Anatomy of a Screw
For Preparing Hardwoods
When drilling the clearance and pilot holes and the countersink in hardwoods, three bits are required.
Drive Designs and Drivers
The three basic screw-drive designs are shown above. The slotted screw was first on the scene. Once screws became standardized, slots did, too, and screwdrivers followed suit. Next came the square drive, developed by Peter Robertson in 1907. The corresponding driver was crafted for an exact fit — within 1,000th of an inch. In 1937, Henry Phillips was granted a patent for his cruciform recess design, which also has a firm fit.
In softwoods, a tapered-bit-and-countersink combo (above) can be used.
Cam out can strip the screw head, making the screw difficult to remove without a special extractor bit.
Devices such as Rockler’s Insty- Drive System let you move fast without re-chucking.

It’s rare anymore to come across what once was the standard wood screw. We now are confronted with a variety of head types, shank types and thread types. But, regardless of the differences in detail, they all work in the same way.

A screw is a wedge whose thread gradient is wrapped around a cylinder. It’s used to join either two pieces of wood or a piece of wood and a piece of metal, such as the leaf of a hinge. The piece closest to the head of the screw is called the captive piece, and the piece in which the threaded end enters is the anchor piece. As the head of the screw seats on the captive piece, it is resisted by the top edges of the helical thread embedded in the anchor piece, and the pieces are pulled together tightly.

No matter what type of screw you are using or what power source — muscle or tool — you are using to drive it, the aim is to seat the screw without causing damage to the slot or recess of
the screw. This damage is called cam out, and the key to preventing it is to exert enough pressure to keep the driver engaged and to control the speed of driving so that you stop when the screw is seated.

Another key to avoiding cam out is proper preparation of the material to be joined by the screw.

This involves cutting a pilot hole in the anchor piece and a clearance hole and a countersink in the captive piece. For best results, the clearance hole in the captive piece should be large enough for the screw to freely pass through, and the pilot hole in the anchor piece should be the diameter of the core of the screw, not including the thread.

For hardwoods — such as maple, oak, and cherry — you need three bits to make the countersink, the clearance hole and the pilot hole. If you are going to drive a brass screw into hardwood, it’s good practice to drive a steel screw into the hole first to avoid breaking the softer brass fastener.

For softer woods, such as poplar or pine, the three bits may be substituted by a single tapered bit with attached countersink. Adjust the length of the tapered bit using the stop collar. Also available are systems that combine multiple drives, taper countersinks or even self-centering bits.