Screws May Seem To Look Alike, But There Are Lots Of Differences

May 31, 1997|By Nick Harder, The Orange County Register.

Probably the most confusing thing about screws is they don't all look the same.

Not all have points on the bottom.

They all seem to have different heads.

A few don't have any heads at all.

The one constant of screws is their advantage over nails. That far outweighs the disadvantages.

- Screws have much more holding or fastening power than most nails. They are able to hold hardwoods together as well as composite materials such as particle board.

- Screws can be installed more easily, with more accuracy and often with less damage than nails. A screwdriver or power driver can drive a screw. Use a hammer and nails and there's more chance for a misdirection and damage to the material. Here is a guide to choosing the screw most appropriate for your home project.

- Classification:Screws are classified in several ways. Among the criteria are length and thickness. Most screws available in hardware stores and home centers are from 1/2 inch to 4 inches long. Longer screws can be ordered.

Most screws are classified by a gauge number. This refers to the thickness -- the diameter -- of the screw. Screw gauges run from 2 to 24. Those used most often have diameters from 5 to 14. The lower the gauge number the smaller the diameter.

When you look at screws, especially those in packages, you will notice two numbers. One number refers to the length of the screw, the other to its thickness. For instance, if the numbers are 1 and 10, the screws are 1-inch long and have a moderate thickness.

- Shape: While most screws have a point at the bottom and a head at the top, not all do. A few look like bolts because they do not have a point. These types of screws are different from bolts. They're meant to be inserted where no nut is required to fasten.

Then there's something that's called both a screw and a bolt. It's the lag screw (or bolt). It's pointed and has threads and a shank. It also has an octagonal head that must be turned by a wrench or socket bit on a power driver. Generally, lag screws (or lag bolts) are used to fasten heavy or thick materials. They're available in much larger sizes than the rest of the screws.

Then there's the dowel screw. It has points on both ends and threads all the way through. It's screwed into a main surface with one of the sharp ends and a dowel worked on the other sharp end.

Last but not least is the thumb screw. More like a bolt because it does not have a point, its head is vertically flat. It is used to prevent something from slipping, such as a wall-mounted flagpole standard in which a thumb screw often is used to tighten the pole lodged in the standard.

- Head configurations: A variety of head configurations -- the shape of the heads -- also can be confusing, but the differences are absolutely necessary for the wide variety of tasks the different heads must perform. Among the configurations are pan head, round head, bugle head, flat head, square head and oval head. Some of these styles are self-descriptive; some can easily be confused with one another.

1. Flat. Shaped as its name denotes. Most-used of all the screws. Head can be installed flush with a surface or buried slightly using countersink (a drill bit the diameter of the screwhead which burrows into and near the surface).

2. Bugle. Flat head tapered to the thickness of the threaded shaft. Found on deck and drywall screws. Bugle head screws are usually self-tapping (they cut their own holes as they go).

3. Round. Shaped as its name implies. Used in situations where the head does not have to be buried beneath or set even with the surface.

4. Oval. A little bit of the round head screws. A small portion beneath the head (but larger than the shank) is buried beneath the surface. It provides excellent holding power. It appears as a round head on top of the surface but has a deeper slot than a flat head for increased turning power.

5. Pan. Very much like a round head but threads all the way. No shank.

6. Wafer. Extra-wide flat head used mostly in drywall. Like having a built-in washer.

7. Square. This is a bit of a misnomer. Actually "square heads" are six-sided.

- Head styles: This refers to the hole style in the head. There are several types, but two are used more than any other in the United States: slot and Phillips.

1. Slot. Flat-blade screwdriver fits this head. Uniform depth of its groove.

2. Phillips. Two grooves that cross at right angles, with their depth greatest at center. Considered easier to drive than slot.

If you're not confused enough already, consider this. Screws also come in a variety of metals. Among them are steel, brass, stainless steel and aluminum. Steel screws are generally the least expensive and withstand even tough driving. They're coated with a variety of materials, zinc being the most common.

Brass screws are a bit softer and often used with decorative materials and almost always inside.

Stainless steel screws are meant to withstand weather. They're frequently the most expensive, but they never rust or corrode.