General Practice in Drilling Holes
In the construction and production of practically all machinery, the process of hole
making is extensive. Fasteners such as bolts, studs, machine screws, rivets needed for
assembling such machinery all require holes. Drilling is one of the most efficient
methods that we have for removing metal and while its is a simple process in
application, achieving this efficiency requires a degree of competency. These
competencies include; drill type selection, sharpening, point selection, drilling sequence
and speed or RPM of rotation.
The twist drill does most of the cutting with the tip of the bit. There are flutes to carry
the chips up from the cutting edges to the top of the hole where they are cast off.
Some of the parts of a drill bit are diagramed below as viewed from the cutting tip of the
Some other features of the drill bit are shown below for a side view of the drill bit,
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Typical parameters for drill bits are,
1. - Material is High Speed Steel
2. - Standard Point Angle is 118°
3. - Straight shank - held in chuck
4. - Tapered shank (Morse) – held in sleeve and or a machine spindle.
A drill drift is required for removal.
Drill sizes are typically measured across the drill points with a micrometer.
Typical drill sizes are,
1. - FRACTIONAL - 1/64" to 3 1/4" dia. in 1/64" steps
2. - NUMBER - #1 = 0.228" dia. to #80 = 0.0135" dia.
3. - LETTER - A = 0.234" dia. to Z = 0.413" dia.
4. - METRIC - 0.4mm dia. to 50mm dia.
Additional types of drills include:
1. - Core drills - a drill with a small helix, and 3 or 4 flutes. This is used
for light drilling, such as opening holes in castings.
2. - High helix - When drilling a deep hole in a soft material these drills
are used to help remove chips
3. - Straight fluted - Used to drill soft metals and plastics. The straight
flutes prevent the bit from digging in.
4. - Center drills - A drill with a small entry tip, and a widening profile.
The result is a hole that has a conical shape on the outside, that
may be used to mount the part between centers, or to act as a
guide for a larger drill
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High helix drills are used for drilling deep holes in soft material, the high helix helps
remove chips quickly and prevent packing and subsequent binding.
Straight fluted drills are used along with low helix to drill plastics. The ejection of chips
is the most important problem to overcome drilling plastic. Chips tend to stick and pack
in the flutes causing rubbing and heating. Most plastics are comparatively low-
temperature compositions. Straight flutes provide the most clearance for chip
evacuation from the drill point.
When metal parts are to be machined on centers as in turning or cylindrical grinding, it
is always necessary to Drill and Countersink the center holes to serve as seats or
bearings for the machine centers. Two types of drill-countersinks are available: Regular
type (this is used at CWU) and Bell type. For ordinary run of work the regular type is
satisfactory. If parts are to be machined in numerous operations on center, there is the
danger of marring the edges of the center hole. Bell type drills are beveled to prevent
damage to the center hole itself.
Standard Bell type Drilled extra deep
Center drills are sized with numbers 0 to 8. The numbers do not correlate to a particular
fractional size. Refer to the chart below for size recommendations to provide adequate
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Drill coatings are provided to enhance characteristics of selected drills. For any drill
coating to provide the promise of performance increase, the proper drill must be
This treatment is applied to finished tools and produces a thin black iron oxide surface
coating. It also provides additional tempering and stress relieving. This coating reduces
galling and chip welding and also increases the ability of the tool to retain lubricants.
Recommended in iron and steel drilling applications. It should not be used in non-
ferrous metals such as aluminum because it increases the loading tendencies of the
TiN Coatings (Titanium Nitride)
A hard dense wear resistant coating suitable for a wide range of applications. An
excellent general purpose coating for protecting a wide variety of cutting tools from wear
generated from heat dissipation. TiN is good for use with Iron- based materials. Tin
coated tools are easily recognizable by their bright gold color.
Chromium Carbide (CrC)
A super hard coating (>80Rc) applied to the surface utilizing PVD coating technology.
Same hardness as TiN coating but with an oxidation resistance that is 20–30% higher.
Highly recommended for machining titanium and aluminum alloys
CrN Coatings (chromium nitride)
A very hard, very adherent, abrasion and wear resistant coating. A superior alternative
to conventional chrome plating, applied using the PVD process which, unlike regular
chrome plate, has no negative environmental side effects. This chromium-based coating
has higher hardness, approx. Rc78, offering improved edge sharpness. Particularly
effective in the tapping and drilling of titanium, nickel and copper alloys. The thin nature
of the coating (approx.0001") will not alter dimensional tolerances
TiCN Coatings (Titanium Carbo-Nitride)
A hard dense, adherent wear resistant coating suitable for a wide range of applications.
TiCN has a higher hardness value than TiN which provides excellent heat resistance for
more abrasive materials. The lower oxidation temperature limits the use of TiCN to
workpiece materials where moderate temperatures are generated at the cutting edge.
Tools coated with TiCN have a platinum/blue sheen.
TiAIN Coatings (Titanium Aluminum Nitride)
An ideal coating for high speed and high temperature applications. Available with
standard and increased aluminum levels. TiAIN performs very satisfactorily in materials
that generate high cutting temperatures during machining. The key to its success is the
addition of Aluminum to TiN, which increases its hot hardness capability and oxidation
stability. When this coating is exposed to high cutting temperatures, TiAlN forms a hard
aluminum oxide layer that has low thermal conductivity. This layer acts as an insulator
to the heat and rejects more heat into the chip. Tools coated with TiAlN have a purple
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CrAITiN Coatings (Chromium Aluminum Titanium Nitride)
A tough multilayer coating suitable for applications where high temperature and high
chemical wear resistance is required.
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