Why grit needs sorting at all
A press run produces a mixed harvest. The crystals that come out are not all the same size, not all the same shape and not all equally strong. A grinding tool made from a random blend would wear unevenly, cut inconsistently and might scratch a workpiece with a few oversized particles. Grading is the step that turns a mixed output into predictable material.
Three properties are usually sorted: particle size, particle shape and strength. Size is the most visible, but the other two often decide how a grit actually performs.
Sorting by size
Size is traditionally expressed in one of two ways. Coarse and medium grit is described using mesh numbers, which refer to the openings of the sieves the particles pass through or are held on. A higher mesh number means a finer grit. National and international standards, such as those from FEPA in Europe and ANSI-style conventions in the United States, define the ranges, and the labels differ between systems. A grade is not a single size but a narrow band between an upper and lower sieve.
Very fine powders, below what sieving can sensibly handle, are described by their size in micrometers. For these, methods based on settling in a liquid, on light scattering or on image analysis are typically used to measure the distribution. A good grade has a tight spread: most particles near the stated size and few oversized ones, because a handful of large particles can ruin a polished surface.
Sieving is simple in principle, with stacked screens on a shaker, and finer grades are separated by sedimentation or air classification, where the speed at which particles fall or are carried by a stream of air depends on their size.
Sorting by shape
Two crystals of the same sieve size may look very different. Some are compact, blocky and well formed. Others are elongated, flaky or irregular. The difference matters because shape affects how a particle sits in a tool bond, how it breaks, and how sharp its cutting points are.
- Blocky, compact crystals tend to be strong and are often preferred where the tool takes heavy loads, as in sawing or drilling.
- Sharper, more irregular or friable particles tend to break in small pieces during use and expose fresh cutting edges, which suits some grinding jobs.
- Needle-like or flat particles are usually less desirable because they are weaker and pack poorly.
Shape sorting can be done mechanically. A vibrating, slightly inclined surface lets rounder particles roll off while flatter ones stay behind, so shape sorting is possible without a camera. Image analysis, where a camera records each particle’s outline, is also used in more advanced setups. The exact methods vary among producers.
Sorting by strength
Strength matters because grit in a tool is under repeated impact and heat. Producers typically test it by crushing a sample under a defined load, or by repeatedly impacting it, and then measuring how much of the sample is broken into finer pieces. A commonly reported measure is a toughness index, where a higher value means more of the grit survived. A related thermal test heats the grit first, to see how well it holds up after exposure to high temperatures, since heating can weaken crystals that contain metal inclusions from the growth process.
The metal remaining in some crystals can matter in another way. Grit made in metal solvent presses can contain small inclusions that are magnetic. Magnetic separation can therefore be used as a sorting step, because grit with more inclusions tends to be more magnetic and often behaves differently under heat. Strength and inclusions are linked, though not perfectly.
Differences in strength come from the growth conditions. Fast-grown crystals tend to hold more defects, and slower growth often gives stronger crystals, a point that ties strength back to the crystal growth conditions in the press. The grading step simply measures the result.
How grades map to uses
The grade a buyer picks depends on the job, whether that is diamond tooling for stone or grinding wheels for carbide. A few general patterns hold, though specific tool makers have their own preferences.
| Grit character | Typical use direction |
|---|---|
| Coarse, strong and blocky | Sawing and drilling hard stone and concrete |
| Medium, moderately friable | Grinding wheels for hard materials such as carbide |
| Fine, tightly sized | Lapping, polishing and finishing |
| Ultra-fine powder | Polishing compounds and some composites |
This table is a rough guide, not a rule. For example, how well grit is held depends on the bond types used in the tool, and a tool maker may choose a weaker grit specifically so it breaks and refreshes itself.
Coating and further treatment
Some grit is treated after sorting. A thin metal coating, for example, can improve how well the particle is held in a resin or metal bond. Others are cleaned to remove graphite or leftover metal. These steps are applied to specific grades, so the grade label alone does not tell the whole story, and suppliers typically publish data sheets with detail.
Quality control and labeling
Consistency from batch to batch matters as much as the nominal grade. Producers typically sample each lot, check the size distribution and strength, and record the results. Buyers who care about repeatability often look at documentation and the quality standards a supplier follows rather than just the label.
In practice, grading is a balance. A tighter distribution costs more because more material is rejected or reprocessed, while a looser one is cheaper but less predictable. Tool makers choose the grade that gives the performance they need without paying for precision the job does not require.