Machining of trapezoidal thread screws
Trapezoidal thread screws are key components in mechanical transmissions and are widely used in machine tools, lifting equipment, and other fields. The quality of their turning directly impacts the performance of these equipment. Machining trapezoidal thread screws is a complex process that requires mastering the correct machining methods, selecting appropriate tools and cutting parameters, and focusing on precision control during the machining process.
Before turning a trapezoidal thread screw, thorough preparation is required. First, the appropriate material should be selected based on the screw’s design requirements. Common materials include 45 steel and 40Cr. Screws with higher precision requirements also require tempering or aging treatment to eliminate internal stress and improve hardness and wear resistance. Second, the blank must be pre-processed, including cutting and centering. The quality of the center hole is crucial, as it serves as a positioning reference for subsequent processing. Any deviation in the center hole can cause the screw to bend or eccentric during turning. Furthermore, the appropriate turning tool must be selected based on the thread parameters (such as nominal diameter, pitch, and thread angle). The geometric parameters of the trapezoidal thread turning tool must meet the requirements. The tool’s rake angle, clearance angle, and rake angle must be appropriately selected based on the material and cutting conditions to ensure smooth cutting and accurate thread profile.
Trapezoidal thread screws are typically turned in stages, beginning with rough turning to remove most of the stock, followed by finish turning to ensure thread accuracy and surface quality. The primary goal of rough turning is to quickly remove excess material, so a larger depth of cut and lower cutting speeds can be used. To improve rough turning efficiency, either the left-right or oblique feed method can be employed. The left-right method involves continuously adjusting the lateral position of the turning tool to cut from both sides of the thread profile. This method protects the tool from simultaneous high cutting forces and reduces tool wear. The oblique feed method feeds the tool obliquely along one side of the thread profile and is suitable for threads with larger pitches. During rough turning, care must be taken to control the thread’s pitch diameter to allow for an appropriate machining allowance for finish turning, typically between 0.5 and 1mm.
Finish turning is a key step in ensuring the quality of trapezoidal thread screws, and requires strict control of various thread parameters, including mean diameter, pitch, tooth profile angle, and surface roughness. During finish turning, a smaller cutting depth and a higher cutting speed should be selected to improve the surface quality of the thread. The cutting edge of the finish turning tool should be sharp, and the tool can be ground if necessary to reduce extrusion and friction during the cutting process. During the finish turning process, a trial cutting method can be used for processing, that is, a small amount of thread is turned first, and then a thread gauge or other measuring tool is used for measurement. The position of the turning tool is adjusted according to the measurement results until the thread parameters meet the requirements. For screws with higher precision requirements, multiple measurements and adjustments are required during the finish turning process to ensure that the mean diameter tolerance, pitch error, and tooth profile angle error of the thread are all controlled within the allowable range.
When turning trapezoidal thread screws, it’s important to address various issues that may arise during the machining process. For example, the screw is prone to bending and deformation during turning, primarily due to excessive cutting forces, internal stress release in the material, or improper clamping. To prevent this, a steady rest or steady rest can be used to increase the rigidity of the screw. The steady rest should be mounted on the lathe’s slide and move with the slide, providing support for the screw. Longer screws also require multiple aging treatments during machining to eliminate internal stresses. Furthermore, thread buckling is a common problem during turning. This is primarily caused by the screw’s pitch not being an integer multiple of the workpiece’s thread pitch, or by a change in the relative position of the tool and thread profile during retraction and refeed. To prevent buckling, a reverse turning method can be used for retraction, or a pulley can be installed on the lathe spindle to ensure the required speed ratio between the screw and the workpiece.
After turning, the trapezoidal thread screw requires necessary inspection and finishing. Inspections include the thread’s pitch diameter, pitch, tooth angle, surface roughness, and screw straightness. These can be performed using measuring tools such as the three-needle method, pitch gauges, and tool microscopes. Any unsatisfactory areas require appropriate finishing, such as fine grinding or manual filing to adjust the thread parameters. The screw must also be cleaned and treated to prevent rust, removing surface oil and iron filings. Anti-rust oil should be applied to prevent rust during storage and transportation. Strict turning processes and quality control ensure the excellent transmission performance and service life of the trapezoidal thread screw.
