A set screw is normally used without a nut (which distinguishes it from a bolt), being screwed instead in a threaded hole drilled in only one of the two objects to be secured. A set screw is often headless and threaded along its entire length, so that it will sit entirely inside that hole; in which case it may be called a grub screw or blind screw.
Set screws are not always the best way to resist the torque of driven shafts. To reduce the chance of slipping and to increase load capacity, a detent (often called a "flat") may be milled or ground at the part of the shaft where the set screw's point contacts. The detent, however, must be closely aligned with the threaded hole prior to fastening. The operator can often feel the screw push the flat into final alignment as they make the last half- or quarter-turn that tightens the screw. Set screws can successfully hold against heavy-torque applications if the details are right.
For longer life, set screws are usually made of alloy steel and case hardened. Hardened set screws often leave a plastic deformation, in the form of a circular or semicircular mark, in the shaft that the screw sets against. This has both pros and cons. On the pro side, such deformation increases the holding power (torque resistance) of the joint, as the screw is essentially "making its own detent" on a small but effective scale. On the con side, if one is aiming to have a blemish-free cosmetic finish on the part being set against, one must take actions to prevent the circular marks. In most set screw applications, this consideration is irrelevant. Another con is that this plastic deformation can cause corresponding high points in the shaft surrounding the circular mark. This can result in difficulties during dis-assembly if a bearing or other tightly toleranced part needs to slide past this area.
