What you should definitely know about flank angles

Every thread profile consists of a detailed set of precisely coordinated dimensions and angles. There’s more to it than you might think. The good news is that you don’t need to master all of them to cut perfect threads. Manufacturers handle the complex calculations for you, ensuring the thread dimensions align perfectly in the end. However, there are certain variables within the thread profile that you should be aware of—especially if you're selecting the right tap for your project. One of these critical variables is the flank angle.

The flank angle is a key factor that defines the thread profile. While it remains consistent for some common thread types, its significance is sometimes overlooked. But when dealing with foreign standards or specialized areas like motion threads, getting the flank angle right becomes unavoidable. To help you stay prepared, we’ve compiled all the essential details about flank angles.

Where is the Flank Angle Located in the Thread Profile?

Let’s break it down step by step. What exactly is the flank angle? To identify it, picture a thread profile in your mind. Imagine slicing a tap or screw lengthwise (mentally, of course) and examining the edges of the cut surface. The thread profile resembles a jagged line or a series of peaks, much like a mountain range. Each peak has two inclined planes, which are referred to as flanks. The flank angle is the angle formed between two opposing flanks.

For conventional pointed threads, this visualization works well to show where the flank angle is located. But what about other thread types like trapezoidal or round threads, which don’t form such a clean, continuous profile? In those cases, simply extend the thread flanks in your imagination until they meet at a point. The angle between them is the flank angle.

What Effects Does the Correct Flank Angle Have?

The flank angle plays a crucial role in determining the friction and self-locking properties of threads. In practical terms, the flank angle affects how easily a screw can be tightened or loosened. A smaller flank angle results in smoother threading. Whether you want less friction or stronger self-locking depends on the application. For instance, threads used in motion systems typically have a small flank angle since excessive friction isn’t desirable. On the other hand, standard fastening threads require good self-locking to prevent loosening, so they use a larger flank angle.

Common Flank Angles Used Today

Flank angles are standardized and play an integral part in thread design. They’re the product of precise calculations and extensive experience. Despite this, only a limited number of flank angles are used in modern threads. It’s vital to pay close attention to the flank angle you’re working with, as even minor deviations can make threads incompatible.

Flank Angles on Pointed Threads

The most commonly encountered flank angle is 60°, found in pointed threads and fastening threads like metric standard and fine threads, as well as American imperial threads (UNC, UNF, UNEF). Be cautious when dealing with British imperial standards like BSW and BSF threads, which have a flank angle of 55°. Similarly, British pipe threads, including the widely-used British Whitworth pipe thread G, also feature a flank angle of 55°.

Flank Angles on Motion Threads

Unlike the higher flank angles used in fastening threads, motion threads typically have much smaller angles, usually around 30°. Examples include the metric ISO trapezoidal thread Tr (30°), the metric saw thread S (33°), and the round thread Rd (30°). Beware of international variations though—American ACME trapezoidal threads, for instance, have a flank angle of 29°, slightly smaller than their metric counterparts.

Special Flank Angles

While most threads fall under the categories mentioned above, there are exceptions and historical curiosities worth noting. For instance, saw threads often have two distinct flank angles due to their asymmetrical profile, designed for one-sided loading. Metric saw threads typically have a flank angle of 30° + 3°.

Some older thread types feature unusually large or small flank angles. For example, steel armor pipe threads have a flank angle of 80°. Historical threads even included ones with a 0° flank angle, resembling a right-angled profile. These were precursors to today’s trapezoidal threads but were abandoned due to manufacturing challenges and susceptibility to wear.

No matter the flank angle you need, rest assured that BAER’s online store offers the perfect tap or die for your requirements. Browse our selection and feel free to reach out with any questions or concerns!