The mentioned feature line family is also of great significance to study the machining principle of gear profile, because any machining method of involute profile directly obtains any two families of surface feature lines and finally constructs the whole profile. It can be called the configuration principle of involute profile. For high-precision large involute gears, especially helical gears, almost all adopt the hobbing method. In order to understand the error characteristics of large involute gears, it is necessary to discuss the configuration principle of hobbing:
The so-called configuration principle of gear hobbing mainly refers to how the gear hobbing machine constructs the machined tooth profile under a certain motion condition. The configuration principle of hobbing spur gear is relatively simple. It mainly discusses the situation of hobbing helical gear, and ignores the error caused by intermittent cutting between the blade and the blade.
As shown in figure a, the workpiece and hob rotate according to the strict speed ratio, which is guaranteed by the gear splitting transmission chain of the machine tool. In addition, the workpiece and hob have relative movement along the workpiece axis, and the workpiece still has additional rotation in order to roll out the helical gear. This is realized through the differential chain. Therefore, the basic principle of gear hobbing machine is involute spiral meshing principle with differential.
In order to study the configuration principle, the hob and workpiece can be rotated first, but there is no axial feed and differential. Therefore, according to the spiral meshing principle, the hob will roll out a contact trace on the tooth profile surface. Secondly, if the hob and workpiece do not rotate, there is only axial feed and differential, so the hob will scratch a trace on the contour of the workpiece, which is a spiral composed of points Q1, Q2 and Q3, which is called the starting line.
It can be seen that the workpiece profile will be constructed by contact trace network and spiral network. The specific construction process of workpiece profile can also be given. If the contact points between the workpiece and the hob during tool setting are Q1 and Q1 points (only one side of the tooth profile is drawn in Fig. B, and Q1 and the other side of the tooth profile are not drawn). When the hob and workpiece rotate, the contact trace 1 can be rolled out; When the workpiece rotates for one cycle, due to axial feed and differential movement, the contact point of hob and workpiece is Q2, and contact trace 2 can be rolled out: by analogy, other contact traces can be rolled out and the whole tooth profile surface is formed. Q1, Q2, Q3… Are called starting points. Hob and workpiece have common relative positions at these points.