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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.
==Turning==
On turned parts an undercut is also known as a ''neck''. 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 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.〔.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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