The ordinary bugle head screw will pull down flush in softwoods, but it will remain slightly above the surface in hardwood. Some bugle-head
screws have sharp nibs below their heads to cut away the wood below the
head as you drive the screw. This type will pull flush even in hardwood.
Roundhead screws have a flat bottom, so they don’t have to be countersunk. The
rounded top extends above the wood surface. Roundhead screws are usually used
for attaching hardware. Panhead screws are similar to roundhead screws, but the
head is flattened on top so it doesn’t extend as much above the surface.
Screws are commonly available with recesses in their heads to fit the following three types of screwdriver:
straight blade, Phillips, and square (Illus. 10-8). Screws that take a straight-blade screwdriver are often called slotted-head screws. This is the oldest type of screw. Slotted-head screws work fine when you are driving screws by hand, but they are hard to use with a power screwdriver.
The Phillips-head screw has a cross-shaped recess in its head. This type is much easier to drive with a power screwdriver because the bit doesn’t jump off the screw head as easily. Phillips-head screws have become increasingly popular as more and more people use power screwdrivers.
The square-head screw has a square recess in its head. Although it is not as
widely available as the Phillips-head screw, it is increasingly popular among
wood- workers because it offers an even better grip for the screwdriver bit.
There are several other types of screwdriver designs, but at present they are
mostly used in industrial applications.
The traditional wood screw thread pattern is an old design that was based mostly
on the limitations of thread-cutting machinery of a century ago. It has two disadvantages.
First, it requires a two-step pilot hole, one size for the threaded portion and
a slightly larger size hole for the shank. Second, the threads are fairly shallow,
so the screw’s holding power is limited.