Tap is a tool used for cutting internal threads, featuring grooves along its axial direction. It is also referred to as a screw tap. Based on their shapes, taps are categorized into straight flute taps, spiral flute taps, and point taps (also known as tip taps). Straight flute taps are easy to manufacture and have slightly lower precision, but they are produced in large quantities. They are generally used for threading on conventional lathes, drilling machines, and tapping machines, with relatively slow cutting speeds. Spiral flute taps are mostly used on CNC machining centers for drilling blind holes, offering faster processing speeds, high precision, good chip evacuation, and excellent alignment. Point taps have chip pockets at the front and are used for processing through holes. Most taps provided by tool manufacturers are coated taps, which significantly improve tool life and cutting performance compared to uncoated taps. Taps with unequal pitch design distribute the cutting load reasonably, resulting in high processing quality, but they also have higher manufacturing costs. Taps for trapezoidal threads often adopt an unequal pitch design.

Machine taps and hand taps are standard taps for cutting general-purpose threads. In China, it is customary to refer to high-precision high-speed ground thread taps as machine taps, while taps made of carbon tool steel or alloy tool steel with rolled or cut threads are called hand taps. In reality, the structure and working principles of both are essentially the same. Typically, a tap consists of a working part and a shank. The working part is further divided into the cutting section and the calibration section. The former is ground with a cutting taper and is responsible for the cutting work, while the latter is used to calibrate the size and shape of the thread.

Taps are used for cutting general internal threads on nuts or other machine components (i.e., tapping). Machine taps usually refer to high-speed ground thread taps suitable for tapping on machine tools; hand taps refer to taps made of carbon tool steel or alloy tool steel with rolled (or cut) threads, suitable for manual tapping.

Taps are tools for processing various small and medium-sized internal threads. They have a simple structure, are easy to use, can be operated manually or on machine tools, and are widely used in production.

For small-sized internal threads, taps are almost the only cutting tool available. Types of taps include: hand taps, machine taps, nut taps, form taps (extrusion taps), etc.

Tapping is a relatively difficult machining operation because the tap is almost buried in the workpiece during cutting. The load per tooth is greater than that of other tools, and the contact area between the tap and the workpiece along the thread is very large. When cutting threads, it must accommodate and remove chips. Therefore, it can be said that taps operate under very harsh conditions. To ensure smooth tapping, potential issues should be considered in advance, such as the properties of the workpiece material, selection of tools and machines, cutting speed, feed rate, etc.

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Tapping on Special Workpiece Materials

The machinability of the workpiece material is key to the difficulty of tapping. Tap manufacturers are now primarily focused on developing taps for machining special materials. Based on the properties of these materials, the geometry of the tap's cutting part is modified, especially its rake angle and hook (the degree of concavity on the face). The maximum processing speed is sometimes limited by machine tool performance. For smaller taps, the spindle speed required to achieve the ideal speed [rpm = (sfm × 3.8) / tap diameter] may exceed the maximum spindle speed. On the other hand, high-speed cutting with larger taps generates significant torque, which might exceed the horsepower provided by the machine tool. Using internal cooling tools at 700 psi, cutting speeds can potentially reach 250 sfm. On machine tools without internal cooling facilities, cutting speeds are limited to about 150 sfm. Taps differ from most metal cutting tools because their contact area with the hole wall of the workpiece is very large, making cooling crucial. If a high-speed steel tap overheats, it can break or burn. The geometric characteristics of NORIS company's high-performance taps include a large relief angle and back taper.

The aforementioned tap geometry, combined with special surface coatings (such as TiN, TiCN, CrN, or TiAlN), can significantly extend tap life. These heat-resistant, smooth coatings reduce cutting forces and allow tapping at higher speeds. In fact, the development of newer high-performance taps has greatly contributed to the increase in machine tool spindle speeds and power.

Increase in Carbide Taps

Similar to how carbide tools are gradually replacing high-speed steel tools in turning, carbide taps are also being used more frequently for threading holes. Compared to high-speed steel, carbide has higher hardness but greater brittleness. Using carbide taps for tapping presents challenges with chip handling. Nevertheless, carbide taps perform very well in machining cast iron and aluminum alloy materials, with mechanical wear being the main failure mode.

As the automotive industry processes large quantities of cast iron and aluminum alloy parts, carbide taps are used to achieve longer tool life. When machining these materials, carbide taps last longer than high-speed steel taps. In the automotive industry, reducing tap change times is clearly an important factor, and the long life of carbide taps minimizes changeover times. Using coated carbide taps with a low helix angle for tapping aluminum alloy workpieces with 8%-12% silicon content works very well. Taps made from submicron-grain carbide increase tool toughness without reducing hardness, performing excellently when cutting hardened steel, plastics, and difficult-to-machine nickel-based alloys. The DL15 Ni series of nickel alloy specific taps produced by NORIS can, under certain conditions, continuously tap over 200 threads in Inconel 718 nickel-chromium-iron alloy, whereas previously, regrinding was necessary to achieve this.

Functional Characteristics

Taps are usually single or supplied in sets. Small and medium-sized through-hole threads can be tapped with a single tap in one operation. When processing blind holes or large-sized threads, sets of taps are commonly used, i.e., two or more taps are used sequentially to complete a threaded hole. Tap sets have two designs: equal pitch and unequal pitch. In equal pitch designed sets, each tap differs only in the length of the cutting taper; in unequal pitch designed sets, the thread dimensions of each tap are different, and only the final tap has the full thread form.

Selecting Thread Standards

Common general-purpose threads have three standards: Metric, Imperial (British Standard Whitworth), and Unified (also known as American Standard). Metric threads use millimeters as the unit and have a 60-degree thread angle. For example: M8X1-6H indicates a metric fine thread with a major diameter of 8 mm, a pitch of 1 mm, and a 6H internal thread tolerance zone.

Imperial threads use inches as the unit and have a 55-degree thread angle. For example: BSW 1/4-20 indicates a major diameter of 1/4 inch, with a coarse pitch of 20 threads per inch. This type of thread is rarely used now. The Unified thread system uses inches as the unit and has a 60-degree thread angle.

For diameters smaller than 1/4 inch, numbers are often used for designation, ranging from No. 0 to No. 12, representing diameter specifications from 0.06 inches to 1/4 inch. The Unified thread system is still primarily used in the United States.

Selecting Tap Types

Commonly used types are: straight flute taps, spiral flute taps, point taps, and form taps (extrusion taps), each with its own strengths.

Straight flute taps are the most versatile, suitable for both through and blind holes, and for both non-ferrous and ferrous metals. They are also the least expensive. However, they are less specialized; they can do everything but are not the best at any specific task. Spiral flute taps are more suitable for machining blind hole threads, as the chips are discharged backwards during machining. Due to the helix angle, the actual cutting rake angle of the tap increases with the helix angle.

To improve tap cutting efficiency, enhance chip accommodation and evacuation, and reduce chipping and breakage, modern taps have various new structures.

1. Point Taps (Spiral Point Taps): The cutting section is ground with oblique grooves, forming a negative inclination angle (see cutting tools). During cutting, chips are pushed forward, suitable for through holes.

2. Spiral Flute Taps: The chip flutes are spiral-shaped. When machining blind right-hand threads, the tap is made with right-hand spiral flutes to push chips forward and prevent scratching the threads.

3. Fluteless Form Taps (Extrusion Taps): These form threads through plastic deformation of the hole wall metal, mainly used for machining ductile materials like aluminum alloy and copper, but also suitable for low-carbon steel and stainless steel. The front end of the tap has a tapered forming section. To reduce friction and lower the extrusion force, the tap's cross-section is made polygonal. Extrusion taps have high strength and are particularly suitable for machining small threaded holes below 6 mm in diameter.

4. Jump Flute Taps (Interrupted Thread Taps): Every other tooth along the thread helix is ground away, increasing chip thickness, which is beneficial for chip breaking and evacuation. Used for machining workpieces like stainless steel.

5. Internal Chip Evacuation Taps: Chips are discharged through the internal hole of the tap, used for machining large-sized threads.

6. Self-Retracting Taps: After tapping is completed, the tap teeth automatically retract inward for quick withdrawal.

7. Broach Taps: Essentially a broach with cutting teeth distributed along a helical line, often used for machining trapezoidal and square threads.

8. Carbide Taps: Mainly used for machining cast iron and non-ferrous metals, offering high cutting efficiency and tool life.