Gear manufacturing – hobbing: During gear hobbing, which components rotate during the generating process?

Introduction / Context:
Hobbing is a versatile generating process for cutting spur, helical, and worm gears. Understanding the kinematics is vital for setting up the correct module, number of teeth, and helix angle on a hobbing machine.

Given Data / Assumptions:

• Standard gear hobbing machine and worm-type hob.
• Goal: identify which elements rotate during cutting.
• Generating method (not form milling) is assumed.

Concept / Approach:
In generating processes, the final tooth form results from the relative rolling motion between tool and workpiece, not from a stationary form template. In hobbing, the hob rotates continuously while the gear blank also rotates; their angular velocities are linked by a precise gear ratio proportional to the number of teeth and module. Feed motions (axial or radial) advance the hob through the blank to complete the tooth spaces.

Step-by-Step Solution:

Verification / Alternative check:
Machine kinematic diagrams show a change-gear train or electronic synchronization (in CNC hobbers) coupling hob and worktable rotation.

Why Other Options Are Wrong:
“Only one rotates” would be a shaping or fly-cutting scenario; “neither rotates” cannot generate involute teeth.

Common Pitfalls:
Confusing gear form milling (form tool, no generating roll) with hobbing; mis-setting the ratio causes the wrong tooth count.

Final Answer:
Both hob and gear blank rotate (timed to a gear ratio)