What is thread milling?

In engineering, a “thread” is a spiral groove on an external cylindrical or hollow cylindrical geometry that can be anything from a small screw to a custom part for a large machine assembly. Cutting, tapping, and milling are the key processes for producing threads. Thread milling is generally used to produce threads on medium to large workpieces.While this mechanical structure may seem simple, it has a wide range of uses, such as fastening, motion transmission, radial load bearing, and sealing. It is also complex and challenging to manufacture accurately.

This article will provide basic information on the process of CNC milling threads in metal and plastic workpieces, and we will also compare it with tapping at the end of the article.

What is thread milling?

Thread milling is an economical and versatile process that can cut a variety of threads, parts, and workpiece materials on the same machine. Our solid carbide thread milling cutters, multi-tooth indexable insert tools, and Threading Wizard software make thread milling easy and produce high-quality threads with excellent tooth shape and surface.

In general, thread milling produces an excellent burr-free surface and reduces tool inventory costs. Shops can use the same tool for both left-hand and right-hand threads and different thread tolerances. In addition, the same tool can be used to thread mill a wide range of materials and hole diameters. Thread milling differs from tapping in that it can produce full thread depths with first-rate accuracy, even in hardened materials.

Depending on your needs, you can use a milling machine to produce both internal and external threads. The tool moves in a spiral-like circular motion along the side of the workpiece, removing material and forming the spiral structure. For internal threads, the tool follows a spiral path along the outside of the cylindrical workpiece, and when the tool enters the hole, it follows a similar path along the inside wall.This method is suitable even for large holes and thin-walled parts. A single tool can be used for different projects with different pitch diameters, providing flexibility and customization. As a result, you can mill threads in hard materials such as tool steel (> 45 HRC), titanium alloys, and Inconel.

Terms of Mechanical Threading

In the engineering and manufacturing fields, there are specific terms used to describe the design and production process. Before we go any further, let’s understand these terms.

• Major Diameter: The outside diameter (or the diameter of the shaft on which the thread is formed)
• Minor Diameter: The inside diameter (minimum diameter) after the helix is created/the diameter of the hole before threading (if it is an internal thread)
• Crest: The top (top edge) of the helical shape.
• Root: The bottom surface between the two thread flanks.
• Flank: The surface of the thread located between the crest and the root.
• Thread Angle: The angle formed by two adjacent thread flanks.
• Pitch: The pitch is the distance between two adjacent crests.
• Pitch Diameter: It is the diameter of the male/female component that is properly mated with it.
• Lead: Refers to the length/distance that the thread travels per turn when mated.

Types of CNC Machined Threads

There are many types of threads according to different classification standards. In the unified thread system, UNF fine pitch threads and UNC coarse pitch threads are standard threads. But generally speaking, there are two types of threads: internal threads and external threads.

• Internal threads: Internal threads are machined using single-edge thread cutters. In addition to this single-edge tool, traditional threading methods can also help cut internal threads. It is important to note that cutting internal threads only occurs on concave surfaces. So, when do you need internal threads? If the workpiece needs to have screws inserted, internal threads are needed. Typically, machinists use hand taps or machine taps to cut internal threads.

• External threads: External threads are used for screws, bolts, studs and plug gauges, and using a lathe is one of the most effective ways to make external threads. Another method is to cut external threads manually using round dies. The round dies used in this case are still in a fixed mold library. In addition to round dies, there are hexagonal dies, which are ideal for interlocking connections.

Thread Milling Workflow

The key mechanism is that the thread milling cutter moves along a spiral path, which is controlled by the CNC program. Typically, the workpiece remains stationary while the pointed milling cutter rotates and its cutting edge cuts the material. Unlike tapping, the milling cutter geometry is not directly related to the thread geometry. Instead, the movement along the X and Y axes determines the diameter, and the Z axis controls the pitch.The steps of CNC thread milling are as follows;

Step 1: Machine Setup and Tool Selection

First, select the appropriate type of CNC milling machine or dedicated thread milling machine. The workpiece is pre-machined, and holes or outer surfaces are drilled or ground where threads are to be machined. Therefore, it is equally important to choose the right tool: single-edge milling cutters are suitable for small-size precision thread machining, multi-tooth milling cutters are suitable for high-speed machining, and indexable milling cutters are suitable for large-size thread machining. The choice of tool size depends on thread parameters, such as major and minor diameters.

Step 2: CNC Programming

After the tool is mounted on the spindle and the workpiece is aligned with it, G and M codes control all tool movements. They control the tool path, number of passes, material removal rate, depth, etc. The CNC program also defines the entry and exit arcs of the cutting edge, the thread type (e.g., M10x1.5, UNC), pitch, tool clearance, and feed rate.

Step 3: Thread Cutting Process

At this stage, the CNC program runs and the machine tool starts cutting threads. The tool enters at a predetermined arc and cuts along a spiral path, and retracts after the last operation is completed.

Thread Mill Types

There are many types of thread mills available that vary in geometry, chip evacuation mechanisms, and ability to cut specific thread types. Let’s discuss them briefly.

• Straight Flute Thread Mill: As the name implies, this thread mill has straight grooves that evacuate chips as the teeth engage and remove material. The cutting teeth continuously engage and cut the material, and the straight grooves between the teeth evacuate the chips. Straight flute mills are more commonly used to cut threads in easy-to-machine materials such as aluminum and ABS.

• Spiral Flute Thread Mill: A spiral flute mill has a set of teeth on the circumference of the tool that are arranged in a spiral path with spiral grooves between the teeth. The helix angle is usually between 15 and 30°. This geometry is better for chip evacuation and faster cutting speeds than a straight flute mill, and it produces a better surface finish.

• Single-Flute Thread Mill: This thread mill cuts the thread groove in one pass (or one thread at a time). It is primarily used for internal threading; the teeth engage and move forward in the material while the spiral grooves evacuate the chips. Although speeds are lower, you get more flexibility with a single-profile cutter.

• Single-profile thread mills: Unlike single-profile cutters, single-profile cutters have multiple cutting edges that can cut multiple threads simultaneously while maintaining a consistent thread profile. You can cut threads of the same pitch and size faster and more accurately. However, these cutters are less flexible and are only suitable for the specific thread size they were designed for. For example, a single-profile cutter designed for M6 × 1 will not cut M6 × 0.75.

• Polypron thread mills: Whereas single-profile cutters can only cut one thread groove in one pass, polypron cutters can cut multiple pitches using teeth with axial spacing that matches the corresponding pitch size. Polypron cutters are also used to quickly machine threads of specific thread types, sizes, and pitches.

When should you choose thread milling?

Thread milling creates threads by helical ramping of a rotating tool. The axial movement of the tool in one revolution creates the pitch. Although not as widely used as thread turning, thread milling can achieve high productivity in certain applications. Thread milling should always be chosen when:

• When machining asymmetric/non-rotating parts
• When machining materials that have chip breaking and chip evacuation issues
• When machining tough materials that generate high cutting forces
• When cutting into shoulders or near the bottom of blind holes
• When machining thin-walled parts
• When the part clamping is unstable
• When you need to minimize tool inventory
• When you don’t want to risk tap breakage when machining expensive parts – the thread mill can always be completely removed from the part
• You need a machine that can move in the X, Y and Z axes simultaneously

Benefits of thread milling

Flexibility: A single thread cutter can perform a variety of thread milling types in different sizes, such as internal and external threads.

• Customization: Tools/cutters can be designed to specific sizes and can create identical threads over a wide range.
• Fast production: Automation in CNC thread milling reduces production cycle times and allows for fast cutting while minimizing deviations in size and quality.
• Various material options: Thread mills are compatible with both soft and hard materials; you can even create threads in difficult-to-machine materials with the right tool.
• Minimal wear: The cutting tool acts only on the area that needs to be threaded, not on the entire circumference. As a result, wear on the workpiece is minimal.
• Precision: CNC milling machines can achieve as low as

Disadvantages of thread milling

Not suitable for small hole machining: Advanced machinery and time-consuming setup may not be economically viable for simple threads and small production runs. Thread milling cutters are not suitable for small holes; usually requires a hole diameter of ≥ 6 mm.

• Complex setup: Thread milling generally requires a more complex setup than conventional tapping. This includes selecting the right tool, machine calibration, and programming, which increases preparation time.
• Tooling costs more: Thread milling cutters are generally more expensive than taps, especially when you need multi-edge tools for high-performance applications.
• Slower speeds: Compared to tapping, thread milling generally has slower cutting speeds, especially when processing larger thread sizes. This can lead to longer cycle times, especially in large-scale production.
• Limited material compatibility: While thread milling can machine hard materials, it may not work well with certain alloys or extremely hard materials such as hardened steel. In these cases, tapping may be more effective.
• Smaller thread depths: For shallow or medium-depth threads, thread milling is generally more effective. For deep threads, tapping may be more effective, especially in terms of tool wear and cycle time

Differences between thread milling and tapping

Tapping is a traditional thread cutting method that uses a tap that rotates and feeds into the material. It is generally used to produce threads with smaller diameters and is not as flexible as thread milling in terms of thread customization. Tapping is another method of producing threads inside a hole (internal bore) and requires the use of a special tool called a “tap”. The tapping tool enters the hole and forms a thread that is the same size as the hole itself. Tapping tools can be operated manually or by CNC machines and are capable of producing threads in easy-to-machine materials such as aluminum, steel and ABS.

Differences between thread milling and threading

Thread milling and threading (tapping) differ in terms of process, flexibility and material suitability. Thread milling is more versatile and can handle a wider range of thread types and materials, while tapping is generally faster for small standard threads. The choice between thread milling and tapping depends on the specific application requirements. Thread milling is often a better choice for large diameter threads, hard materials, or custom profiles. Tapping may be better suited for smaller threads or high-volume production where speed is a priority. To determine whether thread milling or tapping is right for your application, consider factors such as material type, thread size, and production volume. Consulting with an Xtproto expert can also help you make an informed decision based on your specific needs.

Common Thread Cutting Methods in CNC Machining

There are a variety of thread cutting methods in CNC Machining. These different methods each have their own characteristics and advantages. Here, we will discuss the common methods.

• Tapping: Tapping holes are very suitable for making internal threads. It is an economical and efficient thread cutting method. This CNC method is suitable for threaded holes that do not require particularly high precision and holes with small apertures. The cutting structure of tapping is simpler than other methods. In addition, tapping is a high-speed cutting process that effectively improves productivity and cutting efficiency. This cutting method uses cheaper cutting tools, thereby reducing manufacturing costs. It is also a versatile process with a wide range of applications.

• Thread milling: This is another method for making threads. This process uses a milling cutter and a CNC machining center. Thread milling is suitable for creating threads for large parts or high-value materials. Thread milling is characterized by fast processing speed and high accuracy. The milling cutter used usually contains carbide. In addition, the versatility of the milling cutter tool reduces the need to obtain different tools for different milling operations. For example, the same tool can cut left-hand and right-hand threads with the same pitch. Threads produced using milling have a high-quality surface. This method is ideal for making thin-walled parts, machining blind holes, and asymmetric or non-rotating parts.

• Thread turning: Cutting threads on a lathe is another common thread cutting process with a wide range of applications. CNC lathes help produce high-quality threads. Using this method, machinists can create multiple tapered threads, lead threads, and pitches. Rigid tapping and single-point thread cutting are two of the most common methods used for thread cutting on CNC lathes. Single-point thread cutting uses a tool with an indexable insert whose shape and size corresponds to the finished screw head.

• Thread grinding: This method is ideal for thread cutting hardened workpieces. There are two main types of grinding wheels used in this process: multi-line grinding wheels and single-line grinding wheels. The pitch accuracy of the single-line grinding wheel is 5~6 levels. It also has a surface roughness of R1.25~0.08 microns. Single-line grinding is ideal for making precision screws, worms, thread gauges, relief hobs, and small-batch thread cutting.Multi-line grinding is divided into plunge-cut and longitudinal grinding methods. The main difference between the two methods is the width of the grinding wheel. In the longitudinal grinding method, the width of the grinding wheel is less than the length of the thread being ground. On the other hand, the width of the grinding wheel used in the plunge-cut method is greater than the length of the thread.

Need a CNC Thread Milling Partner?

As we discussed, thread milling can process various types of screws and threads, including both external threads for shafts and internal threads for drilled holes. But to achieve precise results, the right tools, milling parameters, and CNC programs need to be used.Xtproto’s CNC milling services offer a variety of operations, including CNC thread milling. We have multi-axis CNC machines and experienced engineers to ensure that your project is completed smoothly. We provide comprehensive solutions, from design optimization for manufacturability to surface treatment, at very competitive prices. If you are looking for a reliable and professional partner to process the threads and prepare your design, Xtproto can provide you with a solution.