dundonald wrote:

I've just secured a bookcase to the wall with two screws. The screw on the
left just keeps on turning when flush to the wood whereas the screw on the
right has become tight. I've often wondered:

1. why does a screw sometimes keep on turning?

Because the thread is stripped.


2. if a screw does keep on turning, what is the impact on the security of
the fixing?

Yes, the fixing won't be able to take as much load before failing.

If the screw is into a wall plug, then you need to pack the wallplug,
say with a smaller wallplug, or with a match, to ive the screw something
to grip.

--
Grunff