Next, from the accuracy and productivity of gear processing to compare the technological characteristics of gear hobbing and gear shaping.
Machining accuracy
The movement accuracy of gear hobbing is high, but the profile and surface finish of gear shaping are good. This is because:
1) The transmission chain of the mechanical gear shaper is more complex. The outstanding point is that there is one more tool worm gear pair than the gear hobber, that is, there is more transmission error. The reciprocating motion of the tool spindle and the moving part of the table are easy to wear. Since the tooth profile of the gear to be machined is obtained when meshing with the gear shaper without clearance, the accumulated error of the pitch of the gear shaper must also be reflected on the gear, so that the kinematic accuracy of the gear shaper is generally lower than that of the gear hobbing. In order to process gear with high precision, it is necessary to use gear shaper and gear shaper with high precision. However, with the popularization of gear shaper, this problem has been well solved, because the axis of CNC gear shaper is independent, the transmission chain is greatly shortened, and the transmission error is greatly reduced.
2) The number of tangents to form the profile envelope during gear shaping is determined by the circumferential feed, which can be selected, while the number of tangents to form the involute envelope during gear hobbing is only related to the number of grooves of the hob and the number of heads of the helix, and the number of tangents of the envelope cannot be increased or decreased by changing the processing conditions. Therefore, the surface finish of gear shaping is much higher than that of gear hobbing, and the tooth profile error is also smaller.
3) The installation error of gear shaper cutter has little influence on the tooth profile error, while the installation error of hob will cause large tooth profile error of workpiece.
The shape of gear shaper cutter is like spur gear or rack. Compared with hob, the manufacturing process is simple, easy to manufacture more accurately, so the machining accuracy of gear is higher. Therefore, from the perspective of machining accuracy, gears with low requirements for kinematic accuracy can be processed directly by gear shaping, without shaving or grinding. However, gear and pre shaving gear, which have higher requirements for kinematic accuracy (shaving can not improve kinematic accuracy), are better to use gear hobbing.
Productivity
Generally, the productivity of gear hobbing is higher than that of gear shaping. Only when the gear with small module, large number of teeth and small tooth width is machined, gear shaping has the ability to compete with gear hobbing. This is because:
1) The hob uses rotation to cut, with higher cutting speed and less loss of empty stroke. The gear shaper cutter is reciprocating, which limits the increase of cutting speed, and the actual cutting stroke length only accounts for about the total stroke length, and the empty stroke loss is too large.
2) Multiple hobs can be used to improve the efficiency of rough hobbing.
3) The moving time of gear hobbing is related to the number of teeth of the workpiece, while the moving time of gear shaping only needs to consider the diameter of the workpiece. That is to say, gear with large diameter and small module is suitable for gear shaping.
In general, when processing gears with a module of more than 5, the productivity of gear shaping is not as high as that of gear hobbing due to the need to remove a large number of redundant metals. When processing small module gears with a module of less than 2.5, the efficiency and accuracy of gear shaping are higher than that of gear hobbing due to the very small number of metals removed. When processing gears with a medium module (2.5mm-5mm), the productivity of gear shaping is equivalent to that of gear hobbing, In this case, gear hobbing is preferred. For some special gears with large module (such as sector gear, herringbone gear, rack, etc.), the productivity of gear shaping is not lower than that of gear hobbing.
As mentioned above, due to the characteristics of gear shaper structure and tooth movement, the technological field of gear shaper is wider than that of gear hobber. All gears (except worm gear) that can be processed by gear hobbing machine can be processed by gear shaper. In other words, gear shaper can process internal gears or special shaped workpieces. Gear hobbing machine may not be able to process them. For example, gear shaper uses special tools and clamps, and can also process rack, involute spline, shorter spline shaft and parts with special profile on the circumference. In the processing of spiral gear, because of some special requirements (such as small empty cutter slot) and special forms such as multiple gears and internal gears, the hobbing machine can’t do anything but finish the processing on the gear shaper. Of course, there are many advantages in gear hobbing, such as machining left and right spiral gears with different helix angles, gear hobbing only needs to swing the hob, while gear shaping also needs to design and manufacture special spiral guide rail and corresponding gear shaper. In this respect, the omnipotence of gear shaping is not as good as that of gear hobbing.