When it comes to fasteners there are many options and variables at play. Coarse vs fine thread? Being a common question. There are many differences between the two and even the way they are measured. Screws and bolts are fasteners with external threads that mate with an internal thread. There can be some variations such as the handedness of a fastener.
Handedness in reference to a fastener is the direction needed to drive it. Most commonly used are right-hand threads. Left-hand threads although not common are used in special applications. They are also used when an external force can cause a right-hand threaded fastener and nut to loosen, the left bike pedal on a bicycle for example.
Fine threads
Fine threads have many positive aspects to them. A fine thread fastener of the same size as a coarse thread fastener would be stronger in tension and shear because of their higher tensile stress area. This is because the fine thread fastener has a higher minor diameter. Fine threads are also are less prone to self-loosening when under vibration, and shock. They can resist stress corrosion cracking better than coarse threads. They can also take higher torque before yielding.
Fine threads are great for fine adjustment when compared to coarse threads. This is because the amount a nut would travel for one revolution of the fastener is less when compared to a coarse thread fastener. A major setback for fine threads is the higher chance of galling. It is more likely to occur because of the greater distance a fine thread fastener would have to travel compared to a coarse thread fastener.
Coarse threads
Coarse threads also have advantages its counterpart doesn’t have. Firstly they are less likely to suffer from stripping or galling. A coarse thread fastener with the same major diameter and length as a fine thread fastener would travel less than the fine thread fastener. Ultimately decreasing the chance for galling.
When coarse threads are filled with dirt they can still be used depending on the amount of dirt in the threads. Fine threads have a harder time allowing this. You might have to clean out the dirt between fine threads for it to mate properly. In most applications, a fastener with a major diameter 1″ or greater should use coarse threads.
Conclusion Coarse vs fine thread?
Both fine threads and coarse threads have their advantages and disadvantages. There is no ultimate answer when choosing between the two. Coarse vs fine thread? It all depends on your application. Most industrial applications do use coarse threads. You can use a drill and tap chart for metric here. You can also use a drill and tap chart for SAE here.