Gear machining process, cutting parameters and tool requirements if the gear is too hard to be turned and the machining efficiency needs to be improved

Gear is the main basic transmission element in the automobile industry. Usually, each automobile has 18~30 teeth. The quality of gear directly affects the noise, stability and service life of the automobile. Gear processing machine tool is a complex machine tool system and a key equipment in the automotive industry. The world’s automobile manufacturing powers such as the United States, Germany and Japan are also gear processing machine tool manufacturing powers. According to statistics, more than 80% of automobile gears in China are processed by domestic gear making equipment. At the same time, the automobile industry consumes more than 60% of gear processing machine tools, and the automobile industry will always be the main body of machine tool consumption.

Gear processing technology

1. casting and blank making

Hot die forging is still a widely used blank casting process for automotive gear parts. In recent years, cross wedge rolling technology has been widely promoted in shaft machining. This technology is especially suitable for making billets for complex door shafts. It not only has high precision, small subsequent machining allowance, but also has high production efficiency.

2. normalizing

The purpose of this process is to obtain the hardness suitable for the subsequent gear cutting and to prepare the microstructure for the ultimate heat treatment, so as to effectively reduce the heat treatment deformation. The material of the gear steel used is usually 20CrMnTi. Due to the great influence of staff, equipment and environment, the cooling speed and cooling uniformity of the workpiece are difficult to control, resulting in large hardness dispersion and uneven metallographic structure, which directly affect the metal cutting and ultimate heat treatment, resulting in large and irregular thermal deformation and uncontrollable part quality. Therefore, isothermal normalizing process is adopted. Practice has proved that isothermal normalizing can effectively change the disadvantages of general normalizing, and the product quality is stable and reliable.

3. turning

In order to meet the positioning requirements of high-precision gear processing, the gear blanks are all processed by CNC lathes, which are mechanically clamped without regrinding the turning tool. The processing of the hole diameter, end face and outer diameter is completed synchronously under one-time clamping, which not only ensures the verticality requirements of the inner hole and end face, but also ensures the small size dispersion of mass gear blanks. Thus, the accuracy of gear blank is improved and the machining quality of subsequent gears is ensured. In addition, the high efficiency of NC lathe machining also greatly reduces the number of equipment and has good economy.

4. hobbing and gear shaping

Ordinary gear hobbing machines and gear shapers are still widely used for gear processing. Although it is convenient to adjust and maintain, the production efficiency is low. If a large capacity is completed, multiple machines need to be produced at the same time. With the development of coating technology, it is very convenient to re coat hobs and plungers after grinding. The service life of coated tools can be significantly improved, generally by more than 90%, effectively reducing the number of tool changes and grinding time, with significant benefits.

5. shaving

Radial gear shaving technology is widely used in mass automobile gear production because of its high efficiency and easy realization of the modification requirements of the designed tooth profile and tooth direction. Since the company purchased the special radial gear shaving machine of Italian company for technical transformation in 1995, it has been mature in the application of this technology, and the processing quality is stable and reliable.

6. heat treatment

Automobile gears require carburizing and quenching to ensure their good mechanical properties. Stable and reliable heat treatment equipment is essential for products that are no longer subject to gear grinding after heat treatment. The company has introduced the continuous carburizing and quenching production line of German Lloyd’s, which has achieved satisfactory heat treatment results.

7. grinding

It is mainly used to finish the heat-treated gear inner hole, end face, shaft outer diameter and other parts to improve the dimensional accuracy and reduce the geometric tolerance.

The gear processing adopts pitch circle fixture for positioning and clamping, which can effectively ensure the machining accuracy of the tooth and the installation reference, and obtain the satisfied product quality.

8. finishing

This is to check and clean the bumps and burrs on the gear parts of the transmission and drive axle before assembly, so as to eliminate the noise and abnormal noise caused by them after assembly. Listen to sound through single pair engagement or observe engagement deviation on comprehensive tester. The transmission housing parts produced by the manufacturing company include clutch housing, transmission housing and differential housing. Clutch housing and transmission housing are load-bearing parts, which are generally made of die-casting aluminum alloy through special die casting. The shape is irregular and complex. The general process flow is milling the joint surface → machining process holes and connecting holes → rough boring bearing holes → fine boring bearing holes and locating pin holes → cleaning → leakage test and detection.

Parameters and requirements of gear cutting tools

Gears are severely deformed after carburizing and quenching. Especially for large gears, the dimensional deformation of carburized and quenched outer circle and inner hole is generally very large. However, for the turning of carburized and quenched gear outer circle, there has been no suitable tool. The bn-h20 tool developed by “Valin superhard” for strong intermittent turning of quenched steel has corrected the deformation of carburized and quenched gear outer circle inner hole and end face, and found a suitable intermittent cutting tool, It has made a worldwide breakthrough in the field of intermittent cutting with superhard tools.

Gear carburizing and quenching deformation: gear carburizing and quenching deformation is mainly caused by the combined action of the residual stress generated during machining, the thermal stress and structural stress generated during heat treatment, and the self weight deformation of the workpiece. Especially for large gear rings and gears, large gear rings will also increase the deformation after carburizing and quenching due to their large modulus, deep carburizing layer, long carburizing time and self weight. Deformation law of large gear shaft: the outer diameter of the addendum circle shows an obvious contraction trend, but in the direction of tooth width of a gear shaft, the middle is reduced, and the two ends are slightly expanded. Deformation law of gear ring: After Carburizing and quenching, the outer diameter of large gear ring will swell. When the tooth width is different, the direction of tooth width will be conical or waist drum.

Gear turning after carburizing and quenching: the carburizing and quenching deformation of gear ring can be controlled and reduced to a certain extent, but it can not be completely avoided For the deformation correction after carburizing and quenching, the following is a brief talk on the feasibility of turning and cutting tools after carburizing and quenching.

Turning the outer circle, inner hole and end face after carburizing and quenching: turning is the simplest way to correct the deformation of the outer circle and inner hole of the carburized and quenched ring gear. Previously, any tool, including foreign superhard tools, could not solve the problem of strongly intermittent cutting the outer circle of the quenched gear. Valin superhard was invited to carry out tool research and development, “Intermittent cutting of hardened steel has always been a difficult problem, not to mention the hardened steel of about HRC60, and the deformation allowance is large. When turning the hardened steel at high speed, if the workpiece has intermittent cutting, the tool will complete the machining with more than 100 shocks per minute when cutting the hardened steel, which is a great challenge to the impact resistance of the tool.” Chinese knife Association experts say so. After a year of repeated tests, Valin superhard has introduced the brand of superhard cutting tool for Turning Hardened Steel with strong discontinuity; The turning experiment is carried out on the gear outer circle after carburizing and quenching.

Experiment on turning cylindrical gear after carburizing and quenching

The large gear (ring gear) was deformed seriously after carburizing and quenching. The deformation of the outer circle of the gear ring gear was up to 2mm, and the hardness after quenching was hrc60-65. At that time, it was difficult for the customer to find a large diameter grinder, and the machining allowance was large, and the grinding efficiency was too low. Finally, the carburized and quenched gear was turned.

Cutting linear speed: 50–70m/min, cutting depth: 1.5–2mm, cutting distance: 0.15-0.2mm/ Revolution (adjusted according to roughness requirements)

When turning the quenched gear excircle, the machining is completed at one time. The original imported ceramic tool can only be processed many times to cut off the deformation. Moreover, the edge collapse is serious, and the use cost of the tool is very high.

Tool test results: it is more impact resistant than the original imported silicon nitride ceramic tool, and its service life is 6 times that of silicon nitride ceramic tool when the cutting depth is increased by three times! The cutting efficiency is increased by 3 times (it used to be three times of cutting, but now it is completed by one time). The surface roughness of the workpiece also meets the user’s requirements. The most valuable thing is that the final failure form of the tool is not the worrying broken edge, but the normal back face wear. This intermittent turning quenched gear excircle experiment broke the myth that superhard tools in the industry can not be used for strong intermittent turning hardened steel! It has caused a great sensation in the academic circles of cutting tools!

Surface finish of hard turning inner hole of gear after quenching

Taking the intermittent cutting of gear inner hole with oil groove as an example: the service life of the trial cutting tool reaches more than 8000 meters, and the finish is within Ra0.8; If the superhard tool with polishing edge is used, the turning finish of hardened steel can reach about Ra0.4. And good tool life can be obtained

Machining end face of gear after carburizing and quenching

As a typical application of “turning instead of grinding”, cubic boron nitride blade has been widely used in the production practice of hard turning of gear end face after heat. Compared with grinding, hard turning does greatly improve the work efficiency.

For carburized and quenched gears, the requirements for cutters are very high. First, intermittent cutting requires high hardness, impact resistance, toughness, wear resistance, surface roughness and other properties of the tool.

overview:

For turning after carburizing and quenching and for end face turning, ordinary welded composite cubic boron nitride tools have been popularized. However, for the dimensional deformation of the outer circle and inner hole of the carburized and quenched large gear ring, it is always a difficult problem to turn off the deformation with a large amount. The intermittent turning of quenched steel with Valin superhard bn-h20 cubic boron nitride tool is a great progress in the tool industry, which is conducive to the wide promotion of the “turning instead of grinding” process in the gear industry, and also finds the answer to the problem of hardened gear cylindrical turning tools that has been perplexed for many years. It is also of great significance to shorten the manufacturing cycle of gear ring and reduce the production cost; Bn-h20 series cutters are known as the world model of strong intermittent turning quenched steel in the industry.