Everything you need to know about thread length

Picture this: you've finally picked out the perfect tap for your threading project, set up your workpiece, adjusted all the settings on your machine, and you're all set to begin. You’re ready to go, right? Not quite yet! One crucial detail might still be unclear—how deep do you really need to cut the thread into the hole? While the question of thread length is often overlooked because, let’s admit it, it’s not typically a top priority in the DIY world, ignoring this aspect can lead to serious issues down the line. Thread length isn’t just a minor detail—it plays a significant role in ensuring the stability and longevity of your bolted connections.

To truly grasp the mechanics behind threads—the forces involved, the factors influencing them, and how this translates into the appropriate thread length—you’d probably need a degree in mechanical engineering. But before you dive headfirst into university courses, let’s simplify this complex topic into a few key points. This will suffice for most personal projects while adding an extra layer of reliability and safety to your threaded joints. After all, who wouldn’t want that?

Differentiating Between Thread Length and Usable Thread Length

The portion of the tap where the threads are formed is called the thread length. However, this doesn't equate to the actual thread length you'll create with the tool. There's a critical distinction to make here.

Let’s start with the easier scenario: cutting a thread in a through hole. There are numerous possibilities here, but in most cases, the thread length will depend on the thickness of the workpiece. You can’t create threads where there’s no material, so in cases like sheet metal or metal plates, the actual thread length will generally be shorter than the maximum thread length the tap could potentially produce.

Blind holes, however, are a whole different ball game. Since these terminate within the material, the tap can’t exit at the other side. Here, you’ll need to decide consciously on the thread length, which also affects the depth of your pilot hole. This is where the difference between thread length and usable thread length becomes vital. The usable thread length is always the smaller of the two values, as you must subtract the chamfer of the tap. In this region, the threads aren’t fully formed and thus don’t contribute to the overall thread length. Therefore, the usable thread length for blind holes equals the thread length minus the chamfer.

The Connection Between Screw-In Depth and Thread Length

Now, why is all of this so important? The thread length directly impacts the minimum possible screw-in depth. And screw-in depth is a critical parameter when it comes to the security of screw connections.

Screw-in depth refers to the area where the threads of the screw overlap with the nut threads. It’s typically expressed as a multiple of the diameter. A screw-in depth of 1.5d means one and a half times the diameter. This value needs to be sufficient to distribute the resulting stresses across all threads, preventing them from tearing out. Contrary to outdated beliefs, all threads—not just the first three—are load-bearing. To ensure the connection remains intact, there must be adequate threads present.

Of course, achieving the desired screw-in depth hinges on the thread length. Without threads, there can be no overlap, and the screw connection may not be tight enough. This can have severe implications, especially for safety-critical joints.

Consequences of an Incorrect Thread Length

So, what’s the ideal thread length? This is where those mechanical engineering lessons come in handy. The minimum screw-in depth—and hence the required thread length—depends on several factors, including the strength class, the thread diameter of the screw, and the shear strength of the material. The intended purpose of the joint is also crucial, as screw-in depth is particularly important under load.

Perhaps you’re thinking that longer threads are better. Unfortunately, that’s not always true. While overly short threads can compromise strength, excessively long ones are equally problematic. Long threads are challenging to produce, prone to causing issues during tapping, and can make assembly cumbersome. They also increase the risk of jamming or seizing.

Thus, the rule of thumb when it comes to thread length is: as much as necessary, as little as possible.

Determining the Required Thread Length

Wondering how much thread length you need? Online resources offer plenty of screw-in depth charts. Calculators are also available, allowing you to input specific dimensions and materials to get tailored recommendations. Just remember to consider the actual usable thread length when working with blind holes, as the chamfer reduces the effective length.

Be cautious if the chart or calculator suggests unusually high screw-in depths. Avoid thread lengths exceeding three times the diameter. In such cases, switching to alternative materials that provide robust and stable connections even with shorter threads might be wiser.

Once you’ve figured out the correct thread length, head over to our BAER online store to find the perfect tap for your project. If you have any questions or encounter challenges, feel free to reach out—we’re here to help!