The turning of double-start trapezoidal threads on curved and conical surfaces is a highly challenging process in machining. The threads possess the spatial form of curved or conical surfaces, the symmetrical structure of double-start threads, and the load-bearing characteristics of trapezoidal threads. They are widely used in precision transmission mechanisms, helical pairs of pressure equipment, and other applications. Compared with ordinary cylindrical threads, the turning of such threads requires simultaneous control of radial, axial, and circumferential motion accuracy. In particular, the intersection of the generatrix of the curved and conical surfaces and the thread helix must strictly conform to the designed trajectory. Slight deviations will result in poor thread engagement or uneven load bearing. Experimental data from a heavy machinery research institute show that if the machining error of curved double-start trapezoidal threads exceeds 0.03mm, it may cause a drop in transmission efficiency of more than 15% and even induce mechanical vibration.
Tool preparation for turning double-start trapezoidal threads on curved and conical surfaces requires balancing multiple technical requirements. Trapezoidal threads have a 30° profile angle, and the tool’s cutting edge angle must be precisely ground, with both edges maintaining symmetry to ensure consistent lead. For curved or conical surfaces, the tool’s nose radius must match the surface curvature or cone half-angle to avoid overcutting or undercutting. For hard materials (such as 40Cr quenched and tempered parts), carbide forming turning tools should be selected, with a tool body constructed of high-strength alloy structural steel to ensure resistance to deformation under high cutting forces. When machining non-ferrous metals (such as brass), high-speed steel tools combined with specialized cutting fluids can reduce tool sticking and maintain thread surface roughness within Ra1.6μm. A precision instrument manufacturer successfully achieved a profile half-angle error of ±15° when machining curved double-start trapezoidal threads using a customized adjustable-angle turning tool.
The turning process of double-start trapezoidal threads on curved and conical surfaces requires precise control in stages. First, the workpiece surface must be pre-machined to create a base shape for the curved or conical surface. For curved surface machining, the profiling method or CNC interpolation method can be used to ensure the accuracy of the main line. For conical surfaces, the taper must be set by adjusting the lathe’s small slide angle or using the CNC program. Thread line division is then performed. Double-start threads must ensure that the two lines are symmetrically distributed 180° on the circumference. Traditional lathes can achieve this through the spindle indexing mechanism or pulley adjustment, while CNC lathes complete line division by setting the spindle positioning angle through the program. Thread cutting uses a layered progressive method. The first pass must determine the starting position and helix angle of the thread. Each subsequent pass uniformly reduces the back cutting amount radially. The feed rate in the rough turning stage is 0.3-0.5mm, and the fine turning stage is controlled at 0.1-0.2mm. When a hydraulic equipment factory processes conical double-start trapezoidal threads, it controls the thread lead error within 0.02mm/100mm through the process route of “conical surface first, then thread, rough division first, then fine turning”.
Motion linkage control during turning is key to ensuring precision. Turning curved threads requires the lathe to achieve a combination of spindle rotation and tool motion along the curved surface’s generatrix. Traditional lathes require mechanical linkage via a profiling device, while CNC lathes can precisely calculate the displacement for each pulse by combining G02/G03 circular interpolation with the G33 thread cutting command. Conical threads require synchronized control of the ratio of axial and radial feeds to ensure uniform thread profile distribution across the conical surface. During machining, special attention must be paid to the transition between the thread’s starting and ending points. A circular transition should be used at the interface between the curved surface and the thread to avoid stress concentration. The major and minor end diameters of conical threads must be strictly calculated according to the taper ratio to ensure a uniform thread height variation across the entire length of the double-start thread. An aerospace machinery manufacturer uses a five-axis CNC lathe to machine curved double-start trapezoidal threads. By real-time compensating for dynamic errors in the spindle and feed axes, the cumulative error of the helix is kept to within 0.015mm.
Quality inspection of double-start trapezoidal threads on curved and conical surfaces requires specialized methods and tools. Lead accuracy can be verified using a universal tool microscope coupled with an image measurement system, measuring the deviation between the actual lead and the designed value at different cross-sections. The symmetry of double-start threads is checked using a thread splitter, and the relative error between the two thread tips should not exceed 0.02mm. Form errors on curved or conical surfaces are analyzed by scanning the thread generatrix using a coordinate measuring machine and comparing it with a theoretical model. For load-bearing threads, thread strength verification testing is also required, applying axial force using a specialized testing machine to observe thread deformation. The inspection specifications established by a nuclear equipment manufacturer require that three parts from each batch of curved double-start trapezoidal threads undergo a comprehensive inspection, including thread accuracy, surface quality, and load-bearing capacity. Only qualified parts can be released to storage. With the advancement of digital machining technology, the combination of online inspection and adaptive machining systems is gradually resolving the machining challenges of these complex threads, providing technical support for high-end equipment manufacturing.
