Puncher machines, like the other coal cutters, are used for under-cutting the coal, but they dull, often twice or three times per shift. The disc is mounted on a flat arm, and is driven by a gear-wheel which engages in radial teeth inside of the cutting edge. This construction ,permits the disc to enter the coal nearly its full diameter. The cutting wheels are some times arranged to be tilted so as to follow irregularities or avoid obstacles. Either com pressed air or electric motors may be used. The machines are heavy, weighing one to two tons, and are usually run on a track laid along the face, which also acts as a guide to resist the tendency of the machine to swing out of line from the action of the cutter wheel. Some times the machine slides on the floor without rails, and is guided between the face of the coal and the first row of props. The machine is pulled forward by a wire rope anchored at one end and attached to a windlass on the machine.
The machines require from 15 to 30 horse power and average 8 to 12 square yards of undercut per hour, 3 to 7 feet deep. The kerf is 3,/, to 51/2 inches high and 3 feet to 5% fect deep and is frequently made in thc underclay. The tendency seems to be toward the use of heavier and more powerful machines and to the use of motors permitting a heavy overload. At times disc machines may use as much as 50 to 100 horse power from friction on the disc or bind ing from the weight of the coal. The coal as it is undercut is supported at frequent intervals by sprags to prevent the cutting wheel from being caught and held by the premature settle ment of the coal.
Bar cutters have been much improved in recent years, and are now powerful and efficient machines. While generally driven by electric motors, compressed air has been used. The cutter bar, from 5 to 8 feet long, is tapered from 3 inches to 7 inches in diameter and has spiral ribs which act as a screw conveyor to remove the cuttings. The bar revolves 300 to 400 times a minute and has a reciprocating motion of two inches. The steel cutting bits are pointed and are inserted at intervals in the spiral rib. Sometimes they are inserted in the hollow between the Tibs 'when a narrow kerf is desired. The kerf in any case is wider than with a disc machine. Sometimes this may be an advantage. The machines weigh frorn one to two tons and have motors of from 18 to 26 horse power. In some of the English machines the bar can be raised and lowered while at work and can be swung through an arc of 180 degrees. The American machine* in Fig. 7 is a short-wall machine and is shown loaded on its truck for transportation. At work it slides on the floor
and is moved about by its own windlass and chain. Bar machines have about the same capacity as disc machines, 5 to 15 square yards per hour. A bar machine permits close sprag ging and is not so likely to be caught by the weight of the coal as a disc machine, and will free itself with less expenditure of power. Bar machines follow irregularities and avoid ob stacles in cutting, more readily than disc ma chines. They can be used for undercutting in narrow rooms or entries. Bar machines operate with less noise than any other type of coal cutters.
Chain machines practically divide the field with puncher machincs in this country, with a tendency toward the adoption of the chain for new installations. In these machines the cutting bits are inserted in alternate links of an endless chain, which runs in guides on the cutter arm. As in other coal cutters, provision is made for easily replacing worn bits. In the breast chain machines the cutting takes place at the end of the cutter bar, in other types of chain machines at the side of the cutter bar. Fig. 8 is a chain breast machine. It is mounted on a fixed frame, at right angles to the face of coal to be under cut, and slides forward cutting its way into the coal for the desired depth. The machine is then withdrawn, the frame moved to a new position and a second undercut made alongside of the first. This type of machine is best adapted for narrow work in small rooms and entries, but even for this work is being super seded by the short-wall machine.
Figure 9 is a chain breast machine as modi fied for a vertical or shearing cut. It is raised and lowered on the standard by its own power. A coal cutting and loading machine* has been developed which consists of a breast machine for undercutting, combined with two shearing machines and a battery of puncher machines. The machine is mounted on a carnage running on rails and is provided with an endless belt conveyor and car loader. This machine will cut and load into mine cars at one operation a block of coal 5 feet wide, 7 feet deep and tne slides and swings about on the floor of the working place by its own power. A separate power driven truck is used to move the machine from one working place to another. The load full height of the coal seam, up to 7 feet. It is said to have a capacity of 75 to 100 tons per eight hours: It can be used for driving entries 10 feet or less in width at the rate of 20 feet per shift of eight hours.