We previously discussed the roughing of shafts. Roughing is the foundation for shaft finishing. For parts that require finishing, sufficient, but not excessive, machining allowance should be left to ensure the smooth progress of finishing.
Finish turning requires a high level of skill and should be performed by skilled workers on precision lathes. If the center hole of the axle is worn after rough turning, it must be re-corrected to obtain a precise machining positioning datum before finish turning can proceed.
When precision machining, it is essential to ensure the dimensions between the two bearing supports, also known as the bearing shoulder, as its accuracy requirements are quite strict, and it serves as the reference for most other bearing supports.
After the shaft is precision turned, the bearing seat, shaft extension and other parts need to be ground. Similarly, during the precision turning process, appropriate machining allowance should be left in the parts that need to be ground. While ensuring that the dimensions are in compliance, the grinding process should ensure that the machined surface meets the roughness requirements.
For particularly large motor shafts, due to the size control requirements of the equipment, the parts that should theoretically be ground are finished by precision turning, but this requires high equipment precision and skilled operators.
Precision turning of motor shaft
Finish turning requires a high level of skill and should be performed by skilled workers on precision lathes. If the center hole of the axle is worn after rough turning, it must be re-corrected to obtain a precise machining positioning datum before finish turning can proceed.
Dead centers provide a more stable workpiece mounting, but they experience greater wear. Moving centers have lower rigidity but prevent wear between the workpiece and the center; therefore, they are widely used in high-speed cutting.
The clamping method for finish turning is roughly the same as that for rough turning. During finish turning, except for the steps that need to be ground, which allow for grinding ( 0.3 ~ 0.5 mm), the diameter and length of all other shafts are turned to the dimensions specified in the drawing. The end face chamfer and grinding wheel runout groove are also machined at the same time.
For small motors, to prevent shaft bending and deformation during press-fitting, the core is press-fitted after rough knurling, followed by finish machining of the bearing seats, shaft extensions, and core outer diameter. Alternatively, the armature, commutator, shaft extension, and fan sections can be finish-machined first, with the bearing seats enlarged by 0.3-0.5 mm according to the drawing dimensions . The rotor is then press-fitted, followed by finish machining of the bearing seats, armature outer diameter, commutator outer diameter, and other parts to ensure coaxiality of the rotor components.
When precision machining, it is essential to ensure the dimensions between the two bearing supports, also known as the bearing shoulder, as its accuracy requirements are quite strict, and other bearing supports are often referenced to it.
To improve turning productivity, the focus should be on increasing the manufacturing precision of the blank to reduce machining allowance; increasing the turning depth and reducing the number of passes to shorten tool path time and auxiliary time. Advanced technologies employed include high-speed cutting, heavy-duty cutting, multi-tool multi-edge machining, and contour turning.
To improve machining accuracy in turning, it is necessary to analyze the factors affecting machining accuracy based on specific machining conditions and take corresponding measures. The workpiece and tool clamping must be accurate and reliable; the rigidity of the machining system must be good; the cutting edge of the tool must be carefully ground; smaller depth of cut and feed rate result in smaller cutting forces, thus smaller elastic deformation of the machining system and improved surface finish.
When precision turning shafts, a smaller depth of cut and feed rate are often used to achieve higher machining accuracy and surface roughness.
Shaft grinding
The bearing mounting areas at both ends of the motor shaft and the shaft extension section require a high degree of surface roughness. While precision turning can meet these requirements, it results in low productivity and high cost. A more economical machining method is to use semi-precision turning followed by grinding of the outer diameter.
During grinding, the workpiece is placed between two dead centers and rotated using a chuck and a rotary dial. The purpose of using dead centers is to reduce the clearance between the centers and the shaft, eliminate vibration, and thus achieve the required machining accuracy.
When grinding the outer diameter, longitudinal feed (longitudinal grinding) is usually used. The grinding wheel performs the main cutting motion, while the workpiece rotates and reciprocates linearly with the worktable (longitudinal feed). The grinding allowance is removed through multiple longitudinal feeds. At the end of each reciprocating stroke, the grinding wheel makes a transverse (radial) cut. The longitudinal grinding method is characterized by high precision. For small motor shafts, due to the short machining area, transverse feed (transverse grinding) is sometimes used. The grinding wheel only performs transverse feed. The width of the grinding wheel should be slightly larger than the length of the part being ground (generally about 5-10 mm larger). During rough grinding, the transverse feed is 0.025-0.02 mm/workpiece per revolution; during finish grinding, the transverse feed is 0.001-0.012 mm/workpiece per revolution. The transverse grinding method is characterized by high efficiency, but the cylindricity deviation of the outer diameter is relatively large, and the grinding wheel shape must be frequently dressed with a diamond cutter.
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