Gear hobbing without differential (non-differential hobber): The feed change gear ratio is best described as

Introduction / Context:
In gear hobbing, kinematic relationships among spindle speed, hob rotation, work rotation (indexing), and axial feed determine tooth geometry. On non-differential hobbing machines, the change gears that set indexing have a defined relation to the feed gearing, especially for helical gears, to maintain the correct lead and tooth advance.

Given Data / Assumptions:

• Non-differential hobber (no differential mechanism to compensate for helix).
• Conventional gear trains implement work indexing and feed.
• Objective: maintain proper axial feed per work rotation according to tooth lead.

Concept / Approach:
On non-differential machines, helical cuts require coordinated motion: the feed per work revolution must match the lead relation. Thus, the feed gearing cannot be chosen independently; it is tied to indexing gearing so that the helix angle and lead are correct. Any mismatch distorts tooth geometry.

Step-by-Step Solution:

Verification / Alternative check:
Hobber manuals show feed and index gear trains are linked for helix without differential; differential machines decouple these via a compensating train.

Why Other Options Are Wrong:

• Independence claims ignore the helix synchronization need.
• Speed ratio alone does not set axial advance relative to work rotation.
• “Fixed by hob lead only” omits the kinematic coupling to work indexing.

Common Pitfalls:
Setting feed for a correct mm/rev value but forgetting it must correspond to the actual work rotational speed imposed by index gearing.

Final Answer:
inter-related to index change gear ratio