Tools for gear hobbing are particularly suitable for forming obliquely cut teeth in conical surfaces of different workpieces, e.g. for the manufacture of crown gears, pinions, and similar details of the type that, among other things, is found in toothed gearings and trains of gears. A primary advantage of gear hobbing is that the tool and the workpiece both can be rotated in a synchronous relationship during continuous indexing of the workpiece and thereby continuous forming of the requisite tooth slots. In such a way, the machining time is reduced most considerably in comparison with older gear milling methods, in which the tooth slots were milled one by one after indexing in steps of the workpiece in relation to the tool. U.S. Pat. No. 7,736,099 discloses a gear hobbing tool, which in a front or upper side of a pulley-shaped milling cutter head includes detachably mounted chip removal units of the initially generally mentioned kind. In this case, each chip removal unit is equipped with a milling insert, which in the mounted state and protrudes above the front side of the head of the milling cutter and is fixed in the appurtenant holder by means of a screw, which extends through a through hole in the milling insert and is tightened in an upper part of the holder, more precisely in a threaded hole that mouths in a tangential support surface included in the part. This means that the screw is oriented essentially parallel to the front side of the head of the milling cutter, as well as being located in the immediate vicinity of the last-mentioned one. In such a holder, the tangential support for the milling insert is a flat surface and the axial support is two V-shaped arranged partial surfaces, which individually form an acute angle in relation to the flat tangential support surface, and which together form a wedge-shaped bottom in which a congruent, rear part of the milling insert can be accommodated, in order to, upon tightening of the screw, press the back side of the milling insert against the tangential support surface. Therefore the hole in the milling insert has to have a greater diameter than the threaded portion of the screw.
The different milling inserts are identical and co-operate in pairs in such a way that two consecutive milling inserts along a cyclo-palloid curved path machine opposite tooth flank surfaces in a tooth slot to be made. If a milling insert running ahead in the slot is right-hand cutting, i.e., machines a right flank in the tooth slot to be made, the one following behind is left-hand cutting.
It should also be mentioned that lower parts of the holders of the individual chip removal units are countersunk in hollow spaces situated below the front side of the head, and removably fixed by means of screws, which include key grips accessible from a peripheral, circumferential surface of the pulley-shaped head.
In the above-mentioned patent, it is asserted that an advantage of the disclosed tool construction would be the possibility of dismounting (and mounting, respectively) the individual milling inserts individually, without needing to dismount the appurtenant holder from the milling cutter head. This supposed advantage should, however, be of theoretical, rather than realistic, nature. Because the screws for the milling inserts are oriented parallel to and placed near the front side of the head of the milling cutter it makes the accessibility to the key grip of the screws significantly more difficult. For instance, it is impossible to use traditional T-wrenches. The possibility of releasing the milling insert running ahead of each pair of co-operating milling inserts is, therefore, considerably impaired. Even more, the possibility of individually releasing the milling inserts following behind the same pair should be practically non-existent.
Another disadvantage of the known tool is based on the dimensional accuracy of the spatial locations of the different milling inserts in relation to the geometrical fixed points, i.e., the center axis and the front side of the head, respectively, of the head of the milling cutter is of vital importance to a good machining result. Thus, in practice, there is required—within modern milling technology—a dimensional accuracy of 0.005 mm (or better). For this reason, extensive measuring and adjustment operations are required before the tool can be considered operable. Normally, the replacement of an individual milling insert does not involve any time-consuming measuring and adjustment operations, but if also the appurtenant holder would need to be dismounted and remounted, the tolerance chain between the milling insert and the fixed points of the head of the milling cutter is lost. In other words, time-consuming measuring and adjustment operations are then required, if only for just one chip removal unit.
US 2011/0164931 discloses a gear milling tool having a milling cutter head that in a front side includes a plurality of protruding cutting bodies, which are held in place by means of wedges and screws, the key grips of which are accessible from above. More precisely, the wedge holds a lower (large) part, countersunk in the head, of the individual cutting body pressed against a surrounding wall in a hollow space in which the same is mounted, the wedge acting approximately radially (inward or outward) against the bottom part of the cutting body. This means that the cutting body stability will be mediocre since the cutting forces act tangentially rather than radially against the protruding part in which the cutting edge is included. In this connection, the part of the cutting body, i.e., the one protruding and including the cutting edge, lacks any form of tangential support that could carry the cutting forces. In addition, the wedge, like the lower part of the cutting body, is countersunk in the head, and is included in a tightening mechanism that furthermore includes a plurality of additional components, which make replacement of the cutting bodies more difficult and easily may destroy the tolerance chain between the cutting edge and the milling cutter head.