I was looking to aluminium.

Could you direct me to a site explaining how to calculate the cutting power, I searched on Google but didn't find anything of use.

Thanks for the link to that spindle, but I imagen it would be way out of my price range. I am looking for a very cheap temporary spindle for my home-made CNC mill. I need to mill PCBs which requires the high speed, however I also want to mill aluminium. I would be happy with a 3mm endmill for the aluminium, but for cost reasons I want to run them both from the same spindle.

The air spindle I linked to costs around £20 posted so obviously the price is ideal for me - I was just hoping that I could get away with using it for the aluminium too. Maybe after I look at the cutting power formula it may be acceptable ? I will find out how much power the spindle provides.

Thanks.