Trapezoidal thread turning
Trapezoidal threads are a common thread type used in mechanical transmissions (such as machine tool lead screws and jack screws). Their thread profile is an isosceles trapezoid (with a 30° thread angle), offering high transmission efficiency and strong load-bearing capacity. Turning them requires guaranteed thread profile accuracy, lead accuracy, and surface quality. Compared to conventional triangular threads, trapezoidal threads have a larger thread height (h=0.5P+ac, where P is the pitch and ac is the clearance), a larger machining allowance (total allowance 3-5mm), and often feature a large pitch (P≥4mm). This results in high cutting forces during turning, which can easily generate vibration, leading to thread misalignment or surface roughness. For example, when machining a Tr40×7 trapezoidal thread, if the lead error exceeds 0.03mm/100mm, the clearance between the nut and the screw will be uneven, affecting transmission stability. Therefore, turning trapezoidal threads requires specialized processes encompassing tool design, cutting methods, and parameter optimization.
The geometric design parameters of trapezoidal thread turning tools must strictly comply with profile standards. The tool’s profile angle must be 30°, with an tolerance of ≤±10′. Calibration can be performed using a dedicated angle template. During sharpening, the rake angle should be 0°-5° (for rough turning) or 0° (for finish turning) to avoid distortion of the profile angle due to excessive rake angles. The clearance angle must take into account the influence of the thread lead angle (φ): the clearance angle in the feed direction is 6°-8° + φ, and the clearance angle in the run-off direction is 6°-8° – φ. Ensure the actual working clearance angle is ≥3° (for example, for a Tr50 thread with a 10mm pitch, φ is ≈3.6°, and the feed-direction clearance angle is 10°). The tool tip width must be equal to the root width (W = 0.366P – 0.536ac). For example, if P is 10mm and ac is 0.5mm, the tool tip width is 3.66 – 0.268 = 3.392mm, with an tolerance of ≤±0.03mm. The tip of a roughing tool can be ground to a 0.5-1mm transition radius to enhance strength. Finishing tools must remain sharp, with a straightness error of ≤ 0.01mm/100mm. A machine tool manufacturer, through precision grinding of trapezoidal thread turning tools, has reduced the tooth angle error to within ±5°, improving thread fit accuracy by 40%.
The turning method for trapezoidal threads depends on the pitch. Small-pitch trapezoidal threads (P ≤ 4mm) can be machined using the straight-in method: the tool feeds directly radially, cutting to the tooth height in one go. This method is highly efficient but also results in high cutting forces, making it suitable for workpieces with high rigidity. Large-pitch threads (P ≥ 6mm) require a layered cutting method, including left-right and oblique-in methods. The left-right method uses a small slide to adjust the tool angle, turning each side of the thread separately, with a radial feed of 0.1-0.3mm per cut, before finishing the groove bottom. The oblique-in method feeds the tool obliquely along one side of the thread profile, turning one side and the groove bottom simultaneously. This method is 30% more efficient than the left-right method, but requires precise control of the oblique angle. For multi-start trapezoidal threads (such as the double-start Tr32×6), the lines must be divided using a graduated disk with a line division error of ≤ 0.01mm. Either the circumferential division method (using the spindle scale) or the axial division method (using the leadscrew feed) can be used. When a heavy machinery factory processed P12mm trapezoidal threads, it adopted the “oblique feed method for rough turning + left-right method for fine turning”, which reduced the processing time to 60% of the original time and achieved the required thread profile accuracy.
Optimizing cutting parameters requires balancing machining efficiency and thread quality. For rough turning, the cutting speed is 40-60 m/min (for steel) or 30-50 m/min (for cast iron). The feed rate is equal to the thread pitch, and the back-cutting depth is distributed in a decreasing order (e.g., 1 mm for the first cut, 0.8 mm for the second, and 0.3 mm for the finishing allowance). For finish turning, the speed is reduced to 20-40 m/min to reduce vibration. The feed rate is 0.1-0.2 mm/r (normal), and the back-cutting depth is 0.05-0.1 mm. Ensure a surface roughness Ra ≤ 1.6 μm. Controlling cutting forces is crucial. Excessive radial force can cause workpiece bending (e.g., long lead screws). This can be reduced by reducing the back-cutting depth and increasing the lead angle. If necessary, use a toolholder (the support block is made of cast iron, and the contact pressure with the workpiece is 0.02 MPa). An experiment shows that when the rough turning speed is reduced from 50m/min to 40m/min, the lead error of the Tr40 thread is reduced from 0.025mm to 0.015mm, but the processing time increases by 20%, which needs to be weighed according to the accuracy requirements.
Cooling, lubrication, and quality inspection are crucial aspects of trapezoidal thread turning. Extreme-pressure emulsion (10%-12% concentration) is used during cutting. This fluid is injected into the base of the thread profile through a high-pressure nozzle (0.4 MPa) to reduce friction between the tool and the workpiece and flush away chips (especially those within the grooves). For precision trapezoidal threads (such as machine tool lead screws), sulfurized cutting oil is used to enhance lubrication and reduce surface roughness. Quality inspection includes: checking with a trapezoidal thread gauge (go gauges ensure insertion; stop gauges require insertion of ≤2 threads); measuring the thread angle, pitch diameter, and pitch using a tool microscope; the pitch diameter error is ≤±0.02mm, and the cumulative pitch error is ≤0.05mm/300mm; and measuring the Ra value of the thread surface using a surface roughness tester. After machining, aging treatment (200°C for 4 hours) is required to eliminate cutting stresses and prevent deformation during use. A precision machine tool factory has extended the service life of trapezoidal thread screws to more than 15 years through strict testing and aging treatment, and maintained the transmission accuracy within 0.01mm/1000mm.
Common problems and solutions for trapezoidal thread turning require a thorough understanding. Tooth profile asymmetry is often caused by skewed tool installation (not perpendicular to the workpiece axis), requiring calibration with a square. The perpendicularity error should be ≤0.01mm/100mm. Excessive mean diameter tolerances may indicate a feed rate miscalculation, requiring recalculation and adjustment of the cutting depth. Surface ripples are often caused by vibration, requiring checking the lathe spindle clearance (≤0.01mm), reducing the cutting speed, and increasing tool rigidity. For hardened trapezoidal threads (such as 40Cr tempered to HRC35-40), CBN tools are required for precision turning at a cutting speed of 80-100m/min to avoid tool chipping. By summarizing solutions to common problems, a fastener manufacturer has increased the pass rate for trapezoidal threads from 85% to 99%, significantly reducing scrap rates and production costs.
