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Wood & Machine Screw Information & Specifications

Screw Configurations -- A Lesson in Screwology

The general size of a screw is given a number. As the number increases, so does the size of the entire screw, both head size and shaft size--but not length. Therefore, a #8 screw is about twice the size of a #4 screw, but may be the same length. Wake Up!-- This is important.

Most wood screws have a common "pitch" to the threads, but some have a thread with a steeper incline. We simply call this a "fast" thread, but they are technically Type A screws.Think of this as a road going up a mountain; the steeper the road, the sooner you get to the top. Most of the screws for mounting hinges are self-tapping (they tap their own mating threads in wood) type AB (they have more threads per inch and are more effective in brittle materials like wood than Type A). For more info on this, see screw types section.

Also in reference to threads you will see "8-32". This is the common knob and pull screw thread. The "8" refers to the size (diameter) of the screw, and the "32" means it has 32 threads to the inch. The diameter is measured at the shank of the screw.

Screws are sized by gauge number and length. For example, an 8 gauge screw with 32 threads per inch and 1" in length would be written as: 8-32 X 1". However, most wood screws do not include the threads per inch measurement and would just be listed as 8 X 1". Thus, a 6 gauge screw with 15 threads per inch and 1 1/4" in length would be written as: 6-15 X 1 1/4". If the gauge number is not known, simply measure the diameter of the shank in inches and round to nearest listed number on chart below for screw number identification.

Wood Screw Size Chart

The length of screw is taken from the surface of the material to the point of the screw. See illustration below.

How to Measure Length of Screw

Shank and Root of Screw

SCREW TYPES

Wood screws are classified by the type of drive, the shape of head, its length, and whether it is designed for wood or metal; this page refers to wood screws only.

Types of Drives

Driver refers to the indented shape on the screw head used to turn the screw. There are many different types of drives. Here we are only covering the four most used drives. These include slotted (flat head), phillips (cross head), square, and pozidriv. A brief description of each drive is below with a picture at the bottom to illustrate each ones unique design.

SLOTTED/FLAT HEAD: This is the original screw drive. You find these everywhere, though the practice of using screws with slotted drives is on the decline because the screwdriver slips out of the slot, particularly when you are applying heavy torque to really tighten down (or loosen, for that matter) these types of screws.

PHILLIPS/CROSS HEAD: This screw drive type is very popular - and again, you find them in a very wide range of applications. Common sizes are Phillips #1, #2 and #3. The most common Phillips size is #2.

SQUARE HEAD: Square recess are being used more and more as they are very resistant to cam-out, which is a fancy way of saying the tip of the tool does not slip out and mar the screwhead very easily. Commonly found in two sizes: Square #2 and #3.

POZIDRIV: This screw head isn't seen very often in the U.S.A., though it is very common in Europe. It looks a lot like a Phillips screw head, but it includes 4 more contact points. Common sizes are Pozidriv #1, #2 and #3.

Screw Drive Diagrams

HEAD TYPES & SHAPES

Screw head types refer to the shape of the head at the top of the screw. There are many different head types as well. Again, we will adequately cover head types, but only the most generally used.The shape of the screw head can be described as flat (countersunk), oval, round, pan, truss, button (dome), etc. Detailed descriptions for each of the ones listed are below. Looking at the drawings should make the designs of each self-explanatory.

FLAT/COUNTERSUNK: Supplied to standard dimensions with an 80' to 82' angle to be used where finished surfaces require a flush fastening unit (concealed below woods surface). The countersunk portion offers good centering possibilities.
Screw Flat Head Diagram

OVAL: Fully specified as "oval countersunk", this head is identical to the standard flat head. but possesses. in addition, a rounded, neat appearing upper surface for attractiveness of design.

Screw Oval Head Diagram

ROUND: Not recommended for new design (see pan head). The round head rests on the surface of wood. This was the most universally used in the past.

Screw Round Head Diagram

PAN: Recommended for new designs to replace round, truss and binding heads. Provides a low large diameter head, but with characteristically high outer edge along the-outer periphery of the head where driving action is most effective for high tightening 'torques. Slightly different head contour where supplied with recessed head.

Screw Pan Head Diagram

TRUSS: Also known as oven head, stove head, and oval binding head. A low, neat appearing, large diameter head having excellent design qualities, and as illustrated can be used to cover larger diameter clearance holes in sheet metal when additional play In assembly tolerance is required.Suggest pan head as a substitute.

Screw Truss Head Diagram

BUTTON/DOME: Cylindrical with a rounded top.

Screw Button Head Diagram

THREADING & POINT TYPES

Most wood screws have a common "pitch" to the treads, but some have a thread with a steeper incline. We simply call this a "fast" thread, but they are technically Type A screws.Think of this as a road going up a mountain; the steeper the road, the sooner you get to the top.

Also in reference to threads you will see "8-32". This is the common knob and pull screw thread. The "8" refers to the size (diameter) of the screw, and the "32" means it has 32 threads to the inch. Most of the screws for mounting hinges are self-tapping (they tap their own mating threads in wood) type AB (they have more threads per inch and are more effective in brittle materials like wood than Type A)
Self Tapping Screw Diagram

Threading on the shank is designed specifically for wood; wood threads have a tapped screw while sheet-metal screws have mainly a parallel thread. Wood type screws are also normally used for securing into wall plugs. Screws for chipboard usually have 2 threads the full length of the shank.

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