The combination-socket (Figs. 178 and 179) accomplishes the same class of work as the slip-socket, and has even greater strength. It differs in construction from it in having either three or four slips, filling a complete circle on the inside and held down by a coil spring instead of being part of a rein. The larger number of slips permits the lost tool to be grasped more fully, and, as in the slip-socket, the hold of the slips increases with the strength of the pull applied. When using the combination-socket, however, the exact size of the body to be caught must be known, -because of the close fit of the slips, while a considerable range of sizes may be caught with the same slips in a slip-socket. For this reason, when doubt exists as to the size of the tool to be caught it is preferable to use the latter. A further advantage of the slip socket is that when the lost tools have been pulled from the hole, the rein-slips are much more easily disengaged from their hold than are the slips of the combination-socket.
These sockets are both of the bulldog type, i.e., when they have once taken hold of the lost tool they are not easily released. However, in many cases this may be done, when it has been found impossible to move the lost tool and it is desired to release the fishing-string, by what is known as 'jarring both ways.' The walking-beam is given such a stroke that a jar is applied at the contact of the slips with the lost tool on both the up and down strokes of the fishing-string, eventually either pulling the socket free from the lost tool or smashing one of the slips, thereby loosening the hold.
When this does not succeed in loosening the fishing-tools and it is considered advisable to withdraw the drilling line, leaving the tools in the hole, the line may be cut by one of the several forms of rope-knives. These are run into the hole on the end of the sand-line, and are simple affairs that consist essentially of a frame, surrounding the line to be cut, and a strong chopping blade. When the frame has been lowered until the tool rests on the rope-socket of the fishing-string the blade is driven into the line by raising and lowering the sand-line, which drops a metal block on the blade, forcing it diagonally across the drilling-line.
A tool used especially when the drilling-line has pulled com pletely out of the rope-socket, instead of having broken off at the top of it, is the tongue-socket (Fig. 180), containing a mandrel with slip to run into the opening from which the wire-line has escaped, and a slip inside the main body of the tool for grasping the neck of the socket.
Occasionally one of the joints between the tools in a drilling string may become unscrewed, leaving the pin of a stem, sinker or set of jars pointing up. In such a case either the combination or
slip-socket may be run, unless the body of the tools occupies so much of the space inside the casing that no room remains for the socket to pass over and grasp it. Tin-slips' to be used in a combina tion-socket are made for such a condition, with an inside thread conforming exactly to the threads on the pin of the lost tools. When the socket is lowered, the slips fall around the threads on the pin, meshing with these, and hold it while, the tools are pulled out. This method is not applicable when the tools are lodged so tightly that they must be jarred before they „become free to move.
When the pin-slips will not pull the tools, or the latter have broken at a point where they occupy the entire inside of the casing, it is necessary to cut away an outside portion of the top of the lost tools with a milling tool (Fig. 181). This is run in on tubing, which is suspended from the surface on a specially-constructed jack that holds it as casing is held by a spider and slips, and at the same time permits it to turn readily on a set of rollers. The tubing is turned by a large wheel driven by power, and is gradually lowered by means of the jack as fast as the exterior of the lost tool becomes milled, until a sufficiently long pin has been cut to permit an ordinary socket to grasp it (Fig. 182).
The points that become weakened and break most frequently in a string of drilling-tools are at the joint of the drilling-bit with the stem and directly above this a few inches, where the box of the stem is welded to the stem proper. Breaks of this kind are liable to cause considerable difficulty when the top of the lost tool has become burred and damaged by the subsequent blows delivered before the accident is detected, and also because the bit, or box-end of the stem if the break occurred at that point, is below the bottom of the casing and tends to fall off to one side of the hole (Fig. 183). For this reason it is preferable to use as long bits as possible, many operators never running them when they are worn down to a length of 4 ft. If the bit fortunately remains erect it may be recovered with a slip or combination-socket, provided the top has not been deformed by the pounding to such an extent that it will not pass up inside the slips. If this has happened, a side-rasp (Fig. 184), or two-wing rasp (Fig. 185) must be swung up and down on the end of the fishing-string until the irregularities have been milled away.