Thread milling: procedure and special features

It's well known that thread cutting isn't the only way to create threads. There are other methods like turning, whirling, shaping, rolling, and even erosion. While we're passionate about thread cutting, it's always good to explore alternative techniques. That’s why we’re diving into a method that’s not often mentioned: thread milling.

If you've ever used a thread cutting tool, you'll find many similarities between that process and thread milling. Both involve removing material, and the tools share some design elements. However, thread milling works on a completely different principle. Let’s take a closer look at what makes this technique unique and how it functions.

Key Features of Thread Milling

Thread milling has several distinguishing characteristics. Here are some of the most important ones.

Design of a Thread Mill

At first glance, a thread mill might look similar to a thread cutter. Both have a shaft and a head with grooves. But if you look closely, you’ll notice that a thread mill doesn’t have a thread itself. Instead, it features profiled grooves spaced at the pitch distance.

Thread Milling Requires 3-Axis CNC Control

To create a thread using a thread mill, the machine must be involved. The tool moves in a helical path inside the drilled hole, gradually forming the thread. The final thread diameter is larger than the tool itself. This precision requires a 3-axis CNC machine to ensure accurate movement along the pitch.

Thread Milling Offers Great Flexibility

Because of its design and operation, a thread mill is more versatile than traditional thread cutting tools. It can produce both left-hand and right-hand threads, as well as through-holes and blind holes. It can also handle internal and external threads. Some tools can even accommodate multiple pitches and thread sizes, reducing the number of tools needed for the job.

Thread Milling Produces Small Chips

One of the major benefits of thread milling is the chip shape. Instead of long spiral chips, the process produces small fragments, which reduces chip clogging and makes removal easier.

Thread Milling Can Handle Large Diameters

Due to its design, thread milling is not limited by thread size. This means it can efficiently machine very large diameters that would require huge tools in traditional cutting methods.

How to Create Threads Using a Mill

The thread milling process depends on the type of tool being used. Some tools can simplify the process. For clarity, we’ll break down the applications into two main types: with and without pre-drilling.

Thread Milling with a Pre-Drilled Hole

Some thread mills require a pre-drilled hole. Unlike thread cutting, there's no fixed formula for the core hole diameter—it can vary depending on the application. Once the hole is drilled and countersunk, the thread mill is inserted to the required depth. The process begins with a 180° lead-in helix, followed by a helical motion to cut the thread. A 180° lead-out helix then allows the tool to exit cleanly.

Another option is the thread chamfer mill with a chamfering stage, which eliminates the need for a separate countersink step. The chamfering is done during the milling process itself.

Drill Thread Milling

Some thread mills can operate without pre-drilling. These drill thread mills include a chamfering section and a drilling tip. They drill their own core hole and chamfer it before proceeding with the thread milling process.

Important Considerations for Thread Milling

Precision is essential in thread milling. Even when pre-drilling is involved, the hole must be accurately positioned and sized. The workpiece should be securely mounted close to the milling area to reduce vibration and improve accuracy. The machine must also have high rotational precision.

Cutting speed and feed rate can be adjusted independently, but the feed rate needs careful calibration due to the circular motion between the tool’s edge and axis. Otherwise, the machine might interpret the speed incorrectly, increasing the risk of tool breakage.

The depth of thread that can be milled is limited. Typically, the maximum depth is around three times the tool’s diameter. Additionally, with small thread angles and coarse pitches, there may be slight deviations in the thread profile—especially in non-standard thread forms like trapezoidal or buttress threads.

Is Thread Milling Right for Your Needs?

Thread milling offers many advantages under the right conditions. However, if you're not working in mass production or don’t have access to a well-equipped workshop, a simpler method might be more practical. Many hobbyists lack the necessary CNC equipment, and programming a CNC machine can be challenging—even for experienced users.

If you occasionally need to make threads or require them for a specific project, traditional thread cutting tools might be a better choice. With BAER’s extensive range of screw taps, you’re sure to find the right solution. Plus, our expert customer service is here to help!