Selection Course and Fine Threads

Coarse Threads

• Coarse threads are commonly used where rapid assembly or disassembly is required.
• If corrosion or damage from handling or use is likely.
• 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

• 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 of a 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.
• Fine threads are generally easier to tap. Since the thread height is shallower, the chip load per tooth and chip volume are lower, resulting in less tapping torque and breakage, particularly in difficult to machine materials.
• Less chip volume also means that more lubrication will reach the cutting teeth resulting in longer tap life.
• Fine threads require larger tap drill than for coarse threads, which improves the performance of the drill and tap.