Page 338 - Gear Technology Solutions

23.6 Summary

Blade stepping and top slope angles are means which should ensure a smooth
root fillet without steps and fins. In face hobbing of generated members, both
elements are required to achieve this goal. However, the natural slot with taper
of face hobbed pinions and gears often prohibits the perfect elimination of a
fin. The reason is the difference between the tracks of the outside blade and
the inside blade. Those tracks begin at the toe end of the root with a perfect
match. As the tracks move to the midface position and continue to approach
the heel end of the slot, they deviate. Each blade moves towards the flank it is
cutting which creates a geometrically balanced slot width, which splits the
tooth thickness and the slot equally. This in turn means that the blade tops,
which overlapped at the toe perfectly, are apart in the heel position with the
potential to create a disturbing fin.

The blade stepping of generated face hobbed parts is calculated from the
length crowning cutter tilt angle component, the blade pressure angles and the
blade point radii. The complex part of the calculation is retrieving the length
crowning tilt angle component from the basic settings. The length crowning tilt
component k can also be directly used as top slope angle.

Face hobbed Formate ring gears are not stepped because length crowning is
generally not used in face hobbed gears. If in spite of this statement, small
amounts of length crowning are introduced for Formate ring gears, then no
stepping but only the top slope angles are calculated from the difference
between the blade angles and the pressure angles of the workpiece.

In face milling, regardless if generated or Formate, only top slope angles are
calculated. The blade stepping is implied by building the outside blade (on its
higher clearance side edge) to the same height above the cutter head face.
This procedure leads to the correct top slope angle and a perfect stepping due
to the arrangement shown in Figure 5.

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Also, the blades in Coniflex Plus cutters receive a top slope angle because the
cutter blade represents one side of the generating profile and therefore has
90° plus the pressure angle between cutting edge and root. If the blade tilt
angle and the cutter dish angle are factored in, then the top slope angle
required to cut a flat root bottom is calculated as gear pressure angle – blade
slot tilt angle + dish angle (for example –(20°–18 °+3°) = –5°.

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