cylindrical
hole, the piston-rod has been shaped in a both spiral and flat manner,
and, as it passes through a movable " hub" or boss of a ratchet wheel,
the back stroke turns the ratchet every time to the extent of one
tooth, or an average of one-tenth of an inch, similarly as is done by
the miner holding and turning the drill for the striker. As the hole
gets deeper, either a longer and sharper drill needs to be inserted at
the chuck, or, if the end of the cylinder is close to the rock, the
motive-power is turned off and the cylinder with drill, &c, is
"fed" backwards to the full extent of the screw provided, and boring
can again be commenced. The " feeding " forward is of very great
importance, and requires very careful attention on the part of the "
boss " in charge, because he must feed in accordance with the exact
nature of the rock passing through, and any want of care will prevent
his getting full duty from the motive-power expended in working these
drills. Upon the character of the rocks about to be perforated depends
also the velocity given to the blows given per minute by the piston,
and they can be delivered to the number of 500 or 1,000 if required.
With
all these machines it is a matter of grave importance that their inner
parts, which are subjected to extraordinary wear and tear under great
pressure, are solid and as few as possible in number ; that the drills
should at the same time be portable, and easily fixed for boring at any
conceivable angle, the same as a miner is required to do with hand
drills. By way of comparison and explanation, I would state that the
English Burleigh drill consists of 120 pieces, and it weighs about 230
lbs.; the Ingersoll of the same size weighs 185 lbs.; and the National
weighs but 168 lbs., the latter being composed only of but 80 principal
and interchangeable parts.
These
drills are either mounted on a tripod to suit the bottom of workings,
however uneven, when they are held down by heavy weights, to withstand
the enormous concussions in drilling underfoot; in other workings they
are geared to a hollow iron "bar," by means of an adjustable and
massive clamp, which is furnished with a knuckle, or universal joint,
enabling the drill to be fixed in any given direction by means of set
screws. This bar is made of strong wrought-iron tubes, which at one end
has a strong movable double anchoring bit riveted on, for the purpose
of gripping in the " hitches" cut for that purpose in one side of the
workings. At the other end, and inside the bar, a square-threaded screw
works in a thread of the same description—the screw thread varying from
1/4 inch to § inch in proportion to the size of the drills —and of such
a length as will permit the extension of the bar considerably over its
ordinary length, by means of a lever that is inserted into the lower
end of the screw ; this end can be made to work either in an iron
socket or, if the nature of the rocks renders it necessary, in a block
of soft wood, so that the greatest possible strain can be put upon this
wooden block in order to withstand the concussions put upon the bar and
drill combined.
General Directions for Working Pomer Drills.
1. Before starting, all parts of the drills ought to be carefully lubricated.
2.
The bar should then be rigidly fixed by means of the lever, and, should
it slacken as the work progresses, "lever up" to take up the slack as
required.
3.
Let the piston drop down on the upper end of the drill-bit, and let the
"bit" touch the rock where the hole is to be bored. If,after the first
stroke, the piston stops, the valve gland must be tightened to prevent
the valve moving back over the port, and thus cut off the
motive-power—which in future shall be compressed air—then start again,
and attention with accu-
