Gear manufacturing – hobbing: During gear hobbing, which components rotate during the generating process?
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AOnly the hob rotates
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BOnly the gear blank rotates
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CBoth hob and gear blank rotate (timed to a gear ratio)
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DNeither hob nor gear blank rotate
Answer
Correct Answer: Both hob and gear blank rotate (timed to a gear ratio)
Explanation
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:
Recognize hobbing as a generating process requiring relative rotation.Recall that the hob acts like a worm; the blank must roll with it.Therefore, both the hob and blank rotate with synchronized speeds.Select the option that states both rotate.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)