Countersinking refers to boring a large diameter, partial-depth hole at an existing hole location. The starter hole is sized for the body of a standard fastener (bolt or lag screw), typically 1/16" larger than the fastener diameter. The countersink diameter is large enough to fit a washer for that size bolt. For example, a ¾"ø bolt location will first be drilled 13/16"ø, then partially countersunk at 2¼"ø to fit a standard 2"ø washer.
Bolts are countersunk for two common reasons. First is if the bolt axis is at an angle to the face of wood. Rather than use a wedge shaped washer, it is almost always simpler to counterbore the hole to create a flat surface for the bolt head and washer to bear. Second is to countersink bolt heads and insert wood plugs to create connections without exposed steel, usually involving kerf plates. Bolts are also countersunk just to create a flush finished surface, without a bolt sticking out, for safety in the finished structure or to simplify installation of other materials.
*Wednesday's Word Of The Week is a feature on Unalam's Wood Times Blog. Each Wednesday our structural engineer, Rik Vandermeulen, will discuss a new term associated with glulam manufacturing. He will do this until we run out of words. If there is a timber or glulam term that you have heard of and want to know more about, let us know in the comments.
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