Trapezoidal thread turning tool
Trapezoidal thread turning tools are specialized tools for CNC machining trapezoidal threads. Their tooth profile matches the typical trapezoidal thread form (a 30° isosceles trapezoid). They are widely used in threaded pairs used to transmit power, such as machine tool lead screws, jacks, and presses. Trapezoidal threads offer strong load-bearing capacity, high transmission efficiency, and adjustable clearance after wear. The quality of the tool’s CNC machining depends directly on the tool’s geometry, grinding accuracy, and handling. The design of a trapezoidal thread turning tool must balance cutting performance and tooth profile accuracy. The appropriate tool material and construction must be selected based on the material being machined and the required precision to ensure dimensional accuracy and surface quality.
The material selection for trapezoidal thread turning tools depends on the material being machined and the cutting conditions. Commonly used tool materials include high-speed steel (HSS) and cemented carbide. High-speed steel (such as W18Cr4V) offers a sharp cutting edge, excellent toughness, and the ability to withstand high impact loads. This makes it suitable for CNC machining plastic materials (such as steel and copper alloys) and low-speed cutting (10-30 m/min). It also facilitates sharpening of complex tooth forms, making it the preferred choice for single-piece, small-batch production. Cemented carbide (such as YT15 and YG8) offers high hardness (HRA89-93) and excellent wear and heat resistance, making it suitable for CNC machining high-strength steel, cast iron, and other materials at high cutting speeds (50-100 m/min). However, its brittleness makes sharpening more challenging, making it suitable for mass production. For trapezoidal thread CNC machining in high-strength alloy materials (such as 40CrNiMo), coated carbide (such as TiAlN coating) with a coating thickness of 3-5 μm can increase tool life by 2-3 times. In addition, the tool handle material must have sufficient rigidity. Generally, 45 steel with quenching and tempering treatment (220-250HBW) is selected. The cross-sectional size of the tool bar is determined according to the major diameter of the thread, usually (10-15) × (15-20) mm, to reduce vibration during cutting.
The geometric design of trapezoidal thread turning tools is crucial for ensuring tooth profile accuracy and cutting performance. The selection of the rake angle (γ₀ ) depends on the material being machined. When CNC machining plastic materials like steel, a rake angle of 5°-10° is recommended to reduce cutting forces and friction. When CNC machining brittle materials like cast iron, a rake angle of 0°-5° is recommended to enhance cutting edge strength. The clearance angle (α₀ ) must take into account the thread lead angle (ψ). Because trapezoidal threads have a relatively long lead (typically 2-20mm ), the lead angle reduces the tool’s actual clearance angle. Therefore, the clearance angle in the feed direction is 8°-12° , and the clearance angle in the non-feed direction is 3°-5° to prevent friction between the tool’s flank and the thread flank. The primary rake angle (κᵣ ) is equal to the thread profile half-angle ( 15° ) to ensure that the tool’s tooth profile is consistent with the thread profile. The secondary rake angle (κᵣ ‘ ) is 1° -2° to reduce friction between the secondary cutting edge and the machined surface. The tool tip radius (rε) should be smaller than the minimum arc radius of the thread root, generally 0.1-0.5mm, to prevent overcutting. Furthermore, the tip width of a roughing tool should be smaller than the thread groove width (0.366P-0.536mm, where P is the pitch). The tip width of a finishing tool must be strictly equal to the groove width to ensure proper thread clearance.
The quality of sharpening a trapezoidal thread turning tool directly impacts the profile accuracy of the thread. During sharpening, ensure symmetry between the two cutting edges (tolerance ≤ ±10°). Calibration can be performed using an angle template. The profile angle (30°) must be controlled within ±30°. For high-precision thread turning tools, this tolerance must be within ±10°. This can be inspected using a tool microscope. The cutting edge must be sharp and free of chipping, with a roughness Ra ≤ 0.4μm. A 120-180 grit alumina grinding wheel can be used for sharpening, followed by grinding with an oilstone to remove burrs and microcracks. For wide-edge trapezoidal thread turning tools (for threads with a pitch of P ≤ 4mm), ensure a straightness of the cutting edge of ≤ 0.01mm/100mm. For trapezoidal thread turning tools with chip grooves (for threads with a pitch of P > 4mm), the chip grooves should be symmetrically distributed, 0.5-1mm deep, and 1-2mm wide to improve chip evacuation. After sharpening, the turning tool needs to be tested and cut. By measuring the tooth angle and tooth thickness of the thread, the sharpening error can be corrected until it meets the requirements.
The installation requirements of trapezoidal thread turning tools are strict to ensure the processing accuracy of the thread. When installing the turning tool, the tool tip must be at the same height as the workpiece axis. If the tool tip is higher than the axis, the thread profile angle will become larger; if it is lower than the axis, the thread profile angle will become smaller. The error can reach 1°-2°, and it needs to be calibrated by adjusting the tool holder height. The center line of the turning tool must be perpendicular to the workpiece axis and can be calibrated with a right-angle ruler. Excessive deviation will cause asymmetry in the thread profile on both sides, affecting the fitting accuracy. For high-speed cutting or large-pitch threads, an elastic tool rod is required. When the cutting force exceeds a certain value, the tool rod will bend slightly to avoid tool chipping. After installation, the tool holder screws need to be tightened to prevent the turning tool from loosening during processing. For multi-tool processing (rough turning tools and fine turning tools are installed separately), the tool tip position of each tool must be consistent. It can be calibrated by a tool setter, and the position error is ≤0.02