For example, structures such as bridges, communications towers, and buildings use many fasteners of several different types in their construction.

Products used in medicine, sports, transportation, and piping use fasteners.

Three methods are commonly used for fastening:

Mechanical fastening-a process that uses a manufactured device to hold parts of an assembly together. Mechanical fasteners include threaded fasteners and nonthreaded fasteners, such as rivets, keys, pins, snap rings, and clips.

Bonding-a process that uses a material to hold parts of an assembly together. Bonding is a permanent fastening method and includes such processes as welding, soldering, brazing, and gluing.

Forming-a process that relies on the shapes of components to hold them together. This is the least expensive fastening method and may or may not be permanent. Heating and air conditioning ductwork is held together by forming the sheet metal.

Initially no thread standards were available, so nuts and bolts from different producers could not be interchanged. When you bought a bolt, you had to buy the matching nut. Eventually, standards began to emerge. In the 1800s, Joseph Whitworth developed the English standard screw threads. The United States adopted its own screw thread standard, based on the thread proposed by William Sellers, in 1864.

The United States standard thread was different from England’s, which later caused problems in World Wars I and II, because spare equipment parts could not be interchanged. In 1948, the Unified Screw Thread Standard was established, allowing the United States, England, and Canada to interchange parts. Also, in 1946, the International Organization for Standardization (ISO) began development of the current international system of standard metric screw threads.

A rivet is an example of a permanent fastener, because it can only be removed by destroying it.

Adjusting is the process of locating or modifying the position of a part.

Screw jacks and certain types of gears use screw threads to transmit power, such as for lifting an automobile.

The terms described in this section are the ones used most for representing threads on engineering drawings.

Chamfer-the angular relief at the last thread, to allow easier engagement with the mating part.

Crest-the peak or top surface of a screw thread.

Depth-the distance between the crest and root of a thread, measured normal to the axis.

Die-a tool used to form external threads.

External thread-the screw thread on the outside of a cylindrical or conical surface.

Internal thread-the screw thread on the inside of a cylindrical or conical surface.

Lead-the distance a screw will travel when turned 360 degrees, or one revolution.

Major diameter-the largest diameter on an internal or external thread.

Minor diameter-the smallest diameter on an internal or external thread.

Pitch-the distance between corresponding points on adjacent thread forms, measured parallel to the axis. The pitch is equal to the number divided by the number of threads per inch. For example, a screw thread with 12 threads per inch would have a pitch of 1/12.

Pitch diameter-the diameter of an imaginary cylinder that is located equidistant between the major and minor diameters.

Root-the bottom of a screw thread cut into a cylinder.

Screw thread-a continuous and projecting helical ridge on a cylindrical or conical surface.

Side-the screw thread surface that connects the crest and root.

Tap-a tool used to make threads in holes.

Tap drill-a drill bit used to make a hole in metal before tapping an internal thread.

Thread angle-the angle between the surfaces of two adjacent threads.

Thread form-the profile or shape of a thread cut into a cylinder.

Thread series-the number of threads per inch for a given diameter.

Threads per inch-the number of threads in one inch, measured axially (parallel to the axis); the reciprocal of the pitch.

Thread form

Thread series

Major diameter

Class of fit

Threads per inch

Coarse series fasteners are used for quick assembly or disassembly of cast iron, soft metals, and plastic, and are designated NC or UNC.

Fine series fasteners are used when a great deal of force is necessary for assembly, and are designated NF or UNF. These fasteners are used extensively in the aerospace and automotive industries.

Extra fine series fasteners are used when the length of engagement is short and the application calls for high degrees of stress.

Class 1-a loose fit where quick assembly is required and looseness or play between parts is acceptable.

Class 2-a high-quality, general purpose, commercial class of fit for bolts, nuts, and screws widely used in mass production.

Class 3-a very high-quality threaded fastener with a close fit, used for precision tools and for high stress and vibration applications.

Major diameter, in three--place decimal form, followed by a dash. Fractional sizes are permitted. If a standard number designation is used, the decimal equivalent should be given in parentheses, such as No. 10 (.190)–32 UNF–2A.

Number of threads per inch, followed by a space.

Thread form designation.

Thread series designation, followed by a dash.

Thread class designation (1, 2, or 3).

Internal or external symbol (A is for external threads, B is for internal threads), followed by a space.

Qualifying information, such as:

LH for left-hand threads.
If the thread is right hand, RH is omitted.
DOUBLE or TRIPLE for multiple threads.
Thread length.
Material.

Thread notes can also provide information about tap drill depth and size, drill and thread depths, countersinking, counterboring, and number of holes to be threaded.

Metric bolts and screws use a numerical coding system.

The basic designation for a metric thread.

A complete metric thread note should contain the following information, in the order given:

Thread form symbol. The letter M is used to designate the metric profile. The J profile class is a modified M profile.

Nominal size (basic major diameter) in millimeters, followed by an X.

Pitch in millimeters, followed by a dash. The pitch can be eliminated for coarse threads, but it is preferred in the American use of the standards.

General purpose tolerance, which for external metric threads is 6g, and for internal metric threads is 6H. For closer fits, 6H is used for internal threads and 5g6g is used for external threads. The tolerance class designation, then includes:

Pitch diameter tolerance: grade, position.

Minor diameter tolerance: grade, position.

Simplified method of representing external threads.

Simplified method of representing internal threads.

A bottom tapped hole is one in which the tapping tool cuts threads all the way to the bottom of the drilled hole.

Approximating pitch and minor diameter measurements.

Schematic representation of internal threads for through, blind, and bottom tapped holes.

Graphic representation of tapered pipe threads can include the tapering angle.

Bolted joints should provide a strong, stable connection that minimizes undue stress.

Using a shared CAD database makes DFA and DFM much easier. Since everyone on the design team has access to the database, and since design teams today include representatives from all stages of the design and manufacturing process, coordinating the need for ease of assembly is greatly facilitated.

Bolt—a mechanical threaded device with a head on one end and threads on the other end. Bolts are paired with nuts.

A nut is a mechanical threaded device used on the end of a bolt, a stud, or a machine screw.

Stud—a rod that is threaded on both ends and joins two mating parts. A nut may be used on one end.

Cap screw—a mechanical threaded device with a head on one end and threads on the other end. Cap screws join two mating parts and have longer threads than bolts. Also, cap screws can be made with slotted heads.

Machine screw—a mechanical threaded device with a head on one end and threads on the other end. The threaded end may screw into a mating part, or may be used with a nut. A machine screw is similar to a cap screw, but it is normally smaller.

Set screw—a mechanical threaded device with or without a head, used to prevent rotation or movement between parts, such as a shaft and a collar. Set screws have different types of points and heads for different applications.

American National standard bolts have either hexagon or square heads. Square head bolts are not available in metric. The nuts used on bolts come in several variations, depending on the application or on design considerations.

Nominal size (major diameter)
Threads per inch
Length
Name
Material
Protective finish

Examples of thread notes for bolts and nuts are as follows:

.500–13 UNC X 2 Hex Cap Screw
1/2–13 UNCX 2.5 Square Bolt, Steel, Zinc Plated

Hexagon bolts are either regular, which are used for general purposes, or heavy, which are used for applications that require more strength. The thickness of the head is the major difference between a regular and a heavy bolt. Square bolts are available in the regular form only.

Bolts are further categorized as either finished or unfinished. A finished bolt has a finished surface called a washer face, which is 1/64" thick and provides a smooth bearing surface. An unfinished bolt has a head without machined surface.

Normally, standard bolts are not drawn on engineering drawings, except for assemblies.

ANSI B18.2.1-1981 hex head bolt standards.

Guide for determining bolt lengths.

Drawing a square head bolt.

ANSI B18.2.1-1981 square head bolt standards.

Other standard types of nuts.

Drawing a hex head nut.

Drawing a square head nut.

The ends of all caps screws are flat, with chamfers that are drawn at 45 degrees.

A shoulder screw is widely used in industry to hold machine parts together. Shoulder screws have toleranced, unthreaded shafts so that they can be used as pivots and linkages.

Self tapping screws are hardened fasteners, with various types of heads, that will form their own tapped holes in softer materials.

Wood screws are threaded fasteners used to hold wood products together. Lag bolts, screw hooks, and flat, oval, and round head screws are varieties of wood screws.

Wing and thumb screws have special types of heads that can be turned by hand, for special assembly purposes. Refer to manufacturers’ catalogs for other types of fasteners and their dimensions.

HELICAL SPRING LOCK WASHER–5/16 HEAVY–ALUMINUM–ZINC ALLOY.

HARDENED GROUND MACHINE DOWEL—STANDARD SERIES, 1/4 X 2, STEEL

Rivets are generally used for sheet metal, such as attaching the skin of an aircraft to the frame, or ship parts.

On engineering drawings, rivets are represented by standard symbols.