Best Thread for Your Specific Needs

Bolt threading has a huge impact on the performance of a fastener. There are many options when it comes to threading. But let start with the basics and explore the different threading options Extreme Bolt offers.

Fine and Coarse Threads
Each diameter fastener has a different thread pitch number signifying the number of threads there are per inch. Typically, bolts come in coarse thread and fine thread. An example is a ½” diameter bolt. Coarse thread for a ½” diameter bolt is 13. This means for every inch of the bolt shaft there are 13 threads. For the same ½” diameter bolt in fine thread, the pitch would be 20. – meaning there are 20 threads per inch for a fine thread. That means for every inch of the shaft there are 20 threads for a fine thread pitch, compared to 13 of the coarse.

Coarse threads are denoted by the standard UNC. A bolt with coarse threads has less threads per inch than a fine thread – so the threads are further apart. Coarse threads work for most applications and are the most common.

In contrast, fine threads offer more threads per inch. They are denoted by the standard UNF. Bolts with fine threads provide more grip as the threads are closer together and there are more threads to hold the bolt in place. Fine threads are typically required for applications that require greater security and tighter tolerances.

Understanding Thread Tolerance and Classes
Bolts have specified thread dimensions for each diameter bolt and the thread pitch. The tolerance of a thread is the specified amount the threads being produced can deviate between these dimensional limits. A looser tolerance means it can deviate more from the specified dimensions. If you want a tighter tolerance bolt means the threading must be more precise to the limits assigned by the that diameter and thread pitch.

Threads are categorized into 3 classes that denote the thread tolerance for the pitch: 1, 2 and 3. Class 1 has the loosest tolerance whereas 3 has the most precise. Class 2 is often most common for standard applications. In addition to the classes, there are letters assigned to denote internal verses external threads.

The letter A is assigned to external threads so threading on bolts, screws, rod and studs: classes are 1A, 2A and 3A. The letter B is assigned to internal threading for nuts, which include 1B, 2B and 3B.

• Class 1A or 1B thread pitch is for the loosest tolerance. They provide the largest amount of play or clearance in assembly and are best used when easy/rapid assembly and disassembly of bolts is required.
• Class 2A or 2B is the most common thread class. Class 2 provides a combination of interchangeability, quality and fit.
• Class 3A and 3B are the highest tolerance thread class. These are mostly used in precise applications where tolerances are most critical, and a tight fit is required.

Rolled Threads Compared to Machined Threads
Depending on the material and dimensions, Extreme Bolt can produce threads that are both rolled and machined (or cut). But what is the difference?

Machined threads refer to the internal root and flank of the thread being cut away to create the thread form. In contrast, rolled threads means the threads are formed by pressing the parts between two round dies or flat plates that already have the thread pitch machined onto their face. When the parts move, or roll, through the dies, the threads are formed by the applied pressure without cutting or removing any material.

When threads are rolled, it creates a rounded root radius. Also, the cold working process of the thread rolling operation increases the strength of the material within the threads. For non-heat-treated bolts – rolled threaded parts have higher strengths than cut threads in the area where the thread was formed. However, for heat treated parts, rolled or cut threads have the same strength as the heat treatment dictates the strength. Therefore, rolled threads are a way off adding strength to the threads in non-heat treated materials.

Specialty Threads
“R” Pitch Threads
An “R” pitch can apply to UNRF (unified rounded fine thread) and UNRC (unified rounded coarse thread).

The only difference between UNC threads and UNRC threads (or UNF and UNRF threads) is that the root of the thread – the valley that is between the thread “peaks” – is curved/rounded rather than angular. These roots are rounded with an approximate curvature between 0.108 and 0.144 times the pitch. The UNRC and UNRF makes the roundness between threads mandatory, whereas with the UNC and UNF threads the roundness is optional.

The reason UNRC threads are produced is because this rounded root helps to improve fatigue strength. UNR stands for unified rounded thread. UNC and UNRC threads are compatible. So, if you are mixing a UNC nut with a UNRC bolt – the threads will still maintain a proper fit.

“J” Pitch Threads
Extreme Bolt can offer off J threads. The UNJ external threads have a larger root radius than a standard thread pitch. This ultimately improves the tensile stress area of the fastener, making the threads less susceptible to cracking. Many aerospace applications use these types of threads because of the increased durability.

The biggest trick about J threads is that internal J threads on a nut can be used with standard UNC or UNF threaded bolts, whereas the reverse is not the case. Remember you CANNOT use an external J threaded bolt, screw or stud with a standard threaded nut due to the interference at the minor diameter.

12 Point Screw Availability

• 17-4 PH
• 254 SMO
• 310SS
• A286
• AL6XN
• Alloy 20
• Alloy 330
• Alloy 926
• Aluminum 7075
• Duplex Steel
• Hastelloy
• Incoloy
• Inconel
• Monel
• MP159
• MP35N
• Nickel 200
• Nimonic
• Nitronic
• Titanium
• Waspaloy