【正文】 to as 39。shifting39。, occurs continuously in the diagonalhobbing method, as here the feed consists of an axial and tangential ponent. The leading dimension s and setting values of a hob are shown in Fig. in which the hob is shown in the actual cutting position. The angular setting (angle of incidence, η) of the hob is dependent upon the direction and value of the helix angle β of the tooth to be cut (if any) andthe helix angle γ of the worm on the hob. With any one hob, gears of any given number of teeth and helical tooth angle, as well as all kinds of tooth profile modifications, may be produced by differing machine settings, provided the teeth are of the same module and pressure angle. Any limitation is only as a result of the working capacity of the machine. A simplified layout of the drive of a hobbing machine is shown in Fig. . The main motor drives the hob directly and also the work table through the pickoff change gear train and the intermediate telescopic worm and worm wheel drive. The choice of ratio in the pickoff change gear train coordinates the rotations of the tool and work, which are dependent upon the number of teeth to be cut on the gear and the number of start s of the helix on the hob. The worm drive drives the feedchange gear train or, as shown in Fig. , the rotation of the axial feed spindle through a stepless variable drive. For the manufacture of helical teeth and diagonal hobbing, the gear blank to be cut is subjected to an additional rotary movement relative to the hob feed, provided by the differential drive. The differential cage is released and set into motion by the differentialchange gear train, having selected the appropriate gears. A detailed representation of the kinematics of such a hobbing machine is shown in Fig. , which also shows how the rotary motion for the radial feed spindle is obtained. Figure shows a general picture of a hobbing machine. On conventional machines, the column is bolted on to the machine bed. The workspindle slide and its support centre are driven by the radialfeed spindle and moved horizontally to obtain a radial feed. The hob spindle can be moved along the hobbing slide through a tangential drive and a tangential spindle, and may also be angularly adjusted to acmodate the setting of the required helix angle for the gear to be cut. The hob drive and the feed drive are located in the column. On a machine of recent design, the table is stationary and the column slides along the machine bed. Only the main drive shaft and the feed shaft are in the column. All other drive elements are in a separate drive box on the left side of the machine in order to improve heat dissipation. In order to provide backlashfree drives, modem machines have all feed shafts fitted with preloaded, recirculating ball nuts. The table drive may be made backlashfree either by the employment of two axially opposed prestressed worms or by the use of a so called 39。double worm39。 which has a slightly different pitch on its right and left flanks, enabling it to be adjusted in the direction of the slightly thicker helix when wear has taken place. Figure shows a crosssectional view of such a constructional device. Bevel gears Contrary to the exclusively continuous mode of the hobbing of spur wheels, bevel gearhobbing machines can operate either continuously or with an indexing action. The cutting tools are sideand facetype milling cutters, insertedtooth cutters or bevel gear hobs. Bevelgear indexhobbing machines usually employ two large cogtype, interlocking, radialinserted toothcutter heads, with easily interchangeable cutter tips. The cutting edges of all the cutters represent a tooth of a given crown wheel against which the gear blank to be cut is rolled. Figure pictures the working area of such a machine with both work stations and the interlocking cutter heads. For small module s up to m = 6 mm, the two cutters first sink a slot into the stationary gear blank (plunge milling)。 the gene rating process then mences. The cone of the gear blank thus rolls against the gene rating plane. Each tooth space is pleted in a single cut, and when finished the work is indexed. For the cutting of modules of m 7 mm, two operations are carried out。 roughing by plunge milling and finishing by a gene rating action. Normally, it is not necessary to pro vide a feed in the direction of the tooth length, as due to the large diameter of the cutter head, the largest blanks within the capacity of the machine can be produced without the roots of the teeth being unduly hollow. Plunge and gene rating feeds are infinitely variable on modem machines. Moreover, the generatingfeed rate can be changed in accordance with the generating path. The cutters of the cutter head are very largely universal. Within a certain range of modules, the same set of cutters may be used, which enables these machines to be economically employed even for smallscale production. On a different construction of machine for indexhobbing of bevel gears, the cutter takes the form of a facemilling cutter head, which permits the manufacture of curvedtoothed bevel gears. In this variation of the technique, the cutting edges of all the cutters again represent one tooth of the 39。idea!39。 crown wheel. The gear blank makes a generating motion against the rolling plane of the crown wheel, from which the tooth profile develops. The possible movements of such an inserted toothcutter head bevelgearhobbing machine is shown in Fig. . The generating motion during the process is executed about the axes of the gene rating drum (rocker) (2) and the gear blank (1)。 the cutting action occurs about the cutter head axis (4). The plunge movement is obtained through the movement of the slide (8). Adjustments on the cutterhead slide (3) enabl