Grinding and Abrasive Processes

Introduction to Grinding and Abrasive Processes

Grinding is a machining process that removes material using abrasive grains bonded together to form a grinding wheel. Unlike turning or milling, where the tool has a defined geometry, the grinding wheel is a tool with multiple, random cutting edges. Each abrasive grain acts as a micro-blade that removes a tiny chip. This chapter covers the fundamental principles, types of grinding wheels, operating parameters, common operations, speed calculations, and essential safety rules for the Red Seal exam.

Understanding grinding is crucial for the machinist, as it applies to surface finishing, tool sharpening, cylindrical and surface grinding, as well as cut-off operations. The candidate must master wheel nomenclature, abrasive selection, cutting speeds, and the geometric tolerances achieved.

Fundamental Principles of Abrasion

Material Removal Mechanism

Material removal by grinding occurs through three distinct mechanisms:

8.Micro-shearing: the abrasive grain acts as a negative rake cutting tool, deforming and shearing the metal.

9.Ploughing: the grain pushes material to the sides, creating plastic deformations.

10.Rubbing: worn or dull grains rub against the surface, generating heat.

The proportion of each mechanism depends on the wheel hardness, cutting speed, and depth of cut. Effective grinding maximizes micro-shearing and minimizes rubbing.

Cutting Parameters

The three fundamental parameters are:

Wheel peripheral speed (V): expressed in metres per second (m/s) or feet per minute (ft/min). It is calculated using the formula V = π × D × N, where D is the wheel diameter in metres and N is the rotational speed in revolutions per second.

Workpiece speed (v): feed rate of the workpiece under the wheel, in m/min.

Depth of cut (a): thickness of material removed per pass, in millimetres.

The relationship between these parameters determines the maximum chip thickness and the quality of the finished surface.

Heat and Lubrication

Grinding generates intense heat. Approximately 60 to 80% of mechanical energy is converted into heat. Inadequate heat management causes:

Grinding burns (discolouration of the workpiece).

Surface cracks or micro-cracks.

Thermal expansion affecting dimensional accuracy.

Wheel deterioration (grain sintering).

The use of a cutting fluid is essential. Soluble fluids (emulsions) are the most common for grinding. They provide cooling, lubrication, and chip evacuation. The flow rate must be sufficient to cover the entire contact zone.

Types of Abrasives and Bonds

Conventional Abrasives

Abrasive	Chemical Formula	Knoop Hardness (kg/mm²)	Typical Applications
Aluminium oxide (Al₂O₃)	Al₂O₃	2000 – 2500	Carbon steels, alloy steels, tool steels
Silicon carbide (SiC)	SiC	2500 – 3000	Cast iron, non-ferrous metals, carbide, ceramics
Zirconia alumina	ZrO₂-Al₂O₃	1600 – 1800	Steel roughing, heavy stock removal

Aluminium oxide is the most widely used abrasive for steels. It comes in several varieties: white (more friable, for hardened steels), pink (for high-speed steels), and brown (for general purpose). Silicon carbide is harder but more brittle; it suits materials with low tensile strength.

Super-Abrasives

Cubic boron nitride (CBN): hardness close to diamond, but stable at high temperatures. Ideal for hardened steels and high-speed steels. Does not react chemically with iron.

Diamond: the hardest of all abrasives. Used for tungsten carbide, ceramics, and glass. Not suitable for steels (chemical reaction at high temperatures).

CBN and diamond wheels use specific bonds (resin, metal, vitrified) and require higher rotational speeds.

Bonds (Binding Agents)

Bond	Symbol	Characteristics	Application
Vitrified	V	Porous, rigid, heat-resistant	Standard grinding wheels
Resinoid	B	Flexible, impact-resistant	Cut-off, rough grinding
Rubber	R	Very flexible, finishing	Thin wheels, regulating wheels
Metallic	M	Very strong, grain retention	CBN and diamond wheels
Silicate	S	Soft, cool-cutting	Large-diameter wheels

The vitrified bond represents approximately 70% of grinding wheels. Its porosity allows for chip evacuation and cooling.

Wheel Marking and Identification

The standardized marking system (ISO 525 standard and CSA B44.1 standard) consists of a sequence of letters and numbers. The exam requires complete reading of this code.

Example: A 46 K 5 V 12

Position	Meaning	Example
1	Abrasive type	A = aluminium oxide, C = silicon carbide
2	Grain size	46 = medium grain (mesh)
3	Hardness (grade)	K = medium hardness
4	Structure (porosity)	5 = medium structure (1 to 15)
5	Bond type	V = vitrified
6	Manufacturer's mark	12 = internal code

Grain Size

Grain size is indicated by a number corresponding to the number of meshes per linear inch of the screen. The higher the number, the finer the grain.

Grain Number	Average Diameter (mm)	Application
10 – 24	2.00 – 0.71	Roughing, cut-off
30 – 60	0.60 – 0.25	General grinding
70 – 180	0.21 – 0.09	Finishing, sharpening
220 – 600	0.07 – 0.02	Superfinishing, polishing

Wheel Hardness

Hardness (grade) indicates the strength of grain retention by the bond, from A (very soft) to Z (very hard). A wheel that is too hard for the application becomes loaded and burns the workpiece; a wheel that is too soft wears quickly and loses its shape.

Rule of thumb: for hard materials, choose a soft wheel (grains dull quickly and must break away). For soft materials, choose a hard wheel.

Cutting Speeds and Calculations

Wheel Peripheral Speed

Peripheral speed is the linear speed at the circumference of the wheel. It is determined by the manufacturer and must never be exceeded.

Formula: V = π × D × N

Where:

V = peripheral speed (m/s)

D = wheel diameter (m)

N = rotational speed (rev/s)

Conversion to revolutions per minute: N (rpm) = (V × 60) / (π × D)

Example: A 200 mm diameter wheel must run at 30 m/s. What is the rotational speed?

N = (30 × 60) / (π × 0.200) = 1800 / 0.628 = 2865 rpm

Recommended Speeds

Operation	Peripheral Speed (m/s)
Surface grinding (vitrified wheel)	20 – 30
Cylindrical grinding	25 – 35
Tool sharpening	20 – 25
Cut-off (resinoid wheel)	45 – 80
CBN wheels	45 – 80
Diamond wheels	25 – 35

Caution: the maximum speed is printed on the wheel (e.g. "MAX 35 m/s"). Never exceed this value. A wheel that bursts can cause serious injury.

Workpiece Speed and Feed

Workpiece speed in surface grinding is typically 10 to 30 m/min. The cross-feed (wheel width per pass) is 1/4 to 1/2 of the wheel width for finishing, and 1/2 to 3/4 for roughing.

Depth of Cut

Roughing: 0.025 to 0.075 mm per pass

Finishing: 0.005 to 0.015 mm per pass

Superfinishing: less than 0.005 mm

Grinding Operations

Surface Grinding

Surface grinding produces a flat, parallel surface. The workpiece is held on a magnetic chuck (for steels) or in a vise. The wheel makes successive passes.

Typical parameters:

Wheel: A 46 K 5 V

Wheel speed: 25 m/s

Depth of cut: 0.01 – 0.03 mm

Cross-feed: 1/4 of wheel width

Flatness control: use a straight edge and feeler gauges. Typical tolerance is 0.005 mm over 100 mm.

Cylindrical Grinding

External cylindrical grinding is performed between centres or in a chuck. The workpiece rotates at a slow speed (20 to 40 m/min) while the wheel removes material.

Types of cylindrical grinding:

Plunge grinding: the wheel advances radially into the workpiece.

Traverse grinding: the wheel moves axially along the workpiece.

Internal grinding: used for bores. The wheel is small in diameter and the peripheral speed is limited by the wheel size.

Tool Sharpening

Sharpening cutting tools (drills, milling cutters, lathe tools) requires high precision. Cutting angles must be respected:

Twist drill: point angle 118°, clearance angle 8 – 12°.

Lathe tool: rake angle 5 – 15°, clearance angle 5 – 10°.

Sharpening is generally done on a dry wheel or with light lubrication. The wheel must be dressed frequently to maintain a flat surface.

Cut-Off and Rough Grinding

Cut-off uses thin wheels (2 to 5 mm thick) with resinoid bonds. Rough grinding (snagging) is performed with cup wheels or coarse-grain wheels.

Dressing and Truing of Grinding Wheels

Dressing

Dressing consists of restoring the wheel's geometric shape and concentricity. It is performed with a diamond mounted on a holder. The diamond must be positioned slightly below the wheel centre (1 to 2 mm) and moved at a constant speed.

Dressing parameters:

Dressing pass: 0.02 to 0.05 mm

Diamond feed: 0.1 to 0.2 mm per wheel revolution

Lubrication: abundant

Truing

Truing (or cleaning) consists of cleaning the wheel of embedded metal particles and opening up the pores. It is performed with a truing stone or an abrasive stick. Truing is necessary when the wheel "burns" the workpiece or vibrates.

Wheel Balancing

An unbalanced wheel causes vibrations, poor surface finish, and premature spindle wear. Balancing is performed:

99.Statically: on balancing knives or a balancing device.

100.Dynamically: with vibration measurement equipment.

Wheels larger than 150 mm in diameter must be balanced before mounting. Balancing flanges with adjustable weights are used.

Safety and Regulations

Fundamental Safety Rules

104.Maximum speed: never exceed the maximum speed printed on the wheel.

105.Inspection before mounting: check for cracks (ring test: a sound wheel produces a clear ring).

106.Guards: wheels must be equipped with guards conforming to CSA Z432.

107.Face shield and safety glasses: mandatory.

108.Protective guard: the distance between the wheel and the workpiece must be minimal.

109.Mounting: flanges must have a diameter of at least 1/3 of the wheel diameter.

110.Blotting paper: use paper washers between the wheel and flanges to distribute pressure.

Canadian Electrical Code

The Canadian Electrical Code, Part I (CE Code), applies to the electrical installations of machine tools. Rule 8-200 concerns the grounding requirements for machinery. Grinding machines must be properly grounded to prevent electric shock.

Relevant CSA Standards

CSA B44.1: Grinding wheels and grinding machines (safety requirements).

CSA Z432: Safeguarding of machinery (guards and protectors).

CSA W117.2: Safety in welding (if applicable to related work).

Grinding Defects and Corrections

Defect Observed	Probable Cause	Correction
Burns (discolouration)	Wheel too hard, speed too high, insufficient lubrication	Choose a softer wheel, reduce speed, increase fluid flow
Vibrations	Unbalanced wheel, improper mounting, workpiece speed too high	Balance the wheel, check mounting, reduce workpiece speed
Rough surface	Grain too coarse, wheel too soft, feed too fast	Choose a finer grain, a harder wheel, reduce feed
Loaded wheel	Soft material, wheel too hard, speed too low	Choose a softer wheel, increase speed, true the wheel
Wheel wears too quickly	Wheel too soft, speed too high	Choose a harder wheel, reduce speed

Tolerances and Surface Finishes

Finishes Achieved by Grinding

Operation	Roughness Ra (μm)
Roughing	1.6 – 3.2
Finishing	0.4 – 0.8
Fine grinding	0.1 – 0.4
Superfinishing	0.025 – 0.1

Dimensional Tolerances

Surface grinding: ± 0.005 mm

Cylindrical grinding: ± 0.0025 mm

Internal grinding: ± 0.0025 mm

Cutting Fluids for Grinding

Types of Fluids

Type	Concentration	Application
Soluble emulsion	3 – 5%	General purpose
Semi-synthetic fluid	5 – 8%	Precision grinding
Synthetic fluid	3 – 10%	Superfinishing, CBN
Neat oil	100%	Thread grinding, CBN

Fluid Requirements

Cooling: ability to absorb heat.

Lubrication: reduce friction between the grain and the workpiece.

Flushing: evacuate chips and worn grains.

Anti-corrosion: protect the machine and workpiece.

The fluid flow rate must be at least 20 L/min for a 200 mm wide wheel.

Pitfalls to Avoid

136.Confusing wheel hardness and grain hardness: hardness (grade) concerns grain retention by the bond, not the hardness of the grain itself.

137.Forgetting to recalculate speed after dressing: the wheel diameter decreases with wear, which changes the peripheral speed at constant rotational speed.

138.Using a silicon carbide wheel on steel: SiC reacts chemically with iron at high temperatures, causing rapid wear.

139.Neglecting the ring test: a cracked wheel can burst during operation.

140.Confusing plunge and traverse grinding: in plunge grinding, the wheel advances radially; in traverse grinding, it moves axially.

141.Ignoring CSA B44.1: the safety requirements are detailed there.

142.Forgetting to balance large-diameter wheels: vibrations damage the machine and workpiece.

143.Choosing too fine a grain for roughing: productivity drops and the wheel becomes loaded.

144.Not accounting for thermal expansion: a hot workpiece measures larger; measurement must be done at a stable temperature.

145.Using an unsuitable cutting fluid: a fluid that is too diluted does not cool sufficiently.

Summary

Grinding is an abrasive machining process using bonded abrasive grains.

The three material removal mechanisms are micro-shearing, ploughing, and rubbing.

The main abrasives are aluminium oxide (Al₂O₃) for steels and silicon carbide (SiC) for cast irons and non-ferrous metals.

Super-abrasives (CBN and diamond) are reserved for specific applications.

Wheel marking follows the format: abrasive – grain – hardness – structure – bond.

Peripheral speed is calculated using V = π × D × N and must never exceed the maximum printed value.

Typical speeds are 20 to 35 m/s for conventional grinding.

Dressing with a diamond restores shape and concentricity; truing cleans the pores.

Balancing is mandatory for wheels larger than 150 mm.

Safety relies on respecting speeds, inspecting wheels, using guards, and wearing personal protective equipment.

CSA B44.1 and CSA Z432 govern the safety of grinding machines.

Common defects (burns, vibrations, roughness) are corrected by adjusting the wheel, speed, or fluid.

Typical tolerances are ± 0.005 mm for surface grinding and ± 0.0025 mm for cylindrical grinding.

Cutting fluids provide cooling, lubrication, and flushing.

Self-Assessment Questions

162.Calculate the rotational speed of a 250 mm diameter wheel for a peripheral speed of 30 m/s.

163.Interpret the following marking: C 60 L 7 V 15.

164.Which abrasive would you choose to grind a tungsten carbide workpiece? Justify your answer.

165.What are the three main causes of grinding burns?

166.What is the difference between dressing and truing a wheel?

167.Name two CSA standards applicable to grinding machines and their main purpose.

168.Why does a wheel that is too hard cause burns on a hardened steel workpiece?

169.What is the role of flanges in mounting a wheel?

170.Calculate the total depth of cut required to remove 0.15 mm of material in finishing (0.01 mm per pass).

171.What are the three material removal mechanisms in grinding?

Answers: 1) 2292 rpm; 2) Silicon carbide, grain 60, hardness L, structure 7, vitrified bond; 3) Diamond (carbide is too hard for Al₂O₃ and SiC); 4) Wheel too hard, speed too high, insufficient lubrication; 5) Dressing restores shape, truing cleans the pores; 6) CSA B44.1 (grinding wheels and machines), CSA Z432 (safeguarding of machinery); 7) Dull grains rub instead of cutting, generating heat; 8) Distribute clamping pressure and transmit torque; 9) 15 passes; 10) Micro-shearing, ploughing, rubbing.

Grinding and Abrasive Processes