A Guide to Choosing the Right Grit Size for Precision Grinding

In the realm of precision grinding, the choice of grit size in diamond and CBN (Cubic Boron Nitride) grinding wheels is critical for achieving the desired surface finish on a workpiece. Whether working with hard materials like carbide and tungsten carbide or honing high-speed steel (HSS), the right grit size can make a significant difference in both the efficiency of the grinding process and the quality of the finished product. This article explores the relationship between different grit sizes and the resulting surface finishes, providing insights into how to select the ideal grit for your specific grinding needs.

Choosing the Correct Grit Size for Specific Materials

Grit size refers to the size of the abrasive particles embedded in the grinding wheel. It is a crucial factor that influences the wheel's cutting ability, stock removal rate, and the surface finish of the workpiece. Grit sizes are categorized into coarse, medium, and fine ranges. Coarse grits, with larger abrasive particles, are typically used for rapid material removal and rough grinding. Medium grits strike a balance between material removal rate and surface finish, while fine grits are used for achieving a smooth, high-quality surface finish.

Selecting the Right Diamond and CBN Grinding Wheel Types

Diamond and CBN are two of the hardest materials available for abrasive grinding wheels, known as superabrasives. Diamond wheels are optimal for grinding non-ferrous materials and hard, brittle materials like ceramics and carbide. CBN wheels, on the other hand, are ideal for ferrous materials, including HSS and tool steels. The selection between diamond and CBN grit depends on the material of the workpiece and the desired outcome of the grinding process. For example, grinding tungsten carbide typically requires a diamond wheel, while HSS is best ground with CBN.

This introduction sets the stage for an in-depth exploration of how grit size affects surface finish, with subsequent sections delving into specific grit types, surface finish requirements, and application-based grit selection. You can continue to build upon this foundation, integrating the graphical comparison of Ra and RMS with USA mesh and FEPA grit sizes, and discussing the various factors and considerations in selecting the appropriate grit size.

Selecting Diamond Grit for Non-Ferrous and Brittle Materials

Diamond is the hardest known natural substance, making it the premier choice for grinding materials that are hard and brittle but non-ferrous. When selecting the right diamond wheel for your application, keep in mind that the grit is measured by mesh numbers—a system where the grit number is inversely related to the actual size of the diamond grain.

The logic of grit size selection is straightforward: the finer the grit (represented by a higher number), the more points of contact the wheel has with the workpiece. This makes fine grits the ideal grit for achieving a smooth, mirror-like finish on sensitive components. Specifically, synthetic diamond grits are unmatched for processing:

• Cemented Carbides and Tungsten Carbide: Diamond penetrates these ultra-hard materials without dulling, maintaining sharp cutting edges for longer production runs.
• Ceramics and Glass: Fine diamond grits reduce micro-chipping on delicate, brittle substrates, ensuring a superior surface finish.
• Composites and Non-Metallic Materials: Its exceptional hardness allows for high material removal rates while maintaining strict form tolerances.

Note: Diamond is not suitable for grinding steel or iron. At high speeds, the carbon in the diamond reacts chemically with the iron, causing the grit to degrade rapidly into graphite.

Leveraging CBN Grit for High-Speed Steel and Ferrous Metals

While diamond grinding is the standard for non-ferrous materials, it is critical to use Cubic Boron Nitride (CBN) when working with iron-based alloys. Selecting a grit wheel with CBN is the only way to achieve optimal grinding on ferrous metals because, unlike diamond, CBN is chemically stable and will not react with the carbon in the steel.

This makes it the superior choice for:

• High-Speed Steel (HSS): CBN's extreme thermal conductivity prevents "burning" the delicate cutting edges of tools during high-speed sharpening.
• Hardened Tool Steels: Selecting the right grit for materials like A2, D2, or M2 steel ensures that you maintain high grinding performance without sacrificing the structural integrity of the workpiece.
• Hardened Cast Iron: Its resistance to abrasive wear allows it to handle the tough, "crusty" surface of cast components while maintaining precise form retention.

Determining the Correct Grain Size for Production Speed

When selecting the appropriate grit size for your production line, the grain size must be matched to your required material removal rate. Coarser CBN grit (such as 100 Grit) is used for heavy stock removal where speed is the priority, whereas finer micron sizes (400 Grit and finer) are reserved for superfinishing and achieving strict geometric tolerances.

Surface Finish Requirements

Understanding surface finish is critical when selecting the right grit size for grinding. Surface finish refers to the texture and quality of the metal surface after grinding and is often measured in Ra (Roughness Average) and RMS (Root Mean Square) values. These values are inversely related to the grit size of the grinding wheel; finer grits result in lower Ra and RMS values, indicating a smoother surface. The forthcoming graphic will illustrate how different USA mesh and FEPA grit sizes correlate with specific Ra and RMS finish requirements, providing a practical guideline for selecting the appropriate wheels.

Surface Finish Correlation Table

When selecting the right diamond or CBN wheel, it is vital to understand how the grit size affects the final texture of your workpiece. In the superabrasive industry, diamond grit size refers to the number of openings per linear inch in the sizing screen.

The following table provides a variety of grinding tools guidelines for expected finishes using standard resin bond wheels. Note that choosing the ideal grit for your application involves balancing the material removal rate with your specific surface finish requirements.

Technical Variables in Surface Quality

It is important to remember that a grit wheel alone does not guarantee a specific finish. Several grinding performance variables will influence these results:

• Bond Hardness: A harder bond holds the grain size longer, which can result in a finer finish but may lead to "wheel loading" or burning if not managed.
• Coolant Delivery: Proper coolant use significantly improves the desired surface finish by flushing away swarf that could otherwise cause deep scratches.
• Machine Rigidity: Vibrations in the grinding machine can cause chatter marks, effectively negating the benefits of a fine grit.

Factors Affecting Surface Finish

Several factors influence the surface finish achieved during grinding, including:

• Bond Type: The bond of the grinding wheel (resin bond, vitrified, etc.) affects its wear rate and cutting efficiency. Resin bonds, for instance, are known for their good surface finish and are often used with fine grits. Vitrified bonds offer a rigid structure ideal for precision grinding.
• Coolant Use: The choice between wet and dry grinding has a significant impact. Wet grinding, using a coolant, typically results in a better surface finish and longer wheel life due to reduced heat and friction. Dry grinding, without coolant, might be used for less heat-sensitive applications or where coolant is not feasible.
• Grinding Wheel Speed: High-speed grinding can lead to better finishes as it may prevent the wheel from loading up, but it also requires wheels designed to withstand the higher forces.

Grit Selection for Specific Applications

Choosing the right grit size is application-specific. For instance:

• Cylindrical Grinding and Centerless Grinding: Medium grits are usually employed to achieve a balance between material removal and surface finish.
• Honing and Fine Surface Grinding: Fine grits are necessary to achieve the high-quality finishes required in these applications.
• Coarse Grits for Roughing: When rapid material removal is the priority, such as in stock removal or shaping of a workpiece, coarse grits are the go-to choice.

Additional Considerations

In addition to grit size, other factors to consider include:

• Material Removal Rate (MRR): The speed at which material is removed from the workpiece. Higher MRR can be achieved with coarser grits but may sacrifice finish quality.
• Tolerances: Tighter tolerances might require finer grits to achieve the necessary precision.
• Dressing Tools: The condition of the wheel's surface is maintained by dressing tools, which can also impact the effectiveness of the grit.
• Micron Sizes for Superfinishing: In applications where ultra-fine finishes are required, micron-sized grits are used to achieve the necessary precision and smoothness.