Threaded inserts are cylindrical metal and plastic shapes placed in existing holes to provide a threaded track for bolts, screws and other fasteners. The outer shell of a threaded insert allows it to lock in place once put in the hole, and its inner cavity is threaded to allow for the insertion of fasteners.
Threaded inserts are available in different shapes according to their function. Key-locking inserts are useful in situations where an existing thread has become stripped. The keys in these inserts cut into the original threads and lock the insert in place. Molded inserts are designed to be placed in a mold cavity before the forming of a plastic component. Knife-thread and hexdrive inserts are effective solutions in most types of wood. Press inserts are another effective means of adding threads to plastic parts, although they are added after the plastic is molded. Self-tapping threaded inserts are also used, even for metal. These inserts cut their own threads as they are placed in the work-piece.
Insertion of threaded inserts typically requires specific tools. Heat insert drivers and ultrasonic transducers are used on plastic work-pieces. Press fitting is also an option, although it generates far more stress on the insert and requires a firmer boss design. For metal inserts, there are a variety of handheld and stationary tools available. These are regularly classified according to brand and function. Machining capabilities and insert size and shape vary between devices, although a number of tools allow for the installation of multiple types of threaded inserts. Power drivers, hand-turned chuck attachments and other drive tools can also be used.
Brass threaded inserts are regularly used for plastic applications, although inserts are also made from a wide range of other materials. Steel and stainless steel inserts perform well in many metals, and can be modified by a variety of finishes, such as zinc plating.
