Grinding burns ruin metallurgical properties — and result in batch rejection. Most shops think the solution is 'use more coolant'. They're wrong. The real solution is matching coolant velocity to wheel speed.
This article explains the physics of grinding burns and provides practical solutions for prevention.
What Are Grinding Burns?
Grinding burns are localized areas of metallurgical damage caused by excessive heat during grinding:
1. Rehardening Burns: Localized heating above austenitizing temperature followed by rapid cooling creates brittle, untempered martensite.
2. Tempering Burns: Heating above tempering temperature (but below austenitizing) causes localized softening.
3. Surface Oxidation: Severe burns cause visible discoloration from oxide formation.
Consequences of Grinding Burns:
Why Burns Happen: The Air Barrier Problem
Grinding burns are caused by coolant not reaching the grinding zone:
The Physics:
A rotating grinding wheel drags air with it, creating a boundary layer. At typical grinding speeds, this air layer has significant momentum and deflects incoming coolant away from the grinding zone.
The Critical Factor:
If coolant velocity is lower than wheel velocity → coolant gets deflected → burn happens.
Common Misconception:
More coolant flow rate doesn't solve the problem if the coolant velocity is too low. A high-volume, low-velocity stream just creates a bigger splash without reaching the grinding zone.
The Engineering Formula That Solves Grinding Burns
The solution is matching coolant jet velocity to wheel speed:
The Formula:
Vkss ≈ Vc
Where:
Calculation Example:
For a 400mm wheel running at 1500 RPM:
Achieving This Velocity:
Use the nozzle pressure formula: V = √(2P/ρ)
Where P = pressure (Pa) and ρ = coolant density (~1000 kg/m³)
For 31.4 m/s, you need approximately 5 bar (500 kPa) nozzle pressure.
Coolant System Requirements
Effective grinding coolant systems include:
1. High-Pressure Capability: 10–20 bar minimum pump pressure.
2. Adequate Flow Rate: Enough volume to maintain pressure through the nozzle.
3. Coherent Nozzle Design: Round, coherent jet nozzles maintain velocity over distance.
4. Proper Nozzle Size: Match nozzle diameter to required flow rate at target velocity.
5. Filtration: Fine filtration (<20 μm) prevents nozzle clogging and surface scratches.
Nozzle Positioning and Alignment
Even with correct velocity, poor positioning defeats the system:
1. Tangential Alignment: Coolant jet must enter tangent to the wheel, not perpendicular.
2. Correct Distance: Nozzle should be as close as practical to the grinding zone (typically 20-50mm).
3. Impact Point: Aim at the wheel-work interface, not the wheel face.
4. Stable Mounting: Rigid nozzle mounting prevents vibration-induced misalignment.
Additional Burn Prevention Methods
Beyond coolant velocity, several techniques reduce burn risk:
1. Use Softer Wheels: Counterintuitive but correct. Softer wheels self-dress, maintaining sharp cutting action and reducing rubbing heat.
2. Dress Wheels Frequently: Regular dressing maintains wheel sharpness and prevents glazing. A glazed wheel rubs instead of cutting, generating excessive heat.
3. Reduce Grinding Forces: Lower infeed rates and traverse speeds reduce heat generation.
4. Optimize Wheel Specification: Porous, open-structured wheels provide better coolant access to the grinding zone.
5. Use Appropriate Abrasive: CBN wheels run cooler than conventional abrasives for hardened bearing steel.
Detecting Grinding Burns
Early detection prevents shipping defective parts:
1. Visual Inspection: Severe burns show discoloration, but mild burns may not be visible.
2. Nital Etch: Standard metallurgical etch reveals burn patterns as light or dark areas.
3. Barkhausen Noise: Non-destructive magnetic testing detects subsurface stress and metallurgical changes.
4. Eddy Current: Detects conductivity changes from metallurgical damage.
5. Hardness Testing: Softening burns show reduced hardness; rehardening burns show increased (brittle) hardness.
Process Monitoring for Prevention
Proactive monitoring catches problems before they create defects:
1. Power Monitoring: Grinding power correlates with heat generation. Spikes indicate potential burns.
2. Acoustic Emission: AE sensors detect grinding behavior changes in real-time.
3. Temperature Measurement: Infrared sensors can detect thermal events during grinding.
4. Coolant Flow Monitoring: Ensure coolant system maintains pressure and flow throughout the cycle.
Final Takeaway
Burns disappear when coolant velocity meets wheel velocity. This is the #1 solution used by top bearing manufacturers. Invest in proper coolant system design, maintain wheel sharpness, and implement detection methods to ensure burn-free production.