(1) Processing methods of gear teeth
There are many processing methods for gear teeth, such as cutting, casting, rolling, stamping, etc., among which the cutting processing method is commonly used. Cutting processing methods are divided into two categories: forming method and forming method.
1. Forming method
The forming method uses a forming milling cutter with an involute tooth profile to directly cut out the tooth shape on a milling machine. There are two commonly used types of disc milling cutters and finger milling cutters. During gear cutting, the milling cutter rotates and the wheel blank moves along the gear axis. After milling one tooth slot, turn the wheel blank through 360°/z, and then mill the second tooth slot. The rest can be deduced in this way.
This gear cutting method is simple and does not require special machine tools, but it has low productivity and low precision, so it is only suitable for single-piece or small batch production and gear processing with low precision requirements.
2. Fan Cheng method
Fan Cheng method is a method that uses the meshing principle of gears to process gears. This method forces the tool to move relative to the workpiece (wheel blank) and cut simultaneously. The kinematic relationship between them is the same as the meshing of a pair of gears, so as to ensure the correct tooth shape and uniform tooth division. Gears with the same module m and pressure angle a but different numbers of teeth can be processed with the same tool.
The commonly used tools for cutting teeth using the Fancheng method are as follows.
1) The top of the gear inserting tool is c*m higher than the normal teeth in order to cut out the tooth top clearance.
During gear shaping, the inserting knife makes a reciprocating cutting motion along the axis of the wheel blank, and at the same time, the inserting knife and the wheel blank are forced to rotate at a certain transmission ratio, imitating a pair of gear transmissions, until all tooth spaces are cut.
The tooth profile of the gear shaping cutter is a precise involute, so the inserted gear is also an involute. According to the correct meshing conditions, the module and pressure angle of the cut gear must be equal to the module and pressure angle of the inserting knife, so the gears cut with the same inserting knife can mesh correctly.
2) Rack inserting tool Figure 3-36 shows the use of rack inserting tool to process gears. When the number of teeth of the gear shaper increases to infinity, the radius of its base circle becomes infinite, the involute tooth profile becomes a straight tooth profile, and the gear shaper becomes a rack shaper. Figure 3-37 shows the shape of the tooth profile of the rack inserter. Its top is c*m higher than the transmission rack in order to cut out the radial gap during transmission. Because the tooth profile of the rack is a straight line, it can be seen from the figure that whether they are on the center line or on any other straight line parallel to the center line, they all have the same pitch p (pm), the same module m and the same Tooth profile pressure angle a. For rack cutters, a is called the tool angle, and its size is equal to the pressure angle on the gear index circle
3) The principles of the above two processing methods of gear hobs are based on the meshing principle of gears. The machining accuracy is high, but it can only cut intermittently, and the productivity is low. Gear hobs are currently widely used, which can cut continuously and have high productivity. The shape of the hob is very much like a spiral, and its axial section is a rack with a straight tooth profile. When the hob rotates, it is equivalent to the movement of the rack, so that the involute tooth profile of the wheel blank is cut out according to the principle of Fan Cheng. In addition to rotating, the hob also feeds along the axial direction of the wheel blank in order to cut out the entire tooth width. When hobbing spur gears, because the helix of the hob is inclined, in order to make the helix direction of the cutter teeth consistent with the direction of the gear teeth to be cut, when installing the hob, it is necessary to make its axis and the end face of the wheel blank at a hob lift angle
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