Drill Bit SizeRecommendations
Drill bit selection, for square drive screws and other fasteners, should take into consideration the material properties. Harder materials, or “soft” screws, require larger pilot holes. This table is a starting point – a quick technique is to hold a drill bit up to the shank of the screw. If you’re breaking screws, try enlarging the pilot hole – even 1/64” will make a difference. Nominally, the proper bit is the same size as the shank between the threads. In practice, use a slightly larger bit for hardwoods, a slightly smaller one for softwoods. The root diameter of a Square Drive screw is smaller than that of a Standard “Wood Screw”; pilot recommendations are also smaller.
What Length Do I Use?
Unfortunately, there is really no hard and fast rule concerning the appropriate screw length. Sometimes you don’t have much choice as to length because of the joint configuration. In general, though, the screw should always go through the thinner piece first, and thread into the thicker piece.
In choosing length, try to have about 2/3 of the screw threaded into the “secondary” piece, as shown. As the thickness of the lumber or fastener length increases, though, this ratio can be reduced so that only 1/2 of the shank is threaded into the secondary piece. Not coincidentally, only 2/3 of the shank is threaded on most of our 1” and longer screws.
Having said that, our new ProMax® screws are designed with the perfect thread configuration for common joint configurations. In general, ProMax® screws have a longer than standard unthreaded shank length to ensure that the screw doesn’t crossthread at the joint.
For more information on this new development, please see the ProMax® screw section of the Web site.
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