7
steady penetration and a constant, even rotation speed with no
jamming (see table).
N�B. It is important that the feed force be adapted to suit the
weight of the drill string. When drilling deep holes, this requires
control facilities for “negative feeding”, a so-called “holdback”
function.
Rotation speed
In hard rock, the rotation speed for COP 32/42 should be set
between 20-35 r/min, depending on the hammer size and bit
diameter (the larger the bit diameter, the slower the speed). The
upper limit generally produces the best penetration rate. In very
abrasive rock formations, however, the rotation speed should be
reduced to avoid excessive wear of the drill bit.
When drilling in softer rock or in the case of the COP 42, when
drilling with high air pressure (above 16 bar) in non-abrasive for-
mations, higher rotation speed may be used. The following should
be noted:
Rotation speed – recommendations ( r/min)
Penetration rate
Rock characteristics
COP 32
ø 90 mm
COP 42
ø 115 mm
0,7 - 1 m/min.
Soft, abrasive rock
Soft, non abrasive rock
(Air pressure above 16 bar)
40-55
–
35–50
35–70
0,4 - 0,7 m/min.
Medium-hard,
abrasive rock
non abrasive rock
(Air pressure above 16 bar)
30–40
–
25–35
25–55
<0,4 m/min.
Hard, abrasive rock
Hard, non abrasive rock
(Air pressure above 16 bar)
25–35
–
20–30
20–50
Too high a rotation speed will cause increased wear to the drill bit,
hammer and drill pipes. Stresses to the feed and rotation unit will
also increase.
Too low a rotation speed results in a poor drilling output and
uneven operation.
Flushing — Air-blowing
To avoid wasteful re-crushing and the risk of
jam ming, drill cuttings must be flushed out
of the drill hole at the same rate as new
cuttings are produced. It is good drilling
practice to clean out the hole at regular
intervals by means of air-blowing. This is
especially import ant in non-consolidated
formations and when there is a danger of the
hole wall collapsing. Air-blowing is done by
lifting the hammer off the bottom of the hole
(fig. a) and running the feed back and forth.
N�B� Rotation must always be to the right.
The impact mechanism stops as soon as the
hammer is lifted and the bit drops down
-
wards, which causes a considerable volume
of air to flow through the hammer and flush-
out the hole. When the hammer is lowered
back on to the bottom of the hole, the bit is
pressed back into the impact position, which
re-starts the impact mechanism.
• Always wear goggles during drilling. The backward-direct-
ed flushing air from the top sub contains drill cuttings and oil
residue which can injure the eyes.
WARNING
Extra flushing
The need for extra flushing is greater when drillingt with lowe
working pressures and in difficult drilling conditions, e.g. when
there is a high flow of water inteo the drill hole.
Extra flushing is obtained by changing the plug in the control tube
to a special "choke plug", which has an air duct passing straight to
the flushing ducts in the drill bit.
Changing the choke plug
The choke plug can be changed when the control tube has been
removed from the DTH hammer. The choke plug is then removed
and fitted as follows:
Place the control tube on a workbench wiith the collar downwards.
Knock out the plug with the aid of a mandrel.
Now turn the control tube so that the collar is uppemost. Tap in the
new plug with the aid of a mandrel. To make it easier to fit the plug
into the control tube, the guide of the plug can be smeared qwith a
soap solution or similar substance.
Air consumption
COP 32 6 bar 12 bar
With choke
plug ø 4 mm
50 l/s
120 l/s
COP 42 6 bar 10�5 bar 18 bar
With choke plug
ø 5 mm
ø 7 mm
71 l/s
85 l/s
132 l/s
149 l/s
250 l/s
280 l/s
Drilling in wet holes
The inflow of water into the drill hole is expected when drilling
water wells, but can also occur when drilling deep holes for other
purposes. Water inflow does not normally create problems for
drilling, although both “too little” and “too much” can be trouble-
some.
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