What you should definitely know about flank angles

When it comes to thread profiles, there’s a lot more going on than meets the eye. Each thread profile includes an intricate combination of precisely measured dimensions and angles. While it might seem overwhelming, you don’t need to worry about understanding every single detail to get the job done. Manufacturers handle the complex calculations for you, ensuring that the final product fits perfectly without any hiccups. However, there are a few key variables within the thread profile that you should be aware of, especially if you're planning a specific project. One of the most crucial elements is the flank angle. The flank angle plays a vital role in defining the thread profile. It’s often overlooked in commonly used threads where the flank angle remains consistent. However, things change when you’re dealing with foreign standards or specialized movement threads. In those cases, getting the flank angle right becomes essential. To help you stay prepared, we’ve compiled all the necessary information about flank angles.

Where is the Flank Angle Located in the Thread Profile?

Let’s break this down step by step. What exactly is the flank angle? To identify it properly, picture the thread profile in your mind. Imagine slicing through a tap or screw lengthwise and looking at the edges of the cut surface. The thread profile will resemble a jagged line, similar to a mountain range. Within this profile, the two inclined planes forming each peak are referred to as flanks. The angle between these opposing flanks is the flank angle. For traditional pointed threads, visualizing the flank angle is straightforward. But what happens with other types of threads, like trapezoidal or round threads? Even though their profiles don’t create a seamless mountain chain, they still have flank angles. Simply extend the flanks in your imagination until they intersect, forming a point. The angle created is the flank angle.

What Impact Does the Flank Angle Have?

The flank angle affects the friction and self-locking properties of threads. In practical terms, the flank angle determines how smoothly a screw can be tightened or loosened. A smaller flank angle results in a smoother thread. Depending on the application, varying levels of friction or self-locking might be required. For instance, threads used in motion systems typically have a small flank angle since excessive locking isn’t desirable. On the flip side, standard fastening threads require robust self-locking to prevent accidental loosening.

Frequently Used Flank Angles

Flank angles are critical and fixed elements of thread standards, resulting from meticulous calculations and extensive experience. Although only a few different flank angles are utilized in modern threads, it’s important to pay close attention to which angle applies to your work. Even minor deviations can render threads incompatible.

Flank Angle on Pointed Threads

The most common flank angle you’ll encounter is 60°. This includes metric standard and fine threads, along with American imperial fastening threads like UNC, UNF, and UNEF. Be cautious when working with British imperial thread standards. The BSW and BSF threads from the UK have a flank angle of 55°. The same goes for British pipe threads, including the widely used British Whitworth pipe thread G, which also features a 55° flank angle.

Flank Angles on Movement Threads

Movement threads typically have much smaller flank 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°). However, there are international differences in movement threads. The American trapezoidal thread ACME has a flank angle of 29°, slightly less than its metric counterpart.

Special Flank Angles

While the majority of threads fall under the previously mentioned flank angles, there are exceptions and curiosities worth noting. For instance, saw threads often feature two flank angles due to their one-sided load design. Their asymmetrical profile resembles a saw tooth, with distinct load-bearing and non-load-bearing flanks. The flank angle is the angle formed by an imaginary line perpendicular to the axis and these two flanks; for metric saw threads, this might be 30° + 3°. Some older thread types, characterized by unusually large or small flank angles, are becoming obsolete. One such example is the steel armor pipe thread, which boasts an 80° flank angle. Historical threads even existed with a 0° flank angle, resembling today’s trapezoidal threads but with a rectangular profile. These early designs proved challenging to manufacture and prone to wear, leading to their eventual replacement by versions with larger flank angles. No matter which flank angle you require, rest assured that BAER has the perfect tap or die for your needs in our online store. Feel free to browse our selection and reach out with any questions or concerns. We’re here to help!