How To Control The Machining Accuracy of Gear Parts

How To Control The Machining Accuracy of Gear Parts

Most of the parts in the gearbox are gears, so the machining accuracy of the gear parts has an important impact on the quality of the gearbox. Recently, the YD13 series gearboxes often have various failures during the test run, which restricts the total output of the whole machine. Check and analyze the commissioning records to understand the main commissioning failures of the YD13 series gearbox. Through dismantling and analysis of these failures, it is found that most of the failures are caused by the processing problems of gear parts. How to improve the machining accuracy of gear parts has become the main means to reduce gearbox failures. The purpose of this paper is to control and analyze the entire machining process of gear parts, and summarize the method to effectively control the machining accuracy of gear parts, and at the same time, it can be extended to the production of other types of gearboxes, which can effectively reduce the cost of gearbox maintenance and improve the production efficiency. Factory gearbox factory rate.

With the development of the national economy, the demand for construction machinery gearboxes in the market is increasing, and gearbox manufacturers have also become busy due to the increase in orders. However, various failure problems often occur in the transmission during the commissioning stage. These problems seriously restrict the output of the transmission, and the produced products cannot be delivered to the users, resulting in a large backlog of orders, which seriously affects the sales of the products. When the technicians analyzed and inspected these test failures, they found that most of the failures were caused by the processing problems of the gear parts. Therefore, in order to improve the delivery rate of the gearbox, the processing quality of the gear parts must be guaranteed. After controlling and analyzing the machining process of gear parts, the methods to effectively improve the accuracy of gear parts are summarized. After a lot of verification, it is proved that these measures are effective in controlling the accuracy of gear parts.

Measures to Control Gear Machining Accuracy.

1.1 Guarantee the quality of gear material

The material for making gears is the basis for ensuring the quality of parts processing, so it is particularly important to control the quality of gear materials. The gears made by our factory are usually 20CrMnTi, 19CrNi5, 20MnCr5 and other alloy steels. When the material enters the factory, in addition to the acceptance of the relevant parameters of these forgings in strict accordance with the enterprise standards, if necessary, a normalizing treatment can be added after the rough machining of the forging to change the material properties, control the subsequent machining and heat treatment deformation, and improve the parts. Processing quality. For example, when our factory manufactures the hydraulic gearbox of YH350 railway maintenance equipment, the important gear parts are subjected to rough turning and normalizing treatment, and the subsequent processing results are very good.

1.2 Control the machining error of gears before heat

The hot pre-processing of gears is divided into two parts, one is the processing of the gear blank, and the other is the processing of the teeth.

1.2.1 Gear blank machining accuracy

In the process of gear blank processing, it is the most critical to control the dimensional accuracy of the positioning inner hole of the gear part. According to the accuracy grade of our gears, the dimensional tolerance of the inner hole of the gear is usually set to 0.03, which not only meets the accuracy requirements of the subsequent tooth processing and positioning, but also does not increase unnecessary costs due to excessive accuracy. 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 direction tolerance items during hobbing. Usually, the inner hole and the end face of the part are required to be processed at the same time to ensure that the end face runout is less than or equal to 0.01. When turning around to process the other end face, it is necessary to ensure that the parallelism between the two end faces is less than or equal to 0.02. At the same time, the radial runout of the outer circle of the tooth and the positioning inner hole should be Controlled below 0.03.

1.2.2 Tooth machining accuracy

In the tooth machining process, the main factors affecting the tooth error are: the precision of the gear machining machine tool; the gear machining tool; the tooth blank positioning fixture and so on.

⑴ gear processing machine tool accuracy.

The precision of the gear processing machine tool largely determines the precision of the machined gear, so the maintenance of the machine tool should be carried out in accordance with the regulations.

Before preparing to machine the tooth of the part, it should be determined that the deviation value of each part of the machine tool meets the requirements: for example, the radial clearance of the machine tool spindle < 0.01, the diameter jump of the tool shaft < 0.005, the movement of the tool shaft < 0.008, the diameter jump and the end of the tool after installation The jump is <0.005, the coaxiality of the upper and lower centers is <0.008, and the radial jump and the end jump at the fixture positioning position after the workpiece fixture is installed on the machine table is <0.01.

⑵ gear machining tools.

When the gear cutter enters the factory, the parameters of the cutter should be checked and accepted according to the design requirements, and the results should meet the requirements of the drawings. The commonly used tool grade is grade A. For some parts with high precision requirements, AA grade tools can be selected; in order to improve the durability of the tool, titanium-coated tools can be selected; after the tool is sharpened, the radial error of the front face should be controlled.

⑶ tooth blank positioning fixture.

The gap between the outer diameter of the mandrel of the fixture and the inner hole of the gear should be as small as possible, usually less than 0.02. The thread on the mandrel should ensure that the verticality is less than 0.01, and the internal thread of the shoulder nut is less than 0.01 vertical to the end face.

1.3 Chamfering of tooth top and tooth end

After the tooth shape processing, the tooth top chamfering and tooth end chamfering should be carried out, in addition to meeting the needs of the gear in the transmission process, it can also avoid the problem of parts bumping. The tooth top chamfer can also be formed by using a chamfered hob with chamfering during tooth profile machining.

1.4 Control the quality of heat treatment

Heat treatment has a great influence on the accuracy of gears. In order to ensure subsequent processing, for parts of ordinary structure, it should be specified that the ovality of the inner hole after the part is heated is not greater than 0.03, and the change in the common normal dimension is not greater than 0.03; for thin parts with a large difference between the inner hole and the outer diameter of the tooth, In addition to the above provisions, the warpage of the end face of the part should not be greater than 0.05; for parts with internal splines, excessive deformation of the internal splines should be avoided.

1.5 Finishing of parts after heat

After the parts are heat treated, the whole is slightly deformed, and the surface quality is reduced, and some important dimensions need to be finished.

When the inner hole of the gear is ground after heat, the inner hole and end face runout should be adjusted to be less than 0.015, and the pitch circle runout of the gear should be checked to be less than 0.04 to ensure that the inner hole and the index circle are concentric. The inner hole and the end face grinding should be carried out at the same time to ensure the verticality of the end face to the inner hole. When grinding the other end face as the positioning face for tightening, ensure that the parallelism between the two end faces is less than 0.015. Parts machined with a table grinder should be demagnetized after machining.

1.6 Tooth finishing

Our factory usually adopts the grinding method of CNC gear grinding machine. At present, there are two types of gear grinding machines, namely forming gear grinding machine and worm grinding wheel gear grinding machine.

For the forming gear grinding machine, before clamping the parts, check whether the front and rear centers of the machine tool are worn and the coaxiality between the two centers; try to keep the parts in the middle position when setting the tool, and check the two teeth during the grinding process. If there is more grinding on one side and less grinding on the other side, the grinding position of the part should be adjusted in time to ensure that the grinding amount of the part is uniform and not eccentric; the grinding parameters and grinding wheel dressing parameters should be set reasonably. , to maintain the sharpness of the abrasive grains of the grinding wheel; replace the used grinding wheel in time to avoid the phenomenon of insufficient length of the gear;

For the worm grinding wheel grinding machine, because its grinding method is different from that of the forming gear grinding machine, in addition to paying attention to the above requirements, it should also pay attention to the following aspects: the worm grinding wheel grinding belongs to the generation method grinding, and at the same time There are multiple teeth involved in grinding, so more grinding heat is generated, and it is easy to cause tooth surface burns and cracks. When setting grinding parameters, a method of less single feed and multiple strokes can be adopted; worm grinding wheel grinding When dressing the grinding wheel, the two sides and the outer circle of the grinding wheel are dressed in two steps. The grinding wheel repaired in this way is likely to cause insufficient length of gear grinding root or grinding, which will cause interference in the subsequent gear transmission. Therefore, in the process of finishing the grinding wheel and starting the grinding of the parts, you should observe the grinding situation of the teeth more and make adjustments in time; due to the special structure of the grinding wheel dresser of the worm gear grinding machine, the tool for dressing the grinding wheel is not The diamond pen is a square block. This kind of diamond block is very easy to wear. The wear of the diamond block should be observed frequently, and the diamond block should be replaced in time to avoid batch quality accidents.

For the grinding tool grinding mandrel, the clearance between the positioning outer circle of the mandrel and the inner hole of the part is usually less than 0.015, the outer circle of the mandrel and the end face run out less than 0.01, and the surface of the outer circle of the mandrel is smooth and without traces; the two top holes of the mandrel There is no wear and foreign matter; the thread part of the mandrel must be ground by a thread grinder to ensure that the verticality is below 0.01, and the tightening nut and the thread are loose.