One of the lessons I have learned the hard way is don't rely on self tapping screws in wood. Over time they will fail. I now use bolts with large washers on the back side of the wood. If there is enough of the bolt protruding thru the nut, I hammer it to distort the threads so that it can't ever come off. Just be sure that your setup is exactly the way you want it prior to doing this because once you mess up the threads, you're pretty much married to that setup.