In which case your figuring chip load per flute and thats going to be very very small(at 12800 rpm give or take). I am not familiar with wolfgang spinldes, so my assumption is that it fairly small, and you have to attach the motor to the input of it directly(via a coupling of some sort). I know enough about about the hobby motors to tell you that they will not last much more then a half an hour at speed or they will burn up, even under forced air flow cooling(don't forget in an airplane its forcing air over the motor). I really think your best bet will be something along the lines of Proxxon or an air spindle. I dabbled in RC aircraft I know that the fellows that were teaching me did not like the electrics because they required a cooling down period.

chris

[LongRat]

in2steam, I suspect things may have moved on significantly since you were involved with RC planes. People are flying 1/3 scales on electric now. I actually think there is mileage in running a brushless motor to drive the spindle. For example, I have a Hacker B50 motor rated to 100,000 RPM which will handle over 1kW with cooling. It will run indefinitely at lower powers, just be sure to run a well matched controller to the load so you don't overheat the ESC with switching losses through part-throttle running.
Chances are, you'll need something in the region of 50W of power to run those cutters based on my experience. I have a Proxxon IB/E spindle which is rated at 100W and slows down slightly when I push it with a 3mm cutter. Much more load than you intend.
I would question whether the Proxxon is not the way to go though. Even a relatively cheap motor/controller combo will cost you more than an IB/E or similar (they are about £70 in the UK).

[philbur]

If you belt it for a 2:1 ratio you are going to have side loads on the motor bearings.

Phil

Originally Posted by GammaWill The brushless hobby motors used in RC airplanes look perfect, but I'm not sure they can handle the continuous operation (up to 12 hours continuous). I would suspect that the bearings would be the part that would fail over time, and I'd like to run it at 20K RPM using a 2:1 belt between it and the spindle, giving me 40K RPM at the cutter. Load would be light, since the 1/32" cutter would be eating at only about 15 IPM max. Thanks, ---Will

[philbur]

Here's a typical result from MePro.

1.0 watts. You could blow the material off with a mild cough.

endmill HSS
diameter (mm) 0.8
flutes 2
cut depth (mm) 0.4
cut width (mm) 0.8

surface speed meters per min 90
rpm 38000
mm per tooth 0.00409
mm per rev 0.00817
mm per minute 317
power required (Kw) 0.001
material removal rate (cc per min) 0.102

Phil

Originally Posted by GammaWill Regarding torque, I wonder how I can calculate that? Does anyone know how I might determine the torque on a 1/32" 2-flute end mill running at 40K RPM, cutting .00625" into 6061-T6 aluminum at 10 IPM? I suspect it is negligible at that rate.

[GammaWill]

Originally Posted by philbur Here's a typical result from MePro. 1.0 watts. You could blow the material off with a mild cough. endmill HSS diameter (mm) 0.8 flutes 2 cut depth (mm) 0.4 cut width (mm) 0.8 surface speed meters per min 90 rpm 38000 mm per tooth 0.00409 mm per rev 0.00817 mm per minute 317 power required (Kw) 0.001 material removal rate (cc per min) 0.102 Phil

Phil - Thanks! I see! 1W of power is consumed when cutting 1/64" deep at 12.5 IPM

Most of the hobby-type brushless motors seem to be able to produce 100W easily. This cheap 3800 KV motor & ESC looks about right, and the motor case doubles as a heat sink (no carbon fiber shell on this one):

http://cgi.ebay.com/3800KV-Brushless...QQcmdZViewItem

Brief Specs:
Motor RPM/KV: 3800
Motor Max Watts: 250W
Motor Max Continous Current: 8-10A
ESC Minimum Voltage: 4.8V
ESC Max Voltage: 14.4V

So, driving it at 5V (and at 100% "throttle") will give me 19K RPM.

From the numbers above, at 5V the motor could generate a MAX of ~30W of power. So, each 1W consumed at 5V would require 200mA current.

I will only need 1W, so I suspect that that running virtually "unloaded" (the cutter will not be pulling many amps @ 12IPM, and 1/64" deep passes), and even with losses through the coupling system (misalignment, friction, heat, etc.) I would be drawing WELL under 1A @ 5V, with 250mA probably being typical for my application.

How's my math, guys? Did I goof up any where?

---Will

[philbur]

Be careful. The program doesn't say but I think the calculated power requirement is at the tool, not the input. How would the program know your spindle drive efficiency.

Regards
Phil

Originally Posted by GammaWill Phil - Thanks! I see! 1W of power is consumed when cutting 1/64" deep at 12.5 IPM Most of the hobby-type brushless motors seem to be able to produce 100W easily. This cheap 3800 KV motor & ESC looks about right, and the motor case doubles as a heat sink (no carbon fiber shell on this one): http://cgi.ebay.com/3800KV-Brushless...QQcmdZViewItem Brief Specs: Motor RPM/KV: 3800 Motor Max Watts: 250W Motor Max Continous Current: 8-10A ESC Minimum Voltage: 4.8V ESC Max Voltage: 14.4V So, driving it at 5V (and at 100% "throttle") will give me 19K RPM. From the numbers above, at 5V the motor could generate a MAX of ~30W of power. So, each 1W consumed at 5V would require 200mA current. I will only need 1W, so I suspect that that running virtually "unloaded" (the cutter will not be pulling many amps @ 12IPM, and 1/64" deep passes), and even with losses through the coupling system (misalignment, friction, heat, etc.) I would be drawing WELL under 1A @ 5V, with 250mA probably being typical for my application. How's my math, guys? Did I goof up any where? ---Will

[Greolt]

Will did you see this thead. Probably worth a look.

Third Machine with 12 volt DC spindle

An Aussie builder put one of those same spindles (I think) on a machine with the same sort of motor as you are looking at.

Greg

[Stepper Monkey]

The little brushless motors for RC cars do lose bearings pretty quick. They are rebuildable and all, but a couple of hundred watts out of a couple ounce motor and something has to give. They are designed for maximum power out of a very small and light package, not longevity. Many of them require rebuilding every 10 hours of run time or less.
If you want a brushless DC motor, with speed controller built in no less, that will handle high side loadings and long runs, try something a bit larger. Those annoying little electric scooters, and electric bicycles as well, have 250W-400W brushless DC motors that have bearings that can handle a LOT of abuse. A PWM speed control board is integrated into the motor housing on many models. Nice bonus is that they often have a brake circuit on the motor as well. (useful for e-stop!)
You can get them brand new uninstalled >very< cheap off of EBAY as the fad is over for scooters. $20-40 bucks will get you a very nice motor.
They are light, but are still 4"-5" in diameter instead of 1", but no worries about overheating or bearing life....