Screws in End Grain Wood
It is well known that screws do not hold well in end-
There are two things you can do to maximize the holding power: drill a pilot hole and don’t over tighten. Sounds simple enough, but many woodworkers violate one or both of these principles routinely without giving it a thought. This article explains the reasoning behind these two items.
Wood tends to fracture along the grain; it splits easily. The wood fibers are very
strong along their length and are able to support impressive loads either in tension
or compression, but the side-
Damaged wood is weak wood.
When you drive a screw into end-
Longitudinal compression occurs because the thickness of the screw thread takes up
part of the length originally spanned by the wood fibers. This compression and the
resulting distortion weakens the side-
Another effect is that the thread of the screw cuts the fibers at regular intervals along their length. Within the volume of the screw thread, the fibers will have been cut into short sections and also compressed. Both of these weaken the wood.
When a pilot hole is provided for the screw, the transverse compression is significantly reduced and the wood just outside the volume of the thread suffers much less damage. The fibers engaged by the screw thread are still cut, but with less compression the sideways bonding between adjacent fibers remains relatively strong.
This, I believe, is why using a pilot hole increases the holding power of a screw
How large should the pilot hole be?
We know it should be at least as big as the root diameter of the screw thread, and perhaps larger, but how much larger? There is probably no exact answer, but we can get an idea by considering the wood that extends more deeply into the region between adjacent screw threads.
It is cross-
I don’t really think it does. I’m suggesting that the pilot hole can be significantly larger than the root diameter of the screw without adversely affecting the holding power. Here’s an example from the real world.
A popular bottle stopper mandrel has a 3/8” x 16 tpi thread. The suggested diameter of the pilot hole is 11/32”. This is only 1/32” smaller than the diameter of the thread, which implies that the depth of penetration of the thread into the wood is only 1/64”. And it works. Granted, bottle stopper blanks are not very large and usually consist of a dry, hard wood, but you get the idea.
Ask, and most people will tell you that the primary purpose of drilling a pilot hole is to make it easier to drive the screw into the wood. This is especially true for side grain where the holding power is not an issue. For end grain, ease of driving the screw translates directly into doing less damage to the wood, and wood that is less damaged will be stronger.
The Best Thread Profile
At this point you can see that a thread with a triangular profile is not the best for holding in wood. Far better is one that is tall and skinny because such a profile produces much less longitudinal compression of the wood fibers. If you examine the threads on decking screws, for example, you will see that the threads are nowhere near triangular.
The photo at right shows the threads of a hex head lag screw, a Woodworm screw insert, and two decking screws.
How tightly should the screw be “tightened down?”
The short answer is, not tight at all. The force applied to the screw in “tightening it down” subtracts from the additional force the screw can withstand before it fails. When driving a screw, it is almost instinctive to give it that last good twist. But doing this in end grain is not good; it actually reduces the ability of the screw to withstand additional load.
To see why, suppose we have a screw driven into end grain that will pull out when the total force on it is 100 lbs. If we tighten the screw until it exerts a force of 40 pounds against the head, the screw can withstand another 60 lbs of additional load before it gives way. If we tighten to 70 lbs, we only have another 30 lbs to go before the screw will pull out. Tighten to 90 lbs and it will pull out when only another 10 lbs are applied.
It is particularly easy to over tighten a screw when using a power driver. My suggestion
is to use the power driver to run the screw in “almost there,” but then to finish
seating it with a hand-
Pilot Holes in Side-
A pilot hole almost always increases the holding power of a screw in end grain, but such is not the case for side grain. In my comparison tests of screws in soft wood (pine, cedar, poplar, etc.), the pilot hole weakened the connection in almost every case.
Why is the holding power reduced by a pilot hole in side grain? In soft wood the fibers can be deflected sideways significantly without breaking. As the screw is driven in, the fibers are displaced to the side to make room for the screw, but a large percentage of the displaced fibers are not broken and will retain their tensile strength. On the other hand, if a pilot hole is drilled, many fibers that otherwise would not break are cut by the drill bit.
The best analogy I can think of is that of driving a screw into the side of a spool of thread. The thread will be displaced, without breaking, to make room for the screw.
In a hard wood like oak, maple, ash, walnut, etc., the question of whether to drill a pilot hole becomes a moot point. Without one, it is almost impossible to drive the screw into the wood. The possibility of breaking a screw makes it unwise not to drill a pilot hole.
The Punch Line (again)
A screw will hold best in end grain wood if (1) a pilot hole is drilled for the screw and (2) the screw is not over tightened when it is driven into the wood.
For screws in soft side-