Gun drilling []
Main article: Gun drill Gun drilling was originally developed to drill out gun barrels and is used commonly for
drilling smaller diameter deep holes. The depth-to-diameter ratio can be even greater
than 300:1. The key feature of gun drilling is that the bits are self-centering; this is what
allows for such deep accurate holes. The bits use a rotary motion similar to a twist drill;
however, the bits are designed with bearing pads that slide along the surface of the hole
keeping the drill bit on center. Gun drilling is usually done at high speeds and low feed
rates.
Trepanning [
edit
]
Trepanned hole in steel plate, with the plug removed and the tool that cut it; in this case the tool-holder is
mounted on a
lathe headstock
while the workpiece is mounted on the
cross slide
.
Trepanning is commonly used for creating larger diameter holes (up to 915 mm
(36.0 in)) where a standard drill bit is not feasible or economical. Trepanning removes
the desired diameter by cutting out a solid disk similar to the workings of a
drafting
compass
. Trepanning is performed on flat products such as sheet metal, granite (
curling
stone
), plates, or structural members like
I-beams
. Trepanning can also be useful to
make
grooves
for inserting
seals
, such as
O-rings
.
Microdrilling []
Microdrilling refers to the drilling of holes less than 0.5 mm (0.020 in). Drilling of holes at
this small diameter presents greater problems since coolant fed drills cannot be used
and high spindle speeds are required. High spindle speeds that exceed 10,000 RPM
also require the use of balanced tool holders.
