Introduction to Thread Forming
Threads are formed in one of two ways. They are either cut or they are rolled. This refers to how the actual threads are made and does not alter the threads per inch (TPI). We will discuss the differences between the two methods as well as the advantages and disadvantages for each.
Cut Thread
Cut thread refers to the process where threads are formed by cutting away material much like a pencil sharpener does. Threads are cut using a thread cutting die or a single tip cutter and the excess material is cut away from the round bar stock. The round bar stock is the same size as the threads you are cutting with the cut thread process. In other words, if you want to cut 1/2-13 threads then you start with 1/2” diameter bar stock. Cut threads will typically be more precise than rolled threads and cut threads can meet any ASTM specification. Threads can be cut on any diameter of material. They are, however, more time consuming and labor intensive to make which results in a longer lead time and higher cost on average.
Rolled Thread
Rolled Thread refers to the process of forming threads by extruding (forcing or squeezing out) steel from under sized bar stock. The steel is squeezed between 2 rollers which form the threads much the same way that if you squeezed putty in your hands the putty would rise up between your fingers. The advantages of rolled thread are that parts can be mass produced quickly which cuts labor time and costs. Rolled threads can be made to meet any ASTM specification with the exceptions of A325 and A490 bolts. Because this process utilizes under sized bar stock the parts use less steel and therefore weigh less than full bodied cut thread parts. This saves money in both raw material costs and shipping costs. Rolled threads do have limitations. Most manufacturers are limited in the diameters and thread lengths that they can roll thread on to. Most rolled threads are on 1” or less diameter bolts with thread lengths of 8” or less. Materials with a tensile strength of 150ksi typically cannot have threads rolled onto them.