External & internal coolant
Methods of coolant supply to the cutting edge are divided into external and internal coolant. External coolant is a method of supplying coolant externally to the cutting point. It does not need a complex mechanism and is easily set up, therefore external coolant is used for many types of drills. Additionally, with the aim of disposal and removal of chips, external coolant can also be supplied to jigs.
Internal coolant is a method of supplying coolant to the cutting edge through the centre of the drill. Typical internal coolant supply methods are side, through spindle and end face. The through spindle method supplies coolant to the cutting edge via the centre of the machine spindle. By passing through the axial centre, the coolant pressure does not drop noticably and is not influenced by the tool rotation. However when
a tool change is carried out the coolant can contaminate the arbor and machine spindle.
High-pressure coolant systems with pressures over 1Mpa are needed for external or internal coolant systems to ensure that the coolant can
reach the cutting edge. It is especially effective for drilling deep holes of depths 5 × D and deeper and when machining difficult-to-cut materials. With the high-pressure coolant method, mist is easily generated necessitating a mist collector, additionally coolants tend to foam when using air. Therefore the use of coolant with a high de-foaming property is advisable.
Comparison: No cooling/external cooling/internal cooling
Here you can see a comparison of the chips developed when using different cooling methods. As it can be seen external cooling shows that the chips are very hot, and that the cutting edge is subject to high temperatures. Internal coolant ensures that the cutting edge is sufficiently cooled.
Dry machining External Internal