Undercut (manufacturing)

In manufacturing, an undercut is a special type of recessed surface. In turning, it refers to a recess in a diameter. In machining, it refers to a recess in a corner. In molding, it refers to a feature that cannot be molded using only a single pull mold. In printed circuit board, construction it refers to the portion of the copper that is etched away under the photoresist. In welding, it refers to undesired melting and removal of metal near the weld bead.

On turned parts an undercut is also known as a neck or "relief groove". They are often used at the end of the threaded portion of a shaft or screw to provide clearance for the cutting tool, and also referred to as thread relief in this context. A rule of thumb is that the undercut should be at least 1.5 threads long and the diameter should be at least 0.015 in (0.38 mm) smaller than the minor diameter of the thread. Strictly speaking the relief simply needs to be equal or slightly smaller than the minor diameter of the thread. Thread relief can also be internal on a bore, and then the relief needs to be larger than the major thread diameter. They are also often used on shafts that have diameter changes so that a mating part can seat against the shoulder. If an undercut is not provided there is always a small radius left behind even if a sharp corner is intended. These types of undercuts are called out on technical drawings by stating the width and either the depth or the diameter of the bottom of the neck.

Undercut - Any indentation or protrusion in a shape that will prevent its withdrawal from a one-piece mold.

Undercuts on molded parts are features that prevent the part from being directly ejected from an injection molding machine. They are categorized into internal and external undercuts, where external undercuts are on the exterior of the part and interior undercuts are on the inside of the part. Undercuts can still be molded, but require a side action or side pull. This is an extra part of the mold that moves separately from the two halves. These can increase the cost of the molded part due to an added 15 to 30% cost of the mold itself and added complexity of the molding machine.

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