Everything you need to know about thread length

Imagine you've finally found the perfect tap for your cutting job, prepared the workpiece, and carefully set up your machine—ready to go. But wait, one important detail might still be unclear: **how deep should the thread go into the hole?** While the concept of **thread length** is often overlooked in DIY projects, it's actually a crucial factor that affects the strength and reliability of your bolted joints. Understanding thread length isn't just about mechanics—it’s about ensuring safety and stability. You don’t need a mechanical engineering degree to grasp this, but having a basic understanding can make a big difference in how well your fasteners perform. ### What's the Difference Between Thread Length and Usable Thread Length? The **thread length** refers to the total length of the tap where threads are cut. However, the **usable thread length** is what actually matters when you're working on a project. This is especially important when dealing with **blind holes**, which don’t go all the way through the material. In a **through hole**, the usable thread length is usually limited by the thickness of the workpiece. In contrast, a **blind hole** requires careful planning because the tap can't exit the other side. That means you have to account for the **tap’s chamfer**, which reduces the effective thread length. So, the usable thread length is always **the thread length minus the chamfer**. ### How Screw-In Depth Relates to Thread Length The **screw-in depth**—how far the screw goes into the threaded hole—is directly influenced by the thread length. It determines how much of the screw’s threads overlap with the nut or internal thread. This overlap is essential for distributing the load evenly across the threads, preventing them from stripping out under stress. A common rule of thumb is that the screw-in depth should be at least **1.5 times the diameter** of the screw. This ensures enough engagement to handle the forces applied. Even though some may think only the first few threads carry the load, in reality, **all threads share the burden**. That’s why it’s important to have enough threads in place to avoid failure. ### The Consequences of an Incorrect Thread Length Getting the thread length wrong can lead to serious issues. If the thread isn’t long enough, the connection might not be strong enough for the job. On the flip side, making the thread too long can cause problems during tapping and complicate assembly. Excessively long threads can also lead to **jamming or seizing**, especially in tight spaces. So, the key takeaway is: **use as much as needed, and as little as possible**. There’s no benefit to overdoing it. A thread that’s too long doesn’t improve performance—it just adds unnecessary complexity. ### How to Determine the Right Thread Length You can find **screw-in depth charts** online, or use **calculators** that take into account the material, screw diameter, and application. Always remember to check the **usable thread length** of your tap, especially if you’re working on a blind hole. If you see recommendations suggesting a screw-in depth greater than **three times the diameter**, consider whether a different material or design might be more suitable. Sometimes, a stronger or more durable material can provide better results even with a shorter thread length. Once you’ve determined the right thread length, you can head to our **BAER online store** to find the perfect tap for your project. If you have any questions, we’re here to help!

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