Brushless DC Motor Used For High Speed CNC Spindle

Brushless DC Motor CNC Spindle

Brushless DC motors are common place in RC Vehicles. They are small, light, fast and can be inexpensive. [Raynerd] wanted a new spindle for his CNC machine and thought that a brushless DC motor would be a great platform to build from.

[Raynerd] started with an off the shelf motor that had an 8mm shaft. This shaft size was important because the motor shaft was to be replaced with an ER16 collet arbor of the same size. A collet is a device used to hold cutting tools by collapsing a segmented ring around the tool. Collets allows for quick tool changes while providing a strong clamping force. ER16 is a designation of one of many collet standards.

The main housing was machined out of aluminum specifically for this project. This housing holds two radial load ball bearings that support the new rotating collet arbor. There’s another bearing in this assembly, a thrust washer this time, that keeps the arbor from moving axially in the housing.

The 12 volt output of a standard ATX power supply was used to power the system for testing purposes. A general RC Vehicle electronic speed control and a servo tester work in conjunction to manually regulate the spindle speed. Check out the bench test video and an exploded photo after the break.

Brushless DC Motor CNC Spindle

 

49 thoughts on “Brushless DC Motor Used For High Speed CNC Spindle

    1. normally the distinction is that a BLDC motor should be driven with square waves and and has a trapezoidal back EMF, just like a “normal” DC motor with brushes.
      An AC motor should be driven with sinewaves and has a sinewave back EMF

    2. Definitely a BLDC motor, the trapezoidal back-emf distinction is not really consistent – a BLAC motor is the same as a BLDC motor except the coils are wound (very) slightly differently to produce ‘natural’ sinusoidal back-emf… Both can be driven by 6-step controllers (hobby esc). The distinction between AC and DC motors is just the power source they are driven from, all motors are AC when you look at the waveform at the motor, even brushed DC motors which just use a mechanical commutator to do the same job as the inverter/esc used for their brushless counterparts.

      1. Its may be in a cnc milling machine, definitly not a modern one though, as that size fly wheel would greatly decrease the acceleration/decceleeration times, leading to slow chip to chip tool changes. Probably an older mahcine with low bandwidth.

        1. Flywheels on milling machine heads are/were also used to dampen spindle vibration and speed fluctuations. I think I have seen pictures of SiP and and Deckel heads of 1950-60′s vintage that had this as a feature, have to dig through the lathes.co.uk archive to find specifics. As I recall, they were meant for high precision work done by the supremely anal retentive sort that purchase horrifically expensive Swiss manufacturing equipment.

    1. Having some experience in the strength of small brushless motors I say go for it – but expect to lose your finger.

      I have used something similar in a homemade mini-multitool setup and it equal a much larger standard multitool (think Dremel) in power. Would still be using it if I hadn’t made a mistake that caused a piece of metal to destroy the internal windings of the motor.

      But for a superman like yourself a Dremel can be stopped by a finger too, right? ;)

  1. The Fabtotum did the same thing only using an ER8 collet. They found that they needed feedback to keep the spindle turning in the optimal range. Otherwise it would bog down while routing.

    I’m not sure why he went with an ER16. An ER11 would handle as big of a bit as that motor could handle.

    1. There are esc’s made for rc helicopters that have a governor feature to keep the speed constant. Could work here too…

  2. I forking love these and the fact that they are available as off the shelf parts. Just a speed controller (again off the shelf) and you’re good to go. I wish more stuff was so easily accessible as this

  3. Is there any Problem with the bearing load because a mill moves *sideways* (sorry for that non-technical description)?

    1. the lateral loading is taken up with bearings in the bright aluminum spindle section below the motor, instead of pressing on the motor’s bearings

      1. a tip is that you could just make the spindle belt driven and use a off the shelf spindle or what not, and then there is very little side load on the motor shaft, just a bit of tension from the belt. or you can still build the spindle, but now it is two seperate assys, which is easier to deal with imho.

  4. Hi, I’m Raynerd who originally made this and took the pictures and vid (!!) I just wanted to say that despite what sceptics may say, the spindle works great! It cuts wood just fine and I’ve used it countless times to cross out brass clock wheels and machine brass clock frames! It’s a great little spindle. If you follow the youtube channel there are recent videos of the spindle cutting out a wooden clock. I’ve changed the £1 cheap bearings once.

    1. I’m a little unclear on how much the motor was modified, but if I were to have my input I have been considering such a project, although I need a bit more power than he does. In my design, the motor belt drives the spindle. this lets me have a hi/lo range selection for metal and plastic/tiny bits but still have digital spindle controll woo!
      I would do it as a belt drive so the motor sees less side load on the shaft. I know you support it with bearings, but by making it two separate assemblys soft-coupled w/ a belt, it is easier to have a good motor design, and a good spindle design, nad not have to compromise. then you don’t need to worry about concentricity etc.

      …oh and maybe I already have a belt driven spindle.

    2. Hi Raynerd,
      We’re not critics, we’re armchair quarterbacks!
      So, if you were going to build another, what would you do the same and what different?

    3. Hi Raynerd! Quick Q? What kind of 3-phase brushless Dc Driver (variable) are you using??? I am in need of one, and would like to ask you your thoughts on where to locate a decently priced one!?! Amazing project and thank you!!!

  5. Wasn’t this discussed before on HAD and the problem was that the axle of the motor is not stable enough for precision ? I think there was some mentioning about the endurance and lateral forces as well…

    1. the motor’s bearings are augmented with a set of heavier-duty bearings in the bright aluminum housing, to take the load off the motor’s bearings

        1. Brogan – try it. They work. Thoughts and opinions whether the bearings are up to the job are irrelevant since the I’ve made it, used it and it runs great! I’ve changed out the cheap £1 bearings once in 3 years! I’ve cut clock wheels, pcbs, clock brass frames…

    2. looks like skate bearings, they are made to handle the weight of a person and then some, they are not going to just fall apart

      1. But your skate wheels are not going to rotate at 10k rpm or more, and they can have a lot of slop.. I would only throw in a needle trust bearing instead of washers, and also important to note, always open your new bearings and pack them with grease, them re-fit the covers.

  6. I think you’re going to have to replace those skate bearings with angular contact bearings to get longevity. Skate bearings are good for strict radial loads, but your milling bit will apply an axial load when pushing into the material and will still have some axial component while milling because of torque. The right bearing for the job in this case is angular contact.

    1. If I understand roller skates, they DO have to support high lateral loads. Like hundreds of pounds, if you happen to jump at all when using them. Explain how this is wrong.

      1. An alternative name for skate bearings is “radial ball bearings”. The intended load is definitely in the radial direction.

        This is from the angular contact bearings on wikipedia:
        “An angular contact ball bearing uses axially asymmetric races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings better support “combined loads” (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load.”

        All I can tell you is that radial ball bearings are not *designed* for high rpm with any axial loads over long periods of time. It has to do with the design of the track race and lack of support in the axial direction.

    2. Er .. I mean axial loads as well. Sure, the load is all lateral when you’re just riding in a straight line, but there is plenty of axial load any time you’re turning.

        1. Are there bearings of that size that could stand up to the kind of abuse a skater puts on them without eventually breaking down though? I mean, having a full grown person land on them at weird angles from relatively high heights puts a ridiculous load on them.

    3. And if you go check out the load and life calculators on SKF.com a correctly installed and preloaded radial contact ball bearing can take 1/4-1/3 of it’s radial load rating as an axial load. Should be plenty for a machining spindle.

  7. What is the motor constant (KV) for that motor? Looks kind of slow, which is good for machine tools, but harder to find slow RC hobby motors.

  8. There is no side load on the motor bearings at all, there are no motor bearings any more, they were removed along with the motor centre spindle in order to fit the rotor onto the chuck shaft. On mine I left the motor intact and stuck it on top but it can’t have side load on the motor either as the motor is floating so is free to move if it needs to. Feedback was necessary in order to keep the speed up but it is easy to pick that up from inside the ESC, the ESC uses it anyway and feeding it into the parallel port so that LinuxCNC could read it was fairly straightforward. The difficult part was making a signal to drive the ESC from the parallel port, that needed a PIC because LinuxCNC and the parallel port hardware did not allow a fine enough signal for the ESC. Mine has a 1600kv motor and will run reliably at 30,000 RPM. It has been running for three years without the need to replace anything.

  9. Been running one of these spindles for months after following the build log from Chris. Only difference is that I used angular contact bearings. Spindle has an ER16 collet as I wanted the ability to use 10mm tools. I have been cutting wood, plastic and Ali all with excellent results. Speed is controlled by mach3 and doesn’t get bogged down at all. Plus it is really quiet compared to a router spindle. Best mod I have made to my router

  10. How much runout do you get with those bearings, in this setup? I’ve been doing a fair bit of spindle measurement recently, and it is an interesting topic. I have some old routers (15 years old) where the collet socket runout is only .0002″. More recent examples are closer to .0006″ to .0008″. Quality seems to be dropping in all brands.

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