Thread tolerances - Fits like a glove!

Have you ever noticed that in your work, perfection is more of an ideal than a reality? Even with the best tools and the most precise measurements, achieving absolute precision is nearly impossible. For everyday use, we can only get close to perfection, not fully reach it. This might sound discouraging, but it doesn’t have to be. The good news is, in many cases, perfection isn’t even necessary.

In most situations, you don’t need to stick to the exact nominal size down to the last decimal—especially when it comes to threading! Achieving such high accuracy would take too much time and effort, and the return on investment wouldn’t be worth it. After all, the main goal is just for the nut to fit onto the screw. Sometimes, a little play is actually preferred or even essential for proper function.

So, a little bit of inaccuracy is okay—but only within certain limits. These limits are called thread tolerances. Like most things in craftsmanship, thread tolerances are standardized. If you buy a tap with a tolerance class of 6H, you can trust that all dimensions fall within precisely defined limits.

But what exactly do these numbers and letters mean? And how do they affect your work? That’s what this blog post is all about: exploring the world of thread tolerances.

The Concept of Thread Tolerance

When your drill says 5 mm, you assume the hole you’ve drilled is exactly 5 mm. But have you ever checked it? It might be slightly smaller, like 4.999 mm. If you think that doesn’t matter, then you already understand the concept of thread tolerance.

That’s exactly what it’s about. Thread tolerance defines the range in which the actual size can vary without affecting the correct fit. This range is known as the tolerance field. The size and position of this field depend on the thread tolerance class. And the specific class you need depends on several factors, including the surface condition. Some coatings, for example, require more space than others.

Understanding Thread Tolerance Designations

Thread tolerance classes are represented by two elements: a number and a letter. The number indicates the degree of tolerance, showing how tight the specifications are. The letter shows the position of the tolerance field relative to the nominal size.

Tolerance Grade

The tolerance grade is shown by a number. According to DIN EN ISO 286-1, there are 20 ISO tolerance grades. However, in the context of threads, only grades 4 through 7 are typically used. Each grade corresponds to a specific amount of allowable variation. The rule is simple: the lower the grade number, the higher the precision.

Tolerance Field

The tolerance field describes where the tolerance lies in relation to the nominal size. Holes are labeled with uppercase letters, while shafts use lowercase letters. So, for thread tolerances, a capital letter refers to an internal thread, and a lowercase letter refers to an external thread.

Tolerance fields are arranged from A to Z, with the nominal size acting as the zero line. For holes, the A-field is above the zero line, and the Z-field is below. For shafts, it's the opposite. Standards ensure that screws are always slightly larger than the nominal size, and nuts are slightly smaller. This prevents overlap and ensures a smooth fit.

By the way, you don’t need to memorize all the fields. For thread tolerances, the commonly used positions are:

• G and H for internal threads
• e, f, g, and h for external threads

Selecting the Right Thread Tolerance

The thread tolerance system doesn’t use all possible ISO grades. Only grades 4 to 7 are typically used. Similarly, only certain tolerance fields are relevant—G and H for internal threads, e, f, g, and h for external ones. Even with this limited set, there are still many combinations you’ll encounter when choosing the right threading tool.

Thread tolerance classes are usually divided into fine, medium, and coarse. Surface conditions also play a role. Higher tolerance grades tend to be coarser. Thick galvanic coatings, for instance, require more space between the tolerance field and the nominal size. With an h or H tolerance field, which is right next to the zero line, coatings can only be applied if the field isn't completely filled.

You can find the exact boundary dimensions for each tolerance class in a tolerance table. These tables provide all the necessary values for different thread sizes and for both internal and external threads.

Applying Thread Tolerances in Real Life

If this has been a bit technical, don’t worry. For most standard applications, you don’t need to dive too deep into thread tolerances. The 6H tolerance class is the most common one you'll encounter regularly.

Only in special cases will you need threads with a lot of play or very tight fits. Or maybe you're using a thick coating that requires a different tolerance class. In such situations, it’s helpful to consult experts. At BAER, a leading tap manufacturer, we’re here to help with your questions and challenges. We offer comprehensive advice on thread tolerances and a wide range of taps across all tolerance classes. Feel free to explore our online shop!