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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > High Speed Spindle for CNC BF30
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  1. #1
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    Aug 2005
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    180

    High Speed Spindle for CNC BF30

    I've now completed by the CNC conversion of my Optimum BF30 and its time to build a high speed spindle so that I can use small cutters / engraving bits.

    The concept is to follow on from the success of others and build the spindle around an RC BLDC motor with an 8mm shaft, an ER11 collet chuck with an 8mm parallel shank, some angular contact magneto-bearings. For controllers I'll use RC testers to start with but may connect up to LinuxCNC later.

    The body of the spindle will be chopped out of 1.5" dia steel bar.
    Attached Thumbnails Attached Thumbnails IMG_0647.JPG   IMG_0649.JPG   IMG_0648.JPG   IMG_0651.JPG  

    IMG_0650.JPG   spindle wire.jpg   spindle solid.jpg  

  2. #2
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    Hmmmm, had a bit of rethink about the design. I think I can get the 4 M4 bolts to fit through the whole body without the need to create the recess. Countersunk screws would make a neat job and only need around 3mm of thickness.

    Heres a link to the motor I'm planning to use. Mine is quoted at 900KV so on 24V this should equate to around 20krpm.

    I've also just finished the prototype PIC code for a tachometer, by using the CCP module and tracking the roll over of the timer I can measure rpm from zero to 100'skrpm. I've run the code on an 18F252 using a function generator as a signal source and predicted rpm vs generator frequency is accurate up to around 900,000 rpm (simulated!!) and somewhat more that I need.

  3. #3
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    Apr 2005
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    How are you going to get 1350W of 24V?

  4. #4
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    Quote Originally Posted by 691175002 View Post
    How are you going to get 1350W of 24V?
    56 amps, motor will handle it in bursts with good air flow. Being er-11 im not sure how often the load will be higher than 500 watts or so though, I run small bits at 24k rpm on my 2.2kw spindle and the amps barely raise from no load. Figure 80% or less efficiency too, watts at the bit will be lower. Lots of routers use 800 watt spindles and 1/8 inch or smaller bits without too much trouble. You can also run higher voltages as long as you keep the amps in check. I would consider using a controller with a governor, think of it as a sensorless vector vfd..... Good luck for comparison I fly a 12 pound heli that peaks at 7.5 horsepower through a 400 gram motor and the motor comes down warm after 5 minutes. German motor and controller though....

  5. #5
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    Aug 2005
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    Hmmm, not sure even an M6 cutter could stand 1350W going though it. I've had a play with a loan motor already and the current draw is pretty small when loaded with calibrated finger pressure, the motor was very difficult to stop - I have the burn to prove it!.

    I'm not sure what diameter prop an M8 shaft is expecting but I assume the torque required to spin a large prop is >> than the torque required to turn a small endmill. As time goes by I'll post up some power data now I have the watt meter thingy.

    Target application is small cutters (<M3 dia), engraving bits and carbide PCB drills and routers so light cuts combined with high axis speeds - I'm not planning to drive an M20 ripper mill hogging 0.5" cuts ).

    I need to think about cooling the motor and bearings maybe I attached an RC prop or something - or even mill a fan from solid.

    I'm quite interested in this project as I've read everything from "this is a great way to get a high speed spindle" to "it was a disaster and I am now beating myself for every thinking of such a thing"

    As I said at the start I'm following in the foot steps of other people, check these links
    Link1

    Link2
    [ame="http://www.youtube.com/watch?v=fGrITuuTZtQ"]Link3[/ame]

  6. #6
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    Apr 2005
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    I think the cooling fan will be absolutely essential. I would consider robbing one from a PC fan, just pull the motor out, turn up a nice hub to replace it and attach it to your outrunner motor.
    LongRat
    www.fulloption.co.uk

  7. #7
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    Dec 2004
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    Another point of reference, my big heli has a 6mm motor shaft and 10mm main shaft, both hardened of course.

  8. #8
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    May 2008
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    1185
    A PC fan at 20,000 RPM?

    A electric fan might be a simple way to cool things and not have a fan spinning at 20K. You already will have a 24V supply so a 24V Panflow would be a good choice.

    Heat will mostly be a function of power and the loads with 1/8" cutters will be light.

  9. #9
    Join Date
    Dec 2007
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    2134
    That linked spindle is absolutely awesome!

    good luck,
    Ian
    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!

  10. #10
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    Feb 2012
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    36
    For engraving, fan may not be necessary for the motor...
    I have been using an outrunner brushless motor on my hobby CNC for over 3 years now. I have cut 0.5" thick aluminum plates with 0.25 endmills at shalow DOCs and the motor would barely stay warm. I think the high efficiency of these motors help keep them cooler.
    Get an ESC with a much higher current rating. Otherwise, you may find that your ESC heating up a lot more than the motor. I had the fan on my ESC rather than the motor.
    As someone here pointed out, an ESC with governor mode will be essential. Like you, I had deveoped a microcontroller code to generate the pulses for controlling the motor. The microcontroller also monitors and displays the RPM.

    When I ran with governor mode off, the rpm fluctuated quite a bit when cutting. with governor mode on, the RPM stayed solid in and out of cuts. Turns out governor mode is a bit more work for the ESC, but that is where having an ESC with a much higher current rating pays off.
    Good luck.

    I am buildinng another CNC for cutting aluminum, and I plan on keeping the BLDC spindle.

  11. #11
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    Apr 2005
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    20k will be no problem for a small PC fan suitable for a motor like this, but it might be an excessive amount of air.
    However, it does make sense to use a separate fan drive seeing as the right voltage is available anyway. I would also consider a method of protecting the motor from the feathery chips that will no doubt be generated. Got to say the build looks good and with the low price of motors and controllers now, a very cheap way of getting what you need.
    LongRat
    www.fulloption.co.uk

  12. #12
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    Mar 2004
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    I'm having similar ideas.. some of those hobby motors are intense

    HobbyKing R/C Hobby Store : T5692 Turnigy Pro Comp Brushless Inrunner Motor 1000kv

    6000watts?!?

  13. #13
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    Aug 2005
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    Okay last night I had fiddle with the motor and my Maplin 30V 3A supply (I managed to kill my Watt meter ). 15V and 3A give a satisfying "whine" I guess the rpm must be around 15K or so. I ran the motor for 15mins and it was barely warm, torque seems okay using my calibrated and now somewhat leathery fingers. The cage of the motor produces a fair blast of air without any additional fan.

    I guess the use case for these motors in aircraft is effectively to short circuit a lipo pack through them (with a bit of control) for fairly short flight times hence the currents of 10's of amps. The CNC application is a bit different in that the current requirements are lower but the run times are longer.

    The thing I'm not really sure about at the moment is the power supply. The Maplin unit is quite good in that it limits the current to a set value. In the case of say a 15V psu the motor will sink as much current (and more) as the psu can supply - the RC guys seem quite okay talking about currents of 60 - 100 of amps flowing into these tiny motors

    So based on this less than scientific assessment I think a good psu would be 12V - 15V with 10A - 20A on tap (ebay have them for 20 - 30 quid), this should give plenty of torque for machining with small cutters with head room to keep things cool.

    Any other thoughts / experiences would be appreciated.....

  14. #14
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    Apr 2005
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    A pair of lead acids would be a reasonable place to start. That way you have basically an infinite amount of current available and you will be able to see what the motor really requires, before ordering the PSU if you decide you need it.
    I fly RC planes and run cars, yes you will see currents getting on for 60+A maybe on a motor like this, and in cars, spikes of 200A and over are common, but they are just transient spikes. Continuously that's totally unsustainable and I don't think you would need that in a milling spindle anyway so I imagine the sort of PSUs you are looking at will probably work well.
    LongRat
    www.fulloption.co.uk

  15. #15
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    Quote Originally Posted by abfa9358 View Post
    Any other thoughts / experiences would be appreciated.....
    My only reservation about using an RC brushless system for a mill is purely the power issue - it can get very expensive fast.

    If you want a theoretical maximum current draw it is actually fairly easy to produce using a milling horsepower calculation: Milling Horsepower Calculator

    Obviously it depends on what kind of cut you are taking, but a 1/4" carbide endmill that is 0.1" deep in aluminium comes out as 0.125-0.3 horsepower (depending on how aggressive the cut is) which would be roughly 16-40 amps after efficiency and such.

    A random set of numbers for 3 flute carbide in aluminium:
    1/8", 0.1" deep = 8-16A at 12V
    1/4", 0.1" deep = 16-40A at 12V
    1/4", 0.2" = 24-60A at 12V

    You can probably use the low end to size a power supply since the aggressive cuts are really heavy (60IPM or faster)

    The numbers could be way off but seem realistic to me.

  16. #16
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    Dec 2004
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    783
    What esc are you using? What max rpm are you looking to run at?

    Sensorless brushless R/C esc's prefer short wires unless you add capacitors to the input wires. Its usually better to have long motor leads than long power leads.

    Use larger gauge wires than you think you need, and braid the 3 motor wires together. The esc needs clean back emf to sense magnet position, and if you use small long wires you can get "cross talk" and the esc will lose sync, which tends to let the smoke out of it....

    If using a switching power supply I would put a large low esr cap on the output. The esc pulses full voltage to the motor, chopped pwm, so the power supply needs to be able to handle the amp spikes.

    Wires longer than 10 inches are considered long btw. Long wires will cause high voltage spikes to the fets on the esc, resulting in heat and eventual failure. It will also overwork the caps on the esc, which are designed for short wiring.

    Going with a higher input voltage and an esc with "active freewheel" and a governor would produce the best results, but those esc's arent exactly cheap. A sinusoidal esc with hall effect sensors on the motor would be ideal, but cheaper to just run an induction spindle and vfd at that point.

    I thought about building a similiar setup but ended up with a water cooled spindle and hitachi vfd, with 0 regrets.

    Also cheap motors are not known for being well balanced, sometimes you get lucky. Higher quality motors are dynamically balanced.



    Dylan

  17. #17
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    Mar 2004
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    94
    I was rooting around for BLDC controllers, and stumbled across this site. Would anyone know if it would be suitable for hobby motors?

    Electric Vehicles Controller - Motorized Bike Parts

  18. #18
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    Aug 2005
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    180
    Thanks all for your input. One question what does a governor on an ESC do? Attempts keep voltage constant by changing current to meet load on motor?

    I have plenty to think about. At the present I'm going to build the mechanicals lash the spindle to the mill and power it with my Maplin PSU and see what happens. At present the motor spins faster than the mill main spindle, seem to have enough torque for small drills and cutter and doesn't get red hot - so at the moment "jobs a good 'un"

    I post more as things progress but as with the CNC conversion it probably wont be quick!

  19. #19
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    Dec 2004
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    783
    Governor uses PID to hold a constant rpm regardless of the load, kind of like a sensorless vector vfd. We use it on heli's, and they can get quite sophisticated with sensor feedback and prediction based on tx stick input.

    Voltage to the motor never changes, just the pwm duty cycle firing each phase does.

    You need some power overhead for it to work properly, usually 65-75% of max load is the sweet spot.

    Electric R/C helis have come a long way in the past 5 years or so. Pretty sure brushless power tools are a result of advancements in the hobby.

  20. #20
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    Aug 2005
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    Ah, okay I understand thanks. I guess that sort of system needs plenty of head room in terms of power.

    At present I am still convinced based on what I seen so far in the shed that the BLDC motor can be made in to a useful spindle for small cutter without breaking the bank and without becoming overly complicated.

    Time will tell.......

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