Inserts are classified as two general types: those held in place by external threads and those that use some means other than threads (knurls, grooves, interference fit) for their holding power.

Externally threaded inserts can be further divided into three types:

Wire-thread inserts: These precision coils of diamond-shaped stainless wire form both internal and external threads. They are screwed into pretapped holes to provide an accurate standard internal thread for screws, bolts, or studs.

Solid self-tapping/self-threading inserts: These thread-cutting or thread-forming external threads are for use in either drilled or cored holes. Thread-cutting types have thread interruptions on the external threads in the form of slots or holes to provide cutting edges similar to a tap. Thread-forming types have external threads shaped in a series of lobes to form threads in the base material. Self-threading types create no chips.

Solid bushing: >These inserts use a number of locking techniques to match most situations. One version has built-in locking keys that are driven in after the insert is in place to provide rotation resistance. Other two-piece inserts have a separate key ring or serrated locking ring to provide the torsional resistance.

Modified or distorted external-thread inserts are used to create an interference fit with the tapped hole through a swaging action. Nylon plugs and strips attached to inserts may also provide locking action.

Inserts without external threads used in drilled or cored holes include:

Molded-in inserts: Used in thermosets, thermoplastics, rubber, and ceramics, they have one or more external knurled sections. Cast-in inserts, which are similar, are used on nonferrous metals.

Pressed-in inserts: These have external retention rings, knurls, or other elements for locking the insert in the base material. Some designs have slotted segments that are forced into the hole when the fastening screw engages the internal thread. Others have integral washers or cones that expand the locking segments as the insert is driven into the hole.

Ultrasonic inserts: Solid bushings with annular and longitudinal grooves prevent both pullout and rotation after installation. These are used only on thermoplastics.

Sandwich-panel inserts: These are used on honeycomb and composite sandwich panels, in either one or two-piece construction. The one-piece type is usually applied on one side of a panel with a potting compound to obtain the required strength. The two-piece construction is usually swaged together with some interlocking feature and applied through both sides of the panel.

Some specialty inserts provide an external male thread or other fastening means. One insert is the stud insert. This may have any of the previously listed holding and locking configurations on one end and a standard thread on the opposite end.