Centreless grinding fundamentals: Where is the workpiece centre located relative to the line joining the grinding wheel centre and the regulating wheel centre?

Introduction / Context:
Centreless grinding is a high-throughput finishing method used for cylindrical parts such as pins, shafts, and rollers. Unlike cylindrical grinding, the workpiece is not held between centres. Instead, it is supported by a work rest blade and rotated between a grinding wheel and a regulating (control) wheel. Knowing the geometric location of the workpiece centre relative to the wheel centres is essential for roundness generation, stability, and process control.

Given Data / Assumptions:

• Conventional through-feed centreless grinding setup with a grinding wheel, a regulating wheel, and a fixed work rest blade.
• Both wheels have their centres on a reference line called the wheel-centre line.
• Goal is to promote self-centering and stable rounding.

Concept / Approach:
In centreless grinding, roundness is generated by the lobing correction mechanism. Placing the workpiece centre slightly above the wheel-centre line, combined with appropriate inclination and speed ratio of the regulating wheel, creates a corrective rolling action. This produces a stable three-point support: grinding wheel contact, regulating wheel contact, and work rest blade. The above-centre position minimizes chatter, reduces lobing error, and ensures a restorative force that guides the part toward circularity.

Step-by-Step Solution:

Verification / Alternative check:
Industry practice and handbooks specify a small “above-centre height,” often a fraction of the workpiece radius. Observing roundness traces with a polar plot confirms reduced lobing when the workpiece is positioned above centre compared with on-centre or below-centre cases.

Why Other Options Are Wrong:

• Below the line: tends to destabilize rounding and can amplify lobing.
• On the line: lacks the corrective geometry; rounding may be slow or unstable.
• Intersection with workrest plane: not a standard geometric criterion for setup.

Common Pitfalls:
Setting the work rest too high can cause chatter; too low can cause burning or poor roundness. Neglecting regulating-wheel inclination and speed ratio also degrades results.

Final Answer:
Above the line joining the two wheel centres