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1、Lesson 18 Cold Hobbing,Cold hobbing is a process in which a hardened steel master hob is forced into a soft steel blank under considerable pressure.,Hobbing is used for the production of cavities which by virtue of their shape would be difficult to die-sink on conventional machine tools.,The basic p
2、rinciple of hobbing is illustrated in Figure 18-1.,Figure 18-1 stages of manufacture of cavity insert by cold hobbing technique,The hobbing process is particularly applicable for the production of cavities on multi- impression moulds.,One master hob can often be used to produce a number of cavities,
3、 thereby saving considerable machining time.,Providing the hob is highly polished, the hobbings too will have a good surface finish, which saves bench finishing and polishing time.,In general a male hob is easier to manufacture than a corresponding cavity of the same form; however some shapes are ve
4、ry difficult to hob and careful consideration must be given to a project before attempting to use this technique.,For ease of hobbing, a low-carbon steel is usually used for the hobbing blank. As stated above, after the hobbing operation the hobbed insert is case-hardened.,This means that the outsid
5、e surface (including the cavity form) is extremely hard whereas the internal core of the steel remains relatively soft.,Thus if in production a localized high force is developed there is a possibility that the surface of the impression may be depressed.,Notes, In general a male hob is easier to manufacture than a corresponding cavity of the same form; however some shapes are very difficult to hob and careful consideration must be given to a project before attempting to use this technique.,通常，对于同一轮廓形状，凸模的加工制造比凹模容易得多，但有些凸模形状也很难挤制成型，在采用这种加工方法前，对其工艺方案必须仔细斟酌。,
