The process of precision screw
As a key transmission component in machine tools, instruments, and other equipment, precision screws are manufactured with a process that directly impacts transmission accuracy, rigidity, and service life. Precision screws typically require high dimensional accuracy (e.g., pitch error within 0.01mm/300mm), geometric accuracy (e.g., straightness within 0.02mm/m), and surface quality (surface roughness Ra ≤ 0.8μm). Therefore, they undergo a complex series of machining steps, from blank preparation to final surface treatment, with strict control of process parameters at every step.
Blank selection and pretreatment are fundamental steps in precision lead screw manufacturing. Because lead screws are subject to alternating loads and torque during operation, high-quality carbon steels or alloy tool steels such as 45 steel, 40Cr, or T10A are typically used. 40Cr achieves excellent overall mechanical properties after quenching and tempering, while T10A is suitable for applications requiring higher wear resistance. Blanks are typically made from hot-rolled round steel or forgings. Pressing for forged blanks refines the grain size, eliminates internal defects, and increases material density, making them the preferred choice for high-precision lead screws. The blank diameter is determined based on the nominal diameter of the lead screw and machining allowance, typically allowing for a machining allowance of 10-15mm. Pretreatment, including normalizing or annealing, aims to eliminate forging stresses, reduce hardness, and improve machinability. For example, after normalizing, the hardness of a 40Cr blank can be controlled to 180-220 HBW, facilitating subsequent machining.
The main task of the rough machining stage is to remove most of the machining allowances and initially form the basic shape of the screw. The rough machining process includes turning the outer circle, end face, drilling the center hole and rough threading. When turning the outer circle, use an ordinary lathe or CNC lathe, select carbide tools , control the cutting speed at 80-120m/min, and the feed rate at 0.2-0.3mm/r, and machine the outer diameter to 3-5mm larger than the design size. The end face machining must ensure that it is perpendicular to the axis, and use type A or type B center drill to drill the center hole with a moderate depth to avoid positioning errors caused by tip wear during use. Rough threading is a key process in rough machining. For trapezoidal threads or rectangular threads, thread turning tools can be used for layered cutting. The pitch error is controlled within 0.1mm, and a finishing allowance of 1-2mm is left for the thread depth. After rough processing, aging treatment is required to eliminate cutting stress and prevent deformation during subsequent processing. Aging treatment can be natural aging (placed for 2-3 weeks) or artificial aging (heated to 500-550℃, kept warm for 4-6 hours).
Semi-finishing and finishing are key steps in ensuring the accuracy of precision lead screws. Semi-finishing includes semi-finishing the external diameter, semi-finishing the threads, and milling the keyway (if applicable). For semi-finishing the external diameter, high-speed steel or coated carbide tools are used at a cutting speed of 60-80 m/min and a feed rate of 0.1-0.15 mm/r. The external diameter is machined to 0.5-1 mm larger than the design size, with a surface roughness of Ra 3.2 μm. Semi-finishing the threads uses a high-precision thread turning tool. Multiple passes are used to correct the pitch error, keeping it within 0.03 mm. The finishing stage includes finishing the external diameter, finishing grinding the external diameter, and finishing or grinding the threads. Finishing grinding is performed on a precision external cylindrical grinder using a white corundum grinding wheel at a grinding speed of 30-40 m/s and a feed rate of 0.01-0.02 mm/r. The external diameter is controlled to IT5 with a surface roughness of Ra 0.4 μm. For high-precision lead screws, thread processing requires the use of a thread grinder for grinding. The grinding wheel is trimmed with a diamond pen to ensure the accuracy of the thread profile. The pitch error can be controlled within 0.005mm/300mm. At the same time, the cumulative error and tooth profile half-angle error of the thread need to be detected.
Heat treatment and surface treatment are crucial processes for improving the performance of precision screws. Heat treatment primarily involves quenching and low-temperature tempering. For tool steel screws such as T10A, the quenching temperature is 760-780°C. After holding, the screws are water-quenched and then low-temperature tempered at 180-200°C. This results in a hardness of 58-62 HRC, improving wear resistance and dimensional stability. 40Cr screws undergo a quenching and tempering treatment (quenching + high-temperature tempering) to a hardness of 220-250 HBW, ensuring sufficient strength and toughness. After heat treatment, straightening is required, using pressure or flame straightening to maintain a straightness within 0.01 mm/m. Surface treatment typically involves oxidation (blueing) or chrome plating. Oxidation forms a dense oxide film, enhancing corrosion resistance. The chrome plating, with a thickness of 0.01-0.03 mm, increases surface hardness and wear resistance. Honing is performed after chrome plating to remove minor imperfections in the chrome plating.
Final inspection and pre-assembly preparation are the final steps in ensuring the quality of precision screws. Final inspection items include dimensional accuracy (external diameter, pitch, thread pitch diameter), form and position accuracy (straightness, coaxiality), surface quality (roughness, cracks, scratches), and hardness. A three-dimensional coordinate measuring machine is used to detect pitch error and cumulative error, a laser interferometer is used to measure straightness, a surface roughness tester is used to determine the Ra value, and a hardness tester is used to test surface hardness. Unqualified screws can be repaired if the error is within the allowable range; those that exceed the tolerance are scrapped. Screws that pass inspection are cleaned, coated with anti-rust oil, and then packaged for storage. Before assembly, the flexibility and clearance of the screw must be rechecked to ensure good meshing with the nut and no sticking, thereby maintaining the transmission accuracy and stability of the equipment.
