I am thinking it may be worth a shot, have a couple of angular contacts and would be simple to make up a housing and shaft. If the box screws up its no big deal as I got 2 of them for £12
So what do you think?

Hard to say without knowing the ratio, but if I am thinking correctly, the original input shaft should be the center pinion and the output would be the planets, so 'arse for tit' the high speed would be the center pinion driven by the planets, which is a good arrangement for a speeder. The gear set will *probably* hold up if the planets are on (or can be put on) needle rollers. I'd say go for it. You are likely to have vibration issues, it will just depend on how bad they are and what kind of effect they have on your finish.

[50,000 RPM is] quite a bit faster than I need, tool I have is rated to 20,000rpm in stainless I think so anywhere between 15 and 20K would suit me.

The point I was making here is that the speed change ratio is minimized if you start with a source closer to the target speed.

Interesting as in funny watching me screw up?

We all get a 'turn in the barrel' so to speak. I specialize in frying electronic components. I get paid to know about things mechanical, so it is more embarrassing when I break the hard stuff.

 Been a busy day but I did get a chance to look at the gearbox, its a 3:1. I put the output shaft in a collet holder and spun up the spindle, first a few hundred RPM to make sure the body was easy to hold still then 1000 then 3000. After about 3 mins the body was too hot to hold so looks like its back to the drawing board.
Best option is obviously a motor that can spin at that speed whether its a separate motor or one of the various spindle that have been mention in this thread.
Think the project will go on the back burner for the time being and will just keep my eyes open for a deal

Hood

I am sure you'll find a good deal - it often looks like they are looking for you Seems you trashed the O-ring idea though...

Daniel

I am just a bit scared that the "O" ring would not have enough power, it would be fine for something like a grinder but for the cutters I use the slightest slippage would snap the carbide insert and at £22 each I wouldnt want that.
Hood

I have acquired a spindle to build my high speed head. It has a 150mm long 12mm OD shaft, .0005" runout and ER11 chuck. The whole shaft has a 5mm thru hole.

This will ride on sealed hybrid ceramic bearings rated for 100,000 RPM.  I'm thinking I will make the casing from a piece of steel DOM tubing so that there will not be expansion issues. I may try an alternate design with an aluminum casing incorporating a spring washer on the upper bearing. 

I have a dynamic balance now, but if it is not fast enough to deal with this spindle I have located a local motor shop that has a professional commercial dynamic balancer and will do a high speed armature or spindle balance for $150. With ER11 and a hollow spindle, I am not anticipating a problem . . unless the thru hole is off center . .  in which case the balance will be hopelessly bad and I will take advantage of the return privilege most likely.

Drive will probably be a flat belt for 50,000 RPM 1/8" bits and tooth belt for 25,000 RPM 1/4" bits.

Anybody else making progress?

No not done anything and probably wont for a while.
Hood