Just FYI....  I have over 100 hours on one of the Harbor Freight tools at continuous duty at maximum speed (15K).  The handpiece is starting to exhibit noticeable side play.  I would no longer trust it with high feedrate for the smaller tools, but its still fine for rough work with a 1/8" ( 3mm apx ) ball cutter.  I have not tried to adjust it yet.  That being said it still has less side play than the stock setting for  Foredom 44T.  I now have a Foredom 25 hand piece which Foredom reccomended for continuous duty production work.  They claim side play is about .00015" ( .004 mm apx ) and that it has much larger bearings to hold up to continuous duty and reduce heat buildup.  It is slightly larger diamter so I need to make a new set of mounting clamps for it before I can test it.  I know these are not as fast as you are looking for at 15K with the HF motor or upto 18K with some of the Foredom motors, but I figured since its what I have direct recent experience with I'ld keep you posted for background information.  

Rich,
 I wanted to do it the way I did as it meant I just had to pt it in the spindle and the spindle would do the driving, just a stabiliser bar needed and full quill travel would be preserved.

Dan
 It was belts and pulleys I had lying around, so not sure what the profile of the teeth was and cant took as its at the workshop. Think the belt is spec'd at 100 deg C but it was the housing and bearings getting that heat that worried me more. Maybe should borrow my friends laser temp gauge and see if it gets hotter on extended runs, maybe just gets to that temp and sticks there.

Belt is 2.5mm pitch and I was running 3000rpm at the spindle and the output would be 11250 at that spindle speed.

Vee belt probably would have been my choice but as I already had the belts and pulleys I decided to use them, not sure how an o ring would do, may give it a go as pulley would be simple to make up. Wont use the cutter I have though for the testing as at  £22 an insert I dont want to break it until I am sure it works.


Simpson

My design process starts usually with what I have lying around or can get cheap    I had a  1/3 sheet of 19mm thick 5083 alu left from a job and the belt and pulleys so that's where I started my design. Not the best way I know but........
The body was made of Alu and this may actually be the reason I feel the heat so quickly and as said above I really need to test a bit more as its possible that it ma not get much hotter with time, or is that a stupid thing to hope for?

Water cooled spindle looks good and I know a few that have them and seem to think they work well but will wait and see what my needs for the high speed are in the future as at the moment it doesn't really justify any more that £100 with the amount I do.

Hood

My design process starts usually with what I have lying around or can get cheap    I had a  1/3 sheet of 19mm thick 5083 alu left from a job and the belt and pulleys so that's where I started my design. Not the best way I know but........

Actually, you have just described probably THE most common design criteria. GM used the same front suspension and steering in full size cars from I think 1955 or so until 1965 and then continued the same setup on the Corvette all the way to 1982. Somebody somewhere was definitely saying "use what we have!". I just finished a very large project that had the same criteria up front. It takes an act of congress to get approval for a new component.

The body was made of Alu and this may actually be the reason I feel the heat so quickly and as said above I really need to test a bit more as its possible that it ma not get much hotter with time, or is that a stupid thing to hope for?

Not at all. The aluminum getting hot is exactly what you want. That means it is drawing the heat out of the part. All you need to do then is take the heat off the aluminum, hence my comments about the exterior arrangement. Fins, fans or fluid, as they say.




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Well I used to be into overclocking computers and got quite handy with the cooling, even had one CPU cooled down to -15 deg C
Going to borrow the thermometer and do some more testing, maybe a waterjacket of some sort will be the easy way, seeing as how I use flood 
Hood

Might as well throw my two cents in, I suppose. If it were me, and I already had a method of driving the spindle, I would go with the collet extender. Everything is pretty simple to make/repair. If a bearing goes out, no biggie, spend a few bucks and get a new set. Better yet, get a few and save on shipping, and when they need replaced, your back in action in a short time. You also have a wide range of collet sizes to choose from. You've already got a lathe and more than enough "know how" to make it, and with your expertise, you can produce a spindle MUCH better than any cheap engraver on the market. If something goes wrong with one of those, your most likely just stuck with replacing it, and putting your project on hold until a new one arrives.
Here's one on the zone Henrik made. http://www.cnczone.com/forums/showthread.php?t=41200&highlight=spindle+design&page=4
Have you thought about using link belting? I've never used them, but have heard great things about them from several sources. Swede from 5 bears is where I got the idea for them. http://www.5bears.com/vfd02.htm If I had to take a guess, since there's less contact area, they probably produce less heat.

Sam
 not quite sure what you are meaning by a collet extender? Making a spindle wouldnt be a big issue for me, its the increasing of the speed that seems to be my problem although after the last few posts maybe just changing the pulley/belt type will solve that problem.

As to the link belt, I use that all the time on the fishing boats I repair, its great stuff and much better than the older style that had metal pins. With the corrosive environment, especially in the smaller craft (under 10m), the steel pins would rust and soon chew their way through the links but the new style has no steel pins so lasts well. For my application though it wouldnt be much use as the pulley diameter is too small

Hood

PreciseBits have a nice ER16 collet chuck on their front page - exactly what you want. 20 mm shaft on it, and TIR very small - 0.0002". Put good bearings top and bottom and drive with a decent motor and a speed-up belt.

Cheers

Did a wee bit of testing this morning, first thing I did was open the box up and replace the toothed belt with an 'O' ring, not ideal as it was driving on the toothed pulleys but was fine for my testing purposes. I never put the mid section back in so I could see what was going on. Spun it up and let it run for 15 mins and there was only a slight increase in the temp. That got me doing a dangerous thing, I did some thinking I was wondering if the air was helping things cool as the pulleys were basically acting as inefficient fans and I could feel the air flow if I put my hand near it. Replaced the 'O' ring with the toothed belt and again gave it a run for 15 mins and it did increase in temp but was nothing like before, I could comfortably keep the back of my hand on it.
I am now thinking I should be able to incorporate some cooling channels in the housing and put the coolant into the top and let it run through the Alu and out at the bottom and direct it via some lok-line onto the cutter, wish me luck

Hood

Round belts ('O'-ring, doughnut, etc) are very good for extreme speed, but I would be hesitant to use them where any swarf or cutting fluid could get on them. The newer tooth belts with curvilinear teeth (GT3, etc) run quiet and cool and have stupidly high power ratings for their size. i.e. you can use a belt that looks (to my old eyes, anyway) far too small to handle the power, but it clocks along for years even with exposure to contaminants. The older trapezoid teeth are a different matter and I don't recommend them for high speed.

The terms 'collet extender' 'collet adapter' and 'collet holder' seem to be used somewhat interchangeably. The item on PreciseBits appears to be 1/2" and not 20mm and also looks too short to use the shaft as an axle . . .  unless I missed something.

O1111 (off topic rant)  
Something that was impossible to miss on the PreciseBits site was the prominent blurb on the company's Christian roots and belief in equality of all people and in the US constitution . . . . . on the same page with a big red banner supporting the new Arizona immigration law which is being challenged as unconstitutional and which clearly presumes all Mexican people to be fundamentally inferior, unwelcome and having criminal intent. Arizona will undoubtedly next pass another new law bringing back the Inquisition.
M99 (End of Rant)
%  

The item on PreciseBits appears to be 1/2" and not 20mm and also looks too short to use the shaft as an axle . . .  unless I missed something.

Oops on the 1/2" bit - I was thinking of another collet chuck. Sorry!

The shank on the PreciseBits one is maybe a bit short, but I have seen others which are longer - up to 160 mm long shank. That might do?
Mind you, with good bearings, even the short one sold by PB would do for anything without huge side-loads.

Cheers