Thread tolerances - Fits like a glove!

Have you ever found yourself striving for perfection in your work, only to realize that it's an unattainable goal? Even with the best tools and meticulous attention to detail, true perfection is rarely achieved. This might sound discouraging at first, but it doesn't have to be. In fact, perfection isn’t always necessary — especially when it comes to practical applications like threading.

In many cases, there’s no need to hit a measurement down to the last decimal place. For instance, when cutting threads, precision beyond what’s necessary can be time-consuming and unnecessary. What really matters is that the nut fits onto the screw, and sometimes a little play is even beneficial for the function of the thread.

So, a small amount of inaccuracy is acceptable — but only within certain limits. These limits are known as thread tolerances. Like most things in craftsmanship, thread tolerances are standardized. If you choose a tap with a 6H tolerance class, you can be confident that all dimensions fall within precisely defined ranges.

But what exactly do these numbers and letters mean? And how do they affect the performance of threaded parts? Let’s take a closer look at thread tolerances and why they matter in real-world applications.

The Concept of Thread Tolerance

Imagine you're using a drill bit labeled 5 mm and you drill a hole in metal. You expect the hole to be exactly 5 mm in diameter. But have you ever actually measured it? It might be slightly smaller, like 4.999 mm. That difference may seem negligible, but it's exactly what thread tolerance is all about.

Thread tolerance defines the allowable range of variation in measurements without affecting the fit or function of a threaded connection. This range is called a tolerance field. The size and position of this field depend on the thread tolerance class. And the choice of class depends on several factors, such as surface condition — some coatings require more space than others.

Understanding Thread Tolerance Designations

Thread tolerance classes are represented by a combination of a number and a letter. The number indicates the degree of tolerance, showing how precise the measurements are. The letter represents the position of the tolerance field relative to the nominal size.

Grade of Tolerance

The grade of tolerance is indicated by a number. According to DIN EN ISO 286-1, there are 20 ISO tolerance grades, but in practice, only grades 4 to 7 are commonly used for thread tolerances. The lower the number, the tighter the tolerance and the higher the accuracy.

Tolerance Field

The tolerance field shows where the tolerance lies in relation to the nominal dimension. Internal threads (like nuts) are marked with uppercase letters, while external threads (like screws) use lowercase letters. For example, an H-tolerance field refers to an internal thread, while an h-tolerance field refers to an external one.

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. This ensures compatibility between threads and prevents them from being too tight or too loose.

Some common tolerance positions include:

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

You don’t need to memorize all of them — the standard specifies which ones are most commonly used.

Selecting the Right Thread Tolerance

While there are 20 possible tolerance grades, only grades 4 to 7 are typically used for threads. Similarly, not all tolerance fields are relevant. For internal threads, G and H are most common, while e, f, g, and h are used for external threads. This gives you a wide range of options depending on your application.

Thread tolerance classes are often categorized as fine, medium, or coarse. The choice also depends on surface conditions. Coatings, for example, may require more clearance. Higher tolerance grades mean coarser threads, and thick layers may necessitate larger tolerance fields.

To find exact boundary dimensions for a given tolerance class, you can refer to a tolerance table. These tables list values for different thread sizes and both internal and external threads.

Thread Tolerances in Real Life

If this has felt a bit theoretical, rest assured — for most everyday applications, you don’t need to dive deep into the details of thread tolerances. The most commonly used class is 6H, which works well for most standard tasks.

However, in special cases — such as when you need a lot of play or a thick coating — you may need a different tolerance class. In those situations, it’s helpful to consult experts. At BAER, we specialize in taps and can help you choose the right thread tolerance for your needs. Our online store offers a wide range of taps across all tolerance classes — feel free to explore!