Processing technology of long lead screw
Lead screws are essential components for achieving linear motion in mechanical transmissions. They are widely used in machine tool feed mechanisms and precision instrument transmission systems. Their machining quality directly impacts transmission accuracy, stability, and service life. Lead screws are characterized by a large length-to-diameter ratio (generally greater than 20), high thread precision, and low surface roughness requirements. Consequently, their machining is complex, requiring multiple steps, including roughing, roughing, semi-finishing, heat treatment, finishing, and inspection. Each step has stringent process requirements.
Preparing the blank for a long lead screw is the first step in the machining process, and its quality significantly impacts subsequent processing and the final product. Blanks for long lead screws are typically made from hot-rolled round steel or forgings, with materials typically selected from 45 steel, 40Cr, or T10A. 45 steel is suitable for general-precision lead screws, 40Cr is suitable for applications requiring higher strength, and T10A, after heat treatment, achieves higher hardness and wear resistance, making it suitable for precision lead screws. The blank diameter should be determined based on the nominal diameter of the lead screw and the machining allowance. It is generally 8–15 mm larger than the nominal diameter and 50–100 mm longer than the finished product to allow for clamping and machining at both ends. Before processing, the blank must be inspected for surface defects such as cracks and folds, as well as for looseness and inclusions within the blank. NDT testing is performed if necessary to ensure acceptable quality. For critical long lead screws, the blank also requires annealing to eliminate internal stresses and improve machining performance.
The primary task of the roughing stage is to remove most of the rough stock and initially establish the basic shape of the lead screw. This involves rough turning the external diameter, facing the end faces, and drilling the center hole. During rough turning, the workpiece is clamped in a three-jaw chuck and tailstock center. A carbide turning tool is used, and machining is performed at high cutting speeds and feed rates. The external diameter is turned to a size 3-5 mm larger than the semi-finished dimensions. Facing ensures that both ends are perpendicular to the axis, paving the way for subsequent center hole drilling. The center hole serves as a crucial datum during long lead screw machining and must be drilled to a standard 60° tapered hole. The accuracy of the center hole directly impacts the screw’s machining precision, so after drilling, a center drill is required to ensure the hole’s roundness and taper meet the required standards. After roughing, the external diameter and end faces of the lead screw should have uniform machining allowances, laying the foundation for semi-finishing. In addition, the screw needs to be aged after rough machining to eliminate the internal stress caused by rough machining and prevent deformation during subsequent processing. Aging treatment generally adopts natural aging (placement for 3 to 6 months) or artificial aging (heating to 500~550℃, keeping warm for 4 to 6 hours and then slowly cooling).
The semi-finishing stage includes semi-finishing turning of the external diameter, rough turning of the threads, and a second aging treatment. During semi-finishing turning of the external diameter, the center holes at both ends are used as positioning references, and a toolholder is used for auxiliary support to increase workpiece rigidity. High-speed steel or carbide turning tools are used to turn the external diameter to a size 0.5-1 mm larger than the finished dimension, with a surface roughness of approximately Ra 3.2 μm. Thread rough turning is a key step in semi-finishing. For trapezoidal thread screws, a forming turning tool can be used. The tool angle is adjusted according to the thread pitch and profile angle, and the general shape of the thread is defined in multiple passes . A finishing allowance of 0.3-0.5 mm should be allowed for the pitch and top diameters of the threads. During rough turning, careful control of the thread pitch error and profile half-angle error is crucial, and a pitch gauge can be used for these checks. After semi-finishing, the screw undergoes a second aging treatment to further eliminate internal stresses and ensure dimensional stability during subsequent processing and use. For precision long lead screws, qualitative treatment, such as ice-cold treatment, is required after the second aging treatment to stabilize the material structure.
The finishing stage is crucial for ensuring the quality of long lead screws. It includes external cylindrical finishing, thread finishing, grinding, and lapping. For external cylindrical finishing, a high-precision lathe is used. Using the center holes at both ends as references, a tool rest and a dial indicator are used for precise alignment. High-speed steel turning tools are used, with a cutting speed of 80-120 m/min and a feed rate of 0.05-0.1 mm/rev. The external cylindrical surface is turned to the designed dimensions, achieving a surface roughness of Ra 1.6μm. Thread finishing is performed on a high-precision thread lathe using precision forming turning tools. Multiple passes are used to gradually adjust the pitch diameter, pitch, and flank angle of the thread. During the finishing process, real-time measurement is performed using a thread gauge or the three-needle measurement method to ensure that the thread meets the required accuracy. For precision lead screws with accuracy grades C3 and C4, the pitch diameter tolerance must be controlled within 0.01-0.02 mm. After precision turning, screws with higher requirements undergo thread grinding, using a thread grinder and cubic boron nitride grinding wheel to reduce the surface roughness of the threads to below Ra0.8μm, further improving the thread’s precision and wear resistance. Finally, both end faces of the screw and the journal are ground to ensure a precise fit.
Lead screw inspection is the final step in the machining process and requires comprehensive testing according to the drawing requirements, including dimensional accuracy, form and position accuracy, surface quality, and thread accuracy. Dimensional accuracy testing includes measurements of the outer diameter, length, major diameter, pitch diameter, and minor diameter of the thread, using tools such as micrometers and vernier calipers. Form and position accuracy testing includes straightness, roundness, and cylindricity. Straightness can be measured using a dial indicator and a flat plate, with the straightness error of the lead screw required to be within 0.1 mm/m. Roundness and cylindricity can be tested using a roundness tester. Surface quality inspection is performed visually or using a surface roughness tester to check for defects such as scratches, cracks, and burns. Surface roughness should meet design requirements. Thread accuracy testing uses thread gauges, a three-needle measuring method, and a tool microscope to examine the pitch diameter, pitch, thread angle error, and cumulative thread error. Lead screws undergo rigorous inspection to ensure they meet design standards before they are put into service. The processing technology of long lead screws is complex and requires strict control of the quality of each link to produce high-precision and high-performance products.
