Coarse threads are commonly used in applications requiring rapid assembly or disassembly and when corrosion or damage from handling or use is likely to occur. They allow for easier starting with less cross threading. In relatively low-strength materials such as cast iron, aluminum, magnesium, brass, bronze, and plastic, coarse threads provide more resistance to stripping than fine or extra fine threads. If subjected to heat, they are less likely to seize than fine threads.
Fine threads are commonly used for nuts and bolts in high-strength applications. While applications vary, in general, fine threads are approximately 10% stronger than coarse threads. They have less tendency to loosen under vibration because the smaller lead or thread helix angle provides better wedging action when the assembly is tightened. Fine threads are also used for fine adjustment and thin-walled applications due to the shallower thread height. The shallower thread height lowers the chip load per tooth and chip volume resulting in less tapping torque and breakage, particularly in difficult to machine materials. Less chip vc also means that more lubrication will reach the cutting teeth yielding longer tap life. Fine threads are generally easier to tap but require a larger tap drill than coarse threads, which improves the performance of the drill and tap.