Improve the precision method of shaft gear hobbing technology

Shaft gear is the most important part of the transmission, and its machining accuracy directly affects the overall quality of the transmission. At present we use the gear tooth part processing method is hobbing - shaving method. To produce high precision gear by rolling and shaving process, it is necessary to bring the rolling and shaving process level into full play. The gear shaving accuracy depends on saddle gear hobbing accuracy to a large extent, so some error items in gear hobbing must be strictly controlled in order to produce high quality gear. Gear hobbing is a common gear processing method, in the high precision gear hobbing machine, using precision hob, can be processed 4-5 precision gear. In the ordinary gear hobbing machine, with ordinary precision hob, only 8 precision gear can be processed. Transmission shaft gear tooth required accuracy of 8-7-7 level, and hobbing processing is mainly based on two center hole and end face as the positioning reference, so it is very important to analyze the error source of hobbing, to master the method to ensure and improve the processing accuracy.

1. Analysis of hobbing machining accuracy

The shaft tooth accuracy is mainly related to motion accuracy, stationarity accuracy and contact accuracy. The hobbing process uses the control common normal length and the gear ring diameter to jump

To ensure the kinematic accuracy, the gear shape error and basal pitch deviation are controlled to ensure the working stability accuracy, and the contact accuracy is ensured by controlling the gear direction error. The following is an analysis of several causes of errors that are easy to occur in gear hobbing:

1.1 Ring radial runout error (i.e. geometric eccentricity)

The radial runout of the gear ring refers to the maximum variation of the probe relative to the axis of the gear tooth when the probe is in the gear groove or on the gear tooth and in contact with the height and middle of the tooth on both sides within one revolution of the gear. It is also the eccentricity of the gear ring relative to the center line of the shaft. This eccentricity is due to the installation of the parts, the two center holes of the parts and the rotary center of the table installation does not coincide or the deviation is too large. Or because the top and the top hole are made poorly, so that the positioning surface is not in good contact, resulting in eccentricity, so the ring diameter jump should mainly be solved from the above reasons.

1.2 Length error of common normal (motion eccentricity)

Gear hobbing is processed by spreading method. The gear splitting transmission chain from cutting tool to gear blank should keep the accuracy of movement according to certain transmission ratio. But these drive chains are made up of a series of drive elements.

Their manufacturing and assembly errors must be concentrated on the end parts of the transmission chain in the process of transmission movement, resulting in the relative movement of the non-uniformity, affecting the machining accuracy of the gear teeth. The variation of common normal length is the largest error reflecting the uneven distribution of gear teeth. This error is mainly caused by the uneven rotation accuracy of worm gear pair of gear hobbing table, the wear of circular guide of gear hobbing table, and the difference between the dividing worm gear and circular guide of the table. In addition, there is a serious bump on the tooth surface of the splitter gear or the bite is too loose or too tight when the wheel is hung, it will also affect the variation of the common normal line.

1.3 Tooth shape error analysis

Tooth profile error refers to the normal distance between two ideal tooth profile (involute) profiles containing the actual tooth profile in the tooth profile working part. It is impossible to get the exact involute tooth shape in the actual machining process, and there are always various errors, which affect the stability of the transmission. The base circle of the gear is the only parameter that determines the shape of the involute tooth. If the base circle produces an error during hobbing, the gear situation will also have an error. The radius of the base circle is R equals PI

Hob movement speed/workbench rotary angular velocity xcosao (ao hob tooth profile Angle original), in the process of gear hobbing machining involute profile mainly between cutter and gear blank tooth to ensure that maintain a certain ratio of points, and thus, tooth profile error is mainly hob tooth profile error, hob grinding quality is bad tooth shape error can occur easily. At the same time, the radial runout and axial movement of the hob in the installation (that is, the installation error) also have an impact on the tooth shape error. Common tooth shape errors are asymmetric, tooth shape Angle error (tooth top becomes fat or thick), the generation of cycle error.

1.4 Tooth orientation error analysis

The tooth orientation error is the distance between the end faces of the two designed tooth orientation lines containing the actual tooth orientation lines within the full tooth width range on the indexing cylinder surface. The main reasons for the tooth direction error are that the vertical feed direction of the machine tool and tool rest is offset from the axis of the parts, or the top center of the tail seat is inconsistent with the rotary center of the table, and the calculation error of the differential hanging wheel is large, and the manufacturing and adjustment error of the differential transmission chain gear is too large when hoof cutting helical gears. In addition, low manufacturing, installation and adjustment accuracy of fixture and tooth billet will also cause tooth orientation error.

1.5 Roughness analysis of tooth surface

Roughness of tooth surface is not good in general there are several phenomena: hair lines, gnawing teeth, fish phosphorus, tear.

The main reasons for the roughness of tooth surface are as follows: insufficient rigidity and large gap of machine tool, cutting tool and workpiece system; The relative position of the hob and the workpiece changes; Hob grinding improper, parts material is not uniform; Cutting parameters are not suitable, etc.