Everything you need to know about thread length

Imagine you've finally found the perfect tap for your cutting job, prepared your workpiece, and carefully set up your machine—ready to go. But wait, one crucial factor might still be unclear: how deep do you want the thread to go into the hole? The question of **thread length** is often overlooked, especially in DIY projects. However, it's not something you can just ignore. Understanding the forces acting on a thread, what influences them, and how this affects the required thread length can be quite complex. While a mechanical engineering degree would help, you don't need one to grasp the basics. For most home users, a simple understanding of thread length can significantly improve the **stability and safety** of your bolted joints—something that’s always worth having more of. ### What’s the Difference Between Thread Length and Usable Thread Length? The part of the tap where the actual threads are formed is called the **thread length**. However, this isn’t always the same as the **usable thread length** you can achieve with that tool. This distinction is important. Let’s start with a **through hole**, where the material is open on both sides. In such cases, the thread length is usually limited by the **thickness of the workpiece**. You can’t cut a thread where there’s no material, so the actual usable thread length is often **shorter than the maximum the tap could produce**. Now, things get trickier with a **blind hole**, which ends inside the material. Since the tap can’t exit, you have to decide how deep you want the thread to go, which also determines the **depth of the core hole**. Here, the **usable thread length** is always **less than the total thread length**, because the **tap’s chamfer** doesn’t fully form the thread. So, for blind holes, the usable thread length equals the **total thread length minus the chamfer**. ### How Screw-In Depth Relates to Thread Length You might wonder why all this matters. Well, the **thread length** directly affects the **minimum screw-in depth** of a bolt. The screw-in depth is the area where the **threads of the bolt and nut overlap**. It’s typically expressed as a multiple of the bolt’s diameter (e.g., 1.5d means 1.5 times the diameter). This value is critical for the **strength and reliability** of the connection. Even though some people think only the first few threads carry the load, in reality, **all threads share the stress**. If there aren’t enough threads, the connection may fail under pressure. To ensure sufficient overlap, the **thread length must be adequate**. Without enough threads, the connection might not be tight enough, which can lead to serious consequences, especially in **safety-critical applications**. ### What Happens if the Thread Length Is Wrong? So, what’s the right thread length? It depends on several factors, including the **material strength**, the **bolt diameter**, and the **intended use**. A longer thread isn’t always better. In fact, it can cause problems during tapping and make assembly harder. Plus, excessively long threads can lead to issues like **jamming or seizing**. That’s why the rule of thumb is: **as much as necessary, as little as possible**. Too short, and the connection won’t be secure. Too long, and you risk complications down the line. ### How to Determine the Right Thread Length If you're unsure, you can find **screw-in depth charts** online or use an **online calculator** that takes into account the dimensions and materials you’re working with. Just remember to check the **actual usable thread length** of your tap, especially when working on blind holes. Also, be cautious if the calculator suggests a very high screw-in depth. Avoid thread lengths greater than **three times the diameter** unless absolutely necessary. In some cases, switching to a stronger material might be a smarter choice, allowing for a shorter but equally reliable thread length. Once you’ve determined the correct thread length, take a look at our **BAER online store** to find the perfect tap for your project. If you have any questions or need help, we’re here to assist you every step of the way.

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