Robertson Screw ad from 1909


A screw is a fastener that is characterized by a helical ridge (or thread) that is wrapped around a cylinder. Screws commonly used in woodworking applications also have a head, of which there are several different styles for a variety of applications. They also tend to have an unthreaded portion of the shank below the head and a tapered point on the end. Screws provide a mechanical connection between parts and provide their own clamping pressure.

Screws come in different gauge sizes which refer to the diameter of the shank, from 0 to 20. Screw gauge charts are available which list appropriate clearance hole and pilot hole sizes for each gauge of screw. A pilot hole is necessary to prevent the wood from splitting and to keep the drive in the head of the screw from stripping out. The pilot hole size is generally the diameter of the shank at the bottom of the threads. The clearance hole is the the diameter of the shank or a little larger, such that the screw drops easily through. A clearance hole is necessary to allow the screw to pull the two parts tightly together. Without a clearance hole the threads of the screw may engage both halves of the work without pulling them tightly together. This is called 'bridging'


The most commonly available screws in Canada have a robertson drive. Other common drives are phillips and slot. Different sizes of drives are used for different gauge screws, and are often colour coded. For robertson screws:


Screw gauge chart:

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