The Gundrilling System - A pressurized coolant flow is pumped through the I.D. of the gundrill. The coolant exits the drill through holes near the cutting edge. The pressurized coolant then forces the chips down an external flute on the gundrill and out of the bore.

The Double Tube System (DTS) - The double tube system involves a boring bar or drill tube which is threaded to a drilling tool. Inside of this outer drill tube is an inner tube which creates a void between the O.D. of the inner tube and I.D. of the outer drill tube. Pressurized coolant is then forced through the void where a portion of the coolant is allowed to exit through slots in the inner tube while the majority of the coolant is directed to the tools cutting edge. These two coolant flows create a vacuum in the inner tube that exhausts the chips through the I.D. of the inner tube. The double tube system can be adapted to standard milling and turning machine tools. The addition of a high volume coolant system is needed for proper chip evacuation.

The Single Tube System (BTA)(STS) - The single tube system (BTA) is similar to the double tube system in that a drilling bar is threaded to the drilling tool and the coolant and chips are exhausted from the bore through the I.D. of the drilling bar. However, the manner in which coolant in introduced to the cutting edge is different. This system incorporates a Pressure Head to force coolant to the cutting edge. Follow this link for more more information on American Heller's BTA Oil Pressure Head.

American Heller Corporation recommends the single and the double tube systems as superior alternatives to gundrilling. Both systems offer greater overall reliability than the gundrilling system. The greater reliability is due to an inherent weakness in the gundrill design -- susceptance to external forces such as torque or thrust due to the nature of the V-shaped flute used to exhaust chips and coolant from the bore. However, the single and double tube systems do differ as illustrated below.

The single tube system (BTA) is the ideal system for drilling very deep holes because it is rigid and is capable of delivering enough pressurized coolant to the cutting edge regardless of the bore depth. This is a limitation in the Double Tube System due to the manner in which the coolant in delivered to the cutting edge and the thinner drilling bar walls required to accept the additional inner tube.

On the other hand the double tube system can be used to convert common machines such as lathes or milling machines to deep hole drilling machines since it uses a drill tube and inner tube to provide coolant to the cutting edge as well as exhaust chips and coolant from the bore. This eliminates the need for a Pressure Head which is required in the single tube or BTA system.