Thread milling: procedure and special features

While thread cutting is a well-known method for producing threads, it's far from the only option. Threads can also be manufactured through processes like turning, whirling, shaping, rolling, or even erosion. Although we at BAER are passionate about thread cutting, exploring alternative methods can often lead to better results. That’s why today, we’re taking a closer look at an often-overlooked technique: thread milling.

If you’ve ever used a thread cutting tool, you’ll find many similarities between that and thread milling. Both are chip-removal processes, and the tools involved share some common features. However, thread milling operates on a completely different set of principles. In this post, we'll uncover what makes it unique and how it works in practice.

Key Features of Thread Milling

Thread milling stands out from other thread production techniques due to several distinct characteristics. Here are some of the most important ones.

Tool Design: A Closer Look

At first glance, a standard thread mill might look similar to a thread cutter. Both have a shaft and a head with grooves. But if you take a closer look, you'll notice that the milling part doesn't actually have a thread. Instead, it has profiled grooves spaced at the pitch distance. This design allows for more flexibility in operation.

Requires 3-Axis CNC Control

To create a thread using a thread mill, the machine must be fully involved. The tool moves in a helical path within the drilled hole, gradually removing material to form the thread. Because of this, the final thread diameter ends up being larger than the tool itself. For precision, the movement must match the thread pitch exactly—something only possible with a 3-axis CNC machine.

Highly Versatile Process

One of the greatest advantages of thread milling is its versatility. Unlike traditional thread cutting tools, a thread mill can handle left-hand and right-hand threads, through-holes and blind holes, as well as internal and external threads. Some tools can even produce multiple thread sizes and pitches with a single tool, reducing the need for multiple cutting tools.

Chip Formation is More Manageable

Another benefit of thread milling is the way chips are formed. Instead of long, spiral-shaped chips, the process produces small, manageable fragments. This reduces the risk of chip clogging and makes chip removal much easier.

Can Handle Large Diameter Threads

Thread mills are not limited by thread size. This makes them ideal for creating large-diameter threads that would otherwise require massive cutting tools. It’s a great solution when working with oversized components.

How Thread Milling Works

The thread milling process varies depending on the type of tool used. Some tools can streamline the process significantly. For simplicity, we'll break it down into two main scenarios: thread milling with and without pre-drilling.

Thread Milling with a Pre-Drilled Hole

Some thread mills require a pre-drilled hole. Unlike thread cutting, there are no strict rules for the core hole diameter—it can vary based on the application. Once the hole is prepared, the thread mill is inserted and begins with a 180° lead-in helix. Then, the thread is milled through a continuous helical motion before exiting with a 180° lead-out helix.

There's also a variation known as the "thread chamfer mill with chamfering stage." This version eliminates the need for a separate countersink, as the chamfering is done during the milling process.

Drill Thread Milling

Another type of thread mill can work without pre-drilling. These so-called "drill thread mills" come with a chamfering stage and a drilling tip. They drill their own core hole and chamfer it before proceeding with the thread milling process.

Important Considerations

Precision is key in thread milling. Even during pre-drilling, high accuracy in both position and dimension is essential. The workpiece should also be securely mounted close to the milling area to minimize vibrations. Additionally, the machine must have excellent rotational accuracy.

When it comes to cutting speed and feed, these can be set independently. However, because of the circular motion between the tool’s edge and axis, the feed rate needs careful adjustment. Otherwise, the machine may misinterpret the speed, leading to potential tool damage.

It's also worth noting that thread mills have limitations in terms of depth. Depending on the tool, the maximum depth is usually around three times the tool’s diameter. In cases of small thread angles or coarse pitches, there may be slight deviations in the thread profile, especially with certain thread types like trapezoidal or buttress threads.

Is Thread Milling Right for You?

Thread milling offers many benefits under the right conditions. However, if you're not engaged in mass production or don’t have access to a well-equipped workshop, simpler methods might be more practical. Many DIYers lack the necessary CNC machines, and programming them correctly isn't always straightforward—even for professionals.

If you occasionally need to make threads or have a specific project in mind, traditional thread cutting tools could still be your best bet. With our wide range of screw taps from BAER, you’re sure to find the right tool for your needs. Plus, our expert customer service is here to help!