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Manufacturing straight and helical tooth evolventcylindrical gears

Profiling

In profiling, a tool appropriate for the tooth groove processes the groove, this can be a disc cutter, cherry cutter or grinder disc. Wjen processing, the tool moves in parallel to the rotational axis of the gear under process on a route length identical to the tooth width, while the gear is stationary. After completing the tooth groove, the gear is indexed one tooth pitch by an index head, and another tooth groove is processed. When manufacturing helical toothing, the tool moves along a spiral relative to the gear.



Hobbing

During hobbing, the cutting edges of the tooth cutter tool envelope the surface of the rack. Three types of hobbing can be differentiated: hobbing, tooth planing with a chasing bar and tooth planing with a cutting roll. The most popular and at the same time most productive gear manufacturing process is hobbing. The sides of the rack forming the tooth grooves are mapped by the cutting edges on the helical surface of the hobbing tool during the relative movement of the cutter and the blank. These movements together may be regarded as if an infinitely long rack would be rolling over the workpiece, forming the gear. The accuracy of the process is increased by multiple cutting edges working at the same time during processing and due to the rolling action, the pitch is continuous. When tooth planing using a chasing bar, the tool performs alternating movements, and in terms of the gear under process it performs tangential and radial movements. In the meanwhile, the blank rolls along the rack of the chasing bar with a rotating movement. The radial movement persists until the required tooth depth is achieved. Tooth planing may be performed using a cutting roll as well. The gear blank performs a rotating movement, while the gear-shaped tool performs alternating and rotating movement and establishes the required tooth depth by moving towards the gear centre at the same time.These hobbing processes are suitable for making helical tooth gears as well, in these cases the tool must be inclined in the required tooth angle. 



Manufacturing bevel gears

Bevel gears may be manufactured, too, using a hobbing process with straight cutting edge tools, similar to the hobbing of cylindrical gears. During such process, the teeth of straight and inclined tooth bevel gears are made by tooth slotting and hob-planing. During the process, the gear under process is rolled together with the matching imaginary plane wheel according to the hobbing process. The engagement is like if the gear under process would be rolling along a rack. The straight cutting edge knife or knife pair, reproducing the tooth of the plane wheel, performs a planing movement to create the tooth’s evolvent profile, that is, the spherical evolvent is approximated by a plane evolvent. Straight tooth and helical tooth bevel gears may be ground with a process based on hobbing. Thanks to grindability, the surface distortion of the gears after heat treatment may be processed, and more accurate gears may be produced, having a positive effect on tooth engagement.



Manufacturing helical tooth bevel gears

The tooth shape of helical gears is defined by the manufacturing process. In terms of the toothing system and the shape of the tooth creator, we differentiate between arc toothing (Gleason), evolvent toothing (Klingelnberg-palloid) and looped epicycloid toothing. The toothing tool used for creating a Gleason arc toothing is a disc-shaped knife-head, with trapezoid shaped knife inserts according to the plane profile. For the large gear, the arc’s tooth direction line is created by broaching or hobbing, and for the small gear this is hobbing. The processing is performed by teeth, that is, after creating a tooth groove, the tool retracts from the bevel gear, the machine indexes the gear by one pitch and the processing of the next tooth groove starts. The evolvent tooth direction toothing is created by hobbing. The trapezoid profile cutting edges are located along the conical helical surface of the manufacturing tool, with a tooth height identical along the cone generatrix. The tool can roll together with the manufacturing rolling cone of the gear under process thanks to its conical shape. The tool used for creating the cycloid-arc toothing is also disc-shaped, with trapezoid knife inserts shaped identical to a plane wheel profile located along multiple spirals. Out of the knives located in each spiral, the first performs roughing, the subsequent two hob the right and left sides of the teeth. The continuous rotating movement and the hobbing process together result in the imaginary tooth direction line on the imaginary plane wheel being a looped epicycloid.



Manufacturing worm drives

The tooth profile of the ZA type Archimedean worm is an Archimedean spiral in the frontal section, and trapezoid in the axial section. It is generally manufactured using a trapezoid profile turning cutter, with cutting edges located in the axis plane, but at the same time it can be manufactured with an inclined tooth evolvent profile cutter disc, too. The tooth profile of the ZI type evolvent worm corresponds to a large inclination angle evolvent toothing. The straight generatrix of the worm surface touches the base cylinder of the worm and the helix on that, and the tooth profile of the worm in the frontal cross-section is evolvent. The worm gear may be manufactured by hob-milling and hob-grinding. The ZN type colvolute worm has a straight profile in the normal cross-section. It may be manufactured by a trapezoid shaped turning cutter inclined by a medium pitch angle located in the normal cross-section of the tooth groove or the tooth. We differentiate between ZN1 and ZN2 type worms, depending on whether the cherry cutter is located in a plane perpendicular to the medium chordal normal helix of the tooth (ZN1) or the tooth groove (ZN2). The ZK type worm may be manufactured by a cherry cutter or a disc cutter. The ZK1 type worm is processed using a cherry cutter on a vertical mill. The ZK2 type worm is manufactured on a horizontal thread mill, with the tool being a trapezoid shape disc cutter.



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