What is a thread insert?

A thread insert is a separate element that forms the thread in a hole instead of the parent material. The best known is the wire thread insert: a stainless steel coil with a diamond-shaped cross-section, specified for metric threads in DIN 8140, in lengths such as 1d, 1.5d and 2d. There are also solid bushes that tap themselves in or are locked in place. Inserts are common in aluminium, magnesium and plastics, in joints that are taken apart often, and to repair a stripped thread.

What does a thread insert depend on?

A wire thread insert needs a larger tapping drill and a tap made for the insert, with its own size and its own gauge; a standard tap of the same nominal size will not do. After winding the insert in, a tanged type has its drive tang broken off and removed from the hole; a tangless type skips that step. How strong the joint becomes depends on insert length, parent material and load. The larger outside diameter of the insert spreads the force over more parent material than a thread tapped directly.

How does a thread insert work in practice?

A hole with an insert usually becomes three steps: drilling, tapping for the insert and fitting, sometimes followed by a gauge check. The insert is a bought-in part, counted per piece, and the tap a special size you manage separately. Fitting can be done on the machine or with hand tools at the bench; at the bench the time falls under a different rate from the machine's. How much this matters depends on the number of inserts per part and the batch size.

What is a thread insert often confused with?

A thread insert is not a press-in nut: that is forced into sheet or plastic with no tapped thread in the hole. Inserts set into plastic by heat or ultrasound are an installation technique of their own. Nor is the insert the thread itself: the drawing asks for an M6, and the insert is one way of making that M6 strong enough.