Q. When we nail near the end of a piece, or nail or screw in the side edge, we often see a small split created by the fastener. Why is this and what can we do? With MDF, the fastener in an edge seems to create a bump in the surface of the board and also create a separation in the wood around the fastener.
A. When you put a nail or screw into the wood, the wood fibers or cells need to compress (or move aside) in order to make room for the fastener. However, wood is a pretty tough material. With woods that are light weight and therefore soft, the wood fibers (which are like miniature soda straws with a big air space in them) can squish fairly easily, especially when higher than 7 percent MC.
However, with dense woods including oak, beech, hard maple, ipe and hickory, the walls of the fibers are much thicker and the air space is much smaller, so compressibility is more difficult. The end result with dense woods is a split radiating out from the fastener; that is, the weak point is to develop a split to make room for the fastener rather than compress the cells.
Stated another way, the force that a nail or screw creates as it enters the wood is so large in dense woods that it may exceed the splitting strength of the wood; wood splitting strength is actually quite low, which is why it is fairly easy to split firewood unless the grain is interlocked. In fact, the truth is that a sharp chisel point on nails and the sharp point of the screw actually enhance the starting of the split.
What to do
Nails. With nails, if we dull the sharp point so that nail actually tends to tear the fibers rather than split them, we can greatly reduce splitting without reducing fastener strength at all, especially when compared to a nail that split the wood.
The best way to deal with the risk of splitting is to predrill the hole for the nail; the hole is called a lead hole. For nails, the suggested lead hole diameter is 90 percent of the nail shank diameter. In fact, a lead hole provides slightly more resistance to withdrawal that a nail driven without a hole.
Screws. For screws in low to medium density solid wood, the lead hole should be less than about 90 percent of the root or shank diameter; the shank or root diameter does not include the threads. In essence, when the hole is almost as big as the shank, it is the threads that are cutting into the wood and “grabbing on.”
For dense woods, the lead hole can be as large as the root diameter, and the withdrawal strength will still be excellent.
For screws in particleboard, lead holes are smaller than with solid wood: 50 to 90 percent of the root; lower density boards need smaller lead holes. I was unable to find guidelines for MDF, probably because special fastening jigs or systems work so much better.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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