When you are working with a bolt or a screw, you may find that a bolt dont fit into a hole even if the bolt has the correct diameter. A bolt may resist turning because the threads on the bolt does not match the threads in the hole. If you use force to turn a bolt that does not match the hole, you will destroy the threads on the bolt and you will destroy the threads in the hole.
Destroying the threads means the bolt and the hole will no longer connect properly, so you must ensure the thread pitch match before you apply force. Thread pitch is the distance between the peaks of the threads on a bolt or a screw. Many different thread pitches can exist for a single diameter of a bolt.
How to Match Bolt Threads
For example, a quarter inch bolt can have a coarse thread pitch or a fine thread pitch. People know coarse threads as Unified National Coarse (UNC) threads, and people use UNC threads for many different tasks because UNC threads allow for faster assembly. UNC threads are also useful because UNC threads can tolerate dirt and imperfections more easy than fine threads can.
You should use UNC threads when you are working with soft material like aluminum or plastics because the deeper threads of a UNC thread provide more surface area for the threads to grip. Fine threads are known as Unified National Fine (UNF) threads, and UNF threads are different than UNC threads because UNF threads have more threads per inch. Because UNF threads has more threads per inch, UNF threads create more contact area and a stronger clamp load.
UNF threads are the preferred choice for precision assembly and for parts that experience constant vibration, such as automotive component or aerospace components. You may also encounter Extra Fine (UNEF) threads, and people use UNEF threads for very specialized applications where the metal walls are extreme thin. Using a UNEF thread in a standard construction project is unnecessary, but using a UNC thread in a delicate application may cause damage to the delicat parts.
In addition to thread pitch, you must consider thread classes, because thread classes determine the amount of clearance between a bolt and a hole. A Class 1 thread fit is a loose fit, and a Class 1 thread fit allow you to spin the bolt easy in messy conditions. A Class 3 thread fit is a tight fit, and a Class 3 thread fit require the parts to be very clean and very precisely aligned.
If you use the wrong thread class, the connection may rattle or the connection may seize. To ensure that you are using the correct thread pitch, you should use a tool call a thread pitch gauge. A thread pitch gauge is a tool that allow you to match the spacing of the threads physically, so you dont have to guess the thread pitch.
You can use a thread pitch gauge to verify your hardware before you apply torque to the bolt. In professional environment, a person may use more advanced tools like ring gauges or micrometers, but a thread pitch gauge is a sufficient tool for many people. If you identify the thread size, the thread pitch, and the thread class before you begin, the metal will behave predictably and the bolt will fit correct.
