ess than 10 years ago the selection of machinery was a system decision; presently continuous indexing (face hobbing) and single indexing (face milling) are unlimitedly available to the user. The continuous indexing method, referred to as “face hobbing” (FH), caused a small revolution in North America a few years ago, in conjunction with dry-cutting. Until then, the classic five-cut method had been state of the art for many decades. This is a single-indexing method, which cuts one after the other gaps in the gear blank. For the pinion, a different cutterhead is used for the convex and concave flanks respectively. Single indexing methods are referred to as “face milling” (FM).
The FM five-cut process is controlled by means of contact pattern assessment. The contact patterns on the drive and coast flanks are moved to the desired shape and position by changing the pinion machine setting. So-called “proportional changes” are used for this purpose. Although the process control is not without problems, all users have learned to adequately handle proportional changes.

Which process is best for you? Depends on the application. Read on for an in-depth examination of the pros and cons of each.

With the introduction of face hobbing, this umphant advance of dry face hobbing possible. condition changed substantially. With dry-cutting, The goal of this article is to make an objecit was possible to reduce the processing time tive comparison between the two gear-cutting per gear set by up to approximately 30 percent in comparison to wet-cutting with the five-cut method. The tool life quantity of the cutter heads was improved by more than factor three. The highcutting speeds of the carbide blades produce an excellent surface quality, which offers the best conditions for downstream lapping operations. methods. All essential characteristics, such as load-carrying capacity, noise behavior and costs, are assessed. To enable this comparison to be made in a practical manner, we have limited the process scope so that a lapping operation always follows with FH gear sets, while FM gear sets are ground after hardening. These economic factors were what made the tri-

Although the tooth forms of FH and FM gears are very similar, there are still some basic differences. The FH method produces an epicycloidal shape in tooth lengthwise direction. The tooth height is the same at both the toe (the inner end of the tooth) and the heel (the outer tooth end). In these cases the root cone

FIG. 1: VARIABLES FOR DETERMINING THE TOOTH-STRENGTH RATING
angle and the tip cone angle are identical. The cone geometry results in a conical tooth tip thickness, and thus a conical trend of the slot width in the gap. With a reasonably sized cutter diameter, the gap at the heel is smaller than at the toe. If the cutter diameter used is too small, the inner tooth end becomes thicker than the outer. Presently, FH gearsets are always manufactured with constant tooth height and conical slot width.
Instead of having a constant tooth height with conical gap width, a conical tooth height with constant slot width can be made. In order to achieve this, the tip cone angle and the root cone angle must be coordinated with the cutter diameter to give a constant slot width in the tooth root. This cone geometry is also called a duplex cone.

If a gear in an FM gear set is processed in such a way that both flanks are manu-factured in a single cut, a constant slot width results in the tooth root due to the circular cutter head. This constant slot width of the gear requires a constant tooth tip thickness on the pinion. If one continues this train of thought, then one can easily deduce the basis for the computation of the duplex cone.

In simplified (partly standardized) calculation methods, the tooth root chord ${\mathrm{S}}_{\mathrm{Fn}}$${\mathrm{S}}_{\mathrm{Fn}}$S_(Fn)\mathrm{S}_{\mathrm{Fn}} between the ${30}^{\circ }$${30}^{\circ }$30^(@)30^{\circ} tangent to the root fillet $\rho f$$\rho f$rho f\rho f and