Cutting threads with dies and taps
Thread cutting with dies and taps is the most commonly used threading method in machining. Characterized by ease of operation, low cost, and wide applicability, it is suitable for threading a variety of metal materials (steel, cast iron, non-ferrous metals) and non-metallic materials (plastics, wood), making it particularly well-suited for single-piece, small-batch production and maintenance. Dies are used for external threads, while taps are used for internal threads. Both are standard tools capable of producing a variety of thread types, including common threads, pipe threads, and trapezoidal threads, achieving thread accuracy levels of IT7-IT8. For example, when using an M10 tap to produce internal threads in 45 steel, the fit accuracy meets general connection requirements (clearance of 0.05-0.1mm). However, the processing quality of this method is significantly affected by operator skill, tool quality, and workpiece material. Improper use can result in thread misalignment, out-of-tolerance pitch diameters, or surface roughness. Therefore, mastering correct operation techniques and key process points is crucial.
The selection and installation of dies are crucial for ensuring external thread quality. Dies are categorized by structure as integral, adjustable, and pipe dies. Integral dies (e.g., M6-M30) are suitable for machining common external threads and offer high precision (IT7 grade). Adjustable dies (e.g., round dies) adjust the thread diameter by 0.1-0.3mm via an adjustment screw, making them suitable for dimensional compensation in mass production. Pipe dies are used for machining pipe threads (e.g., G1/2, Rc1/4) and feature a thread angle of 55° or 60°. Die selection should be tailored to the workpiece material: high-speed steel dies (W18Cr4V) are used for machining steel, alloy tool steel dies (9SiCr) for machining cast iron, and tungsten carbide dies for machining non-ferrous metals. When installing the die in the die holder, ensure that the die axis is coaxial with the workpiece axis (coaxiality error ≤ 0.05mm). Use a dial indicator for calibration to avoid skewed threads. A maintenance workshop increased the pass rate of M16 external threads from 85% to 99% by correctly selecting and installing dies.
The type and selection of taps should be determined based on the internal thread requirements. Taps are categorized by purpose as hand taps, machine taps, and pipe thread taps. Hand taps typically come in sets of two (a nose taper and a second taper). The nose taper has a long cutting section (5-7 threads) and is used for initial cutting, while the second taper has a shorter cutting section (2-3 threads) and is used for finishing. Machine taps are single tapers, suitable for use on lathes or drill presses, offering high cutting efficiency. Taps are categorized by thread profile into triangular threads (M, UNC), pipe threads (G, NPT), and trapezoidal threads (Tr). When selecting a tap, consider the hardness of the workpiece material: coarse-thread taps (large pitch) are used for machining soft materials (aluminum, copper), while fine-thread taps are used for machining hard materials (45 steel, cast iron). Straight-fluted taps are used for through-hole threads, while spiral-fluted taps (with upward chip removal) are used for blind-hole threads. The tap’s rake angle (8°-15°) and relief angle (6°-10°) must be tailored to the material. When machining non-ferrous metals, a larger rake angle is preferred to reduce tool sticking. A machinery shop used spiral-fluted machine taps to produce blind hole threads (M12×1.75) in 45 steel, increasing their first-pass yield from 70% to 95%.
The process for cutting external threads with a tapping die requires standardized execution. Pre-tapping workpiece preparation is essential: the outer diameter should be slightly smaller than the major thread diameter (approximately 0.1-0.2mm smaller, e.g., an M10 tapped to 9.85-9.9mm). The end should be chamfered 15°-30° (chamfer length 0.5-1P, where P is the thread pitch ) to facilitate die entry. When manually tapping, the die holder should be kept horizontal. Apply even pressure during initial entry. Rotate the die 1-2 times, then reverse 1/4 turn to break off chips. For machine tapping, maintain a lathe speed of 100-300 rpm (low speed for steel, high speed for non-ferrous metals), with a feed rate equal to the thread pitch. No manual pressure is required. Cutting fluid selection: Emulsion for steel, kerosene for cast iron, and diesel for aluminum alloys. After processing, check with a thread ring gauge to ensure that the go gauge is inserted smoothly and the stop gauge is screwed in ≤2 threads. Through standardized operations, a certain agricultural machinery factory controlled the middle diameter error of the external thread within ±0.02mm, meeting the connection strength requirements.
Key steps and quality control for tapping internal threads. The drill hole diameter must be strictly calculated according to the following formula: Internal thread bottom hole diameter Dhole = DP (D is the major thread diameter, P is the pitch). For example, the bottom hole diameter for an M10×1.5 tap is 10-1.5 = 8.5mm, with a tolerance of ±0.1mm. A larger diameter will result in insufficient thread profile height, while a smaller diameter can easily break the tap. Drilling depth: Blind hole thread depth = effective thread depth + 0.7D (to ensure full tap penetration). When manually tapping, first use the tap head to penetrate vertically, rotate it one turn, then reverse it a quarter turn to break the chips. Once the tap reaches the desired depth, remove the tap head and trim with a second tap. When machine tapping, maintain a spindle speed of 50-200 rpm and use rigid tapping mode (spindle rotation and feed are strictly synchronized) to avoid thread misalignment. Adequate lubrication is essential: extreme pressure cutting oil for steel and sulfurized cutting oil for stainless steel to reduce tap wear. When a precision instrument factory processed M8 internal threads, the thread accuracy was improved to IT6 level by controlling the bottom hole diameter (6.6±0.05mm) and tapping speed (100r/min).
