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Many novice woodworkers make the mistake of drilling screws into boards without first creating a pilot hole, through which the screw can travel with less friction and less risk of cracking the board. The correct drill bit size needs to be chosen in order to create a correctly sized pilot hole, and choosing the right size is not exceptionally difficult. If the wrong size drill bit is chosen, the woodworker runs the risk of splitting the board, or creating a hole that is too large and therefore not able to give the screw enough wood to grip into.
The right drill bit size will depend on the size of the screw, if one is being used, as well as the intended purpose of the drilling. To choose the right drill bit size for a screw, you can either obtain a drilling chart that outlines which size drill bit is most appropriate for the size of screw being used, or you can make the measurement by eye. Start by laying out a few bits that are similar in size to the diameter of the screw shaft. Hold the screw so the tip is facing you, and then hold different sized bits, one at a time, next to the screw. Try to find the drill bit that is slightly smaller than the diameter of the screw shaft, including the threads.
Choosing a bit that is too small will reduce, but not eliminate, the risk of cracking the wood when the screw is inserted. Choosing a bit that is too large will run the risk of preventing the screw from gripping into the wood. The bit should be only slightly smaller than the size of the screw for optimal screwing purposes.
The drill bit size can also vary if a countersink bit is being used. This bit features two cutting elements: the drill bit itself, and another, triangle-shaped cutting edge situated at the high end of the bit. This triangle-shaped cutting surface will cut into the face of the board once the bit has cut into the wood far enough. It will create a larger hole around the smaller shaft hole so a screw's head can be pressed until it is flush with the surface of the board. The length of the bit shaft will therefore be important as well as the diameter, and the diameter of the triangular cutting surface will need to match or come close to the size of the screw head.