The rotary machines for boring blast holes are known as coal augers and are made in many forms, driven by hand, compressed air, or elec tricity, and mounted on bars, columns or other forms of support, or simply held in place by hand.
Figure 1 is an auger drill operated by .com pressed air. The machine itself weighs but 24 pounds and requires 18 cubic feet of free air per minute at 80 pounds pressure. Where the coal is soft it may be operated as a breast drill. For harder coal or soft rock it should be provided with a feed screw and column sup port. These small power drills are also made to be driven by electricity, direct or alternating current, and of different sizes and weights, from 24 pounds to 125 pounds for the machine alone and from one-half to three horse power. These auger drills are not adapted for use in very hard rock but can be used in bituminous coal, anthracite, rock salt, gypsum, fire clay, shale, slate and similar rocks of medium hard ness. In such material they will bore from one to eight feet per minute.
Figure 2 is a one and one-half horse power electric auger drill mounted on a column, with a differential feed screw, by which the rate of boring can be adjusted to the hardness of the rock Figure 3 shows example of the cutting edges of these auger bits; (a) and (b) are the shapes usually employed; (c) is effective in soft material, and (d) is one of the forms used for larger holes four or five inches in diameter. For boring deep holes in contracted workings, the twisted rods are made in sections of con venient lengths with socket joints.
Air hammer drills, with automatic rotation, and twisted cruciform drill steel with cross-bits, have been successfully used for boring blast holes in anthracite coal. These drills are more effective than augers in anthracite and can be used in hard rock. The twisted steel acts as a twist auger for discharging the cuttings from the hole.
The puncher machines (Figs. 4 and 5) are used instead of a pick for cutting coal. They are designed like machine drills, the cutting tool being fastened to the end of a piston rod driven are also used for shearing the undermined block of coal, that is, for cutting a vertical kerf or deep channel in the middle of a long block, or at both ends of a small block of undermined coal. Puncher machines are also to be pre ferred in hard or slaty coal, or where hard lumps of iron pyrite occur in the seam. Puncher
machines are especially well adapted to narrow work, that is the driving of entries or narrow rooms.
Puncher machines for undercutting are mounted on wheels, as in Fig. 5. For shearing, wheels of larger diameter are used. A better method is to mount the machine on a column, with an arc and pinion support operated by a crank by which the puncher can be swung from side to side for undercutting, or up and down for shearing. In England and on the Continent puncher machines are frequently mounted on cars. A heading machine for driving entries has been perfected.* It consists of a large puncher machine mounted on a heavy frame capable of horizontal and vertical swing so as to command the whole face of the entry. A con veyor and car loader disposes of the coal as it is mined. The rate of advance in an average entry is said to be 10 feet per shift of eight hours.
The cutting bits used with the puncher ma chines have three to seven points, either forged from a single piece of steel or inserted in a head. The points have chisel or fish-tail edges. (See Fig. 6).
Disc coal cutters operate like a circular saw. They are used chiefly for undercutting long faces of coal or in long-wall work. Disc ma chines are not often used in this country, but are much favored abroad. The disc is armed ith cutting chisels, which are replaced when at high speed by compressed air. Puncher ma chines are also driven indirectly by electric power, which is used to compress the air, either In a portable compressor which is brought into the working place, or the air is compressed tn the cylinder of the puncher machine itself. The first plan has been more successful.
Coal punchers are practically machine tools and possess great adaptability for work under difficult conditions. About 40 per cent of the coal mining machines now in use in this country are of this type. Disc and chain ma chines are less flexible and can be used only under conditions that are favorable. Disc and chain machines are generally used for under cutting only, that is, for cutting a kerf or deep grove under the coal so that the mass of coal above can be brought down by wedges or by a few blast holes.