General Guideline by Operation
1. For Heavy Stock Removal & Deburring (e.g., removing weld spatter, heavy rust, or reshaping a damaged bolt head):
- Grit Size: Coarse Grits – #24 to #60
- Explanation: These are aggressive grits used on bench grinders or powerful angle grinders with grinding wheels or depressed-center wheels. They remove material quickly but leave a very rough surface. Use with extreme caution on bolts, as it’s easy to remove too much material and weaken the bolt.
2. For General Cleaning, Rust Removal, and Smoothing Grind Marks:
- Grit Size: Medium Grits – #80 to #120
- Explanation: This is a common range for “cleaning up” a bolt—removing light corrosion, smoothing out burrs from cutting, or blending areas. It’s a good balance between speed and surface finish.
3. For Fine Deburring and Surface Preparation (e.g., before painting or for a smoother finish):
- Grit Size: Fine Grits – #150 to #220
- Explanation: Used for final touch-ups, removing light scratches from coarser grits, and creating a uniform surface. Often used with flap discs or fine grinding wheels.
4. For Polishing or Final Finishing:
- Grit Size: Very Fine Grits – #240 and finer (up to #1000+)
- Explanation: These grits are typically used with polishing compounds, abrasive belts, or specialty papers to achieve a polished or near-polished finish on the bolt, though this is more for cosmetic purposes than functional grinding.
Most Common & Practical Recommendations
For the vast majority of bolt grinding tasks in workshops, automotive, and fabrication, the following are the most useful:
- Flap Discs (Extremely Common): A #80 Grit flap disc is arguably the most versatile tool for bolt work. It’s aggressive enough to remove rust and burrs quickly but leaves a relatively controllable finish. Follow with a #120 Grit flap disc for a smoother surface if needed.
- Depressed-Center Grinding Wheels (“Type 27” or “Type 28”): Typically come in #24, #36, or #60 grit. These are for the heaviest work but require more skill to avoid damaging the bolt. #36 is a popular general-purpose coarse grit.
- Abrasive Belts (for belt grinders): #80 to #120 grit belts are excellent for controlled deburring and shaping of bolt ends or heads.
Critical Safety and Practical Notes
- Material Matters: The above applies to standard steel bolts. For stainless steel bolts, you would often use a different abrasive (like zirconia alumina) to avoid contamination and prevent “heat tint” discoloration, though silicon carbide can be used for initial work.
- Heat is the Enemy: Grinding generates intense localized heat which can:
- Draw the Temper: Soften the hardened bolt, making it weak.
- Create Hard Spots: Alter the metallurgy, making it brittle.
- Always use short, controlled bursts and dip the bolt in water frequently to keep it cool.
- Don’t Grind the Threads! Avoid grinding on the threaded portion, as this will compromise the thread integrity and the bolt’s ability to hold torque. Focus on the head, shank (if necessary), and the very end.
- Weakening: Any significant metal removal reduces the bolt’s cross-sectional area and strength. Avoid grinding the shank down to a smaller diameter.
Summary Table
Operation | Recommended Grit Size | Common Tool Format |
|---|---|---|
Heavy Stock Removal | #24 – #60 | Grinding Wheel, Depressed-Center Wheel |
General Cleaning & Deburring | #80 – #120 | Flap Disc (Most Common), Abrasive Belt |
Fine Finishing | #150 – #220 | Flap Disc, Finishing Wheel, Abrasive Belt |
Polishing | #240 and finer | Polishing Compound, Micro-Mesh |
In short: Start with a #80 grit flap disc for most common bolt cleaning and deburring tasks. It offers the best balance of speed, control, and finish. For heavier damage, step down to a #36 or #60 grinding wheel, and for finer work, step up to a #120 or higher grit.
