BTA drilling

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BTA stands for Boring and Trepanning Association, who are the originators of this method for deep hole drilling. The BTA drilling process, also known as STS or single tube system, requires a supply of high pressure coolant and uses a drill head attached to the end of a hollow drill tube.

In this drilling method, high pressure coolant is forced though the area between the newly drilled hole and the outer surface of the drill tube (also known as “oil room”). To facilitate this, a pressure head is used to provide a high pressure seal between the entry side of the work piece and the guide. The flow of coolant not only provides lubrication to the drill head during cutting but forces the metal chips to flow out through the inner diameter of the tube, then out the machine spindle. Because chips are evacuated through the interior instead of the exterior, the drill tube does not require any type of fluting or grooving to carry the chips away. The resulting cross-section increases the rigidity to the system, enabling it to achieve depth to diameter ratios in excess of 100.

The drill head performs the actual metal removal. It consists of guide pads and carbide cutting tool inserts, giving some drill heads an appearance not unlike that of a face milling cutter. The drill head screws onto the end of the drill tube, and is always larger in diameter than the drill tube. Note that the minimum hole diameter for BTA is 20 mm because of the minimum feasible size of the drill tube.

BTA drilling differs from the gun drilling in several respects. For example, in BTA drilling, the high pressure coolant flows outside the tool, carrying the chips away through the inside of the tool; in gun drilling the coolant flows inside the tool and carries the chips away along the outside surface. In gun drilling, the tool is fluted or has a deep V-groove cut into to assist with carrying away the chips, but in BTA drilling this is not needed, making the system more rigid and adapted to cutting holes with higher depth to diameter ratios. One of the advantages of BTA drilling over gun drilling is that the drill head has cutting tool inserts, making it easier to sharpen the cutting surface of the inserts (or replace the inserts, if needed) and continue with the drilling process.

BTA drilling is best used for holes with diameter ranging from 20 mm to 630 mm, especially extremely deep holes or materials that do not produce well-formed chips during machining (such as stainless steel or low carbon steel). Because of its design and stiffness, BTA drilling can achieve high metal removal rates compared to gun drilling. In fact, it works well for high penetration rates, but requires more power than gun drilling.

Capabilities vary according to machine and tooling, but if you are looking at deep holes with a depth to diameter ratio up to 100 and a hole diameter greater than 20 mm, BTA/STS drilling is an excellent option. It can maintain extreme accuracy, with the right equipment, for depth to diameter ratios up to 400.