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Old Oct 31, 2010, 10:29 AM
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RC motor for a CNC Mill

I would like to make a CNC mill. How would a RC motor work for the spindle? I know RC motors are not rated for continuous duty, so the motor would be derated quite a bit, at least 1/3, probably 1/2. I was thinking this motor:
http://hobbyking.com/hobbyking/store...idProduct=5139

This would give a max RPM of around 8600, how does this sound for milling aluminum, with a bit max 1/2" diameter for large material removal? Would I need to bring the speed up by means of a belt? If so, what speed should I aim for?
I want to mill aluminum and PCB's. I know PCB's require a high spindle speed (and not so much power...) so would a belt where I can swap out sprockets be the best bet?

What are rough spindle speeds used for aluminum?

I would probably make my own ESC for continuous duty.

The motor would provide 5.8 pound-foot at full power (7000W)
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Old Oct 31, 2010, 10:57 AM
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The motor you have noted is a bit on the small size. I usually run a 2HP electric motor on my mill, and it has popped the breaker a few times getting stalled while using four flute 1/2" end mills in steel. I also think you have to run way more RPMs on aluminum than steel.


Wm.
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Old Oct 31, 2010, 11:31 AM
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That motor, when derated 50% is 4.7HP. When it is not derated, it is a max of 9.4HP.

What spindle speed do you use for aluminum?
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Old Oct 31, 2010, 01:45 PM
The Junk Man
Jacksonville, Florida
Joined Jul 2006
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With the availability of a host of economical spindle drivers out there, I have no idea why you would use a RC motor... whose bearings are not designed for radial loads.

Tom
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Old Oct 31, 2010, 05:15 PM
B for Bruce
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The 'Wack, BC, Canada
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The spindle speed you should be using is only partly based on the material. It is also based on the size of the cutter.

Steel, aluminium, copper, G10 epoxi board and wood all have cutting rates that they work best with. The values are given as SFM or Surface Feet per Minute. What you do is use the circumference of the cutter you want to use and match this to the SFM range of speeds and from that you can determine the RPM of the cutter. There are charts and online calculators available to do this.

I also second T_om's wonderment as for why you would want to use this motor for this application. First off the motor is not designed for radial loads as he mentioned. Second you will require a high current DC powersupply and a model based ESC or custom power supply and 3 phase variable frequency power converter to control the RPM on this brushless motor. The need for these support pieces will totally blow your idea of a budget head motor right out of the water.
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Old Oct 31, 2010, 06:39 PM
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I will look more into the motors.

I may take that motor and replace the bearings (to help with radial load). a ESC is fairly easy to make, and 200A is not too much (My last project used pulse power at over 100,000A to control), Cost would be under $60 with heavily over rated FET's. I already home some transformers (need to rewind, currently wound for 600A). Just need to rectify (have some 1200A stud mount diodes) and filter (have some large caps too).

Along with the radial load; Direct drive does not have to be present and belts can be used, and the shaft would be supported by external bearings.

I ask because I saw this:
http://www.cnczone.com/forums/genera...dle_motor.html

Thanks for the explanation of SFM.
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Old Oct 31, 2010, 07:49 PM
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You say an ESC is easy to make, but brushless ESCs are a lot more complicated than brushed ESCs. You need to be sensing motor position via back-EMF from the windings to energize the three phases at the right times.

Just mentioning it, you may be an arduino-wizard or something
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Old Oct 31, 2010, 08:08 PM
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Yea, Im away of that. I am into embedded electronics (though I never touched an arduino, first processor was a 4 core 1.6 gigahertz beast)
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Old Oct 31, 2010, 09:20 PM
B for Bruce
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It sounds more and more like you've got the background to make this work after all.

Even with higher grade bearings in the motor it still won't be good for direct drive. The shaft on this motor may be humungous by model airplane standards but it's a weak little toothpick by machine tool standards. And the bearings, while suitable for typical prop vibrations, would not last long at all with the shock loads of cutters. Far better to isolate the motor from the actual tool bearing shaft with a multispline drive belt. Then your tool head output shaft can be set up with bigger and more suitable bearings and a collet holder suitable for the tool bits and do it all wiht a lot less overhang.

The size of the actual tool holding shaft and bearings on it will do more to determine how big a cutter you can use than the motor itself. Chatter and bearing load carrying are bigger factors than the power from the looks of it. If you were limiting yourself to wood or plastics and up to perhaps 1/4 inch cutters then the motor itself could be set up with a holder and it would likely be fine. But since you're after cutting of metals you're going to want a far more burly cutting head and to isolate the motor from the shaft that mounts up the cutters.

Did you get that bit I wrote in my first post about how to determin the RPM of the cutters? Also there's another spec for metals in particular. This is called the feed rate per tooth per revolution. A lot of cutters cut best if they are taking off enough but not too much material. Try to feed them too slowly and it can result in dulling of the cutter much sooner than normal. Feel them too fast and the chips tend to tear out of the base metal or in extreme cases the cutter edges can fracture and leave you with a damaged cutter that isn't cutting well and results in needing excessive pressure to feed. Any good CNC program will take all this into account using built in tables and from asking you what your material and cutter diameter plus configuration is like or it will rely on you to input the RPM and feed rate to keep the cutters working in that portion of the job in their happy place.
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Old Oct 31, 2010, 09:35 PM
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Yes, I will look into that more. Pulley's are looking more and more appealing as I can change it for a higher speed for PCB milling.

I have no idea how to use torque ratings to determine the correct stepper size for the table. Do you know anything about that?
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Old Nov 01, 2010, 11:57 AM
B for Bruce
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Not a clue from me since I enjoy actually doing the milling. I don't see why folks want to let the computer have all the fun...

I can comment that when a shart cutter is cutting well and at the optimum feed rate there is very little noticable cutter load. It's pretty much all just the friction and stiction loads of the table ways and such. But if you're trying to force a dull cutter then you get a noticable build up of resistance in a hurry.

Likely your best bet, as with many such things, is to closely study other similar machines and see what sort of steppers others use and any typical gearing ratios if any.
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Old Nov 01, 2010, 04:07 PM
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I like manual machining too, but love the precision Plus making a normal mill would just seem useless.
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Old Nov 02, 2010, 09:57 AM
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http://www.rcgroups.com/forums/showthread.php?t=1033449

This is thread we had on the same topic but smaller scale. I hope to get around to making my spindle soon.

Greg
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