Successful Screwing

Everything you always wanted to know about screws, but were afraid to ask.
By Robert W. Lang
Pages: 82-85

Many of the tools and fasteners we use in woodworking have been around for thousands of years. The concept of the screw goes back to the ancient Greeks. Archimedes took a wedge, a simple but powerful device, and wrapped it around a cylinder. The helical threads allowed the leverage derived from the wedge to be delivered via a circular motion.

Until the industrial age, screws were expensive, hand-made items. Certain applications justified their use, but in most cases other methods made more sense. In the 18th and 19th centuries, machinery was developed that made the price of screws reasonable; in the 20th century, better methods of driving screws were developed.

Most woodworkers have a love/hate relationship with screws and screwdrivers. They work well, but it seems like cheating. Screws exert a lot of force, but that force is concentrated in two rather delicate areas – the tiny bit of metal where the thread extends from the shank, and the interface between the driver and the head.

When things go wrong, these weak links fail, which is usually the result of trying to force a screw to do something it wasn’t designed to do, using the wrong tool, or using the wrong technique.

The Fuller countersink is driven by the drill bit. When the countersink meets resistance, it tends to slip on the drill bit’s shaft.

The Fuller countersink is driven by the drill bit. When the countersink meets resistance, it tends to slip on the drill bit’s shaft.

A Clamp With a Twist
I like to think of screws as clamps. As the threads bite into one piece of wood, the head pulls the other piece tightly to it. Take a look at the cutaway picture at the bottom left of the next page, which shows two pieces that have been properly screwed together. The threads are gripping in only the lower piece. In that piece, the hole is the size of the unthreaded portion of the screw, which allows the threads to tightly grip the surrounding wood. The hole in the upper piece is slightly larger than the shank of the screw, and the head sits in a countersink.

In the two center pictures below, the hole on the right was made by force-feeding the screw without first drilling a pilot hole. Instead of neatly cut threads, the wood has been torn and crushed. This damage continues beyond the screw, and the surrounding wood is starting to split. The trick is to get the holes the right size so that the threads hold securely without the shank damaging the surrounding wood.

There are a couple of other bad things that can happen as a result of not drilling a pilot hole, or drilling a pilot hole that’s too small. If the threads engage in the upper piece of wood, it can prevent the two pieces from pulling together, sometimes called “bridging.” When attempting to force the pieces together by applying more pressure on the driver, the threads can be stripped, or in harder woods the screw head can be damaged or the screw can snap.

Three Bits in One
The pilot hole, the clearance hole and the countersink can be drilled in one step with a special bit, as seen at upper left. The Fuller countersink has been the standard for years.The big advantage is the tapered bit, which ensures that the clearance hole is big enough and that the threads grip all the way to the end of the screw.

The Amana countersink extends into the drill’s chuck. It is less likely to slip, and has a long-lasting carbide tip but doesn’t come with a tapered bit.

The Amana countersink extends into the drill’s chuck. It is less likely to slip, and has a long-lasting carbide tip but doesn’t come with a tapered bit.

The biggest problem with the Fuller countersink is the attachment of the countersink cutter to the shaft of the bit. The small Allen head setscrews don’t hold well on the round bit. The countersink can slip on the bit when it meets resistance on the surface of the wood.

A newer style from Amana (far right) has larger set screws, and the shaft that fits in the chuck of the drill is an integral part of the countersink. In addition to being less likely to slip, the Amana countersink has a carbide tip that lasts longer, especially when drilling plywood or particle board.

Choose the right diameter drill bit by holding the bit behind the screw. You should be able to observe that the screw threads are wider than the bit, and the bit is about the size of the shank.

I set the depth of the countersink by holding the bit beside the screw, and setting the end of the tapered  bit just short of the point of the screw. If you’re using a straight bit, set the end of the bit to where the taper begins on the screw. Unfortunately, this may cause splitting in hardwoods. I prefer the tapered bit for solid wood, and the carbide countersink for man-made materials.

How Long a Screw
Screw diameters are specified in gauge sizes, with the higher gauge number indicating a larger diameter. For most woodworking applications, #6 is the smallest useful gauge and #12 is the largest. The best general-purpose size is probably #8 gauge. For attaching hardware to wood, smaller #4 or #5 screws are often used.

The right screw length depends on the thickness of the pieces being joined, and the orientation of the parts of the joint. Ideally, the screw should be 2 to 3 times the thickness of the piece being attached. For example, a 1⁄4″-thick drawer bottom or cabinet back should be held in place with a 3⁄4″-long screw. For thicker pieces, like 3⁄4″-cabinet parts, a 13⁄4″ long screw is sufficient.

Longer screws introduce problems of drilling the pilot hole deep enough, and of keeping the hole straight so the screw doesn’t come out the far side of the wood.