What type of fastening device is characterized by a symmetrical head and a smooth, even surface?

The type of fastening device characterized by a symmetrical head and a smooth, even surface is the flat-faced fastener. Flat-faced fasteners, such as flat-head screws, have a uniform surface that allows them to sit flush with the material where they are installed. This design not only provides a clean aesthetic but also ensures that the fastener does not protrude, which can be important for applications requiring a smooth finish or for reducing the risk of snagging on surrounding materials.

In contrast, other types of fasteners serve different purposes. Hexagonal fasteners typically feature a hexagonal head designed for use with wrenches, providing high torque but lacking the smooth surface characteristic of the flat-faced design. Wing nuts have unique "wings" that allow for easy manual tightening and loosening, which does not align with the properties of a flat-faced fastener. Lock nuts are designed specifically to resist loosening under vibration and do not have a symmetrical or smooth design because they are often thicker and feature a specialized shape for friction engagement.