I just screwed on three of four machine screws. The first three went in relatively easy, the last screw went in just a bit and stopped turning. I aligned the screw with the hole to make sure it was going in straight, but it simply will not go in. I don't want to apply too much force because I may screw up the screw and/or thread then it will really get complicated. Are there any ideas on what I can do? Thanks.
If the threads in the hole are slightly mangled, this will happen. You can "chase" the threads using a tap of the correct size, or there are taps called "chases" that cannot tap a new hole, but will clean up mangled threads on an existing hole. If the threads on the screw are OK, you need to chase the threads in the hole, that will fix the problem.