Thread Relief – What Is It Actually?

There’s a lot to consider when manufacturing a thread. Especially in industrial production, it goes far beyond diameter and pitch. A thread can’t be viewed in isolation—it’s part of a bigger picture. And as such, sometimes a few special tricks are needed to ensure the perfect interplay of different components.

One of these tricks is the thread relief. You've almost certainly seen one before, even if you weren’t aware of it. But do you know where and when relief grooves are used in mechanical engineering? If not, keep reading. We’ll explain all the key facts about thread relief.

What Is a Thread Relief?

A thread relief is the reduction at the end of a bolt or nut that follows the actual thread. It’s a smooth-surfaced section on the bolt where no thread is cut. This section also has a smaller diameter than the thread itself.

Relief grooves are not only found on threaded components but are common in the production of turned parts in general. They are standardized under DIN 509, which also defines their exact shape depending on the application. The thread relief is a specific variant of this manufacturing step.

Thread reliefs can also come in different forms and are important for two main reasons. Let’s take a closer look at what those are.

Why a Thread Relief Is Needed

Fundamentally, thread relief is always about creating space. Either for the tool—in this case, the lathe tool—that cuts the thread onto the bolt, or for an adjacent component.

Run-Out for the Tool

Thread relief as a run-out zone for the tool is especially important when turning threads. If you mainly work with thread taps, you won’t need to deal with this.

However, if you’re using a lathe tool, it needs a space at the end of the thread to run out. This clearance is provided by the relief groove. It ensures that the tool tip doesn’t collide with the part and cause damage.

Clearance When Assembling Components

Relief grooves are also important wherever a bolt needs to be screwed into a thread up to a stop. When two parts need to be joined seamlessly, it’s difficult to achieve that without thread relief. There’s a high risk of thread engagement issues, parts jamming, or the entire assembly being damaged.

If the component doesn’t need to be screwed in all the way, then a thread relief isn’t necessary. In such cases, the standard thread run-out is sufficient, which naturally forms when cutting external threads with a die. This term refers to the area where the thread transitions into the smooth bolt.

Thread Relief According to DIN 76 – What It Looks Like

Thread reliefs are standardized in DIN 76. This standard is divided into three parts and includes dimensions for thread reliefs for metric ISO threads, pipe threads according to DIN 228, and trapezoidal, saw, and round threads.

According to DIN 76, there are four forms: A, B, C, and D. Forms A and B are for external threads, while Forms C and D are used for internal threads. A and B, as well as C and D, share the same geometry but differ in length. Typically, the longer thread reliefs A and C are used. The shorter reliefs B and D are only used in special cases where specifically required.

For a bolt thread, the diameter of the thread relief is smaller than the minor diameter, meaning less than the smallest diameter of the thread. Conversely, the diameter of the relief groove in an internal (nut) thread must be greater than the nominal diameter—the largest diameter of the thread. The width of the relief is about three times the thread pitch.

The rounding of the thread relief is particularly important. It has a direct impact on notch effect.

What Thread Relief Has to Do with Notch Effect

The notch effect refers to the phenomenon where notched components experience increased stress at the notch, making these points weak spots prone to fracture. Avoiding sharp edges on components is an effective way to counter this issue. Thread relief, particularly with rounded transitions, helps achieve this. The rounded areas of the relief reduce the notch effect and thus lessen stress on the component.

Where Are Thread Reliefs Commonly Used?

Apart from thread cutting with a lathe tool, thread reliefs are also important in other areas—namely, wherever components need to be screwed together flush. This may be necessary for various reasons, either to increase the stability of a construction or simply for aesthetic purposes.

Are you looking for high-quality threading tools that make your work easier and offer long tool life? Then check out our BAER online shop. Every piece is crafted with care and expertise to deliver precise results. Our knowledgeable customer service team is always available by phone or via our contact form to help with your questions and concerns.