Grinding wheel grade — what does a “hard” wheel mean? A grinding wheel is said to be of which grade if it holds the abrasive grains more securely and releases them less readily during grinding?

Introduction / Context:
Wheel grade expresses the bond’s holding power on abrasive grains, not the abrasive’s intrinsic hardness. Selecting the right grade is essential to balance self-sharpening and wheel wear.

Given Data / Assumptions:

• Standard vitrified or resinoid-bond wheels.
• Conventional abrasives (aluminium oxide or silicon carbide).
• Grinding steels as a representative case.

Concept / Approach:
A hard-grade wheel firmly retains grains; dull grains remain longer before being shed. This is useful when grinding soft or ductile materials that would otherwise cause rapid wheel wear. Soft-grade wheels release grains sooner, avoiding glazing when grinding hard materials.

Step-by-Step Solution:
Define grade: bond strength in holding grains.Relate “holds more securely” to “hard grade.”Select the appropriate option.

Verification / Alternative check:
Manufacturers’ charts correlate recommended grade with workpiece hardness and contact area; harder grades are suggested for softer materials and large contact widths.

Why Other Options Are Wrong:
Soft/medium grades do not hold grains as securely; “open structure” and “porous” refer to spacing/porosity, not bond holding power (grade).

Common Pitfalls:
Mixing up “grade” (bond holding) with “grit size” (grain size) or “structure” (spacing between grains).

Final Answer:
Hard grade