Machining of triangular threads
As the most widely used thread type in mechanical connections, triangular threads require precise turning of parameters such as thread angle, pitch, and pitch diameter to ensure reliable connections and interchangeability. Triangular threads can be categorized by thread angle as standard threads (60°), imperial threads (55°), and pipe threads (55°). Standard threads (such as M10×1.5) are the most widely used in industry. While their turning process is mature, it requires stringent detail control. From tool preparation to final inspection, every step must comply with standard specifications. Otherwise, problems such as thread misalignment and dimensional deviations can easily occur.
The selection and sharpening of triangular thread turning tools are fundamental to ensuring thread profile accuracy. Tool parameters must be determined based on the thread type and precision requirements. The tool tip angle for standard thread turning tools must be exactly 60°, while that for imperial thread turning tools is 55°. This can be checked using an angle template, and the tolerance must be within ±10°. Tool material selection must be compatible with the workpiece material. When machining steel, high-speed steel (such as W18Cr4V) or carbide (such as YT15) should be used. High-speed steel tools have a sharp cutting edge and are suitable for low-speed fine turning, while carbide tools offer excellent wear resistance and are suitable for high-speed rough turning. When machining cast iron, YG-type carbide should be used to reduce tool edge chipping. Tool geometry parameters require optimization. The rake angle is typically between 0° and 10°, with a larger value being used when machining plastic materials to minimize cutting deformation and a smaller value being used when machining brittle materials to enhance cutting edge strength. The clearance angle is between 6° and 10° to prevent friction between the flank and the thread surface. The tool tip radius should be smaller than the minimum radius of the thread root (approximately 0.125 times the pitch for standard threads) to prevent overcutting. After sharpening, the rake and flank faces should be smoothed with an oilstone, maintaining a surface roughness of less than Ra0.4μm to ensure a smooth cutting process.
The choice of turning method depends on thread accuracy, pitch size, and production batch size. Common methods include straight-in, left-right, and oblique cutting. The straight-in method uses radial feed to cut threads. It is simple to operate and highly efficient, suitable for fine threads with a pitch less than 2mm. Its characteristic is that both sides of the tool cut simultaneously, resulting in high profile accuracy. However, this method is characterized by high cutting forces and is prone to vibration, so a lower feed rate (≤50mm/min) is required. The left-right cutting method combines radial feed with axial movement, making one side of the tool the primary cutting edge and the other the secondary cutting edge. It is suitable for coarse threads with a pitch greater than 2mm. This method reduces cutting forces and vibration and prevents tool chipping. However, it requires precise control of the left-right movement using a dial to ensure profile symmetry. The oblique cutting method feeds the tool diagonally along one side of the thread profile, cutting only on one side. It is suitable for rough turning. It is efficient, but the profile accuracy is lower, requiring correction using the straight-in method during finish turning. For multi-threads, the phase difference of each thread must be ensured through an indexing mechanism. For example, a double-thread thread needs to be accurately indexed 180°, which can be achieved using a dial on the lathe spindle or a special indexing fixture.
Cutting parameters, primarily including cutting speed, feed rate, and depth of cut, must balance machining efficiency and thread quality. The feed rate must be equal to the thread pitch (for single-start threads) or lead (for multi-start threads). For example, when turning an M12×1.75 single-start thread, the feed rate must be strictly set to 1.75 mm/r. This is determined by the transmission ratio between the lathe’s leadscrew and spindle. Incorrect feed rates can result in thread misalignment. The cutting speed is determined by the tool material. High-speed steel tools are used for turning steel, with a speed of 8-15 m/min, while carbide tools are used at 30-60 m/min. Speeds are reduced by 20% when machining cast iron. For finish turning, speeds can be increased by 10-20% to reduce tool-workpiece contact time and reduce surface roughness. The cutting depth should be measured using a layered feed method. The total depth is calculated based on the thread profile height (0.6495 × pitch for standard threads). The first feed is 0.3-0.5mm, and the subsequent feeds are gradually reduced, with the final feed not exceeding 0.1mm to ensure profile accuracy. For example, turning a standard thread with a 2mm pitch, with a total depth of 1.299mm, can be completed in 5-6 feeds, with feeds of 0.4mm, 0.3mm, 0.2mm, 0.15mm, 0.1mm, and 0.049mm, respectively.
Proper thread turning practices are crucial to avoiding machining defects, with particular attention paid to tool start-up, tool setting, tool retraction, and thread misalignment prevention. When starting, the tool tip must be flush with the workpiece axis and aligned using a template to ensure the tool tip angle is symmetrical with the workpiece axis. Errors in tool setting can cause deviations in the thread profile half-angle, affecting thread fit. When retracting, a tool retraction groove (width ≥ pitch, depth ≥ thread profile height) should be provided at the end of the thread, or a reverse retraction method should be used to prevent tool gnawing at the end of the thread. For threads without a tool retraction groove, the tool should be retracted before reaching the end point, and the last 1-2 threads should be machined manually. The key to preventing thread misalignment is ensuring that the tool accurately enters the machined groove with each pass. For lathes with a split-nut drive, thread machining must be completed in a single setup, and the split-nut must not be opened midway. For CNC lathes, precise synchronization can be achieved using a spindle encoder, ensuring accurate tool position even when the machine is stopped midway. In addition, the spindle speed must be kept stable during the turning process to avoid pitch errors caused by speed fluctuations, especially the speed must not be changed between rough turning and finish turning.
The quality inspection of triangular threads must cover dimensional accuracy, form and position accuracy, and surface quality, employing a combination of comprehensive gaging and individual measurements. Comprehensive inspection uses a thread ring gauge (for external threads) or a plug gauge (for internal threads). A go gauge must be inserted smoothly, and a stop gauge should be inserted no deeper than 2-3 threads (short threads are allowed to penetrate 1/3 of their full length). If the go gauge fails to penetrate, the pitch diameter is too large, while a stop gauge that is too loose indicates the pitch diameter is too small. Individual measurements include: measuring the pitch diameter with a thread micrometer, ensuring the error is within tolerance (e.g., the pitch diameter tolerance for a Grade 6 precision external thread is 0.224mm); measuring the thread pitch with a pitch gauge or tool microscope, with a cumulative error of no more than 0.05mm/100mm; and checking the thread angle with a goniometer, with a deviation of ≤±30°. Surface quality is inspected visually and using a roughness tester. The thread surface must be free of defects such as cracks, burns, and burrs, with a surface roughness of Ra ≤ 3.2μm (finish turning) or Ra ≤ 6.3μm (rough turning). For unqualified threads, minor defects (such as excessive surface roughness) can be repaired through manual grinding, while serious defects (such as excessive pitch diameter or incorrect thread profile) require scrapping and reprocessing. Through strict inspection and control, we ensure that triangular threads meet the requirements for connection strength and interchangeability, ensuring the safe operation of mechanical equipment.
