Drill bit vs. screw size

Written by kelvin o'donahue
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Drill bit vs. screw size
Wood screws require a pre-drilled pilot hole to function properly. (screw image by timur1970 from Fotolia.com)

A wood screw has a thread wrapped around a cylindrical shaft. For proper wood joinery, a hole must be drilled in the work piece so that the screw doesn't force its way into the wood.

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Screw Anatomy

A wood screw has three parts. The thread is an inclined plane wrapped around a cylindrical shaft. The head is the exposed portion of the screw that contains a slot or other shape to engage a driver. Between the head and the thread is a plain, unthreaded section called the shank. Screws are identified by a gauge number, such as #8 or #12, and their length. The higher the gauge, the greater the diameter of the screw.

Pilot Holes

A "stepped" pilot hole should be drilled for each wood screw. The pilot hole's diameter should match the diameter of the shaft in the threaded portion of the screw. The outer end of the hole should be enlarged to provide room for the unthreaded shank. A pilot hole in soft wood (pine) can be slightly smaller than a hole for the same screw in hardwood (oak, maple).

Function

The screw thread is wrapped around the shaft in a helix, much like a spiral staircase. The thread carves its way into the wood and, because it is continuous, draws the shaft deeper as it turns. When properly sized, the threaded section engages only one piece of wood, drawing the two pieces together tightly.

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