02.May.2012 Threads and Fasteners: Part 2
It is useful to be able to read and produce proper engineering style thread notations. This formal style of referring to threads is commonly used in component data sheets, for communicating with manufacturers and in a shortened form when shopping for fasteners.
The basic components of a thread call out are the major diameter of the fastener and the thread pitch. Additional information such as thread form and class of fit are also commonly added.
In inch fasteners, the major diameter may be referred to by either a decimal number or a fraction.
Additionally small sized inch fasteners are referred to by number series. The even numbered sizes between #0 – #12 are commonly used but odd numbered and larger numbered screw sizes exist. After #12, most modern fasteners now jump up to .25 inch major diameters and continue with standard fractional sizes.
To prevent confusion, metric fasteners are preceded by the letter M.
Thread Forms: A wide range of threads exist with different uses. Some threads such as acme thread are optimized for load transferring and used in lead screws in CNC applications. The majority of threads are either Unified National or metric.
Unified National Threads are often called out with the following abbreviations:
UNC: Unified National Coarse
UNF: Unified National Fine
UNEF: Unified National Extra Fine
Metric has two primary thread pitches, fine and course. Since all metric standard fasteners are governed by a uniform thread standard, only the thread pitch is called out. In practice, it not uncommon for the thread form to be omitted from documentation involving common hardware unless it is an odd thread form such as Acme or pipe thread. In these cases it is usually assumed that the thread form is either Metric or Unified National.
While noting the thread form may often seem like a redundant step, this practice has its roots in the drafting standards which trace their origin to the industrial revolution where several competing and non compatible thread forms were in common production. Additionally specifying the thread forms is a simple insurance policy against mistakes and miscommunication which can be extremely costly in the production environment.
Class of Fit: The class of fit is the tolerance to which the fasteners are produced.
In inch fasteners 3 primary grades exist.
Class 1 is loose fitting and reserved for low tolerance applications.
Class 2 is the most commonly used grade of hardware providing consistent fit and tolerances at a reasonable price.
Class 3 fasteners are high precision fasteners used when tolerances are extremely critical such as in machine tools and engines.
Metric fasteners use a slightly different grading system that divides hardware into general usage or high precision. Metric Fasteners with a class of 6H for internal threads and 6G for external applications are for general usage. High Precision applications commonly use two designators such 5H6G which refers to the tolerance of the internal thread and the external thread.
Precision fasteners and high strength fasteners are rarely seen in design applications and included for completeness only. When the stresses and tolerances demand this level of consideration, engineers need to be actively involved in the decision making process.