Bar Machines. — In this class of machine a tapered steel bar, armed with cutting tools set round its edge, is used for undercutting the coal. The bar is usually fitted with a spiral groove which acts as a conveyor, worming the cut material from the back of the holing. This bar is usually driven by a motor carried on the frame of the machine, and is made to revolve from 400 to 500 times a minute. In some cases a slight to and fro motion is imparted to the bar, which produces a chipping action with the cutters. The drum carrying the rope by which the machine hauls itself along the face is driven by means of a ratchet and pawl, the feed of which can be varied to suit the conditions of the cutting. The bar is usually fixed to a turntable, which allows it to be swung in and out from the face of the coal, and thus the machine is enabled to cut its way into the coal at the commencement of the run, and out again at the end. This type of machine is well adapted for working in a soft, friable coal, which tends to fall readily when holed, or for cutting at different heights above the floor of the seam. A machine of this type is shown in Fig. 42.

Disc Machine. — In this type of machine the cutting tools. are carried on the periphery of a wheel or disc, varying from 3 feet to 6 feet in diameter. This is carried by a suitable bracket that projects from the side of the frame which carries the engine or motor and the necessary speed reduction gear. A feed gear consisting of a small drum, usually driven by means of ratchet gear, carries a length of steel wire rope, which passes to a prop firmly fixed between roof and floor. This rope is threaded through a small wheel attached to this prop and brought back and fixed to the machine. When the machine is set in motion the cutting wheel or disc revolves, the machine at the same time being dragged forward by the rope. This causes a narrow channel to be cut under the coal, and this cut is carried along as the machine advances. This type of machine is well adapted for hard, continuous cutting either in the coal at the bottom of the seam or in the underlying material forming the floor. A machine of this type, electrically driven, is shown in Fig. 43.

Chain Machine. — In this type a chain is used to carry the cutting tools. The chain is mounted upon a carrying jib, which is fitted to the rear end of the machine, and so supported that it can be swung through an arc of 180 degrees. With this arrangement the cut can be made upon either side of the machine, and by swinging round the jib and turning the machine, it can cut its way into, or out of the coal, in the same manner as the Bar Machine. The feed gear for dragging forward the machine is of the same character as in the Bar and Disc types already described. This type of machine is not well adapted for cutting in hard material, but, given a soft and suitable holing, it performs its work very rapidly, and is not difficult to handle. A machine of this type suitable for longwall work is shown in Fig. 44.

Chain Heading Machines. — This type of coal-cutter is extensively employed in America for holing in pillar and stall workings. Only a small number are in use in British mines, and then usually for development work. The machine consists of a bed-plate which rests on the floor, and is secured in position by screw-jacks set against the roof both back and front. Upon this bed-plate rests a sliding chain-cutter frame, which carries the moving chain on which the cutting tools are fixed. The motor sits upon the top of the bed-plate, and is provided with driving and feeding mechanisms by means of which it drives the chain carrying the cutting tools round, and at the same time carries the sliding frame forward into the cut. After this frame has been extended its full length under the coal, it is withdrawn and the machine moved over its own width and the next cut commenced. A machine of this type is shown in Fig. 45.

Percussive or Punching Machines. — These machines are frequently spoken of as Punchers or Pick Machines, and are made in two different forms. One form is mounted on wheels and provided with handles, by means of which it can be swung round into any desired position while cutting. The other form is fixed to a rigid stand, the holing being performed by the operator moving the machine round in the arc of a circle. The action is the same in each case so far as the actual method of cutting is concerned, blows being struck on the coal by means of a suitable cutting tool, driven by means of a reciprocating piston. Machines of this class are best driven by compressed air. A machine of the latter type is shown at work in Fig. 46.

Conveyors. — These appliances are used for transporting the coal from the working face to a main roadway, where it is automatically loaded into hutches. This mechanical aid to underground work saves the miner much arduous labour, and at the same time prevents breakage of the coal. They are only suitable for use in longwall workings where the roof is good and the floor fairly hard. Sometimes they are worked in conjunction with coal-cutting machines, but as a rule the best results are obtained when they are employed by themselves. In such cases they are most successful in thin seams, where the coal is easily got by hand, and where the backing out or shovelling of it out to the road head entails a large proportion of the total labour of the miner. Under such circumstances their adoption usually means a higher market value for the coal, a considerably increased output, and a smaller cost for ripping and keeping roads, as a face of 100 yards or more may be worked with one main road in the centre and an air inlet at one end with a return at the other, three roads in all. The centre roadway is generally ripped in the floor, and made of such width as will admit of two lines of rails being laid ; those at either side are normal roadways with the usual roof-ripping. The total cost of ripping and upkeep is less than would be required for the same length of face worked in the ordinary manner. In seams of over 2½ feet thick, extra roadways may have to be formed for the purpose of obtaining sufficient material for packing, as this tends to keep down the consumption of timber and prevents sudden collapse of the roof. Various types of conveyors are in use, and are adapted to meet different underground conditions. These types are (1) Endless Belt or Endless Chain Conveyors, (2) Sliding Carriage Conveyors, and (3) Shaking Conveyors.

Allardice Conveyor. — This is an endless chain type, in which a chain with links formed from fiat iron bars 9 inches long is fitted with a scraper every 3 feet. The chain is extended the full length of the face, the top part being carried by props to which supporting pins are fixed, the bottom part drags along the floor. The chain is driven through a sprocket-wheel by means of an electric or compressed air motor, and as it drags along the floor, the coal is thrown between the scrapers, and drawn forward to the roadhead. A good hard floor is an essential to the successful working of this conveyor. Fig. 47 shows this conveyor delivering its coal at the roadhead.

Thompson Conveyor. — This conveyor consists of a number of sledges formed from angle plates about 18 inches wide and having a side 9 inches high, the open side being kept next the face. These sledges are joined together in 6-feet lengths, and drawn along the floor from end to end of the face by means of a wire rope driven by a suitable motor. A bridge is placed over the roadhead, and as the sledges conveying the coal with which they have been loaded on their passage along the face crosses this bridge, the coal is shed into the hutches placed underneath by means of a sloping knife. This conveyor moves at a slow speed, is easily shifted forward and easily kept in repair. Altogether, under favourable conditions, it is one of the best and most successful of its type. The bridge and discharging gear of this conveyor is shown in Fig. 48.

Summerlee Conveyor. — This is the only underground conveyor of this type, and is of very simple construction. It consists of a series of light iron troughs about 20 inches wide and 3 inches deep, made in 6 feet lengths, and joined together by an easily locked joint. The whole series is hung from chains fixed to cross bars set up to the roof, and a quick forward and slow return motion is imparted to them by means of elliptical gear wheels driven by a suitable motor. This conveyor will work on the level or on up or down grades, and delivers the coal, which is filled into the troughs along the face, direct into the hutches. It is best adapted for working in seams of 3 feet or more in thickness. By reversing the direction of the high speed, it can be used for carrying material obtained in ripping the roadway along the face for packing purposes.