The precision machining of the rotor refers to the precision turning of the rotor's outer diameter, a crucial process for ensuring the accuracy of the motor's air gap and its performance. Any defects in this process will ultimately result in uneven air gap, severely impacting motor performance. Today, we will focus on the precision machining requirements of the rotor and how to improve its efficiency.
Rotor finishing requirements
Key points for precision machining of rotors. The requirements for precision machining are that the outer circle of the rotor and the bearing position should be coaxial; the dimensional accuracy and surface roughness of the outer circle of the rotor must meet the specifications of the drawing; the laminations should not have reverse teeth, and the core teeth should not have axial tilt.
The rotor is typically precision machined on a horizontal lathe using a precision cutting tool. To prevent tooth breakage on the laminations, a large rake angle should be selected to ensure a sharp cutting tool. This reduces radial cutting resistance. Similar to the precision machining of the stator core's inner circle, the cutting speed and depth of cut should not be too high, and the feed rate should not exceed the thickness of a single lamination to avoid excessive stress and damage to the laminations.
Because most rotors of small and medium-sized asynchronous motors use semi-closed slots and the rotor conductors are made of cast aluminum, during turning, the cutting tool alternately encounters hard silicon steel sheets and sticky soft aluminum strips, resulting in intermittent cutting and rapid tool wear. This leads to low efficiency in the finish turning of the rotor's outer diameter. Frequent tool sharpening and adjustment make it a weak link in automated machining lines.
To ensure the cutting quality of the rotor core's outer diameter and improve productivity, a special lathe for cutting the outer diameter of the motor rotor and advanced carbide disc cutting tools are used for precision turning. This machine tool is a high-efficiency automatic lathe equipped with automatic measuring devices and automatic compensation mechanisms. It performs automatic measurements during the cutting process and can compensate for tool body thermal expansion and cutting edge wear in both positive and negative directions during high-speed cutting, thereby ensuring the stability of the workpiece dimensions.
How to improve rotor finishing efficiency
Figure 1 shows an example of rotor precision turning using a carbide disc cutting tool. The disc insert 1 is made of carbide, with an outer diameter of 60-100 mm and a cutting edge angle of 6°-7°. The disc insert is clamped to the tool body 2 using hex socket head cap screws 3 and a clamping plate 4. The tool body is fixed to a spindle 5, and three rolling bearings are installed between the spindle and the tool body holder 7, allowing the spindle to rotate freely within the tool body holder. The disc cutting tool is mounted on a tool post using a shank 6.
Figure 1
The cutting edge of a disc lathe tool is dozens of times longer than that of a conventional lathe tool, resulting in reduced tool wear. Furthermore, the rotating cutting edge facilitates heat dissipation, extending the tool's lifespan by more than 30 times compared to a conventional lathe tool.
The disc lathe tool can also be used on a conventional rotor finishing lathe. As shown in Figure 2, during machining, the bearing position of the rotor 8 is supported by the locating bushing 9 to ensure the coaxiality of the outer circle of the iron core and the bearing stop. There should be an inclination angle θ between the tool and the workpiece.
Figure 2
The size of this angle affects the stress on the cutting tool, the wear rate, and the surface roughness of the workpiece. When θ = 0°, the cutting tool cannot be rotated by the workpiece, resulting in rapid wear on the local cutting edge. When θ = 90°, the linear velocities of the cutting tool and the workpiece are the same, making cutting impossible. Practice has shown that the most suitable tilt angle is 18°~20°, at which point the linear velocity of the cutting tool is approximately 25~30% of the linear velocity of the workpiece.
When using a disc lathe tool for precision machining the outer diameter of the rotor core, the required dimensions can be achieved in a single pass, with a surface roughness of no more than 3.2. The feed rate is 4 to 10 times greater than that of a conventional lathe tool, and the cutting speed can reach 250 to 300 meters per minute, increasing productivity by more than 4 times. For motor rotors with a frame size of 5 or smaller, machining the outer diameter of one rotor core takes an average of only 50 seconds.
