Thread milling: procedure and special features

It's no secret that thread cutting isn't the only method for thread production. Threads can also be turned, whirled, shaped, rolled, or even eroded. While our team is passionate about thread cutting, we believe in exploring other techniques to expand our knowledge and improve efficiency. That’s why today we’re taking a closer look at an often-overlooked process: thread milling.

If you’ve ever used a thread cutting tool, many aspects of thread milling will feel familiar. Both are chip-removing processes, and the tools share some similarities. However, thread milling follows a completely different approach. In this article, we’ll break down what makes it unique and how it works.

Key Features of Thread Milling

There are several factors that set thread milling apart from traditional methods. Here are some of the most important ones to consider.

The 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 on closer inspection, you’ll notice that the thread mill doesn’t have a threaded profile. Instead, it features profiled grooves spaced at the pitch distance. This design allows for more flexibility in operation.

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 within the drilled hole, shaping the thread as it goes. Because of this motion, the resulting thread diameter is larger than the tool itself. Precision is key—any deviation in movement can lead to an imperfect thread. This level of accuracy is only possible with a 3-axis CNC-controlled machine.

Thread Milling is Highly Versatile

One of the biggest advantages of thread milling is its versatility. Unlike traditional thread cutters, a thread mill can handle both left-hand and right-hand threads, through-holes and blind holes, as well as internal and external threads. Some models can even produce multiple pitches and sizes with a single tool, significantly reducing the number of tools needed.

Thread Milling Produces Small, Manageable Chips

A major benefit of thread milling is the type of chips it produces. Rather than long, spiraling chips, it generates small fragments, which are easier to remove and less likely to cause issues during the machining process.

Thread Milling Works on Large Diameters

The design of a thread mill makes it ideal for large-diameter threads. Unlike traditional cutting tools, which require massive equipment for large threads, a thread mill can handle these sizes with relative ease, making it a great option for heavy-duty applications.

How to Perform Thread Milling

The exact procedure for thread milling depends on the type of tool being used. Some tools allow for multiple steps to be completed in one pass, while others require pre-drilling. For simplicity, we'll divide the process into two main categories: with and without pre-drilling.

Thread Milling with a Pre-Drilled Hole

Some thread mills require a pre-drilled hole. Unlike thread cutting, where the core hole size is strictly defined, here it can vary depending on the application. Once the hole is drilled and countersunk, the thread mill is inserted and begins with a 180° lead-in helix. The thread is then milled using a continuous helical motion, followed by a 180° lead-out helix before the tool exits the workpiece.

Another variation involves a thread chamfer mill with a chamfering stage. In this case, the chamfering step is performed automatically as part of the milling process, eliminating the need for an extra step.

Drill Thread Milling

Some thread mills are designed to drill their own core hole. These are known as drill thread mills, and they feature a chamfering stage and a drilling tip. They can both drill and chamfer the hole before proceeding with the thread milling process.

Important Considerations

When working with thread milling, precision is crucial. Even during pre-drilling, if it’s required, the hole must be accurately positioned and sized. The workpiece should also be securely and stably mounted, as close as possible to the milling area to minimize vibration and ensure accuracy. The machine itself must have high rotational precision.

Cutting speed and feed can be adjusted independently, but the feed rate must be carefully controlled due to the circular motion between the tool’s edge and axis. Incorrect settings can lead to excessive stress on the tool, potentially causing it to break.

It’s also important to note that thread milling has limitations in terms of depth. Most tools can only mill up to about three times their diameter. Additionally, with smaller thread profile angles and coarser pitches, there may be slight deviations in the final thread form. This is especially true for non-standard thread types like trapezoidal, buttress, or knuckle threads.

Is Thread Milling Right for You?

Thread milling offers many advantages, especially when the right conditions are met. However, if you're not in mass production or don't have access to advanced machinery, it may not be the best fit. Many hobbyists and DIYers lack the necessary CNC equipment. Plus, programming a CNC machine for thread milling can be complex, even for experienced users.

If you occasionally need to make threads or are working on a specific project, traditional thread cutting tools might be more practical. With a wide range of screw taps available from BAER, you’re sure to find the right solution. And our expert customer service is always here to help!