High-pressure coolant is delivered through a duct inside the drill and forced through the center of the gun drill. The coolant provides lubrication and aids in chip removal. Chips exhaust through a V-shaped chip flute along the outside of the drill. Due to the v-groove, the cross-section of the tube occupies .75 of its circumference.
The STS Single Tube System or sometimes referred to as the BTA system: Coolant is supplied to the tool through a pressure head unit which seals and clamps against the part. Also, the coolant is directed to the cutting face via the space between the drill tube and the hole being drilled. This Positive Coolant flow provides both drill head lubrication and proper flow for chip removal. The combination of chips and coolant returns internally through the I.D. of the tool and drill tube and then back to the coolant.
The coolant is supplied at the spindle through a special coolant inducer or connector. In addition, the pressurized coolant flows through the annular space between the O.D. of the inner and the I.D. of the outer drill tube towards the drill head. Most of the coolant is forced through holes in the drill head providing lubrication and chip evacuation through the inner tube. The remainder of the coolant is diverted directly back through the inner tube via the venture slot. This creates a partial vacuum in the inner drill tube and forces the coolant and the chips through the chip exhaust port.