The way the drill motor clutches work is that when you turn that cone shape black plastic "thing" from 1 toward 15 until drill, (can't think of a better word right now... you know, the thing with the numbers around it and the drill bit symbol), it screw inward toward the motor direction.
When that happen, it compress a large spring that's sitting on a series of little metal balls. The harder you compress the spring, the harder it push onto the steel balls, and the harder it engage the ring gear (with little bumps) under it.
So, at the "1" setting, spring is barely pressing onto the ring gear, and that ring gear will be slipping if the drill output shaft is pushing against light resistence. The higher the number goes, the harder the spring compress onto the ring gear, and the more resistence drill output shaft can push against without slipping the ring gear.
That is why at 15 or "Drill", the drill will keep drilling until you stall the motor, or you lose your grip on the drill handle.
So, to completely lock the ring gear in place, there's two ways:
A length tube that replace the spring so the steel balls will constantly engage the ring gear without room to back out,
Or tap the holes (M6) for the steel balls and screw in M6 set screws to constantly engage the ring gear in place.
This way, you won't have to worry about drill slipping out of clutch anymore.
Sorry for the poor description. You should take a look at Dr. Joe's drill advice in the white paper section @
http://www.chiefdelphi.com/forums/papers.php?s= (on page 3)
There are a few pictures in that word file that will help you visualize the set screw method.