Machine Design
A locking device in which the bottom cylindrical portion is recessed to receive the tip of the locking set screw, is called

Castle nut
Jam nut
Sawn nut
Ring nut

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Machine Design
Set screws are

Slotted for a screw driver and generally used with a nut
Similar to small size tap bolts except that a greater variety of shapes of heads are available
Similar to stud
Used to prevent relative motion between parts

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