Can I pick your brains? How would you solve this puzzle....

[m_c]

A rotary union or swivel joint.
There a couple cross-sectional views on this page - http://www.filtonltd.co.uk/products_swivel.php


Basic ones only use two o-rings with glide rings, while more complex/higher duty ones will use ball bearings with lip seals. It all depends on speed/pressure/type of fluid.

[blackburn mark]

im assuming you will be reaching through the spindle bore to get at the opening in the end of the tool ??? might be worth knocking something up for the time being, brass tube with a lose (ish) fit inside the tool coolant bore
if you have a pretty decent compresser it'll keep up with a leaky joint at least till you have proven the concept and start an all out production