|Ask Norm: Knobs Won't Stay Tight|
David: Norm Abram replies: The machine screws might be too short to get a good grip on the knobs. If so, you'll need to buy longer screws. Be sure to take a knob to the store because the threads have to match exactly. If you can't find screws of the right length, buy long bolts and cut them with a hacksaw. Always put a nut on the screw before you cut; it will realign any damaged threads when you twist it off. Or save yourself that tedium and use truss-head breakaway screws. The heads are large and flat, so they won't pull through the wood and loosen, as smaller-head screws sometimes do. Their shanks are scored in ?-inch increments so that you can snap off what you don't need without damaging the threads. I've found that the bolts won't always snap where you want them to, so I snip them at the score using lineman's pliers.
If the knobs still loosen, put a drop of thread-locking liquid on the screw threads before you install the knobs. Mechanics use this to prevent bolts from vibrating loose. It's like a weak glue, so it holds the parts together but not so tightly that they can't be loosened later if you need to remove them.