A round head screw has a bulged head that sits on top of the material held in place by the screw. The broad underside of a round head screw distributes weight over a larger area than other screws types, increasing the strength of the screwed connection. Round head screws-—including machine, wood and self-tapping metal screws—can have a Phillips™, slot or hex indentation to allow the screw to fit numerous requirements for installation. Proper screw and head type selection require knowing the base material holding the screw and the amount of pressure required to install the screw in the material.
Metal parts containing fine threads require round head machine screws with equally fine threads and a broad diameter head to tightly hold a material in a threaded hole while spreading force across the surface of the material. Spreading pressure over a larger area avoids dimpling and tearing in thin metals and other weak materials being held in the connection. This increases the strength of a connection, and adding the properly-sized flat washer under the screw head will increase the holding power of the bolted connection. The threads of a machine screw are too fine to hold in softer materials like wood and plastic. Another type of round head screw provides larger threads for softer materials.
The coarse threads of a round head wood screw grab and hold in softer materials like wood, plastic and composite boards. Round-headed wood screws often have larger-diameter heads than machine screws. This spreads the force of the connection over an even larger area of a soft material, avoiding indentations on the surface of the material or crushing surface fibers of softer woods like screws with tapered heads. Pine boards, plywood and composite boards are all susceptible to surface damage by tapered screw heads. A third type of round head screw also has coarse threads, but is designed for a different type of material.
Self-tapping sheet metal screws often called Parker screws have coarse threads designed to tap and hold in thin sheet metal. The round head of a self-tapping screw has a standard straight slot, but a Phillips™ head recess is common on smaller self-tapping screws. Installing a round head screw with self-tapping threads requires a lot of pressure, allowing the threads to cut into the sheet metal. Using this type of round head screw in other types of material will cause a weak connection that can pull from the base material under pressure.