At what level of thread fineness does a thread qualify as "fine-pitch"? We can define it this way: "coarse-pitch" threads are considered the standard; "fine-pitch" threads are defined in relation to coarse ones. For a given nominal diameter, the number of threads per inch-and thus the thread pitch-differs: coarse threads have a larger pitch, while fine threads have a smaller pitch. For instance, comparing the 1/2-13 and 1/2-20 specifications, the former is coarse-pitch and the latter is fine-pitch; they are designated as 1/2-13 UNC and 1/2-20 UNF, respectively.
Since two different thread specifications can exist for the same nominal diameter, how does one choose between coarse and fine threads?
**Coarse-Pitch Threads**
Coarse-pitch threads are essentially the standard thread type; unless otherwise specified, the stainless steel screws and other fasteners we typically purchase feature coarse threads. They are characterized by high strength, good interchangeability, and adherence to established standards, making them generally the preferred choice. Compared to fine threads, the larger pitch results in a steeper thread helix angle and poorer self-locking capability; consequently, in environments subject to vibration, anti-loosening washers or self-locking devices are required. Their advantages include ease of assembly and disassembly, a wide availability of compatible standard parts, and excellent interchangeability. When specifying coarse threads, the pitch is not explicitly stated (e.g., M8, M12-6H, M16-7H); they are primarily used for fastening connections.
**Fine-Pitch Threads**
Fine-pitch threads are the opposite of coarse threads; they were established to meet specific application requirements that coarse threads could not satisfy. Fine threads come in a range of pitch sizes-all smaller than those of coarse threads-offering superior self-locking and anti-loosening properties. Additionally, the higher number of threads per unit length helps minimize leakage and improves sealing performance. In precision applications, fine-pitch stainless steel screws allow for more accurate control and adjustment.
However, they have lower tensile strength compared to coarse threads and are more prone to thread damage, making frequent assembly and disassembly inadvisable. Furthermore, compatible fasteners (such as nuts) require equal precision; even slight dimensional errors can easily result in damage to both the screw and the nut. They are primarily used for metric fittings in hydraulic systems, mechanical transmission components, thin-walled parts with insufficient structural strength, internal machine components with space constraints, and shaft applications requiring high self-locking capabilities. When specifying fine threads, the thread pitch must be indicated to distinguish them from coarse threads.
How to Choose Between Coarse and Fine Threads
Both coarse and fine-threaded screws are used for fastening purposes; however, fine-threaded screws are typically used for thin-walled components and parts requiring superior vibration resistance. Fine threads offer excellent self-locking properties and strong resistance to vibration-induced loosening; however, due to their shallower thread depth, they have a lower load-bearing capacity for tensile forces compared to coarse threads. Even without additional locking measures, fine threads provide better resistance to loosening than coarse threads, making them ideal for thin-walled parts and vibration-sensitive applications. Fine threads also offer advantages in adjustment applications. The disadvantages of fine threads include unsuitability for materials with coarse grain structures or low strength, as the threads are prone to stripping if excessive tightening force is applied.
