The difference made by using a tool with internal coolant.

Coolant-through holders (also called “coolant-thru holders”) are specially designed toolholders that deliver cutting fluid directly through the tool and onto the cutting edge. This targeted coolant supply improves chip evacuation, reduces heat, and increases tool life compared to external cooling. By keeping the cutting zone cooler and cleaner, coolant-through holders allow higher cutting speeds, better surface finishes, and more consistent machining results—making them a key factor in improving efficiency and productivity in modern CNC operations.

Here are the results using a holder with external refrigeration

Holder with external refrigeration

Here are the results using a coolant thru holder