Dando TERRIER Mk 1, Operating And Maintenance Manual, Page: 48 / 276

The load on the hammer is the actual weight of the rotary head and drill rods
plus the additional hydraulic pressure which is being applied. For drilling at
shallow depths, the requisite load is obtained by applying the necessary
hydraulic pressure, which is set by adjusting the Variable Feed Control -
No.28.
As the depth increases, so the required hydraulic loading can be reduced.
Eventually a position is reached where the combined head and rod weight
exceed the required bit loading. At this point the Variable Feed Control -
No.28 should be turned towards "DECREASE" fully open, the Holdback
Pressure Control - No.30 is also turned towards "DECREASE" fully closed
and the Hoist/Feed Control Lever - No.14 moved to the "UP" position. With
the drill string suspended on the rotary head and hanging just off hole
bottom, the combined head and drill weight will register on Pressure Gauge.
Gravity feed drilling can then continue by slowly turning the Holdback
Pressure Control - No.30 towards "INCREASE" thereby bleeding oil from the
hoist/feed cylinder and transferring the precise requirement of rod weight to
the bit.
The loads equivalent to the pressure indicated on the gauges are given on
the legend plate attached to the control console.
One of the major problems that occurs when carrying out air hammer drilling
is that the cuttings can stick to the sides of the borehole. This prevents the
hammer from being freely withdrawn. This often occurs when there is a
small amount of water sufficient to dampen the cuttings. One method of
overcoming this problem is to fit a retract substitute immediately on top of the
hammer. The over the top rotary head allows the drill rods and hammer to
be continuously rotated, as the rods are withdrawn from the borehole which
enables the retract cutter blades to clear any obstructions.
4.3 CONTINUOUS FLIGHT AUGERS
When operating continuous flight augers, it is important that the auger should
act as a cutting tool and not be allowed to screw itself in to the ground. The
required control can be achieved by using the Holdback Pressure Control -
No.30 under gravity feed, or the Variable Feed Control
– No 28. The actual
system of controlled feed would be entirely dependent on the strata being
drilled.
4.4 ROTARY DRILLING
The major requirement to enable tricone roller bits to drill efficiently is the
application of requisite loading whilst operating within the recommended
speed range. As a general rule, it is essential to make full use of the
hydraulic pull-down system, which is controlled by the Variable Feed Control
- No.28. Turn the control towards "INCREASE" to increase bit loading and
towards "DECREASE" to reduce bit loading. The maximum hydraulic
pressure which can be applied is 2250 psi. This will induce a load pressure
on the drill string of 5 tons. The actual bit loading will be the applied hydraulic
pressure plus the weight of the drill string and rotary head.