Tapping and threading
Tapping and threading are common threading methods in metalworking. Tapping uses a tap to create an internal thread within a workpiece hole, while threading uses a die to create an external thread on a round rod-shaped workpiece. While the two processes operate on different objects, they both adhere to the basic principles of threading and require the proper selection of tools, parameters, and operating methods to ensure thread accuracy and quality. In stamping die and machinery manufacturing, tapping and threading are widely used to connect and secure parts, and are crucial processes for ensuring product assembly performance.
Taps are primarily used as tools for thread tapping. The type and specifications of the tap should be selected based on the type and size of the thread being tapped (e.g., standard, pipe, or trapezoidal). Commonly used taps for standard thread tapping are machine taps and hand taps made of high-speed steel (W6Mo5Cr4V2). Machine taps are suitable for machining on lathes, drill presses, and other machine tools, while hand taps are more suitable for manual tapping. A tap consists of a working section and a shank. The working section includes a cutting section (with a taper angle of 4°-6°) and a leveling section. The cutting section performs the primary cutting work, while the leveling section is used for finishing the thread and guiding the tap. For harder materials (such as quenched 45 steel), carbide taps are recommended for increased wear resistance. For plastic materials (such as aluminum alloys and copper), the tap’s chip flute should be designed with a deeper design (15%-20% of the major diameter of the thread) to facilitate chip evacuation.
Calculating the pilot hole diameter before tapping is crucial for ensuring thread quality. An undersized pilot hole diameter will overload the tap and easily break it; an oversized pilot hole diameter will result in insufficient thread profile height, compromising connection strength. The formula for calculating the pilot hole diameter for common threads is: Dhole = dP for brittle materials (such as cast iron and bronze); Dhole = d – 1.05P for plastic materials (such as steel and aluminum alloys), where d is the major thread diameter and P is the pitch. For example, when machining an M10 × 1.5 common thread, the pilot hole diameter in steel is 10 – 1.05 × 1.5 = 8.4 mm, while the pilot hole diameter in cast iron is 10 – 1.5 = 8.5 mm. The pilot hole depth should be 5-10 mm greater than the effective thread depth to accommodate chips and the overhang of the tap. The cutting speed during thread tapping needs to be adjusted according to the material. Generally, it is 5-10m/min for steel, 8-15m/min for cast iron, and 10-20m/min for aluminum alloy. At the same time, it is necessary to use appropriate cutting fluid (emulsion for steel and kerosene for cast iron) to reduce friction and cool the tool.
The primary tool for threading is the die, which comes in both fixed and adjustable styles. Fixed dies are suitable for mass production, while adjustable dies, which can slightly change the thread diameter via an adjustment screw, are ideal for small, single-piece production runs. Dies are made of similar materials to taps: high-speed steel dies are suitable for general materials, while carbide dies are suitable for high-strength materials. Before threading, the end of the round rod must be machined to a 60° taper angle to facilitate die alignment and entry. The taper angle should be greater than the effective length of the thread. Determining the rod diameter is also crucial. Too large a diameter can easily cause die chipping, while too small a diameter can result in an incomplete thread profile. Generally, the rod diameter should be 0.1-0.2mm smaller than the major diameter of the thread (0.05-0.1mm smaller for fine threads with a pitch under 1mm). For example, for an M8 external thread, the rod diameter should be 7.85-7.9mm.
The threading process parameters must be appropriately set based on the material and thread specifications. The cutting speed is generally lower than that for tapping, with a cutting speed of 3-8 m/min for steel and 5-10 m/min for cast iron to avoid overheating and wear of the die due to excessive speeds. The coaxiality between the die and the round rod must be maintained during threading, with a deviation of no more than 0.05 mm/m. Otherwise, a skewed thread will be produced, affecting the connection accuracy. As with tapping, threading also requires the use of cutting fluid. Extreme-pressure emulsions are used for steel, while sulfurized cutting oils are used for stainless steel to improve thread surface quality and extend die life. For threading on slender rods with an aspect ratio greater than 10, a steady rest or center rest is required to prevent bending and deformation of the workpiece and ensure that the thread straightness error does not exceed 0.1 mm/m.
Quality inspections for tapping and threading include thread dimensional accuracy, tooth profile integrity, and surface roughness. Thread dimensional accuracy is verified using thread gauges. A plug gauge is used for internal threads (a go gauge can be inserted, and a no-go gauge cannot be inserted more than two threads), while a ring gauge is used for external threads (a go gauge can be inserted, and a no-go gauge cannot be inserted more than two threads). Tooth profile integrity must be checked with a magnifying glass or projector. There must be no broken or missing teeth, and the tooth profile angle deviation must not exceed ±1°. Surface roughness is generally required to reach Ra3.2μm, with Ra1.6μm required for critical connection areas. During mass production, taps and dies must be regularly inspected for wear. When the thread pitch diameter error exceeds IT8 or the tooth profile shows noticeable wear, the tool must be replaced promptly to avoid defective products. Strict control of the tapping and threading processes ensures the reliability and interchangeability of threaded connections, meeting product requirements.
