Figuring out the right drill size for screws

For machine screws, this helps you check the size of drill to use for tapping holes in metal. Although, one does often pick a drill that is a bit on the small size for a tap this way, as the proper size drill to use with a tap is usually a little bit bigger than the minor diameter of the thread, whereas with this method, one usually pics a drill that is no bigger than the minor diameter of the screw. For small tap sizes, this means backing out frequently, or risking breaking the tap.

For putting wood screws in hardwood, its also useful. Wood screws actually require surprisingly small amounts of 'bite' to hold well in hardwood. Using a drill that is just the minor diameter of the wood screw often results in twisted off wood screws.

For soft woods, it doesn't matter as much, ad one can often get away without pre-drilling at all in soft woods. Though pre-drilling does make the screws go in much easier, and really cuts down on the probability of splitting the wood.

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