Threading with dies and taps
Threading with dies and taps is the most commonly used manual and semi-mechanized CNC machining method in CNC machining. It’s suitable for small-batch production or repair, characterized by ease of operation and minimal equipment requirements. Dies are used for external threads, while taps are used for internal threads. The combination of these two creates a precise fit and is widely used for threading standard components such as bolts, nuts, and pipe fittings, as well as various other mechanical parts. While this method is less efficient than CNC machining on lathes, milling machines, and other machine tools, it offers significant advantages for threads with smaller diameters (generally ≤M20) or in unusual locations (such as blind holes and threads on housings).
A die is a tool used to produce external threads. It is round or square in shape, with a central threaded hole matching the thread specification. Cutting cones (typically at 40°-60°) are located at either end of the hole and are the primary components of the thread cutting process. Dies are categorized by structure as integral, adjustable, and pipe dies. Integral dies are suitable for processing standard external threads. Adjustable dies, which allow for fine-tuning of the thread diameter (within a range of 0.1-0.3mm) using an adjustment screw, are used for threads requiring higher precision. Pipe dies are used for processing pipe threads (such as G1/2 and Rc1/4). Dies are typically made of high-speed steel (such as W18Cr4V) or alloy tool steel (such as 9SiCr). After quenching and tempering, they reach a hardness of 60-63 HRC to ensure adequate wear resistance and cutting performance. Before use, check the thread accuracy and cutting edge sharpness. Dies with chipped or severely worn edges should be replaced promptly to avoid defects such as rotten or bald teeth in the resulting threads.
A tap is a specialized tool for producing internal threads. It consists of a working part and a shank. The working part includes a cutting section and a calibrating section. The cutting section, ground to a taper angle (typically 5°-10°), performs the primary cutting work, while the calibrating section is used for finishing the thread and guiding the tap feed. Taps are categorized by their usage into hand taps and machine taps. Hand taps typically come in sets of two (a head taper and a second taper). The head taper has a longer 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 the thread. Machine taps are single-piece taps and can be used on machine tools such as drilling machines and lathes. They have a shorter cutting section and greater rigidity. Thread accuracy of taps is graded from H1 to H4, with H1 being the highest and suitable for precision thread production, while H4 is less accurate and is suitable for general use. Taps are made from the same materials as die taps. High-speed steel taps are suitable for CNC machining materials such as steel and cast iron, while carbide taps are suitable for CNC machining difficult-to-machine materials such as high-strength steel and stainless steel.
The operating procedures for CNC machining external threads with dies must be standardized to ensure thread quality. Before CNC machining, the thread blank must be turned. The blank diameter is calculated as the major thread diameter minus 0.13P (P is the pitch). For example, for CNC machining an M10×1.5 external thread, the blank diameter is 10-0.13×1.5, which is approximately 9.8mm. The roundness tolerance of the blank diameter must be ≤0.03mm, and the surface roughness Ra must be ≤3.2μm. The blank is clamped in a bench vise or lathe chuck, calibrated, and secured. A 45° chamfer (the chamfer diameter is slightly smaller than the minor thread diameter) is applied to the end of the blank to facilitate die alignment and entry. The die is held in a die holder, with the die axis aligned with the workpiece axis. Manually rotate the die holder. After the die has penetrated 2-3 threads into the workpiece , check the thread verticality. Any deviation should be corrected immediately. During CNC machining, apply uniform axial force while maintaining horizontal rotation of the die holder. Reverse the die holder’s rotation by a quarter of a turn after every 1/2-1 turn to facilitate chip breaking and removal. For plastic materials like steel, add a cutting fluid (such as emulsion or sulfurized oil). For brittle materials like cast iron, dry cutting or the addition of kerosene is recommended. After CNC machining to the desired length, rotate the die holder in the opposite direction to remove the die and avoid burrs at the thread ends.
The key to tapping internal threads lies in ensuring the correct thread base diameter and tap usage. Before drilling internal threads, the base hole must be drilled. The formula for calculating the base hole diameter is: when CNC machining plastic materials like steel, base hole diameter = thread major diameter – 1.05P; when CNC machining brittle materials like cast iron, base hole diameter = thread major diameter – P. For example, when CNC machining an M10 x 1.5 internal thread, the base hole diameter for steel is 10 – 1.05 x 1.5 = 8.4mm, and for cast iron, it is 10 – 1.5 = 8.5mm. The base hole depth should be 0.7-1D (D is the thread major diameter) deeper than the effective thread depth to prevent the tap bottom from colliding with the hole bottom. After drilling, use a countersink to blunt the hole and remove burrs. When CNC machining with a tap, start with the first tap. Place the tap vertically into the bottom hole. Clamp the shank with a lever, apply light pressure, and rotate clockwise. Once the tap has penetrated 2-3 threads, remove the pressure and simply rotate the lever. Also, pay attention to chip removal, reversing 1/4 turn for every 1 turn. Add a suitable cutting fluid. After the first tap is complete, trim with a second tap. No pressure is required to penetrate the second tap; simply follow the thread guide and feed naturally until the desired depth is reached. When CNC machining blind hole threads, pay attention to the effective working length of the tap to avoid the bottom of the tap extending beyond the thread length, resulting in incomplete CNC machining.
Master the common quality issues and solutions for thread CNC machining with dies and taps. Out-of-tolerance thread dimensions may be caused by die or tap wear, improper blank diameter, or inappropriate bottom hole size. The tool should be replaced and the blank diameter or bottom hole size adjusted. Rough and burred thread surfaces are often caused by blunt tool edges or insufficient cutting fluid, requiring tool sharpening or increased cutting fluid. Ragged threads may be caused by misalignment between the tool and workpiece axes, uneven feed, or uneven material hardness. Correcting the tool position, maintaining a uniform feed, or tempering the material are necessary. Tap breakage is often caused by an undersized bottom hole diameter, poor chip evacuation, or forced feed. The correct bottom hole diameter, timely chip evacuation, and avoiding overloading are essential. Through standardized operation and appropriate tool and cutting parameter selection, dies and taps can produce threads with IT8-IT9 precision, meeting the requirements of general mechanical products.
