Deep Hole Drilling

posted in: Technical Basics | 0

Deep hole drilling is a manufacturing process for drilling holes that a deeper than conventional machining equipment can adequately achieve. This process is used in a variety of industries, such as exploration equipment in the oil and gas industry, engines, plastic injection molding, landing gear for aerospace applications, and more. In this article, we are going to look at what deep hole drilling is and how it is accomplished.

A deep hole has a ratio of hole depth to diameter that is greater than 5. For example, a hole with a diameter of 20 mm and a depth of 150 mm would be considered a deep hole. Without specialized tooling, removal of chips would require several drill cycles which would undoubtedly affect surface finish, tolerances, and straightness. Obviously straightness is a major issue for deep holes, with deeper holes being far more likely to be slightly off. Dimensional tolerances is another challenge, as is maintaining an appropriate surface finish inside the hole. Tools must be specially designed for these types of applications with an eye toward suitable stiffness, chip evacuation, the ability of the cutting surface to hold an edge for an extended period of cutting, and avoiding chatter.

There are three major methods of drilling deep holes: gun drilling, BTA, and ejector. Gun drilling is a very old method and was first used to drill holes in gun barrels (hence the name). The cutting tool itself is called a gundrill, and it consists of a hollow tube with a V-shaped groove/flute along its length. At the tip is a carbide cutting surface that actually cuts its own bushing to guide it as the hole is drilled. To remove chips, high pressure coolant is forced down the hollow portion of the gundrill which in turn flows out (with the chips) along the groove. The machine used in this process is called a gun drilling machine. Gun drilling is used with smaller diameter deep holes (up to 50 mm is common).

In BTA drilling (also known as STS or single tube system), high pressure coolant is forced in around the drill tube at the pressure head and the chips exit through the center of the drill. The drill head consists of guide pads and removable cutting inserts, causing to resemble a face mill at first glance. BTA drilling is typically used for larger holes (20 mm to 630 mm is common).
The ejector method drill tube is made up of an inner tube and an outer tube. Coolant is introduced at the spindle via a rotary connector and passes between the tubes. The rotary connector and the drill head actually create a Venturi effect that draws the chips out through the inner tube. Ejector drilling is considered an alternative to BTA drilling, and its range of diameters is usually between 20 mm and 200 mm. Ejector drilling can be used to retrofit existing machining centers or lathes for deep hole drilling.

Conventional cutting tools, lathes, and machining centers are not appropriate for deep hole drilling. Manufacturers like Botek and TIBO, however, specialize in tools and machines for deep hole drilling applications respectively.