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A Guide to Deep Hole Drilling

Mar. 29, 2018

"What is considered a deep hole?"

In general any hole more than 4 to 5 times its Diameter, is considered a deep hole. With conventional drills such as twist drills several cycles or pecks would be required to clear chips from the flutes of the drill.  With a dedicated deep hole drilling machine and proper tooling and fixturing the hole can be produced in one pass. Depth to diameter ratios of up to 300:1 can be achieved. Good surface finish and hole size may eliminate secondary reaming, or honing operations.


"Which drilling method is right for my application .... Gundrill, BTA, Ejector?"

While many factors are involved in the choice of tooling and machine type Below are brief descriptions and general guidelines.


Gundrilling is an very old process of drilling long or deep holes, first used in the making of gun barrels more than 100 years ago. Today's technology with refined machinery and tool design has made gun drilling a reliable high production method for drilling short holes as well as deep holes. The gundrill consists of a hollow tube with a " V" shaped groove or flute along its length, and a carbide cutting tip designed in such a way as to produce it's own guide bushing as it drills the hole. High pressure coolant is introduced into the center of the drill tube through the spindle of the gun drilling machine to help break and evacuate the chips along the "V" groove of the tool and out of the hole. Gundrilling provides very close tolerance straight holes with excellent surface finish. Gundrilling is able to produce holes as small as .031" 


BTA or STS (single tube system) drilling is a process developed in more recent years, with BTA drilling the coolant is introduced around the drill tube at the pressure head, and chips are evacuated through the center of the drill. The detachable drill head is comprised of individual carbide inserts and guide pads. Very high penetration rates can be achieved with this system along with good surface finish. Diameters as small as .500" (12.7 mm) or as large as 15.00" (381mm) can be produced with this type of system.

Ejector Drilling

Ejector drilling or two tube system is a process similar to the BTA process except in this case the drill tube consists of an inner and outer tube. Coolant is introduced at the spindle via a rotary connector and passes between the inner and outer tube, chips exit through the inner tube. Minimum diameter is limited to about .750 (19 mm) since there is less room for chip removal in the smaller diameter tube. The ejector system performs well in the case where the face of the part is irregular since the design of the rotary connector and drill head create a venturi effect to draw coolant and chips through the inner tube with out relying on a good seal between part and bushing.  The Ejector system is often adapted for use in lathes or machining centers.

Gundrill, BTA or Ejector

What method should I use for my application?

Many factors are involved in the choice of tooling and machine type. In general for diameters below .500" gundrilling is the only choice, although some tooling manufacturers do make BTA tooling slightly smaller than this. 

For diameters 0.500" and above gundrilling may be more economical then BTA strictly on a cost per hole basis, but in the case where very high production rates are required, the number of spindles and initial equipment cost will be as much as three to five times higher with a gundrilling system, since in most cases, one two spindle BTA machine would easily produce the same or more parts per hour as one six spindle gundrill machine.  Other factors to consider are, the number of operators required, time required to change and regrind tools, and even the amount of floor space required.

For diameters larger than 2.00", BTA or Ejector tooling is probably the only choice.

Ejector drilling is limited to diameters above .750" does not require as high as pressure as the BTA system and works well when the part face is irregular or not machined square as it does not require a tight seal between bushing and work piece.