RC Brushless spindle motor

Mostly Printed CNC - MPCNC Troubleshooting
#1

Im looking for a little advice about using an RC brushless motor for a spindle.

My current plan is to machine a housing from aluminum and press a couple of angular contact bearings into each end.
Iv ordered a C10 ER11 100 collet shank, and hopefully I will be able to cut an external thread on the end for a lock nut.
Then I can either go direct drive from the motor, or use a pully.
Will use an arduino to control the speed controller, and probably a separate power supply just for the spindle.

Can anyone recommend suitable spindle speeds for cutting aluminium?
Or better recommend a motor and power supply.

Iv had a quick look on ebay and most of the motors draw a huge amount of current. What power supplies are people using to power these things with?
The biggest 12v led power supply iv found is 600w.

#2

Why did you choose the RC motor?

#3

A number of reasons.

Cost. A variable speed router is about £100. A home built rc motor spindle should be much cheaper.

1/8 inch collet. I can’t find anything other than a dremel that will accept a 1/8 bit over in the UK. Everything more powerfull is 1/4, and I don’t like the look of the adapters.

Noise. Rc brushless should be much quieter.

Repairable. If a bearing fails for example I can easily replace it. I suspect this will be harder with a router.

Speed. I can tailor the motor and or pully sizes to give almost any spindle Speed. Most routers iv seen are limited to a min of 10000 rpm.

Closed loop speed control. I might not ever need this, but i think it would be easy and cool to implement with this setup.

And last but not lease… I like making stuff!

#4

An RC motor was the original plan for me I couldn’t find gears/belts very cheap one set cost about as much as a 660 in the US. In the beginning we used some other tools, and settled on the 660 in the US because of it’s price. The original flex shaft spindle was pretty good, so small and light, but it was expensive. Cost is a rough one, pretty different over the globe.

Keep us in the loop on how it goes. I’d love to see what you have in mind for your closed loop. This alone would make most machines preform so much better regardless of spindle chooice.

As for feeds and speeds there are tons of calculators on the web, the numbers are for different kinds of machines but will give you an idea of what parameter changes what.

#5

I thrown some numbers at G wizard and it recommends 33k rpm and 8mm per second for a 3mm single flute carbide end mill in aluminum.
FSWizard says 38k rpm and 25mm per sec.

Both seam very high spindle speeds.
Im not sure I would feel comfortable with those sorts of speeds on a home built spindle.

This page gives 9000rpm and 7.5mm per sec using 0.002" per tooth and 300 sfm.
http://www.custompartnet.com/calculator/milling-speed-and-feed

That’s massively different!

Im gonna aim for a max speed of between 10 and 15k rpm.

If I find something like 5000rpm works well, do you know if there are any issues with running a brushless motor at half its max speed for long periods of time? Excess heat in the speed controller for example.

I think im gona go for somat like this.
http://www.ebay.co.uk/itm/AGM-GT2815-07-Brushless-Outrunner-GT-Series-Motor-1100kv-for-RC-Plane-UK-Stock-/271641223653?hash=item3f3f1421e5:g:tKEAAOSwDN1URKtj

At 12v i get a theoretical max speed of 13,200rpm.
And has a 5mm output shaft, perfect for a gt2 pulley if I choose to use one. (Still need to do some reading about belts pulleys, not sure how a gt2 belt will cope with 13k rpm!)

How do i know what the power draw is for one of these. It say 36A for a 3s cell, but I assume that’s only whats draw at the given speed for the given prop.

#6

Welcome to cnc…everyone has a different opinion and as you can see, different numbers.

You have a ton of variables to deal with. You should just build something cheap and test it. When I used the Outrunners I had several sizes on hand.

The dewalt we use is 30k rpm.

You don’t have to use 12V. You should be more worried about peak torque than speed. Power is always the concern, don’t forget your gear ratio in the equations and losses. My calculations were off by an embarrassing factor with my senior project using an outrunner.The math is simple but the numbers given can not be trusted, at all. You can adjust to suit your rpm.

On top of all that your need to make sure you have extremely minimal slop/runnout in your entire system. It will be running continuously for many hours at high loads. Don’t forget to cover it to protect it from dust and dirt from cutting wood metal and plastic.

You have a long road ahead of you. It is entirely possible and will be simple when you find the right combination of parts, but I feel you will have a lot of real world experimenting to do.

#7

That’s a very good point about torque been more important than speed.

The only reason I was planning on using 12V was I could use a server power supply with no extra mods.

Im gona leave the motor and power supply for the time tho, and concentrate on the housing and bearings.
Iv got an big slow heavy 24V 150w brushed motor that i use for my mini lathe that i will use for first bench tests of this spindle. The speeds will be no where near what will be seen during operation but will be a good firt test before I splash out on any new electrics.

Do you have any advice on housing design?
Current plan is just a lump of aluminium with a couple of bearings pressed into either end. Just need to make sure the bearing pockets are conentric and square.