How to control the machining accuracy of gear parts

Most of the parts inside the gearbox are gears, so the machining accuracy of gear parts has a significant impact on the quality of the gearbox. Recently, various faults have frequently occurred in the YD13 series gearbox during testing, which has limited the total production of the entire machine. By reviewing and analyzing the test drive records, the main test faults of the YD13 series gearbox were identified. Through disassembly and analysis of these faults, it was found that most of them were caused by machining problems with gear parts. How to improve the machining accuracy of gear parts has become the main means to reduce gearbox failures. The purpose of this article is to control and analyze the entire machining process of gear parts, summarize effective methods for controlling the machining accuracy of gear parts, and promote them to the production of other types of transmissions, which can effectively reduce the maintenance cost of transmissions and improve the factory transmission rate.
With the development of the national economy, the demand for construction machinery gearboxes in the market is increasing, and gearbox manufacturers have become busy due to the increase in orders. However, various malfunctions often occur during the testing phase of the gearbox, which seriously restricts the factory output of the gearbox. The products produced cannot be delivered to users, resulting in a large backlog of orders and seriously affecting the sales of the products. When technicians analyzed and inspected these test failures, they found that most of them were caused by machining problems of gear parts. Therefore, in order to improve the factory production rate of transmissions, it is necessary to ensure the machining quality of gear parts. After controlling and analyzing the machining process of gear parts, effective methods for improving the accuracy of gear parts have been summarized. After extensive verification, it has been proven that these measures are effective in controlling the accuracy of gear parts.
1.1 Ensure the quality of gear materials
The material used to manufacture gears is the foundation for ensuring the quality of machining parts, so controlling the quality of gear materials is particularly important. The materials used for manufacturing gears in our factory are usually alloy steels such as 20CrMnTi, 19CrNi5, 20MnCr5, etc. When the materials enter the factory, in addition to strictly following the enterprise standards for the acceptance of relevant parameters of these forgings, if necessary, an additional normalizing treatment can be added after rough machining of the forgings to change the material properties, control subsequent machining and heat treatment deformation, and improve the machining quality of the parts. For example, when manufacturing the YH350 railway maintenance equipment hydraulic gearbox, our factory carried out rough turning and normalizing treatment on important gear parts, and subsequently achieved good machining results.
1.2 Control gear pre heat machining error
Gear hot processing is divided into two parts, one is the processing of gear blanks, and the other is the processing of gear parts.
1.2.1 Precision of gear blank processing
In the process of gear blank processing, controlling the dimensional accuracy of the positioning inner hole of gear parts is the most critical. According to the precision level of our factory's gears, the size tolerance of the gear inner hole is usually set at 0.03. This value not only meets the accuracy requirements for subsequent gear machining and positioning, but also does not increase unnecessary costs due to high precision, which is in line with production economy. The straightness of the inner hole should be controlled within 0.01.
Control the end face runout and radial runout of the gear blank: The end face runout of the gear blank directly affects the tooth profile tolerance during gear rolling. It is usually required to process both the inner hole and the end face of the part at the same time to ensure that the end face runout is ≤ 0.01. When turning the other end face for processing, it is necessary to ensure that the parallelism between the two end faces is ≤ 0.02, and the radial runout between the outer circle of the tooth and the positioning inner hole should be controlled below 0.03.
1.2.2 Gear machining accuracy
In the process of tooth processing, the main factors affecting tooth errors are: gear machining machine accuracy; Gear machining tools; Several aspects such as tooth blank positioning fixture.
⑴ Precision of gear machining machine tools.
The accuracy of gear machining machine tools largely determines the accuracy of the gears being machined, so maintenance and upkeep of the machine tools should be carried out according to regulations.
Before preparing to process the tooth parts of the parts, it is necessary to ensure that the deviation values of each part of the machine tool meet the requirements: for example, the radial clearance of the machine tool spindle is less than 0.01, the diameter jump of the tool shaft is less than 0.005, the tool shaft displacement is less than 0.008, the diameter jump and end jump after tool installation are less than 0.005, the coaxiality of the top and bottom tips is less than 0.008, and the diameter jump and end jump of the workpiece fixture installed at the fixture positioning position behind the machine tool worktable are less than 0.01.
⑵ Gear machining tools.
When gear cutting tools enter the factory, their various parameters should be inspected according to the design requirements, and the results should meet the requirements of the drawings. The commonly used tool grade is A grade, and for some parts with high precision requirements, AA grade tools can be selected; To improve the durability of cutting tools, titanium coated cutting tools can be selected; After grinding the tool, the radial error of the front cutting edge should be controlled.
⑶ Gear blank positioning fixture.
The gap between the outer diameter of the core shaft of the fixture and the inner hole of the gear should be as small as possible, usually below 0.02. The threads on the core shaft should ensure a perpendicularity of less than 0.01, and the internal threads with shoulder nuts should have a perpendicularity of less than 0.01 to the end face.
1.3 Top chamfer and end chamfer of teeth
After tooth profile processing, the tooth top and tooth end should be chamfered to meet the needs of the gear during transmission and avoid the problem of parts collision. The chamfering of the tooth top can also be formed by using chamfering cutters with chamfering during tooth profile machining.
1.4 Quality control of heat treatment
Heat treatment has a significant impact on the accuracy of gears. To ensure subsequent processing, for ordinary structural parts, it should be stipulated that the ellipticity of the inner hole of the part after heating should not exceed 0.03, and the change in normal line size should not exceed 0.03; For parts that are thin and have a large difference between the inner hole and the outer diameter of the tooth, in addition to the above regulations, the end face curvature of the part should not exceed 0.05; For parts with internal splines, excessive deformation of the internal splines should be avoided.
1.5 Precision machining of parts after heating
After heat treatment, the parts show slight deformation and reduced surface quality, requiring precision machining for some important dimensions.
When grinding the inner hole of the gear after heating, the inner hole and end face should be aligned and the runout should not exceed 0.015. At the same time, the gear pitch circle runout should be checked and not exceed 0.04 to ensure that the inner hole is concentric with the indexing circle. The inner hole and end face grinding should be carried out simultaneously to ensure the perpendicularity of the end face to the inner hole. When grinding the other end surface as a tightening positioning surface, the parallelism between the two end surfaces should be ensured to be below 0.015. Parts processed using a circular table grinder should undergo demagnetization after machining.
1.6 Precision machining of tooth parts
Our factory usually uses CNC gear grinding machines for precision machining of gear parts. Currently, we have two types of gear grinding machines, namely forming gear grinding machines and worm wheel gear grinding machines.
For the gear grinding machine, before clamping the parts, the front and rear of the machine tool should be checked for wear and the coaxiality between the two; When aligning parts, try to keep them in the middle position. During the grinding process, check the grinding condition of both tooth surfaces. If there is more grinding on one side and less grinding on the other side, adjust the grinding position of the parts in a timely manner to ensure that the grinding amount of the parts is uniform and not eccentric; Reasonably set grinding parameters and grinding wheel dressing parameters to maintain the sharpness of the grinding wheel particles; Timely replace the used grinding wheel to avoid the phenomenon of insufficient gear deployment length; After the gear grinding test, it should be sent to the measuring room for inspection of Fa, F β, Fr, Fi ", fi", Fw and other items.
For worm gear grinding machines, due to their different grinding methods compared to forming gear grinding machines, in addition to the above requirements, the following aspects should also be noted: worm gear grinding belongs to the expansion method of grinding, and multiple teeth participate in the grinding, resulting in a large amount of grinding heat, which can easily cause tooth surface burns and cracks. When setting grinding parameters, a method of less single feed and multiple strokes can be adopted; When grinding a worm wheel, the two sides and outer circle of the wheel are trimmed in two steps. The grinding wheel repaired in this way can easily cause insufficient tooth root or unfolded length during gear grinding, resulting in interference in subsequent gear transmission. Therefore, during the process of grinding parts after repairing the wheel, it is necessary to observe the grinding situation of the teeth and make timely adjustments; Due to the unique structure of the grinding wheel dresser in the worm gear grinding machine, the tool used for dressing the grinding wheel is not a diamond pen, but a square block. This type of diamond block is prone to wear and should be regularly observed for wear and timely replaced to avoid mass quality accidents.
For the grinding tool gear core shaft, the clearance between the core shaft positioning outer circle and the inner hole of the part is usually below 0.015, the runout between the core shaft outer circle and the end face is below 0.01, and the surface of the core shaft outer circle is smooth and free of marks; The two tips of the core shaft have no wear or foreign objects in the pointed holes; The threaded part of the core shaft must be ground by a thread grinder to ensure a perpendicularity of less than 0.01, and the nut should be tightened to loosen the thread.
 
 
It is not difficult to control the machining quality of gear parts during the production process. By following the requirements described in this article and adopting a serious and responsible attitude, significant results have been achieved through extensive practical applications. According to the analysis report of the whole machine test run, the improvement of the machining quality of gear parts can reduce the failure rate of the gearbox caused by gear problems, which is of great significance for effectively improving the factory yield of the whole machine.