The Single Tube System (BTA)(STS) - The single tube system (BTA) is similar to the double tube system in that a boring bar is threaded to the drilling tool and the coolant and chips are exhausted from the bore through the I.D. of the boring 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 boring 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.