Types of CNC Thread Taps

Internal threads are a fundamental feature of many mechanical components, used to fasten them together, form a whole, or achieve specific functions. They facilitate secure and adjustable connections between components such as screws and nuts. Threading is a fundamental and critical process in the machining and manufacturing industries. As the primary tool for creating internal threads, the choice of tap directly impacts machining efficiency, thread quality, and tool life. Faced with a wide variety of taps on the market, selecting the right type for specific machining needs presents a challenge for many engineers. This article discusses 13 different types of thread taps, their dimensions, tolerance tables, and professional tips for efficient thread tapping.

What is a thread tap?

A thread tap is a precision tool used to cut internal threads. Taps are typically made of high-speed steel, carbide, or other high-performance materials. They have a threaded appearance, with straight or spiral grooves along the axial direction, forming the cutting edge. The tap’s dimensions and tooth profile must perfectly match the thread being machined, including parameters such as diameter, pitch, and profile angle. This tap works by transmitting cutting force to the workpiece through a rotational motion, gradually removing material and forming the thread groove. Unlike thread milling cutters, taps produce threads through form cutting rather than generating. This reduces the rigidity requirements of the machine tool, but places extremely high demands on the precision and strength of the tap itself. Tapping can be done manually using the tap handle, or it can be automated by integrating these tools into CNC drilling machines. In manual tapping, a tap (size equal to the inside diameter) is rotated against a die, removing surface material and forming the thread. A tool holder, on the other hand, rotates the tap at a fixed speed using a drilling machine. Regardless of the method used, the tapping tool determines the final quality and precision.

Standard Tap Marking

Standard taps have various information engraved on their body through markings or engraving. This information also systematically informs the user of the tap’s performance. The figure below shows a typical marking format.

Nominal Size: The nominal size of a tap refers to the outside diameter of the thread it will cut, or the major diameter of the corresponding thread cutter it will mate with after cutting. You can find information related to this marking on the tool. For example, M6 indicates that the tap will produce a thread with a nominal diameter of 6 mm.

Thread Form Symbol: This defines the type and characteristics of the tapped thread, making it easier to select the appropriate tool for the desired shape, size, and other specifications. These are represented by the corresponding thread designator. For example, the Unified (UN) thread form is a common type, and it has related forms such as UNC, UNEF, UNJC, and NPTR. Other thread form symbols include ANPT, AWWA, BA, and others.

Tap Material: High-speed steel, carbide, cobalt, and other tough materials are commonly used to make different types of taps. The specific tap choice depends on the workpiece material type, the complexity, and precision of the desired thread. Carbide tools can tap stainless steel and titanium alloys, while high-speed steel taps have difficulty tapping.

Pitch Diameter Limitation: The pitch diameter is the diameter of an imaginary cylinder passing through the thread form or diameter, excluding the extended flanks of the thread. The pitch diameter is critical because it determines the tightness of the thread against the corresponding bolt or fastener. Therefore, the maximum allowable pitch diameter is limited based on the function and nature of the thread.

Types of Threading Taps

As previously mentioned, a tap is a specialized tool used to create internal threads in holes, enabling quick fastening of bolts and screws. Various tap types meet different threading requirements through their respective cutting mechanisms. These tap types are designed to optimize threading operations by adapting to different materials and project requirements through specific design features. Proper tool selection facilitates accurate and efficient thread cutting in various work environments. The following lists the main tap types used for threading and their unique application characteristics.

Tapered Taps: Taper taps are the initial tool used to create internal threads in hidden cavities. These tools begin cutting threads with an 8-10 thread chamfer below the full cutting diameter, resulting in a gentle threading effect. Bottom taps cannot reach the bottom of the hole, so a different type of tap is required to achieve deep threads.

Plug Taps: Plug taps are suitable for deep threads and can be used immediately after the taper tapping operation. These taps offer enhanced thread-cutting capabilities because they feature 3-5 thread chamfers extending just beyond their full diameter. Plug taps are effective at achieving the correct depth for metric threads in drilled holes.

Bottom taps: The final tapping operation of bottom taps includes one or two thread chamfers, with a completely disconnected hole. Bottom taps are designed to complete the tapping operation while ensuring full penetration to the bottom of the threaded hole.

Master taps: Master taps are similar to tapered taps, but are designed with additional cutting edges. For example, they typically feature more chip flutes, improving performance in more challenging materials. The additional thread-cutting edges on these taps enable them to machine harder materials. Master taps produce precise, flawless threads and are essential for demanding precision machining.

Pipe taps: These taps are specifically designed for cutting threads in pipes, particularly gas fittings commonly used in hydraulic systems, such as BSPT or NPT threads. This milling cutter is suitable for both cylindrical and conical holes and has the ability to produce effective thread cutting in specific applications with 1-2 thread chamfers.

Machine Taps: Machine taps are used with automatic tapping and drilling machines to minimize the risk of damage to the workpiece and the tap itself. These taps are suitable for industrial mass production facilities, reducing manual intervention and increasing operating speed without compromising thread quality standards. Machine taps are also suitable for threading holes using manual or tap-drilling machines. These taps require 2-4 thread chamfers, making them suitable for repair or thread changeover operations where precision is not required. Furthermore, these tools are effective in all types of standard maintenance operations.

Extended Taps: Due to their longer shanks, extended taps offer better access to hard-to-reach holes, similar in function to hand taps. They are particularly well-suited for accessing narrow holes, such as pulley hubs, which are difficult to reach with standard taps.

Curved Shank Taps: Curved Shank Taps feature a handle design that extends directly from the shank, providing continuous tapping capability. This machine design allows for continuous tapping without interrupting equipment positioning, thereby increasing productivity.

Station Bolt Taps: Tie rod bolt taps used in locomotive manufacturing come in sizes ranging from 15 to 20 inches. These tooling units are designed for threading tie rod bolts. These taps are specifically designed for machining large bores within high-pressure boiler systems.

Spiral Fluted Taps: The spiral design of spiral fluted taps improves chip removal efficiency due to their specific flutes. This tool delivers excellent results when threading softer metals, resulting in a shorter chip shape.

Fluteless Taps: The fluteless tap design eschews standard flutes, utilizing pressure deformation to create threads in soft metals. Fluted taps offer high cutting speeds and excellent thread quality, but are suitable only for soft materials.

How to Choose the Right Threading Tap for Your Project?

Selecting a tap suitable for all thread sizes requires consideration of various factors. Material compatibility, thread geometry variations, and assembly clearances must be considered. The following are key factors to consider when selecting a thread tap tool:

Select by Material Type: First, consider the workpiece material you need to tap. Choose a tap that is capable of processing that material. There are three types of tap materials: high-speed steel, carbide, and nitride-coated carbide. High-speed steel is suitable for soft materials like aluminum and thermoplastics, while hard materials like titanium and hardened steel require nitride-coated carbide taps for threading.

Matching the Required Pitch: Pitch refers to the distance between threads. It is one of the most critical parameters that defines thread characteristics and fit. Therefore, selecting a tap type requires considering the required pitch size and finding the appropriate tap size that can achieve that pitch. For example, if you require a 1.5mm pitch, a tap marked M10 x 1.5 meets this requirement. You also need to consider the fit. In the same example, M10 x 1.5, H2, 5G, and 6H codes meet standard fit tolerances.

Determine whether it is a blind or through hole: Check the type of hole you are tapping: blind or through. While chip evacuation is not a problem for through holes, blind holes require upward chip evacuation to avoid clogging and thermal damage. For blind hole threads, you can choose from spiral point, base, spiral flute, and more.

Consider flute requirements: The tapping edge requirement is generally related to the number of flutes required for effective tapping. Depending on the type of threading tap, it may contain two, three, five, seven, ten, or more flutes. Taps with fewer flutes are suitable for hard materials and blind holes. Taps with more flutes, on the other hand, allow for faster tapping speeds and are best suited for through holes. Furthermore, chip flute geometry influences chip evacuation. Straight flutes direct chips toward the bottom, while spiral flutes push chips forward or upward, depending on the hole type.

Choose based on chip removal requirements: In addition to fluting requirements, consider the length of the hole, chip size and thickness, chip evacuation rate to avoid clogging, and other requirements. For example, spiral flutes or spiral point flutes are best for evacuating long chips, while straight flutes are suitable for evacuating short chips.

Consider right-hand or left-hand threads: Left-hand or right-hand refers to the direction of the thread cutting during tapping. Therefore, you need to consider the direction of tapping, which may depend on the material type, clamping conditions, and other factors. Right-hand taps are best for standard turning needs, while left-hand taps are suitable for projects requiring reverse tapping.

Pro Tips for Efficient Tapping

Here are some proven tips for efficient tapping:

Choose the Correct Drill Size: Drill size directly impacts the tapping process; the hole diameter must be compatible with the tap size you will later create. Therefore, select the tapping drill size corresponding to the desired tapping specifications from the drill size chart.

Choose the Right Cutting Fluid: During tapping, friction between the slot and the hole causes heat buildup, which affects thread quality and can potentially damage the tool. Cutting fluids, such as oil or lubricant-based coolants, are crucial to addressing this issue. Cutting fluids also prevent clogging and chip buildup, promoting an efficient and clean machining process.

Keep the Tap Straight: Maintaining the Tap Vertical refers to the perpendicularity between the workpiece and the tapping head. Any deviation can lead to dimensional inaccuracies and project failure. Ensuring tap perpendicularity requires the use of tap guides, center punches, and tap holding pressure to ensure stable alignment.

Applying the optimal cutting speed: Cutting speed affects the quality and accuracy of chip formation. An inappropriate speed (low speed) can even cause tap breakage and workpiece damage. Therefore, you need to optimize the cutting speed based on the workpiece material type, thread pitch, hole type, and depth, and perform trial runs.

Periodic reverse tapping: The best practice to prevent chip accumulation is to regularly reverse the tap. This involves rotating the tap in the opposite direction after completing a specific number of turns, which helps form fine chips. For example, if you cut right-handed, you should regularly rotate the tap counterclockwise.

Frequently clean the tap: Cleaning the tap is crucial to maintaining its performance and lifespan. Regular cleaning (after a certain production cycle) prevents chip accumulation, rust, and surface degradation.

Conclusion

CNC machining uses different types of taps to create threaded holes in a variety of materials, from thermoplastics to titanium alloys. At the same time, a diverse selection of taps enables manufacturers to tap threads to meet stringent requirements. If you need to select the right tap to meet your design specifications, tap selection depends on the material, hole type, thread size and geometry, and the precision required for a perfect fit during assembly. Furthermore, the quality of the tap itself can impact the final result.

How can I find the right supplier for CNC-machined threaded parts?

If you’re looking for a manufacturer that can process internally threaded parts, Xtproto is a good choice. Our CNC machining services can produce threads on parts processed by milling, turning, laser cutting, drilling, and EDM. Our engineers will analyze your drawings and select the appropriate tap type to efficiently produce precise threads. We can also coat and harden threads or components with threaded features using heat treatment processes. For more details, please contact us today!