How to improve the machining accuracy of Lock Nut?
The machining accuracy of Lock Nut is directly related to its anti-loosening performance, bearing capacity and service life, which is particularly important in the field of precision machinery and high-end equipment. Improving machining accuracy requires scientific control from material control, equipment selection, process optimization to testing system and other processes. In this paper, we will systematically introduce the key technologies and implementation methods to improve the machining accuracy of the lock nut.
First, material selection and pretreatment: build the foundation of accuracy
The stability of material properties and pretreatment quality is to ensure the processing accuracy of the premise, the need to control the material defects from the source of the impact on the accuracy.
1. Selection of high-quality raw materials
should be selected from high-quality steel with uniform chemical composition and low impurity content, such as 8.8 level and above lock nut priority selection of SCM435 alloy structural steel, its sulfur and phosphorus content should be controlled at 0.03% or less, and the level of non-metallic inclusions should not be lower than GB/T 10561 in the 2 level. For high-precision lock nuts (thread tolerance 5g level or more), it is recommended to use forged round steel billet, through the forging process to refine the grain, eliminate internal loosening, the forging ratio is controlled in the range of 2.5 - 3.5, to ensure uniformity of the mechanical properties of the material, to reduce the risk of post-processing deformation.
2. Optimize the heat treatment process
heat treatment needs to take into account the hardness requirements and dimensional stability, tempering treatment should control the quenching temperature (860 ± 10 ℃) and tempering temperature (580 ± 10 ℃), holding time according to the size of the blanks according to the calculation of 25 - 30min / 10mm, to ensure that the hardness of the HRC28 - 32, and hardness uniformity deviation of ≤ 2HRC. For the thread processing of the lock nut, need to carry out low-temperature aging treatment (120 - 150 ℃ heat preservation 4 - 6 hours), to eliminate processing stress, reduce the subsequent use of dimensional deformation, aging dimensional changes should be controlled within 0.01mm/100mm.
Second, processing equipment and process optimization: improve precision control ability
Advanced processing equipment and scientific process parameters are the core guarantee to improve processing accuracy, the need for accurate control of key processes.
1. Key equipment selection and precision calibration
Thread processing should use high-precision thread lathe or thread grinder, with positioning accuracy ≤0.01mm/300mm and repeat positioning accuracy ≤0.005mm. For precision lock nuts (such as 5g/6g thread), thread grinder should be used for processing, with grinding wheel made of green silicon carbide (particle size 80#-120#) and equipped with on-line measurement. The grinding wheel is made of green silicon carbide (grain size 80#-120#) and equipped with online measuring system to monitor the deviation of thread size in real time. The end face is processed by high-precision CNC lathe, with radial runout of spindle ≤ 0.003mm, axial runout ≤ 0.002mm, and positioning accuracy of tool holder ≤ 0.005mm, to ensure the flatness and perpendicularity accuracy of the end face. The equipment should be calibrated quarterly, and the positioning error should be detected by laser interferometer, and the excess items should be adjusted and compensated in time.
2. Optimization of process parameters and tool management
In thread turning, the cutting speed is controlled at 80 - 120m/min according to the hardness of the material, the feed is set according to the pitch, and the amount of back-eating is reduced by a stepwise manner (the first time is 2-3mm, and each subsequent time is 0.5-1mm), so as to reduce the processing deformation. For thread grinding, the grinding wheel linear speed should be stabilized at 30-35 m/s, the workpiece speed should be set at 50-100 rpm, and the feed rate should be controlled at 0.01-0.02 mm per revolution to ensure that the machined surface roughness reaches the accuracy standard of Ra≤0.8μm. The tools should be made of ultra-fine grain carbide (e.g. WC-Co alloy with 8% - 10% cobalt content) or CBN, and the radius of the tip of the thread turning tool should be designed as 0.12 - 0.15 times of the pitch. Tool wear should be checked regularly and replaced when the wear exceeds 0.1 mm, and the tool position should be calibrated with a tool setting gauge to ensure that the radial runout of the tool is kept within 0.005 mm.
3. Process division and margin control
adopts "roughing - semi-finishing - finishing" three-stage machining process, reasonable allocation of machining allowance. Rough machining to remove most of the allowance (leave 0.5 - 1mm allowance), semi-finish machining to leave 0.1 - 0.2mm finishing allowance, to avoid the single processing allowance is too large resulting in stress concentration and deformation. For the lock nut with flange, the internal thread should be processed first, and then the flange end face should be processed with the thread as the reference to ensure the perpendicularity accuracy of the end face and the thread axis (≤0.01mm/100mm). Set up aging treatment link between key processes to eliminate machining stress and reduce the loss of precision in subsequent processes.
Third, testing and quality control: build a whole-process precision guarantee system
Perfect testing means and strict quality control are the last line of defense to ensure processing precision, and the whole process monitoring from raw materials to finished products should be realized.
1. Key dimension inspection methods
Thread accuracy inspection adopts the combination of thread gauge and coordinate measuring machine, thread ring gauge is used for quick inspection of conformity, and coordinate measuring machine (measuring accuracy ≤0.001mm) is used for sampling inspection of parameters such as thread center diameter, pitch and tooth angle, and the proportion of sampling inspection in each batch is not less than 5%, so as to ensure that thread center diameter tolerance is within the range of 5g/6g, and the cumulative error of pitch is within the range of 5g/6g, and the cumulative error of pitch is within the range of 5g/6g, and the cumulative error of pitch is within the range of 5g/6g. To ensure that the tolerance of thread center diameter is within 5g/6g, the cumulative error of screw pitch is ≤0.05mm/m, and the deviation of tooth angle is ≤±0.25°. The end face flatness and perpendicularity test use marble plate and percentage meter, flatness error ≤ 0.02mm/100mm, perpendicularity error ≤ 0.015mm/100mm. surface roughness is detected by roughness meter, the thread surface and end face roughness should reach Ra ≤ 1.6μm.
2. Process quality control measures
Establish process quality traceability system, record processing equipment, operators, process parameters and test results for each process, so as to realize traceability of quality problems. Set up key process control points (e.g. thread processing, heat treatment), carry out full testing of the first product, qualified before mass production, 3 - 5 pieces per hour sampling during production to monitor dimensional stability. For high-precision lock nuts, 100% magnetic particle flaw detection or penetration flaw detection is required to detect whether there are cracks at the root of the threads and stress-concentrated areas, and ensure that there are no defects affecting the strength.
3. Environment and tooling guarantee
Precision machining workshop should control the environment temperature (20±2℃) and humidity (40% - 60%), and the temperature fluctuation should not be more than ±1℃ per hour, so as to minimize the influence of temperature change on machining accuracy. Processing and testing fixtures (such as jigs and gauges) should be made of materials with low coefficient of expansion (such as cast iron or granite) and calibrated regularly, with the positioning accuracy of the jigs ≤ 0.005mm, and the accuracy of the gauges should be higher than the accuracy of the workpieces by 3 - 5 times. Workpiece clamping using flexible clamping (such as elastic collet), to avoid deformation caused by excessive clamping force, clamping force through the torque wrench control to ensure uniformity and stability.
To sum up, improving the machining accuracy of Lock Nut is not the optimization of a single link, but a systematic project involving material pretreatment, equipment selection, optimization of process parameters and inspection control. Through the implementation of scientific control of the factors affecting precision in each link, the dimensional accuracy, shape accuracy and surface quality of the lock nut can be effectively improved to provide solid support for the realization of its excellent performance and prolonged service life, which in turn ensures the long-term reliability and safety of the mechanical connection parts.
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