The question looks simple but it's not simple at all.
Abrasive cutting performance
Abrasive cutting performance depends on many factors. Many relations can be explained, but not all. Sometimes, abrasive A has zero cutting performance on steel B, or causes blade chipping, or extreme stone dishing, or glazing, or balding... We often cannot explain the fact of specific incompatibility, we just accept it.
Grit value is a major factor affecting cutting performance. The intuition tells "coarser" means "faster" (larger particles remove more metal). However, it does not work for extra-coarse grits. In the coarse grit range up to the optimal point called "the diminishing return", the rule is the opposite: "finer" means "faster". The optimal point is unique for a specific combination of abrasive, grit value, surface condition, break-in period, lubrication, steel, heat treatment, scratch pattern, contact spot area, pressure, attack angle, and many other factors. This effect exists in all abrasives, not just diamonds. For power grinders (belt sanders) the optimal point is usually within 24-60 grit. In manual sharpening, where pressure is much lower, the optimal point is usually within 120-400 grit.
Factors for optimal point
- 1️⃣ Break-in period. A new extra coarse stone out-of-the-box has exposed particles protruded at a different height. Only the tallest particles work. The break-in period makes particle height uniform, increases the density of particles in contact. (Demo images below.)
- 2️⃣ Steel hardness. Soft steel lets protruded particles penetrate deeper, it increases the performance of extra coarse stones.
- 3️⃣ Pressure. This is extremely important!!! When you see a low performance for extra-coarse stone, you apply more pressure. By increasing the pressure, you force protruded particles penetrate deeper, it increases the performance of extra coarse stones for a short period of time. Unfortunately, increased pressure leads to many negative effects. Some are hidden (steel weakness, internal structural damage, abrasive grain rounding), others are obvious (very fast stone loading, glazing, dishing, balding).
- 4️⃣ Blade bevel width. Narrow bevel has a small contact spot area, and extra coarse stone may not work.
- 5️⃣ ️Blade bevel roughness. Extra coarse stone may not work on mirror polish - particles will glide. For factory grind, extra coarse stone work much faster.
- ️6️⃣ Binder for diamonds. Only electroplated diamonds have good performance in the extra coarse grit range for most of the steels. For bonded diamonds, CBN, and traditional abrasives, the performance is conditional.
A diamond plate before break-in period:
The same diamond plate after break-in period