The tube that I use on mine is actually a 12mm silver steel bar from Technobots:

http://www.technobotsonline.com/silv...3mm-33173.html

I then cut it to the appropriate length (~100mm) then drilled a hole in either end and tapped it to take an M5 bolt, just to help hold the whole assembly together. There are also 12mm needle roller bearings pressed into the bulkheads.

The output thread/shaft on a drill motor is 3/8"UNF as mentioned. In metric this is approximately equal to 9.52mm. So what I did was drill out the small sprocket to 9.5mm, screwed it on to the drill shaft (it caught the thread very lightly, but not great) and then fitted the reverse thread screw to compress it on. Seemed alright but after one test fire, the sprocket came loose. I've since welded the sprocket on.

If you don't have access to a welder, the next best option would be to drill the sprocket hole out to 9mm and then use a 3/8"UNF tap to thread it and screw it on to the drill motor (this is effectively the same as screwing a blue wheel on) then secure it using the reverse-thread screw. The grub screw option isn't really effective for the 8-tooth sprocket as once you've drilled it out to fit on the shaft, there's not a whole lot of material left to tap.

Hope that helps; usually easier to explain these things in reality than across the internet. More pictures of my axe mechanism can be found here if you want a better look or alternative angles: http://s31.photobucket.com/user/ayrs...?sort=6&page=1