When the standard hob is used to hobbing the super large modified gear, the phenomenon of non involute tooth profile on the top of the top cutter, clamping cutter and tooth tip, short service life of the side cutter and the hob, and even damage of the hob and the machine tool may occur. This paper briefly analyzes it.

When hobbing standard gear and gear with small modification coefficient, the two actual meshing line segments are symmetrically distributed and intersected on the node. When machining involute tooth profile, the cutting is carried out along the meshing line, and the two sides of the cutter’s tooth profile are respectively cut out along the meshing line; When the hob cutting edge is long enough, because the two actual meshing lines intersect, the tooth surfaces on both sides of the gear are cut (enveloped) at the same time in most of the time; When the modification coefficient is bigger and bigger, the addendum circle is bigger and bigger, which will cause the two symmetrical actual meshing line segments to deviate from the node until they do not intersect. The extension line must intersect at the node. The cutter teeth where the extension line segment is located are only the residual tooth root circle and transition curve, and the minimum cutting edge length of the hob is longer.

That is to say, in general, the cutter teeth at the cutting end of the hob cut off most of the metal in the tooth groove tooth by tooth, leaving only a small part of the cutting amount on the cutting surface, which will be cut off by the cutter teeth at the cutting end of the hob; When the modification coefficient gradually increases, most of the cutting amount that should be cut by the middle tooth of the hob is transferred to both ends of the hob, and the cutting amount produced by cutting the metal in the tooth groove and the enveloping tooth shape is also superimposed near the cutting end. Therefore, when the length of the hob cutting edge is enough, the cutting amount that should be cut by other teeth is borne by the cutting teeth at the cutting end of the hob（ That is to say, most of the metal to be cut in the slot is concentrated in this section, so the load of the edge cutter teeth at the cutting end is particularly heavy, which is the fundamental reason for the advanced wear and cutting of the hob edge cutter.

When the standard hob is used to hobble the super large modified gear, because the length of the hob cutting edge is insufficient, the cutter tooth at the cutting end of the hob does not cut off the cutter tooth at the cutting end, and the remaining cutting amount is cut off, which makes the cutter tooth at the cutting end of the hob bear the additional part of the cutting amount that should be cut off by the cutter tooth at the cutting end. This is the main difference between the standard hob and the standard extended hob.

From the above analysis, it is not difficult to understand why the above abnormal phenomenon occurs, even if the hob cutting edge is long enough.