If you are a blacksmith or metal worker, there are times that you are going to have to tap a screw. That means that you are creating screw threads on a screw or pipe or the receptacle so that the two pieces can be connected. Before everything can be connected, you are going to want to make sure that the threads are the correct size, which means you are going to need to use something like a thread gauge to measure that the threads are the right distance apart.
The thread of the screw is what transfers all the energy you put into screwing the screw in from the rotational force to linear force, allowing the screw to dig down into material that the screw is being driven into. Part of what allows this to happen is the lead and the pitch. These things can be the same when it comes to some screws, but they aren't always the same.
Lead and Pitch
The lead is the distance that the thread travels along the axis of the screw. The lead doesn't follow the thread around the axis of the screw; it measures the distance between the two parts of the thread when they are at the same point along the axis in one single rotation. The pitch is the measurement from the top of one crest of the thread to the next crest. In a single start thread or one that has only one thread from the top to the bottom, those measurements are always going to be the same.
However, there are such a thing as double-start screws, and the pitch and the lead are always going to be different with those kinds of screws. In a double-start screw, the lead is going to be measured along the same thread, ignoring the other thread. When you measure the pitch, it's still going to measure the crest from one thread to the next, ignoring the fact that the threads come from different starts.
A thread gauge is a tool that will allow you to measure the pitch. Each of the measuring sticks will have gaps at different spacing, and you measure the lead and pitch by taking the tool until the correct size is found. That will give you the proper measurement and will help you find out whether the screw measurement is metric or imperial.
It's important to make sure that screws and pipes have the proper measurements. A thread gauge will help you find out the pitch and lead of any screw so that you know if it's been tapped correctly.