I often come across screws that just won't budge with a manual screwdriver or even a driver bit in a drill. They are of different screw types and heads in a variety of different materials. What are some techniques I can use to try and remove them?
Twist in the proper direction
Are you turning the right way? Remember, "Righty tighty, Lefty loosy" (which never made sense to me). Most screws are right-handed threads, so you'll turn them anti(counter)-clockwise to remove them. In some situations left-hand thread screws are used (usually to prevent screws from loosening due to rotational forces), so you'll actually have to turn the screw clock-wise to loosen it.
Sometimes tightening the screw very, very slightly, can loosen it up enough that it can be removed.
The first thing to try, is simply to apply more inward force while turning.
If you are using a screwdriver with a hexagonal or square shaft, you can grip the shaft of the screwdriver with an adjustable spanner (wrench) or vise grips. This will allow you to exert more torque, and may put you in a better position to push the screwdriver harder into the screw head.
Sometimes screws seize up due to rust, or other debris. Soaking the area with penetrating oil, may free up the screw enough for you to remove it.
Rapping, Tapping, and Banging
If both methods above have not worked, giving the area a sharp tap with a hammer might break the screw free.
Remember, you're not trying to knock the screw into next week, you just want to loosen it up (though there are situations where you'll have to give the piece a healthy whack).
For really stuck screws, you can use a manual impact driver.
This may not always be the best tool when working with sensitive electronics or delicate equipment, but it's very useful for removing stuck screws.
If you still cannot remove the screw, as a last resort you can drill the screw out.
This is a last-ditch effort, and will destroy the screw (and possibly the threads in the hole, if not done correctly). Use this technique only as your last resort.
Remove the screw, and everything else in a 20ft. radius.
It's a good idea to film the explosion using a high-speed camera, so you can watch it over and over again in super slow-mo.
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Abrasive paste as sold in car repair shops to use when grinding in valves in cylinder heads is great for screw heads that are 'worn'. Place a small dab on the screwdriver head before trying to unscrew the item. The friction from the abrasive in the paste will help the screwdriver 'grab' the worn screw slot..
If its metal on metal you can try heating the area or cooling it with ice. Metal expands and contracts with temperature change (works great on spark plugs too!).
For screw heads starting to strip, a piece of bicycle inner tube or rubberband between the head and driver bit helps it from stripping further.
For screws that held the original hinge in an old wood door, and would not budge, even with the advice above, I aimed a hair dryer at the screw for a few minutes (thinking that perhaps the wood was damp and that was holding the screw in; could also have been something about heat since I often run hot water over stuck screw tops). Anyway, it worked, with only 2-3 minutes per screw of heat.
As an absolute last resort, get a hole saw just larger than the screw head and bore down over the screw, then lever out the screw with a chisel or screw driver. Once out, glue in a plug and bore for another screw.