To tighten a screw, you must have a screwdriver that fits the head of the screw perfectly. If you don't, you can strip the head. This leaves the screwdriver with nothing to grip, so you can't remove the screw. If this happens, you must use a screw extractor drill bit to remove the screw.
Things You'll Need
Screw extractor drill bit
Drill a small hole in the head of the screw you want to remove. Do not drill any further than the head, as you might damage the screw. Make sure the drill bitis no more than half the diameter of the screw head; you need to leave enough head for the screw extractor to grip.
Replace the drill bit in your variable speed drill with a screw extractor drill bit. Use one that is more than half, but no more than two-thirds of the diameter of the screw head. You don't want to risk damaging the screw-hole.
Position your drill so the screw extractor is straight in the screw head and at the same angle as the screw. If you try to drill the screw out at an angle, you could damage both the screw and screw-hole.
Make sure your drill is set to reverse mode and slowly pull the trigger. Your screw extractor drill bit should grip the edges of the screw slots and turn it in reverse, pulling it out of the hole. You can increase the speed of the drill after the screw extractor has gripped the screw.