Twist drill milling
The twist drill is the most commonly used hole-making tool in CNC machining. Its complex structure and unique cutting edge shape make it difficult to guarantee cutting edge accuracy and surface quality using traditional grinding. Milling, however, as a highly efficient and high-precision CNC machining method, has been widely used in twist drill manufacturing. Milling of twist drills primarily involves CNC machining key areas such as the drill tip, spiral grooves, and margins. By properly selecting milling tools, determining milling parameters, and determining the appropriate process path, the accuracy and service life of the twist drill can be effectively improved. Mastering the milling process is crucial for ensuring the drill’s cutting performance and CNC machining quality.
Milling the spiral groove in a twist drill is a key process in twist drill manufacturing. The geometric parameters of the spiral groove (such as the helix angle, groove depth, and groove width) directly affect the chip evacuation performance and cutting efficiency of the twist drill. Spiral groove milling is typically performed on a dedicated spiral groove milling machine or CNC milling machine, using a disc milling cutter or a finger milling cutter. The spiral groove is formed through the rotation of the workpiece and the feed motion of the milling cutter. The helix angle is a key parameter of the spiral groove and is generally determined by the diameter of the twist drill and the material being processed. For smaller diameter twist drills, the helix angle is 18°-30°, while for larger diameters, it is 30°-45°. A smaller helix angle is used when CNC machining brittle materials such as cast iron, while a larger helix angle is used when CNC machining plastic materials such as steel. When milling the spiral groove, the angle between the milling cutter axis and the workpiece axis must be precisely adjusted to equal the helix angle. The milling cutter feed rate and workpiece rotational speed must also be controlled to ensure the lead accuracy of the spiral groove. The material of the milling cutter is usually high-speed steel or carbide. For high-speed milling, carbide milling cutters should be used to improve processing efficiency and tool life.
Milling the twist drill tip is key to ensuring drilling accuracy and cutting performance. The drill tip’s geometric parameters (such as the apex angle, chisel edge bevel angle, rake angle, and clearance angle) significantly impact drill quality. The apex angle is the angle between the two main cutting edges of the drill tip. The standard twist drill’s apex angle is 118°±2°. Adjusting the apex angle for different materials requires adjustment: 118° for plastic materials to reduce cutting forces, and 130°-140° for brittle materials to improve tool life. Drill tip milling is typically performed on a dedicated drill tip milling machine. Two symmetrical milling cutters simultaneously mill the two main flank surfaces to create the desired apex and clearance angles. During milling, the symmetry of the two main cutting edges must be maintained, and the symmetry error should be controlled within 0.03-0.05mm. Otherwise, unbalanced axial forces will result during drilling, causing the drill to yaw and affecting hole accuracy. The chisel edge angle is the angle between the chisel edge and the main cutting edge on the end face of the drill bit, which is generally 50°-55°. During milling, the position of the milling cutter must be adjusted to ensure the size of the chisel edge angle. Too large or too small a chisel edge angle will affect the centering performance and cutting force of the drill bit.
Milling the margin of a twist drill forms the drill’s guiding element and secondary flank. The width and length of the margin significantly influence the drill’s guiding properties and wear resistance. The margin width is typically 0.1-0.5mm (increasing with drill diameter). Excessive width increases friction with the hole wall, leading to overheating; too small a width impairs guiding performance and affects hole straightness. Margin milling is typically performed after spiral groove milling. A small disc milling cutter or grinding wheel is fed along the drill’s axis to remove excess material from the edges of the spiral groove, forming the margin. During milling, radial runout of the margin must be controlled to no more than 0.02-0.03mm to ensure drill stability. The surface roughness (Ra) of the margin should be controlled between 0.8-1.6μm to reduce friction with the hole wall and maximize drill life. For high-precision twist drills, the cutting edge needs to be ground after milling to further improve the surface quality and dimensional accuracy.
Tool selection and parameter optimization for twist drill milling must be determined based on the material and processing requirements of the twist drill. Twist drills are primarily made of high-speed steel and carbide. High-speed steel twist drills can be milled with either high-speed steel or carbide cutters, typically at speeds of 30-60 m/min. Carbide twist drills must be milled with carbide or ceramic cutters, at speeds of 100-200 m/min. The feed rate should be determined based on the milling area and CNC machining accuracy. For rough milling of spiral grooves, the feed rate should be 0.1-0.3 mm/r, while for fine milling of the drill tip, the feed rate should be 0.05-0.1 mm/r to ensure surface quality. The milling depth should be appropriately distributed based on the CNC machining allowance. For rough milling, the majority of the allowance should be removed at once, while for fine milling, a 0.1-0.2 mm allowance should be left to ensure final accuracy. Sufficient cutting fluid must be used during milling. High-speed steel milling cutters use emulsion, and carbide milling cutters use extreme pressure cutting oil to reduce cutting temperature, reduce tool wear, and improve machined surface quality.
The process and quality control of twist drill milling are crucial for ensuring CNC machining accuracy. The typical process is: stock preparation (rolling or forging) → milling the spiral groove → milling the drill tip → milling the land → heat treatment (quenching and tempering) → grinding → inspection. Before milling, the stock must be inspected to ensure that diameter, straightness, and other parameters meet requirements. During milling, parameters such as the helix angle, vertex angle, and land width must be regularly measured to adjust milling parameters. After heat treatment, due to the increased material hardness, final grinding is required to ensure cutting edge accuracy. Key quality control requirements include: the lead error of the spiral groove is no greater than 0.1mm/100mm; the vertex angle error is no greater than ±1°; the length difference between the two main cutting edges is no greater than 0.05mm; and the radial runout of the land is no greater than 0.03mm. High-precision twist drills also require dynamic balancing tests to ensure stability during high-speed rotation. Through reasonable process routes and strict quality control, the milling accuracy of twist drills can meet the design requirements, ensuring good cutting performance and service life in drilling processing.
With the development of CNC technology, the milling process of twist drills has gradually become automated and intelligent. CNC spiral groove milling machines can precisely control parameters such as the helix angle and groove depth through programming, enabling the automatic processing of twist drills of varying specifications. CNC Machining centers can complete multi-step processing of twist drills, reducing clamping errors and improving processing efficiency. The application of online measurement technology allows for real-time monitoring of dimensional changes during the milling process, enabling timely compensation to ensure CNC machining accuracy. Furthermore, the application of high-speed milling technology can significantly improve the processing efficiency and surface quality of twist drills, reducing the workload of subsequent grinding processes. In the future, with advances in materials science and manufacturing technology, twist drill milling will develop towards higher precision, higher efficiency, and more intelligent technology, providing the machinery manufacturing industry with higher-performance hole-making tools.
