Why A Screw Loosens Over Time

I'm having a problem with screws backing out of the slats on my cedar swings and garden benches. Even when I counterbore and cover the screw with a dowel plug and yellow glue, it still loosens and eventually pops out the plug. I can't figure out what I'm doing wrong, and it's driving mc crazy. Any advice?

Pete Hoffman Columbus, OH

O Screws aren't the best choice for seating furniture, or for outdoor furniture. You're much better off with wood-to-wood joints such as dowels or mortises and tenons. Why? It's a wood movement problem.

There arc two wood movement factors at work on your outdoor swings and benches. First of all, seating takes more abuse than almost any other form of furniture. When you sit, the wood flexes and moves under your weight, tugging on the joints and encouraging the screw threads to crush the surrounding wood fibers. The more use the seating gets, the more the screw threads crush the wood, leading to a looser and looser joint. (See drawing.)

Second, outdoor furniture is especially susceptible to wood movement from humidity changes. The wood expands and contracts, but the metal screw does not. As the wood expands, its fibers swell around the screw, with the same crushing effect. When the wood contracts, you're left with gaps again. Retightening the screw only makes the problem worse, since it crushes even more fibers.

Sand at an Angle to the Grain

When I sand coarse-grain wood such as walnut or mahogany, I have trouble telling whether the grit I'm using has erased the scratches made by the previous, coarser-grit sandpaper. I always sand with the grain. I probably sand way more than I need to, because I'm never sure when it's enough. How can I be sure when to stop sanding with each grit? I usually start with 100- or 120-grit.

Bcttc Dusavcl Milwaukee, WI

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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