What is an undercut?

An undercut refers to a feature of any kind that is cut into a component, most commonly a turned or machined shaft, that allows for additional clearance to mating components. This is most often used on shafts that have diameter changes so that a mating part can seat against a shoulder. Another use for undercuts is be to provide clearances for cutting tools during manufacture, referred to as thread relief.

The shape of an undercut will depend on the chosen manufacturing method and the specific tools being used however there are a few best practices which you, the designer, can utilise to improve the manufacture efficiency and reduce overall cost.

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Undercuts

A bore or shaft that requires a tightly controlled diameter tolerance (see our post on limits and fits) will often require a secondary manufacturing step after CNC machining, for example grinding, in order to meet the tight tolerances specified on the drawing.

In order to help meet these tighter specifications, an undercut can be added to help simplify and improve the finishing process. For this application, the component can have an undercut no greater than 0.5mm deep, with lead in and root radii as shown below:

Thread Relief

For threaded portions of a shaft or screw an undercut can provide clearance for the cutting tool to runout close to the shoulder of a shaft.

Strictly speaking, for external threads the thread relief simply needs to be smaller than the minor diameter of the thread and for internal threads, the relief needs to be greater than major diameter of the thread, however some additional clearance is always nice to have 😉

To simplify and standardise your thread undercuts the following values can be used: