Stripped Threads
Anyone who has ever picked up a screwdriver has probably stripped a few threads and probably knows a few of the standard tricks for rectifying the situation.
Some of the most popular: Fill the hole with plastic wood. Fill the hole with epoxy and steel wool. Stuff some glue and a few toothpicks into the hole. Drill the hole out and glue in a dowel. Use a larger screw.
All these tricks work to some extent. For low-stress situations, they will probably get you by. But for more serious repairs, or for prevention, you need something more sophisticated.
One of the most common ways you can strip threads is by overtightening screws driven into end grain. You don’t have to overtighten very much in this case to do the damage. Why? Wood is made up of countless parallel fibers, something like a bundle of soda straws. If you drive a screw down into the end of this bundle, the threads tend to cut the straws into lots of short, stubby pieces that are weak and easily crumble apart.
One way to prevent this problem is to set a dowel into the wood across the grain. Then drive your screw though this dowel. Since the dowel is hardwood, and its grain runs across the direction of the screw, it is much more resistant to stripping than the wood around it.
For even greater strength, you can buy special steel dowels made expressly for this purpose. They are drilled and tapped to accept a 1/4-inch bolt and are typically 1/2-inch long and 13/32 inch in diameter.
To use one, drill a pilot hole for it, slip it into the hole, use a screwdriver to turn it until its tapped hole is aligned properly and drive your bolt into it. Screws driven across the grain typically hold better than those driven into end grain, but even these need reinforcement in certain cases. Knockdown joints (those that must be assembled and disassembled many times) are one example.
For these, threaded metal inserts are a good solution. These come in a variety of sizes, but inside each are threads that will accept common bolts or machine screws. The outside of each insert has deep threads designed to cut deep into the wood and hold securely even under heavy stress.
To use one of these inserts, drill a pilot hole in the wood, then drive the insert in until it is flush or slightly below flush. Three tips:
Inserts often come with directions for the proper size pilot hole, but in my experience, the size called for is not always correct. The ideal is to drill the smallest hole you can drive the insert into, but this size has to be larger in hardwoods than it does in softwoods. I always try a test hole in scrap wood of the same type as my work before I tackle the real thing.
Inserts normally come with slots in the top so you can drive them with a screwdriver. But inserts tend to be brittle and easy to break in the driving process. So instead of a screwdriver, I like to use a bolt of the size that fits the insert. Thread a nut up onto the bolt, thread the bolt into the insert, then tighten the nut down against the insert. Now you can drive the insert home by turning the bolt with a wrench, and there’s no danger of breaking the insert. A little paste wax on the threads makes things even easier.
Inserts tend to chip the wood around the pilot hole when you start them. To prevent this, countersink the pilot hole slightly before you set the insert.
Another device similar to a threaded insert is the T-nut. It’s easier to use, but not as slick. Like the insert, it is threaded inside to accept a bolt. Unlike an insert, it doesn’t screw in place. Instead, you drill a pilot hole for it, then hammer it into the hole from the rear of your work. Four spikes in the flange around the base of the T-nut bite into your stock and prevent the T-nut from slipping.
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Tags: joints, screw, screwdriver, stripped threads