Milling rectangular tooth spline shaft on milling machine
Milling rectangular spline shafts on a milling machine is a common method for CNC machining spline shafts. Rectangular spline shafts feature high centering accuracy, high torque transmission, and easy assembly and disassembly. They are widely used in transmission systems such as automotive transmissions, machine tool spindles, and construction machinery. Rectangular spline shafts have rectangular teeth and can be centered using three methods: large-diameter centering, small-diameter centering, and key-side centering. Small-diameter centering is the most widely used, achieving centering accuracy up to IT6. Milling is suitable for single-piece, small-batch production or repair applications. It can be performed on a universal milling machine or spline milling machine. Using an indexing head, the teeth are evenly divided, ensuring symmetry and uniformity of each tooth to within 0.02mm.
Before milling a rectangular tooth spline shaft, the technical parameters and CNC machining benchmarks must be clearly defined to ensure CNC machining accuracy. The main parameters of a spline shaft include the number of teeth (z), module (m), minor diameter (d), major diameter (D), key width (b), and key length (L). These parameters must be determined based on the design drawings. For example, the parameters of a spline shaft are: z=6, m=3mm, minor diameter d=30mm, major diameter D=36mm, key width b=8mm, and key length L=100mm. The CNC machining benchmarks are usually the center holes and outer diameters at both ends of the spline shaft. The center hole must be ground to a roundness error of ≤0.01mm, and the coaxiality of the center holes at both ends is ≤0.02mm. The outer diameter serves as an auxiliary benchmark and must be pre-turned to size to a roundness error of ≤0.02mm and a coaxiality of ≤0.03mm with the center hole. The blank is made of 45 steel or 40Cr forgings, and is subjected to quenching and tempering treatment after rough turning. The hardness is controlled at 220-280HBW to improve cutting performance and enhance comprehensive mechanical properties.
The tool selection for milling rectangular spline shafts on a milling machine depends on the key width and precision requirements. Commonly used tools include end mills, side and face milling cutters, and form milling cutters. End mills are suitable for CNC machining spline shafts with smaller key widths (b ≤ 10mm). The tool diameter is selected based on the key height, generally 2-3 times the key height to ensure tool rigidity. Tool materials include high-speed steel (such as W18Cr4V) or carbide (such as YT15). High-speed steel end mills are suitable for fine milling, while carbide end mills are suitable for rough milling. Side and face milling cutters are suitable for CNC machining spline shafts with larger key widths (b > 10mm). The tool width should be equal to the key width, allowing both blades to cut the key side simultaneously, improving CNC machining efficiency and key side parallelism. The tool’s outer diameter should be larger than the major diameter of the spline to avoid interference between the tool and the workpiece. Form milling cutters are suitable for mass production. Their blade shape matches the spline tooth groove, allowing tooth groove CNC machining in a single pass and ensuring tooth profile accuracy. However, their manufacturing cost is higher, making them suitable for medium-volume and larger production runs.
Milling rectangular spline shafts on a milling machine involves a two-step process: rough milling and finish milling, gradually ensuring the spline’s accuracy and surface quality. Rough milling aims to remove the majority of the stock, using high cutting parameters: a cutting speed of 30-50 m/min (for high-speed steel tools) or 80-120 m/min (for carbide tools), a feed rate of 0.1-0.2 mm/r, and a finishing allowance of 0.2-0.5 mm per tooth. Rough milling can employ a layered milling method, milling one side of the spline tooth first, then the other, or a symmetrical milling method, milling two symmetrical spline teeth simultaneously, to minimize workpiece deformation. Finish milling is a critical step for ensuring accuracy, requiring high-precision tools, a cutting speed of 20-40 m/min (for high-speed steel tools), and a feed rate of 0.05-0.1 mm/r. After finish milling, the key width tolerance is controlled within the h9 range, and the surface roughness Ra on the key side is ≤ 3.2 μm. During the CNC machining process, precise indexing is required through the indexing head. After each key tooth is milled, the workpiece is rotated 360°/z angle, and the indexing error is ≤±5′. For spline shafts with a large number of teeth, the differential indexing method of the universal indexing head can be used to improve the indexing efficiency.
When milling rectangular tooth spline shafts on a milling machine, the clamping and adjustment must ensure the positioning accuracy and stability of the workpiece, providing the foundation for high-precision CNC machining. The workpiece is clamped using dual ejector pins for positioning, with the center holes at both ends in close contact with the ejector pins. The pressure of the tailstock ejector pins must be adjusted appropriately. Too tight will cause the workpiece to bend and deform, while too loose will produce vibration. Generally, it is best to be able to turn the workpiece by hand without noticeable play. A steady rest or center rest is installed in the middle of the workpiece to support the workpiece and increase rigidity. The support block of the steady rest should be made of wear-resistant cast iron, with a contact area of ≥80% with the workpiece. The support force should be adjusted to a level that does not produce noticeable indentations. When adjusting the position of the milling cutter, ensure that the tool axis is perpendicular to the workpiece axis, with the same height and a deviation of ≤0.02mm. A dial indicator or template can be used for calibration. For spline shafts centered on the minor diameter, ensure that the distance between the side of the milling cutter and the minor diameter is equal to half the key height, with an error of ≤0.01mm. During trial cutting, first mill a key tooth and measure its key width, key height and symmetry. If there is an error, adjust the tool position or dividing head parameters until it meets the requirements.
Quality inspection and error correction for rectangular tooth spline shafts milled on a milling machine must be performed throughout the entire CNC machining process to ensure the spline shaft meets design requirements. Inspection items include: minor diameter (measured with a micrometer, tolerance ±0.02mm), major diameter (measured with a vernier caliper, tolerance ±0.05mm), key width (measured with a key width gauge or vernier caliper, tolerance h9), key tooth symmetry (measured with a dial indicator, error ≤0.02mm), key tooth uniformity (measured with a dividing head and dial indicator, error ≤0.03mm), and surface roughness (measured with a roughness tester, key side Ra ≤3.2μm). Common quality issues and solutions: If the key width dimension is out of tolerance, adjust the milling cutter width or feed rate; if the key tooth symmetry error is too large, check the relative position of the tool and workpiece and recalibrate; if the key teeth are uneven, check the dividing head accuracy and eliminate the indexing gap; if chatter marks appear on the surface, reduce the cutting speed, increase the tool rigidity, or adjust the tool rest support. For spline shafts with higher precision requirements (grade 6 and above), grinding is required after milling to further improve the parallelism and surface quality of the key side to meet the requirements of high-precision transmission.
