Thread tolerances - Fits like a glove!
Perhaps in your work, you've already come to the realization: perfection doesn't really exist. Even if you measure as precisely as possible and use only the highest-quality professional tools. With the means available to us ordinary folks, you can at best approach perfection but never truly achieve it. Sounds sobering? It doesn't have to be. Because, fortunately, perfection isn't always necessary.
In most cases, there's no need to stick to the prescribed nominal size down to the tenth decimal place – not even when thread cutting! Achieving such accuracy would be very labor-intensive and would in no way be proportional to the benefit gained. After all, it's sufficient if the nut fits onto the screw, and often some play is actually desired or essential for the function of a thread.
So, a bit of imprecision is okay, but only within certain limits. These limits are technically referred to as thread tolerances. Like almost everything in craftsmanship, thread tolerances are standardized. If you purchase a tap of tolerance class 6H, you can trust that all dimensions are within precisely specified limits.
But what exactly are these measurements, and what's behind the combination of a number and a letter? We'll explore that in this blog post on thread tolerances.
The Concept of Thread Tolerance
When your twist drill indicates 5 mm and you drill a hole into metal, you assume the diameter of the drilled hole is 5 mm. But have you ever measured it? Maybe the diameter is just 4.999 mm. If you say that doesn't matter, then you've understood the concept of thread tolerance.
That's precisely what it's about. Thread tolerance defines the range in which exact dimensions can vary without jeopardizing a correct fit. This range is also known as a tolerance field. The size and position of this tolerance field are determined by the thread tolerance class. And which thread tolerance class is required depends on various factors. For instance, the surface condition matters because some coatings simply require more leeway than others.
Understanding the Designations for Thread Tolerances
The thread tolerance class is indicated by two parts: a number and a letter. The number indicates the degree of tolerance and shows how precise the measurements are. The letter indicates the tolerance field, signaling how far from the nominal measurement the tolerance range is.
Degree of Tolerance
The degree of tolerance is indicated by a number. DIN EN ISO 286-1 recognizes 20 ISO tolerance grades. But in the realm of thread tolerances, usually only grades 4 to 7 are relevant. Each tolerance grade corresponds to a specific number of tolerance units. Here, the rule is: the smaller the degree of tolerance, the greater the accuracy.
Tolerance Field
The tolerance field indicates the relationship of the thread tolerance to the nominal size. Holes are denoted by capital letters, shafts by lowercase letters. Thus, for thread tolerances, a capital letter indicates an internal thread, and a lowercase letter denotes an external thread.
Tolerance fields range from A to Z on a scale, with the nominal size marking the zero line. For holes, the A-field is above this zero line, and the Z-field below. For shafts, it's the opposite. To ensure compatibility, according to the standard, the screw's measurement is always above the nominal size, and the nut's is always below. This prevents the tolerance fields from overlapping and the nut from being too tight for the screw.
By the way, you don't need to remember all the tolerance fields. For thread tolerances, the standardized tolerance positions are:
- G and H for internal threads
- e, f, g, and h for external threads
Which Thread Tolerance is the Right One?
The system of thread tolerance classes doesn't utilize the full scope of possible ISO tolerances. Of the 20 thread tolerance grades, only grades 4 to 7 are used. For the tolerance fields ranging from A to Z, only G and H for internal threads and e, f, g, and h for external threads are relevant. Yet, even this reduced selection allows for numerous combinations you'll encounter when searching for the right threading tool.
Thread tolerance classes can be categorized into fine, medium, and coarse. Additionally, the surface condition is taken into account. The higher the tolerance grade, the coarser the thread class. Thick galvanic protective layers also require a tolerance field with more distance from the zero line. With an h- and H-tolerance field position, which is directly adjacent to the zero line, coatings are only possible if the tolerance field is not fully utilized. Otherwise there's just not enough space.
The specific boundary dimensions for a tolerance class can be found in a tolerance table. In it, you will find all values for the various thread sizes and for internal and external threads.
Thread Tolerances in Practice
If that was a bit too theoretical, let us reassure you. For most standard applications, you don't need to delve too deeply into the topic of thread tolerances. The tolerance class 6H is the most common one you'll regularly come across.
Only in special cases will you need thread connections with a lot or little play. Or you use a thick coating that necessitates a different thread tolerance class. In such situations, it's handy to have experts by your side. We at the tap manufacturer BAER are happy to help with your questions and challenges and provide comprehensive advice on the topic of thread tolerances. In our online shop, you'll find a wide range of taps from all tolerance classes. Feel free to take a look!