Ejector Drilling

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Ejector drilling is one of three major methods for deep hole drilling (defined by a ratio of depth to diameter that exceeds five), along with gun drilling and BTA/STS drilling. The setup for ejector drilling includes a pair of concentric tubes forming the drill tube, a drill head to perform the actual metal cutting, a rotary connector (sometimes called a coolant inducer) to supply coolant from the spindle through the drill tubes, and an adapter on the front of the spindle through which the coolant and chips exit.

Unlike BTA/STS drilling, which uses a single tube for the drill tube, ejector drilling makes use of a pair of concentric tubes. The flow of high pressure coolant starts at the spindle through a rotary connector. From there, it flows into the annular space between the inner and outer tubes, lubricating the cutting surfaces and forcing the chips up through the inner tube. From the inner the tube, the chips then exit the system adapter on the front of the spindle.

The drill head attaches to the drill tubes using a four-start square thread. Like the drill head used in BTA/STS drilling, it consists of multiple cemented carbide cutting surfaces. Also like BTA/STS drilling, the outer tube does not require any grooves or flutes to facilitate chip removal.

Because of how the rotary connector interfaces with the drill head, a Venturi effect is created which draws the chips and coolants out of the inner tube without requiring a seal between the work piece and the drill tubes, as is the case in BTA/STS drilling. This is the reason why ejector drilling works so well with surfaces that are uneven or irregular (e.g., curved, stepped, etc.), and why it requires less pressure than the BTA/STS drilling method.

Ejector drilling is commonly used to adapt existing machining centers, horizontal boring machines, lathes, or turning centers for deep hole drilling. Machines with a horizontal spindle have been found readily adaptable to ejector drilling, and such retrofits are considered a cost-efficient way to setup deep hole drilling. As long as there is sufficient coolant pressure and filtration available, it does not require extensive retooling to modify a machine for ejector drilling.

Ejector drilling can be considerably faster than gun drilling (between 5 and 20x) and is known for cutting straighter holes with excellent tolerances. One of its limitations involves the type of work piece material: ejector drilling works best with materials that produce well-formed chips, which is the opposite of BTA. This is due to the limited space for chip removal, which is also what constrains ejector drilling to minimum diameters of about 19 mm. Companies like Botek provide ejector drill heads that range from 19 mm up to 200 mm in diameter.

If you are needing to deep drill a hole through a surface that is not flat and need a more cost-efficient manufacturing method than gun drilling, then ejector drilling should be your first choice. It can handle diameters from 19 mm to 200 mm, and ratios of depth to diameter over 100.