Threading is one of the most important aspects of fasteners and part assemblies. You need to choose the proper type of thread to ensure your parts will function properly. Screws, nuts, and bolts are the most common threaded parts, but tons of other parts have internal or external threads to attach with other parts and form a secure assembly.
Threads can be made in any shape and size, but there are two main categories: coarse threads and fine threads.
Coarse threading is when the thread peaks are further apart and bigger. Fine threads are closer together and smaller.
Imperial measurements measure the number of thread peaks per inch. For example, a 2in bolt has 50 peaks, or 25 threads per inch (tpi). Fine threads have a higher TPI, although there isn’t an exact number of threads per inch that tells you if the threading is coarse or fine.
The metric system measures threads by how far, in millimeters, a fastener advances in one turn. If the bolt advances 1 millimeter with each rotation, the thread measurement is 1M. Coarse threaded parts have a higher measurement, because the part moves more with each turn.
Choosing the right type of threading depends on the part’s purpose and environment.
Coarse threads are used for most screws and construction fasteners. The coarse threads create a stronger bond with softer and more brittle materials like wood, drywall, and sheet metal. Coarse threads are also less likely to be cross threaded.
Coarse threads are stronger when they’re not connecting with other threading, but weaker when they are. For example, a coarse threaded screw drilled into plywood is stronger than a fine threaded screw. But, a coarse threaded nut and bolt are weaker than a fine threaded nut and bolt.
Fine threads are used in smaller and more precision fasteners. Finer threading offers a stronger hold because more threads per inch add more surface area to the parts threaded together.
Fine threading also allows fasteners to be tightened with more precision. The downside to finer threading is the higher chance of cross threading and thread galling, when the threads are misaligned with each other. This can also cause damage to the threads.
Fine threads also limit the options to add plating, paint, or other protective coatings to the fastener.
There are many different thread measurements in addition to threads per inch. When you look at different measurements, it can be confusing to figure out what each measurement means.
Thread pitch: the distance between the thread peaks.
Thread length: the distance from the start and end of threading.
Nominal length: the total length of the fastener.
Minor diameter: the smallest diameter of the threaded area.
Major diameter: the largest diameter of the threaded area.
There are a few different common methods to forming part threads.
The first method is called thread rolling. A blank (part without threads) is rolled between two thread dies to form the threading.
The second method used to make threads is CNC milling, or Swiss screw machining. Both of these methods use a cutting tool to remove material and form threads. These machining methods are more precise than thread rolling, but also slower and more expensive.
Single-point thread cutting removes material through a precise cutting tool path. This method creates threads by taking multiple passes, with each pass cutting deeper into the material until reaching the specified thread depth. Carbide cutting tools, combined with rigid machine setups, produce thread tolerances of ±0.005" on Swiss-type CNC lathes.
Thread milling uses rotating cutting tools with multiple points to generate the thread profile. The tool follows a helical tool path while rotating, cutting the full thread form. This achieves surface finishes as smooth as 16 Ra and controls thread depth to ±0.001". Thread milling works particularly well for internal threads in blind holes where traditional taps cannot be used.
The cutting process allows for custom thread profiles and modifications not possible with thread rolling. Programming flexibility enables manufacturing of left-hand threads, variable pitch threads, and multi-start threads. While slower than rolling, precision machining provides tighter control of thread geometry and surface finish.
Small batch sizes and prototype parts benefit from machined threads due to lower tooling costs compared to roll threading dies. The process handles a wide range of materials including hardened steels, stainless steel, titanium alloys, and plastics. Thread milling also allows for thread repair and modifications to existing parts.
For internal threads in blind holes, precision machining remains the only viable manufacturing method. The controlled cutting action prevents material buildup at the bottom of blind holes and maintains thread quality throughout the entire thread depth.
Spex offers a large selection of custom precision machined parts for whatever your project needs. We are an ISO 9001:2015 certified company, and our team specializes in precision machining and supply chain efficiency. Our machined components are available in an array of different materials and finishes and are manufactured to meet all the highest quantity needs.
Reach out to our team to get a quote for custom precision fasteners.
