Splines are mechanisms adopted in parts that transmit power in automobiles, agricultural machinery, ships, and machine tools, transferring large torques with multiple teeth. Splines require high strength and durability, hence materials such as carbon steel S45C, chromoly steel SCM440, and stainless steel SUS330, which are intended for heat treatment, are often used. This article explains the features of splines, typical processing methods, and the challenges and solutions in processing splines.
There are several types of splines, among which involute splines are representative. The term "involute" originates from the involute curve drawn by the end of a thread unwound from a cylinder without slackening. Involute splines, as defined by "JIS B 1603", are characterized by their tooth shape formed based on the involute curve, which allows for high rotational accuracy and smooth operation.
When transmitting rotational motion with teeth formed by the involute curve, the contact points move smoothly along the same curve. This indicates that the center distances between the gears, which affect the accuracy of rotation and the error in meshing, are matched. By combining involute splines, it is possible to achieve high rotational accuracy.
The involute curve has the characteristic that its normals always touch a single circle. By combining involute splines, from the start to the end of meshing between gears, the angle at which pressure is applied remains constant, ensuring that there are no misalignments or play, resulting in smooth operation.
The typical processing methods for involute splines include hobbing (generating method) and milling (forming method).
Traditionally, hobbing was primarily performed using specialized equipment known as hobbing machines. However, in recent years, the use of hob holders has made it possible to conduct hobbing not only on hobbing machines but also on machining centers. Known for its ability to achieve high accuracy at high speeds, hobbing is well-suited for mass production.
In hobbing, a cylindrical tool called a hob cutter is used. By rotating both the material and the hob, teeth are cut into the material. Since a specific hob cutter must be prepared for each gear shape, tooling costs can become high when multiple types of gears are needed. Additionally, hobbing is used only for cutting external teeth and cannot be used for cutting internal teeth.
Milling involves using a common end mill to machine shapes modeled in 3D CAD. It is capable of carving out splines and gears, offering a significant advantage in terms of flexibility in gear shapes. However, since each tooth groove is cut one at a time, the process is time-consuming, making it unsuitable for mass production. It is known as a manufacturing method suited for prototypes and tools, or for small, diverse production batches.
When processing splines, several challenges may arise, including a decrease in machining accuracy and tool anomalies.
Factors such as tool wear and improper installation can lead to a deterioration in spline machining accuracy. A decrease in spline machining accuracy can result in pitch defects and tooth deformation, making it crucial to maintain high machining accuracy.
Tool anomalies can occur due to improper machining conditions or prolonged use of the same tool, leading to accelerated wear or breakage during spline machining. Using a tool that has developed anomalies not only results in defective splines but can also damage the equipment, necessitating caution.
To resolve challenges in spline machining, it is important to consider measures for maintaining machining accuracy and preventing tool anomalies.
To maintain high machining accuracy, it is necessary to check the accuracy during the setup phase before machining. Not only the accuracy of the tool itself but also the precision of the hob's installation, whether the angles are correct, and whether there is any play in the mounting holder, need to be verified. Attention should also be paid to potential issues like workpiece slippage and synchronization between the tool and the spindle.
To prevent tool anomalies, setting appropriate machining conditions is crucial. Conditions such as wet or dry cutting and cutting speed need to be adjusted according to the manufacturer's recommended settings. Additionally, measures to prevent chip entanglement during machining are essential.
This article has explained the features of splines, their typical processing methods, and associated challenges. When machining splines, it is ideal to use the coating and material types recommended by manufacturers to achieve high accuracy. If anomalies occur during spline machining, it is advisable to first investigate the accuracy of the setup phase and whether the machining conditions are appropriate, focusing on these two aspects for troubleshooting.
