Understanding Grit Numbers: The Science Behind the Size

Understanding Grit Numbers: The Science Behind the Size

White fused alumina (WFA) grit sizes follow international standards:

• Coarse Grits (#200 - #400):
  Particles averaging 50–200 microns (similar to table salt). Ideal for rapid material removal.

• Medium Grits (#500 - #1200):
  15–50 micron particles (like fine sand). Balances cutting speed and surface smoothness.

• Fine Powders (#1500 - #8000):
  Particles as small as 1.5–5 microns (flour-like). Used for mirror finishes.

🔍 Key Insight: Grit numbers increase as particle size decreases. For example, #200 grit ≈ 75 microns, while **#8000 grit ≈ 2 microns*.

Matching Grit to Your Application

1. Heavy Stock Removal & Rough Grinding (Grit #200 - #400)

• Best For:

  • Removing weld seams, casting scales, or deep scratches

  • Surface preparation before coating (e.g., thermal spray)

  • Rough shaping of ceramics or refractory bricks

• Why WFA Excels:
  High-purity (≥99.5% Al₂O₃) grains retain sharp edges longer than brown alumina, reducing wheel loading by 25%.

2. Intermediate Finishing (Grit #500 - #1200)

• Best For:

  • Tool and die polishing

  • Medical implant contouring (e.g., titanium knee joints)

  • Automotive valve grinding

• Performance Tip:
  Use #800 grit (FEPA F220) for stainless steel – its blocky grains prevent embedding and yield consistent Ra 0.8–1.2 μm finishes.

3. Precision Polishing & Lapping (Grit #1500 - #8000)

• Best For:

  • Optical lens finishing (e.g., #4000 grit ≈ Ra 0.1 μm)

  • Silicon wafer backgrinding (#2000 grit minimizes subsurface damage)

  • Mirror polishing of sapphire watch crystals

• Critical Advantage:
  Low-iron WFA (<0.03% Fe₂O₃) avoids contaminating sensitive materials like semiconductors or aerospace alloys.

3 Grit Selection Mistakes That Cost You Money

1. Using Coarse Grits for Final Polishing
   → Result: Deep scratches requiring rework.
   → Fix: Always finish 1–2 grit sizes finer than your target Ra.

2. Ignoring Particle Shape
   → Result: Angular grits (#200-#500) cut faster but leave deeper marks.
   → Fix: Use semi-rounded grains (#800-#2000) for uniform scratch patterns.

3. Overlooking Coolant Compatibility
   → Result: Grit clumping in water-based coolants.
   → Fix: Choose hydrophilic-treated WFA for wet operations (common in #1500-#5000).

Pro Tips for Maximum Efficiency

• Sandblasting: Start with #200–#300 grit for steel, shift to #400–#600 for aluminum to avoid pitting.

• Vitrified Wheels: Blend #320 + #800 grits – coarse grains cut fast, fine grains refine the finish.

• Lapping Compounds: For Ra < 0.05 μm, combine #6000 WFA powder with diamond suspension (3:1 ratio).

Your Next Step: Get the Grit Size Calculator

Still uncertain? Download our free "White Alumina Grit Selector Tool" – input your material, finish requirement, and equipment to get instant grit recommendations:

Includes:

• Grit ⇔ Micron conversion charts (JIS/FEPA/ANSI)

• Material-specific starting grit guidelines

• Cost-per-part optimization formulas

FAQ: Quick Answers

Q: Can I skip grit steps (e.g., jump from #400 to #1000)?
A: No – each step removes scratches from the previous grit. Skipping causes uneven finishes and longer processing times.

Q: Why choose white over brown alumina for fine grits?
A: WFA’s higher purity and friability provide better cutting consistency in #1000+ ranges (+30% life in precision work).

Q: Does grit size affect abrasive consumption?
A: Yes. Using #800 instead of #500 on stainless steel reduces consumption by 18% while achieving Ra 0.4 μm.