1. BASICS OF DRILLING-2
I. Satyanarayana,
M.Tech,MBA,MCA
Dy. Manager, Project Planning, SCCL,
isnindian@gmail.com,
visit at
www.slideshare.net/isnindian
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2. Rotary drills
In rotary drilling, the drill bit attacks the rock with
energy supplied to it by a rotating drill stem.
The drill stem is rotated while a thrust is applied to
it by a pull-down mechanism using up to 65% of the
weight of the machine, forcing the bit into the rock.
The drill bit breaks and removes the rock by either a
ploughing-scraping action in soft rock, or a
crushing-chipping action in hard rock, or by a
combination of the two.
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3. Compressed air is supplied to the bit via the drill stem. The
air both cools the bit and provides a medium for flushing
the cuttings from the hole.
Water may be used in addition to the compressed air to
suppress the effects of dust, however, this is normally
found to have a detrimental effects on bit wear.
Blast hole sizes produced by rotary machines vary in the
range of 100 to 445 mm diameter with the most common
sizes being 200, 250, 311 and 381mm.
These drills usually operate in the vertical position ，
although many types can drill up to 25 or 300off the vertical.
Drills are manufactured that can drill horizontal holes used
in overburden stripping where hard bands of material are
located low in the high-wall face.
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4. PRINCIPLES OF ROTARY DRILLING
Rotary drilling can be subdivided into rotary cutting and rotary crushing.
Rotary cutting creates the hole by shear forces, breaking the rock's
tensile strength. The drill bit is furnished with cutter inserts of hard metal
alloys, and the energy for breaking rock is provided by rotation torque in
the drill rod. This technique is limited to rock with low tensile strength
such as salt, silt and soft limestone not containing abrasive quartz
minerals.
Rotary crushing breaks the rock by high point load, accomplished by a
toothed drill bit, which is pushed downwards with high force. The bit,
being of tricone roller type fitted with tungsten carbide buttons, is
simultaneously rotated, and drill cuttings are removed from the hole
bottom by blowing compressed air through the bit. Drill rigs used for
rotary drilling are large and heavy. The downwards thrust is achieved by
utilising the weight of the drill rig itself, and the rotation, via a hydraulic
or electric motor, applied at the end of the drill pipe. Common hole
diameters range from 8 to 17½ in (200-440 mm) and, because adding the
heavy drill pipes is cumbersome, most blast hole drill rigs use long masts
and pipes to accommodate single-pass drilling of maximum 20 m (65 ft).
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5. Electric power is usually chosen for the large rigs, whereas smaller rigs
are often powered by diesel engines. Rotation rates vary from 50 to 120
rpm, and the weight applied to the bit varies from 0.5t/in of bit diameter
in soft rock, to as much as 4 t/in of bit diameter in hard rock.
Recent technical advances include:
Improved operator cab comfort; automatic control and adjustment of
optimum feed force and rotation speed to prevailing geology and bit type
and diameter; and incorporation of latest technology in electric and
hydraulic drive systems. Rotary drilling, which is still the dominant
method in large open pits, has limitations in that the rigs cannot drill
holes off the vertical line. Many rotary rig masts have pinning
capabilities permitting drilling at angles as much as 30 degrees out of the
vertical. However, the inclined hole drilling capabilities in rotary drilling
are limited by the heavy feed force required, since part of this force is
directed backwards. This causes rig stability problems, reduced
penetration, and shorter life of drilling consumables. Consequently, most
blast hole drilling using rotary drill rigs is in vertical holes.
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6. (1) Rotary drill penetration rate
One of the most important factors in drilling is how
fast can drill hole be produced while the machine is
actually drilling.
This factor almost entirely influences productivity
and has a strong influence on unit costs of the hole.
The penetration rate of the rotary drill is about
15m/hr for the size of 250mm hole.
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7. (2) Rotary drill rotation speed
The rotary drive motor turns the drill tool string
thus turning the drill bit at the bottom of the hole.
As the rotary speed increases, so does the number
of contacts and the penetration rate.
The limit to rotary speed is hot bearings in the bit。
Current rotary speeds ranges from 60 to 90 rpm for
hard materials with greater speeds for softer rocks
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8. (3)Rotary drill pull-down weight
A portion of the machine weight is applied through
the pull-down motor via the pull-down chain or
chains, rotary head and drill stems to the drill bit.
As the bit diameter increases, the bearing size
increases thus allowing an increase in the tolerable
load.
Overloading the bit results in severe loss of bit life
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9. (4) Rotary drill bits
Tri-cone rotary drill bit has evolved from the drag
bit and two cone bits.
Mill, or steel tooth bits are used for soft rock with the
cutoff being a medium-strength limestone.
The average footage per bit is about 800 for rock
with compressive strength of 80MPa.
The larger bit sizes, incorporating larger bearings, etc.
can be expected to give higher footages per bit.
This has not always proved the case.
These footages are averages for the highest-grade
tungsten carbide bit of different manufacture, the
ore having variable compressive strengths
approximately 204Mpa. ISN

10. (5) Rotary drill air requirements
Air is used to bail the drill cutting from the hole as
well as cool the bit bearings and, when used, roller
stabilizer bearings.
The air volume is the primary requirement for
bailing cuttings from the hole.
Air velocity up the hole is dependent on the air
volume per minute as well as the hole annulus.
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11. The velocity of the drill cuttings in this air is
dependent on the chip size, density, and shape
At air velocity above this balancing value, the chip
begins to move.
A bailing velocity of 1800mpm is usually adequate
to bail13mm chips.
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12. Choosing the higher velocity has the following advantages:
1) Will bail larger chips;
2) Tend to give higher bit life
3) Will help cater for hole cavities, etc.
4) May give higher penetration rates and possibly lower
cost per ft
5) Reduce the volume of cuttings in the hole for a given
penetration rate.
But they will give increased stabilizer and pipe wear.
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13. (6) Cold weather operation
Drills to be used in cold climatic conditions should
be purchased with a cold weather package.
(7) Single pass drilling
Many new mining ventures selected bench height
and drill equipment to utilize single pass drilling.
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14. Advantages of single pass are:
1) Eliminates adding stems
2) Reduces associated thread damage
3) Reduces machine downtime for rod changing
equipment and tool racks
4) Facilitates the cleaning of boreholes
5) Permits a continuous air flow through the bit at
all times. This is especially important in wet
holes as it eliminates the possibility of
siphoning water and cuttings up into the bit.
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15. Disadvantages:
1) High masts make the drill more unstable, back
brace become almost mandatory,
2) Extra care needs to be taken when moving the
drill over medium or long moves,
3) Pull-down chains become long and may require
special attention.
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16. For operations choosing to reduce bench height in
order to attain single pass drilling, the following
other benefits may also be experienced:
1) Multi-row blasts become easier to blast,
2) Drill cuttings volume is reduced (especially for
the larger holes),
3) Contour areas are easier to drill and blast,
4) Ore grading is easier.
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17. (8) Rotary drill productivity and costs
The larger the sizes of the hole, the greater the productivity
of the rotary drill.
(9) Drilling trends
Rotary drills have increased their dominance of blast-hole
production in open pit mining.
The trend has been to larger sturdier drills, to yield higher
mechanical availability and operating performance.
This increased availability has been achieved through
improvement of crawler track frames, masts, propel chain,
pull-down mechanisms, rotary head drives complete with
automatic lubrication and greasing, on the new models of
machine.
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