Screw thread projecting from a cylindrical surface is known as what type of thread?

A screw thread that projects from a cylindrical surface is referred to as a taper thread. This type of thread is designed such that its diameter decreases along its length, creating a conical form. Such a design allows for a tighter fit between the components being assembled, which is particularly useful in applications where sealing is required. Taper threads are commonly employed in plumbing systems and where components need to be joined securely while minimizing the risk of leakage.

In contrast, straight threads maintain a constant diameter throughout their length and are not designed to offer the same degree of self-locking and sealing characteristics as taper threads. The other options, like thread series and thread shear area, do not specifically refer to the geometry associated with the projecting nature of taper threads. Thus, taper threads are preferred in scenarios where the fit and seal are critical, highlighting their functional importance in machine design.