Engineering Fasteners

Engineering Fasteners

Despite advances, 2D mechanical drawings are still the most popular format for design documentation. Automated extraction techniques allow mechanical drawings to be developed directly from 3D geometric models, simplifying the process. However, some elements of design representation not easily conveyed through the model database and therefore not as easily extracted to 2D drawings. Many of these elements are notational in nature. Some examples include thread specifications, surface finishes, surface quality, and dimension tolerances for fasteners.

Screw threads serve three basic functions in mechanical systems; 1) to provide a clamping force 2) to restrict or control motion, and 3) to transmit power. Geometrically, a screw thread is a helical incline plane. A helix is the curve defined by moving a point with uniform angular and linear velocity around an axis. The distance the point moves linear (parallel to the axis) in one revolution is referred to as pitch or lead. The term internal threads refers to threads cut into the sidewall of an existing hole. External threads refers to threads cut or rolled into the external cylindrical surface of a fastener or stud. The size most commonly associated with screw threads is the nominal diameter. Nominal diameter is a more of a label than a size. For example, a bolt and nut may be described as being ½” diameter. But neither the external threads of the bolt nor the internal threads of the nut are exactly .500 in diameter. In fact, the bolt diameter is a little smaller and the nut diameter a little larger. But it is easier to specify the components by a single size designation since the bolt and nut are mating components.

Standardization of Threads: (Standard Inch Units) To facilitate their use, screw threads have been standardized. In 1948, the United States, Great Britain and Canada established the current system for standard inch dimension threads. This is the Unified thread series and consists of specifications for Unified Coarse (UNC) Unified Fine (UNF) and Unified Extra Fine (UNEF) threads. Metric threads are also standardized. Metric thread specification is given through ISO standards.

Literally a measure of the number of crests per unit of length measured along the axis of the thread. The number of threads/inch for a thread series is given by standard and may be found in thread tables. The Tap Chart shown later in this document gives the number of threads/inch based upon threads series and nominal diameter.

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