Thermal Cracking in Inserts: Coolant Strategy to Prevent

Introduction

Thermal cracking in carbide inserts is a failure mode that catches many machinists by surprise. The insert appears to be performing well, producing acceptable chips and maintaining dimensions, when suddenly the cutting edge spalls or fractures. Upon examination under magnification, a network of fine cracks is visible perpendicular to the cutting edge. These thermal cracks, also known as comb cracks or heat checks, result from cyclic thermal stress and represent one of the most preventable forms of insert failure.

Unlike mechanical wear mechanisms that progress gradually, thermal cracking can appear suddenly after a period of apparently normal operation. The key to prevention lies in understanding the thermal conditions that produce cracking and implementing coolant strategies that keep the insert within its safe temperature range.

The Physics of Thermal Cracking

Thermal cracking occurs when the insert experiences repeated heating and cooling cycles during machining. During the cut, the cutting edge heats to 800-1200 degrees Celsius. When the cut ends or coolant contacts the hot edge, rapid cooling creates tensile stresses in the carbide surface. Since carbide has relatively low tensile strength compared to its compressive strength, these tensile stresses initiate surface cracks.

Each heating-cooling cycle deepens these cracks slightly. After a certain number of cycles, the cracks propagate deep enough to weaken the cutting edge support, and the edge chips or spalls under normal cutting forces. This is a classic fatigue failure mechanism driven by thermal rather than mechanical loading.

Common Causes of Thermal Cycling

Intermittent Coolant Application

The most common cause of thermal cracking is applying coolant inconsistently. If coolant flow starts and stops during the machining cycle, the insert alternates between rapid cooling (when coolant flows) and rapid heating (when coolant stops). Each transition creates a thermal shock event.

Interrupted Cuts with Flood Coolant

When machining features like keyways, flats, or cross-holes, the insert repeatedly enters and exits the cut. During the cut, the edge heats rapidly. During the exit phase, coolant contacts the hot edge and cools it. This creates thermal cycling at a frequency determined by the number of interruptions per revolution.

Coolant Splash and Mist

In some setups, coolant does not reach the cutting zone consistently but instead splashes intermittently onto the insert. This is particularly common in high-speed operations where centrifugal force throws coolant away from the cutting zone, or in setups where the coolant nozzle is too far from the cutting point.

Programmed Dwell or Feed Holds

CNC programs that include dwell commands or feed hold pauses while coolant continues to flow create thermal cycling. The insert cools during the pause and reheats when cutting resumes.

Coolant Strategy Options for Preventing Thermal Cracking

Strategy 1: Continuous, Uninterrupted Flood Coolant

If you choose to use coolant, it must flow continuously and uninterruptedly throughout the entire cutting cycle. This includes:

• Coolant must be flowing before the insert contacts the workpiece
• Coolant must continue flowing through all interrupted cuts without cessation
• Coolant must not be reduced or pulsed during the operation
• Coolant flow must be maintained at a minimum of 10-15 liters per minute for turning operations

This strategy works well for continuous cuts in ductile materials where chip control is not an issue. The constant coolant flow keeps the insert at a relatively stable temperature, preventing the thermal gradients that cause cracking.

Strategy 2: Completely Dry Cutting with Air Blast

For interrupted cuts or operations where consistent coolant delivery cannot be guaranteed, completely dry cutting with compressed air blast is often superior. By eliminating coolant entirely, you remove the cooling phase of the thermal cycle and maintain the insert at a more stable elevated temperature.

Implementation requirements:

• Use insert grades specifically rated for dry cutting (typically PVD-coated with high hot hardness)
• Reduce cutting speed by 20-30% compared to wet cutting parameters
• Direct compressed air (minimum 6 bar) at the cutting zone for chip evacuation
• Ensure adequate chip extraction to prevent recutting
• Monitor insert temperature; the edge should appear dark red to orange, not bright yellow

Strategy 3: High-Pressure Through-Tool Coolant

Modern high-pressure coolant systems (70-350 bar) delivered through the toolholder directly to the cutting zone can prevent thermal cracking even in interrupted cuts. The high pressure ensures consistent coolant delivery that overcomes centrifugal forces and reaches the cutting edge reliably.

Advantages:

• Consistent cooling prevents thermal cycling even in interrupted cuts
• High pressure improves chip breaking and evacuation
• Coolant reaches the chip-tool interface, reducing crater wear
• Allows higher cutting speeds than flood coolant

Requirements: Machine must be equipped with a high-pressure pump system, through-coolant toolholders, and sealed connections rated for the operating pressure.

Strategy 4: Minimum Quantity Lubrication (MQL)

MQL systems deliver a fine mist of biodegradable oil (typically 5-50 ml per hour) to the cutting zone. While MQL provides minimal cooling compared to flood coolant, it reduces friction at the chip-tool interface and does not create the thermal shock associated with flood coolant.

Best applications: MQL works well for aluminum, mild steel, and other materials that do not generate extreme cutting temperatures. It is less suitable for stainless steel, titanium, or hardened materials where thermal management is critical.

Selecting the Right Strategy by Operation

Insert Grade Selection for Thermal Crack Resistance

Some insert grades resist thermal cracking better than others. When thermal cracking is a recurring problem, consider grades with these characteristics:

• PVD coatings: Thinner than CVD coatings, PVD coatings maintain a sharper edge that generates less heat. TiAlN and AlCrN PVD coatings offer excellent hot hardness.
• Fine-grain substrates: Finer carbide grain structures have higher thermal conductivity, distributing heat more evenly through the insert body.
• Tough grades: Grades with higher cobalt content in the substrate absorb thermal stress better than hard, brittle grades.

Diagnostic Checklist

When you suspect thermal cracking, verify these conditions:

1. Are coolant nozzles positioned to deliver consistent flow to the cutting zone?
2. Does coolant flow continuously throughout the entire cycle without interruption?
3. Is the operation an interrupted cut? If yes, is dry cutting or high-pressure coolant an option?
4. Are there programmed dwells or feed holds while coolant continues to flow?
5. Is the insert grade appropriate for the thermal conditions of the operation?
6. Is the coolant concentration within specification (5-10% for soluble oil, as specified for synthetics)?

Conclusion

Thermal cracking is a preventable failure mode that results from cyclic thermal stress on the cutting edge. The fundamental rule is simple: either maintain consistent cooling throughout the entire operation, or eliminate coolant entirely and cut dry. Never alternate between wet and dry conditions, as this creates the thermal cycling that produces cracks. By selecting the appropriate coolant strategy for each operation and ensuring reliable delivery, you can eliminate thermal cracking and achieve predictable, consistent tool life across all your machining operations.