Screw measurement
As a precision transmission component, lead screws are widely used in CNC machine tools, precision instruments, and other equipment. Their accuracy directly determines the positioning precision of the transmission system (e.g., positioning error ≤ 0.01mm/1000mm). Lead screw measurement requires testing parameters such as lead error, pitch diameter error, thread profile half-angle error, and cumulative pitch error. Common methods include tool microscopes, dynamic lead screw measuring instruments, and laser interferometers. For example, the lead error of a ball screw must be controlled within 0.02mm/300mm; otherwise, the machine tool table feed accuracy will be out of tolerance. Therefore, scientific and accurate measurement techniques are key to ensuring lead screw quality. The measurement process must be performed in a constant temperature environment (20±1°C) to avoid measurement errors caused by temperature fluctuations (for every 1°C change in steel lead screw temperature, the length error is approximately 0.011mm/m).
The tool microscope method is a common method for measuring individual screw parameters, suitable for laboratory or small-batch testing. This method uses an optical system to magnify the screw thread profile (10-50x). The screw is placed on the workbench of a tool microscope, the focus is adjusted to clearly image the thread profile, and the parameters are measured using the microscope’s readout. The lead is measured by taking multiple points (at least five) along the screw’s axial direction, and the deviation between the average and the theoretical value is calculated. The pitch diameter is measured by measuring the distance between the two sides of the thread profile and calculating the thread angle. The thread half-angle error is controlled within ±10° by comparing it to a standard sample. The tool microscope method can achieve a measurement accuracy of ±0.001mm, but its efficiency is low (requiring 30-60 minutes per piece), making it suitable for spot checks or arbitration inspections. A precision instrument manufacturer used this method to inspect trapezoidal lead screws and found that the cumulative lead error was primarily concentrated at the ends. By adjusting processing parameters, the error was reduced by 30%.
The dynamic lead screw measuring instrument method enables continuous measurement of lead error, making it suitable for mass production testing. The instrument consists of a spindle drive system, an axial motion system, a grating measurement system, and a data processing system. During measurement, the lead screw rotates with the spindle, while the measuring head moves synchronously along the axial direction. A sensor (such as an inductive probe) collects the thread position signal in real time, compares it with the grating position signal, and calculates the lead error. The dynamic measuring instrument can measure up to 3 meters, with an accuracy of ±0.001mm and a measurement speed of 100-300mm/min, making it 5-10 times more efficient than the tool microscope method. During measurement, attention should be paid to the lead screw’s installation coaxiality (≤0.01mm/m) to avoid error amplification caused by eccentricity. A reasonable sampling interval (e.g., 100 points per revolution) should also be set to ensure data integrity. After introducing the dynamic measuring instrument, a ball screw manufacturer saw an 8-fold increase in inspection efficiency, reducing the lead error rate from 5% to 0.5%.
Laser interferometry is the ultimate solution for high-precision lead screw measurement, achieving accuracy of up to ±0.5μm/m. This method utilizes the high coherence of laser light to measure the axial displacement of the lead screw through the changes in interference fringes, then compares this with the theoretical lead to calculate the error. During measurement, the laser interferometer’s reflector is mounted on the lead screw nut and moves synchronously with the nut. The laser head is fixed to the machine tool bed. As the lead screw rotates, the difference between the actual and theoretical displacements is recorded to generate a lead error curve. The laser interferometer method can measure lead screws up to 10 meters long, making it suitable for inspecting large precision lead screws (such as those used in gantry milling machines). It also enables automated data processing and error reporting. However, this method carries high equipment costs (approximately 500,000 to 2 million yuan) and has strict environmental requirements (vibration ≤ 0.1μm, humidity 40%-60%). It is primarily used for factory inspection of high-precision lead screws or calibration by metrology departments. An aerospace equipment manufacturer used laser interferometers to inspect precision lead screws, improving positioning accuracy from 0.01mm to 0.005mm, meeting the requirements of satellite attitude adjustment mechanisms.
Error control and result analysis in lead screw measurement are crucial for quality improvement. Ambient temperature is the primary source of error. Before measurement, the lead screw must be maintained in a constant-temperature laboratory for at least four hours to maintain a temperature difference between the screw and the ambient temperature of ≤0.5°C. Temperature compensation is also performed on the measuring instrument, and the measured value is corrected based on the material’s linear expansion coefficient (11.5×10^-6/°C for steel). Installation errors must be controlled by adjusting the center coaxiality (≤0.005mm/m) and eliminating axial play (≤0.002mm). Measurement results require data processing: lead error must be calculated as both single-turn error and cumulative error. Single-turn error is ≤0.01mm, and cumulative error is ≤0.05mm/1000mm. Pitch diameter error must be controlled within ±0.01mm. Error curve analysis can identify machining defects (such as periodic errors caused by machine tool lead screw backlash) and guide process improvements. A machine tool factory analyzed measurement data and found that uneven feed during screw grinding caused errors. After adjusting the servo parameters, the error was reduced by 40% and the product quality stability was significantly improved.
