Installed a new fan on my K40 and it really cleans out the laser. Quiet and came with a speed controller.
is that this one - http://www.amazon.com/VenTech-Inline-Exhaust-Variable-Controller/dp/B005GJ7TFE
I am looking to do the same thing. I am going to try to use a single unit for 2 machines and thinking of going with the 6" one. Are you running carbon?