Lathe headstock operation – effect of engaging the backgear: When the backgear train is engaged in a back-geared headstock, what happens to the spindle speed?

Introduction / Context:
Back-geared headstocks provide low-speed, high-torque ranges for heavy turning, large diameters, or threading. Understanding their purpose is essential for safe setup and correct surface speed selection.

Given Data / Assumptions:

• Conventional cone pulley with backgear train on an engine lathe.
• Backgear engagement inserts an extra gear reduction between the cone pulley and spindle.
• No change in motor speed.

Concept / Approach:
Engaging the backgear introduces a large reduction ratio. Speed decreases substantially, while torque at the spindle rises in proportion (neglecting losses). This enables cutting at the low surface speeds required for large diameters or tough materials.

Step-by-Step Solution:

Verification / Alternative check:
Nameplate speed charts on lathes show two ranges; the backgear range lists much lower rpm for the same pulley step.

Why Other Options Are Wrong:
Speed does not increase; it changes substantially, not “unchanged.” Direction does not reverse simply by engaging backgear; torque and speed are coupled by the reduction.

Common Pitfalls:
Engaging the backgear without disengaging the bull gear lock pin can lock the headstock and damage components.

Final Answer:
Agree – the spindle speed reduces considerably