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I often find myself guessing based on what I have on hand. Are there some objective guidelines for how to select the proper size? I'm usually thinking about three attributes:

  • Length

    Length is probably the easier dimension to eyeball, but I imagine there is some logic there I could be applying to be more consistent.

  • Diameter

    Diameter is more difficult. Generally the gripping strength is best achieved by increasing the depth of the hole (+ fastener length), meaning a long skinny screw will hold tighter than a fatty short one (provided we don't go to the extremes). My best guess is to use thicker screws when shear resilience is needed, where the weight of the workpiece or the way in which it is to be used will result in lateral force against the screws. But again, I always make my best guess.

  • Thread Pitch/Depth

    This one is easy at a high level. Harder woods need a narrower thread pitch, softer woods need a wider thread pitch (and deeper threads). Usually I only have two choices (sometimes fewer), but perhaps every type of wood has its ideal thread profile.

But really this all boils down to guestimation, which I already do. What about more objective metrics I could put on a cheat sheet somewhere in my shop (or better yet, something I could remember)?

8

One major issue you should take into account is the weight being supported by the fastner. Most manufacturers will publish weight/load tables for their fastners, such as Quik Drive's table. That gives the various strengths of the fastner. You should then calculate how much strength you need - for example, you have a deck that is going to be big enough for 10 people to stand on, plus maybe 100 pounds of lumber; make sure your fastners combined (number plus individual strength) support 2500 pounds or more. (For decks, I imagine local code would tell you how much to assume, but for other uses like chairs or tables, you can typically figure it out.)

If your load strength suggests a longer or thicker fastner than your project allows for, you may need more fastners and/or a higher quality fastner that will hold up better to the stresses; or consider a different design.

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