14.10 Abrasive Machining, Grinding & Superfinishing

The Abrasive Cutting Mechanism

A grinding wheel is a multi-point cutting tool in which thousands of randomly oriented abrasive grains act as individual cutting edges. Each grain removes a very small chip, and because the grains present highly negative rake angles — often minus 60 degrees or more — the specific cutting energy is far higher than in single-point turning. Most of that energy becomes heat, which is why grinding is a hot process requiring copious coolant.

The distinguishing capabilities are:

• Machining of hardened materials that cannot be cut with conventional tools.
• Very close tolerances, routinely 5 micrometres and better.
• Excellent surface finish, 0.2 to 1.6 micrometres Ra typically.
• Very small depths of cut, making it a finishing rather than a bulk removal process.

Self-sharpening

A grinding wheel differs fundamentally from a solid tool: as grains dull, the increased cutting force fractures them or tears them from the bond, exposing fresh sharp grains beneath. A correctly specified wheel is therefore self-sharpening. Everything about grade selection follows from managing this behaviour.

Standard Wheel Designation

Wheels are marked in a fixed sequence, which the exam expects to be decoded:

$\textbf{Abrasive} - \textbf{Grain size} - \textbf{Grade} - \textbf{Structure} - \textbf{Bond}$

1. Abrasive type

The rule to remember: aluminium oxide for steel, silicon carbide for cast iron and non-ferrous. Silicon carbide is harder than aluminium oxide but reacts chemically with iron at grinding temperatures, so it is unsuitable for steel despite its hardness.

2. Grain size

A number denoting the mesh count: coarse 10 to 24, medium 30 to 60, fine 70 to 180, very fine 220 to 600. Coarse grains give fast removal and rough finish; fine grains give the reverse.

3. Grade

A letter from A (softest) to Z (hardest). Grade does not describe the hardness of the abrasive — it describes the strength with which the bond holds the grains. A soft-grade wheel releases grains readily; a hard-grade wheel holds them firmly.

4. Structure

A number from 1 (dense) to 15 (open), describing the spacing of grains and the volume of pores. Open structures provide chip clearance and coolant access; dense structures give better form-holding.

5. Bond

Example. A wheel marked A 46 K 5 V is aluminium oxide, medium 46 grain, soft-to-medium grade K, medium structure 5, vitrified bond — a general-purpose steel grinding wheel.

The Grade Selection Rule

This is counter-intuitive and therefore heavily examined:

Use a soft wheel for hard materials, and a hard wheel for soft materials.

The reasoning follows from self-sharpening. Grinding a hard workpiece dulls the grains quickly, so they must be released quickly and replaced by fresh ones — hence a soft grade with a weak bond. Grinding a soft workpiece keeps the grains sharp for a long time, so releasing them early would waste the wheel — hence a hard grade.

Related selection rules follow the same logic:

Grinding Machine Types

Centreless grinding deserves attention because it is the highest-production cylindrical grinding method. The workpiece is supported on a blade between a fast grinding wheel and a slower regulating wheel, which controls the rotational and axial feed. Tilting the regulating wheel through a small angle produces through-feed. No chucking or centring is required, so cycle times are very short — the standard method for producing bearing rollers, pins and shafts in quantity.

Wheel Faults and Their Correction

Dressing versus truing

These two operations are distinct and the distinction is examinable:

• Dressing restores the cutting ability of the wheel by removing dull grains, chips and bond to expose fresh sharp abrasive.
• Truing restores the geometry of the wheel, making it concentric and giving it the required profile.

Both are commonly performed with a single-point diamond dresser, and truing usually dresses as a side effect, but the purposes differ.

Grinding Wheel Safety

A burst wheel is lethal, so safety practice is examinable in its own right. Wheels must be ring tested for cracks before mounting, blotters fitted between wheel and flanges, flanges of at least one third the wheel diameter used, and the maximum operating speed marked on the wheel never exceeded. The wheel guard must remain in place and the work rest on a bench grinder set within about 3 mm of the wheel.

Superfinishing Processes

Where grinding leaves 0.2 to 1.6 micrometres Ra, four further processes reach lower.

Honing is worth distinguishing carefully: unlike lapping, it uses bonded abrasive sticks and it corrects out-of-roundness, taper and waviness in a bore, not merely the surface texture. The characteristic cross-hatch pattern it produces in a cylinder bore is functional — it retains oil and helps piston ring bedding, which is why engine and compressor cylinders are always honed rather than merely ground.