Grinding technology — specifying wheel hardness In abrasive machining, the hardness grade of a grinding wheel is designated by a letter of the alphabet rather than by conventional indentation scales. Which notation is used to specify grinding wheel hardness?

Introduction / Context:
Grinding wheels are engineered composites, and their ‘‘hardness’’ refers to the bond's holding power on the abrasive grains, not the intrinsic hardness of the abrasive mineral. This question checks whether you know the industry-standard way to specify wheel grade (hardness) versus material hardness scales used for metals and polymers.

Given Data / Assumptions:

• The wheel is a bonded abrasive tool (e.g., aluminium oxide in vitrified or resin bond).
• Hardness here means bond retention strength, not grain hardness.
• Common material hardness scales (Brinell, Rockwell, Vickers) are for metallic indentation hardness.

Concept / Approach:
Wheel grade is standardized by letters from soft to hard (e.g., A = very soft through Z = very hard). This alpha code communicates how readily dull grains are released to self-sharpen. Indentation scales (Brinell, Rockwell, Vickers) measure resistance to plastic indentation and are unsuitable for a porous, multiphase wheel.

Step-by-Step Solution:
Identify what is being specified: grinding wheel grade (bond strength).Recall standard nomenclature: grades indicated by letters, often in the wheel specification string (e.g., 46A 60 K 7 V).Conclude that the correct notation is a letter of the alphabet, not a numeric indentation scale.

Verification / Alternative check:
Any wheel spec sheet lists the grain size, grade letter, structure number, and bond type; the grade position is always a letter that maps to soft/medium/hard categories.

Why Other Options Are Wrong:
Brinell, Rockwell, and Vickers measure metallic indentation hardness and cannot represent a bonded wheel’s grain-retention property. Shore hardness is primarily for elastomers and some plastics.

Common Pitfalls:
Confusing abrasive hardness (e.g., diamond harder than alumina) with wheel grade; a ‘‘hard’’ wheel can still contain a very hard abrasive but release it slowly, which affects grinding behavior.

Final Answer:
Letter of the alphabet