Thread milling or hole milling is a metal machining process, in which threads of various sizes are cut by the circular ramping motion of a rotating tool, with the thread pitch being produced by the side movement in one revolution. Unlike tapping, thread milling is mainly used for large objects. However, solid carbide thread mills can also produce small threaded holes. The same machine can be used for left and right 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 to achieve the perfect hole axis? What are internal threads? We write about this and many other things below!
Benefits of Thread Milling
The main advantages of thread milling include:
- Better thread quality: Thread milling can provide better thread quality than tapping, as it ensures better chip evacuation. Tapping uses the same tool as the thread size, pushing the chips through the thread.
- Flexible tools: the same mill can be used for internal and external threads and is also suitable for tool holders. You can also use the same tool to make threads of different diameters, as long as the pitch is the same.
- Safe processing: Short chips and low cutting forces compensate for the minimal risk of downtime in processing. In case of tool failure, since the diameter of the thread mill is smaller than the thread, the tool will not jam, so there is a lower probability of damaging the processed object.
- Thread turning in difficult-to-machine materials: Excellent cutting conditions allow for thread milling in hardened steel up to HRC 65, titanium, and other materials considered difficult to machine.
- Threading of blind holes: Thread milling will provide a full thread profile from top to bottom of the hole. But during threading, you have to drill deeper, because the tap does not start creating a full thread profile until the third thread.
Learn more about how to improve the quality of threaded holes, the key differences between threading and thread milling, and choosing the right tool for a given application by reading the rest of the article!
Thread Milling Tools
Thread milling cutter: this tool is designed for cutting threads by milling. A wide range of thread milling cutters or internal, external, medium, and large thread cutters. Thread milling cutters can be used on CNC machining centers with simultaneous, three-axis, and helical interpolation functions. Thread milling using solid carbide thread milling cutters or steel holders with cutting inserts. Solid carbide thread milling cutters: for smaller holes, they are more expensive than replaceable cutters, but can be cost-effective depending on the required quality.
Thread milling cutters with interchangeable inserts: Not suitable for holes with a diameter of 0.625 inches or smaller. Once the body of the thread milling cutter is ready, the cost of thread production only includes the replacement of inserts.
How does a thread milling cutter work?
The process of milling threads can replace thread cutting with tool path interpolation. A thread milling cutter looks similar to a tap, but it serves a different function. It allows you to insert a 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° around and returns to the starting position. When the threading tool makes a circular motion, it must move towards the top edge of the hole or one step along the Z axis of the machine to create a thread. The depth of the thread is usually no more than one and a half times the diameter of the hole to minimize deflection.
Thread Milling Applications
Thread milling is the appropriate machining process for the following applications or requirements:
- Thin-walled components.
- Asymmetrical/non-rotating components.
- Need to reduce tool inventory.
- Unstable component configuration.
- Thread tolerance adjustment.
- Cutting multiple odd threads with one tool.
- The process allows for the re-phasing of the hole.
- Material processing generates large cutting forces and can easily lead to problems with chip removal.
Thread milling and threading - differences
Below we present the basic differences concerning thread milling and threading:
- Threading is typically used for creating threads in small holes after drilling, while thread milling tends to create threads in larger holes.
- Threading uses a tool that is the same size as the thread, pushing out the chips from the thread, while thread milling uses a tool that is smaller than the thread, so thread milling provides better thread quality than threading.
How to mill threads?
Below we present a few tips on how to professionally carry out milling:
Choosing the right tool
High-quality and suitable thread mill is very important to avoid failures and problems during machining. You can choose from several qualified tool manufacturers, compare their pros and cons, and choose the one that is closest to your requirements. It is necessary to master the types of thread mills (thread mills with spiral grooves, thread mills with straight grooves, single-tooth thread mills, thread mills with replaceable inserts, solid carbide thread mills, etc.) forms, such as diameter.
Perform several radial passes
Dividing the thread milling operation into several radial passes ensures higher quality, improves thread tolerances, ensures tool safety, and prevents tool cracking in difficult materials, making it suitable for challenging applications such as long reaches and unstable conditions.
Appropriate chip breaking and removal strategy
Use the appropriate tool or strategy to break and remove chips on time. Consider using coolant and lubrication to avoid re-cutting chips, save time, and ensure thread tolerance.
Determining the tool path and thread depth
Apply left or right thread depending on the chosen tool path. Choose the more efficient method based on your experience. Essentially, the depth of the thread should not exceed one and a half times the diameter of the hole. Because the longer the knife, the more likely it is to bend, which can lead to inaccurate threads.
It's important to know how to choose the best cutting parameters, as this will allow us to 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-thread milling. Let's also pay attention to what materials we are machining. On hardened materials, thread milling will be performed differently than on classic steel. Additionally, during concurrent milling, the direction of the tool's feed movement is consistent with the direction of rotation. Regardless of which concept we choose, it's worth reading the above guide!