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Thread milling, or hole milling, is a metalworking process in which threads of various sizes are cut using the circular ramping motion of a rotating tool, with the pitch of the thread produced by lateral motion in one revolution. Unlike tapping, thread milling is mainly used for large workpieces. However, solid carbide thread mills can also produce small threaded holes. The same machine can be used for left and right-hand threads, different thread tolerances, different materials and hole diameters, producing threads with high precision and excellent surface finish. Although thread milling is not as widely used as thread turning, it can be very useful in some applications. How do you bring a hole into perfect alignment? What are internal threads? We write about this and much more below!

Benefits of thread milling

The main advantages of thread milling are:

  • Better thread quality: Thread milling can provide better thread quality than tapping because it provides better chip evacuation. Thread milling uses the same tool as the thread size, pushing the chips out through the thread.
  • Flexible tools: the same cutter can be used for internal and external threads and is also suitable for tool holders. You can also use the same tool for threads of different diameters, as long as the pitch is the same.
  • Safe machining: Short chips and low cutting forces compensate for minimal machining downtime. In the event of a tool failure, since the diameter of the thread mill is smaller than the thread, the tool will not jam, so there is less chance of damaging the workpiece.
  • Turning threads in materials that are difficult to machine: Excellent cutting conditions allow thread milling in hardened steel up to HRC 65, titanium and other materials considered difficult to machine.
  • Threading blind holes: Thread milling will provide a full thread profile from top to bottom of the hole. But you’ll need to drill deeper when tapping, because the tapping machine doesn’t begin to create a full thread profile until the third thread.

Learn more about how to improve the quality of threaded holes, the key differences between tapping and thread milling, and choosing the right tool for your application by reading the rest of the article!

Thread milling tools

Thread milling cutter: is a tool designed for cutting threads by milling. A wide range of thread milling cutters or cutters for internal, external, medium and large threads. Thread milling cutters can be used on CNC machining centers with simultaneous, triaxial and helical interpolation functions. Thread milling with solid carbide thread mills or steel chucks with cutting inserts. Solid carbide thread mills: for smaller holes, they are more expensive than interchangeable milling cutters, but can be cost-effective depending on the quality required.

Thread mills with replaceable inserts: They are not suitable for holes with a diameter of 0.625 inches or less. When the body of the thread milling cutter is ready, the cost of manufacturing the thread includes only the replacement of the inserts.

How does a thread milling cutter work?

The thread milling process can replace thread machining with helical interpolation of the tool path. A thread milling cutter looks similar to a thread cutter, but has a different function. With it, you can insert the thread cutter into the hole along the spindle axis until the desired full thread depth is reached, the controller moves the thread cutter to the aperture position and cuts the thread on the side walls of the hole, then moves 360° in a circle and returns to the starting position. When the threading tool makes a circular motion, it must move toward the top edge of the hole, or one stroke along the machine’s Z-axis, to form the thread. The depth of the thread is usually no more than one and a half times the diameter of the hole to minimize deflection.

Application of thread milling

Thread milling is an appropriate machining process for the following applications or requirements:

  • Thin-walled components.
  • Asymmetrical/non-rotating components.
  • Need to reduce tooling inventory.
  • Unstable component configuration.
  • Adjustment of thread tolerances.
  • Cutting multiple odd threads with a single tool.
  • You can re-chamfer the hole with the process.
  • Machining the material generates high cutting forces and can easily lead to chip evacuation problems.

Thread milling vs. threading – the differences

Here are the basic differences that apply to thread milling and threading:

  1. Threading is typically used to create threads in small holes after drilling, while thread milling tends to create threads in larger holes.
  2. Threading uses a tool that is the same size as the thread, pushing chips out of the thread, while thread milling uses a tool that is smaller than the thread, so thread milling provides better thread quality than threading.
  3. Thread milling can be done by hand or by machine. Thread milling usually requires a CNC machine with at least three axes for helical interpolation.
  4. Thread milling requires less cutting force than thread machining.
  5. With thread milling, it is not necessary to have the right hole size, for thread milling you can use one tool for different hole sizes. When thread milling, each hole to be tapped requires a different size tapping tool.
  6. Thread milling makes it easy to produce pipe threads without the troublesome, sticky shavings created during the threading process.
  7. Thread milling provides better thread quality and smoother surfaces than tapping. Thread milling allows users to design custom threads, which can be very expensive and have long lead times.
  8. Thread milling does not allow adjustment of the thread fit. Once the hole is tapped, the size and position of the thread is determined. While thread milling can control the fit. Machine operators are able to adjust the thread size using a strategy similar to using a face mill instead of a drill bit to make the hole.

How do you mill threads? 

Here are some tips on how to perform milling professionally: 

Choosing the right tool 

A high-quality and suitable thread mill is very important to avoid malfunctions and problems during machining. You can choose several qualified tool manufacturers, compare their advantages and disadvantages and choose the one that is closest to your requirements. It is necessary to master the types of thread milling cutters (spiral groove thread milling cutter, straight groove thread milling cutter, single-tooth thread milling cutter, indexable insert thread milling cutter, solid carbide thread milling cutter, etc.) forms, such as diameter. 

Make several radial passes

Dividing thread milling operations into multiple radial passes ensures higher quality, improves thread tolerances, ensures tool safety and prevents tool breakage in difficult materials, and makes it suitable for difficult applications such as long overhangs and unstable conditions.

The right strategy for breaking and chip removal

Use the right tool or strategy to break and remove chips in a timely manner. You can consider using coolant and lubrication to avoid re-cutting chips, save time and ensure thread tolerance.

Specify tool path and thread depth 

Apply left or right thread depending on the tool path you choose. Choose the more efficient method based on your experience. As a general rule, the thread depth should not exceed one and a half times the diameter of the hole. Because the longer the cutter, the more likely it is to bend, which can lead to inaccurate threads.

It is useful to know how to choose the best cutting parameters, as this will make the correct thread diameter. Cutting speed, thread diameter, hole bottoms and other factors affect the quality of production. Let’s remember that there is also concurrent thread milling and counter-rotating thread milling. Let’s also pay attention to what kind of materials we are machining. On hardened materials, thread milling will be performed differently than on conventional steel. In addition, during co-rotating milling, the direction of the tool’s feed motion follows the direction of rotation. Remember to pay attention to terms such as internal thread, external thread, and thread pitch when milling threads, which will help the whole process. Regardless of which concept you choose, it is worth reading the above guide!