Grinding wheels — The perceived hardness (softness or hardness) of a grinding wheel depends on the type and amount of bonding material used. For general-purpose cutter and tool grinding (i.e., to keep the wheel self-sharpening as edges dull), which grade of grinding wheel is normally preferred?

Introduction:
In metal cutting and finishing, “wheel hardness” refers to how strongly the bond holds abrasive grains—not the mineral hardness of the grains themselves. For cutter/tool grinding, you want a wheel that releases dulled abrasive quickly so that new, sharp grits are exposed. This question tests understanding of wheel grade selection for general-purpose cutter grinding.

Given Data / Assumptions:

• Application: general-purpose cutter and tool grinding.
• Objective: maintain a self-sharpening wheel face.
• Wheel “soft” vs “hard” indicates bond holding power, not abrasive type.
• Typical work materials are relatively hard (e.g., HSS, tool steels).

Concept / Approach:
Harder work materials tend to require softer wheel grades. A soft-grade wheel sheds dulled grains more readily; the wheel “breaks down” at the surface to reveal sharp grit, keeping grinding forces and temperature lower and preventing glazing. Conversely, a hard-grade wheel holds grits too long, causing rubbing, heat generation, and poor surface quality when grinding hard tools.

Step-by-Step Solution:
Identify goal: keep the wheel cutting freely on hard tool materials.Link to grade: choose a grade that encourages timely grain release.Soft grade accomplishes the self-sharpening behavior required for general-purpose cutter grinding.

Verification / Alternative check:
Practical shop guidance: “Hard material → soft wheel; soft material → hard wheel.” Wheel selection charts from manufacturers show soft to medium-soft grades commonly recommended for HSS cutter/tool sharpening operations.

Why Other Options Are Wrong:

• Hard: tends to glaze on hard tools, raising temperature and reducing finish.
• Silicon carbide / Aluminium oxide: these describe abrasive type, not grade; you can have either abrasive in many grades.
• Medium with resin bond only: bond type matters, but the question asks for grade; “soft” is the correct grade choice.

Common Pitfalls:
Confusing abrasive hardness with wheel grade; believing a harder wheel always lasts longer—glazing can actually reduce productivity.

Final Answer:
Soft