Try using a penny instead of a screwdriver. You can still tighten it up well enough, but you can't put as much "torque" on a penny as you can on a screwdriver, so that should avoid over-tightening. If a penny isn't big enough for you to use, try using a metal disk (or any shape made of metal) that will fit into the slot. Again, you won't have that screwdriver end to really twist down on, but it certainly should tighten things well enough to prevent a needle falling out. I've been using a penny, and I've never had a needle fall off, or broken any machine parts. Just a thought!