Just as a heads up…

Tests have shown that small brushless dc motors (BLDC), more usually used with RC models, (mainly because of their high power to weight / size ratio) can make ideal engraving spindles.
The construction of just one such spindle is detailed here; http://openbuilds.com/builds/890kv-ox-rc-spindle.411/
To facilitate BLDC motor speed control and operation from Mach3 I have now made a simple interface circuit which takes the Mach3 generated PWM signal and converts it to the necessary pulse stream for operation the motor’s ESC and this is detailed here; http://openbuilds.com/builds/software-speed-control-of-a-brushless-dc-bldc-motor-from-mach3.762/

Tweakie.

Nice little project you've done.
I would love to have the skills to tackle something like this.

One thing you wrote caught my eye and I thought I'd suggest a possibility.
Quoted from your site.
"It should be remembered that supply voltage as well as shaft loading will affect the Outrunner’s speed quite considerably so, perhaps as another project I will look at a feedback system (measuring actual RPM) in an attempt to maintain constant RPM under varying load conditions – should be bit of a challenge".

Would it be possible to use a speed controller that has the governor facility.
I used to fly R/C Heli's when electic machines were just starting to evolve. A couple of the guys were using gov/esc's and they simply set the rotor head speed via the transmitter. The head speed would hold a very acurate rpm when loaded up.

Just a thought.

Hi Ozymax,

Thank you for your excellent suggestion - I will investigate the possibility of using a 'gov/esc' (I did not even know they existed for BLDC motors) but it sounds like a brilliant idea to me and would be a very easy solution to maintaining constant RPM.

Tweakie.

Tweakie,
You are talking about a small spindle, but those motors go to over 5KW continuos power ratings!  So if you don't mind a power supply that has to put out 200 amps DC it would work pretty well I would think. Welder anyone?

Hi Gary,

I think you would be able to do quite successful MILLING with a 5kW BLDC motor (although gear reduction may be necessary to reduce the speed). 

Quite obviously, a much smaller motor is required for engraving work (which is exactly what this thread relates to) although I quite like the idea of a 'combined engraving spindle and spot welder'. 

Tweakie.

Tweakie,
Those motors have a RPM/V rating.  The lower the number typically the more poles the motors have and the slower they run.  I've seen some with 28 poles, which allows swinging a large prop by direct drive.  So a motor like that might only need a small belt reduction ratio to be useful for milling. Also for small diameter carbide tooling you typically want high RPMS anyway, However I think the spindle loads should get carried by a separate set of bearings, not the motor bearings.