Fig. 183.
Fig. 184.
In ordinary work, breakages of the boring rod generally take place in the iron, and more particularly at the part screwed, as that is the weakest part. In the case of breakages, the tools usually employed for picking up the broken ends are a conical screwed socket, shown in [Fig. 183], and a crow’s foot, shown in [Fig. 184]; the socket being made with an ordinary V-thread for cases where the breakage occurs in the iron; but having a sharper thread, like a wood screw, when used where the breakage is in one of the wood rods. In order to ascertain the shape of the fractured end left in the bore-hole, and its position relatively to the centre line of the hole, a similar conical socket is first lowered, having its under surface filled up level with wax, so as to take an impression of the broken end, and show what size of screwed socket should be employed for getting it up. Tools with nippers are sometimes used in large borings, as it is not advisable to subject the rods to a twist.
When the boring tool has detached a sufficient quantity of material, the boring rod and tool are drawn up by means of the rope O, [Fig. 158], winding upon the drum Q, which is driven by straps and gearing from the steam-engine T. A shell is then lowered into the bore-hole by the wire-rope U, from the other drum V, and is afterwards drawn up again with the excavated material. A friction break is applied to the drum Q, for regulating the rate of lowering the boring rod down the well. The shell shown in [Figs. 186, 187], consists of a riveted iron cylinder, with a handle at the top, which can either be screwed to the boring rod or attached to the wire-rope; and the bottom is closed by a large valve, opening inwards. Two different forms of valve are used, either a pair of flap-valves, as shown in [Fig. 186], or a single-cone valve, [Fig. 187]; and the bottom ring of the cylinder, forming the seating of the valve, is forged solid, and steeled on the lower edge. On lowering this cylinder to the bottom of the bore-hole, the valve opens, and the loose material enters the cylinder, where it is retained by the closing of the valve, whilst the shell is drawn up again to the surface. In boring through chalk, as in the case of the deep wells in the Paris basin, the hole is first made of about half the final diameter for 60 to 90 feet depth, and it is then enlarged to the full diameter by using a larger tool. This is done for convenience of working; for if the whole area were acted upon at once, it would involve crushing all the flints in the chalk; but, by putting a shell in the advanced hole, the flints that are detached during the working of the second larger tool are received in the shell and removed by it, without getting broken by the tool.
Figs. 185-187.