Why does the CNC milling cutter turn black during use?

1. High temperature oxidation (main cause)
Cause: The high temperature generated during cutting (especially when processing difficult-to-process materials such as stainless steel and titanium alloy) will cause the coating or base material (such as high-speed steel, cemented carbide) on the surface of the milling cutter to react with oxygen in the air to form a black oxide layer (such as ferroferric oxide).

Performance: The tip or cutting edge is blue-black or dark brown.

Solution:

Reduce the cutting speed or feed rate to reduce heat accumulation.

Use coolant (such as emulsion, oil-based coolant) to fully cool down.

Choose high-temperature resistant coated tools (such as TiAlN, AlCrN coating).

2. Built-up edge (chip adhesion)
Cause: During the cutting process, chips accumulate on the blade and weld to the tool surface under high temperature and high pressure to form a black carbide layer.

Performance: The blade is partially blackened, accompanied by increased cutting force or worse surface roughness.

Solution:

Increase the cutting speed or reduce the feed rate to avoid chip retention.

Use sharp tools or increase the rake angle to improve chip removal.

Choose coatings with good anti-stick properties (such as diamond coating).

3. Inadequate lubrication or improper cooling
Reason: When dry cutting or insufficient coolant concentration occurs, friction heat cannot be effectively dissipated, resulting in tool oxidation.

Symptom: The tool is black as a whole, and may be accompanied by smoke or odor.

Solution:

Increase the coolant flow or replace high-performance coolant (such as extreme pressure cutting fluid).

Check whether the cooling system is aligned with the cutting area.

4. Tool material or coating failure
Reason: The substrate or coating of low-quality tools has poor heat resistance and rapidly oxidizes and peels off at high temperatures.

Symptom: The substrate is black at the place where the coating falls off, and the cutting performance drops sharply.

Solution:

Replace high-quality tools (such as carbide coated tools from well-known brands).

Avoid overload cutting.

5. Influence of processing materials
Reason: Some materials (such as titanium alloys and high-temperature alloys) have poor thermal conductivity, and heat is concentrated on the tool, aggravating oxidation.

Solution:

Use special milling cutters suitable for difficult-to-process materials (such as high-cobalt carbide tools).

Use high-pressure coolant to flush the cutting area in a directional manner.

6. Unreasonable cutting parameters
Reason: Excessive cutting depth (ap) or feed rate (f) causes a sudden increase in heat.

Solution:

Refer to the cutting parameters recommended by the tool manufacturer and optimize them step by step.

Inspection steps:
Observe the blackened area (tip, edge or whole).

Check the chip state (whether it is continuous, whether there is adhesion).

Measure the cutting temperature (infrared thermometer) or monitor the cutting noise (abnormal noise may indicate overheating).

Preventive measures:
Check tool wear regularly and replace it in time.

Adjust cutting parameters and cooling methods for different materials.

Give priority to high-temperature resistant and high-hardness coated tools.

If blackening is accompanied by rapid tool wear or deterioration of workpiece surface quality, it is necessary to stop the machine immediately for investigation to avoid further losses.