The amount of work a machine is capable of performing in a given time can be expressed in tons only when the thickness of the vein and the amount of impurities in the shape of partings, bony coal, or slate, etc., are known. A better method of designating the amount of work the coal-cutter is capable of performing in one clay is by giving the number of cuts it can make, or the number of sq. ft. it can undercut. This daily work, of course, varies some what with the nature of the coal, whether the latter is hard or soft, or contains sulphur or bastard, the width of the workings, and the territory to be covered by one machine. The largest record so far made with rotary coal-cutters is said to have been 52 cuts m ten hours, or 950 sq. ft. undercut. The average work in the same mine in wide workings is 35 cuts, or 645 s . ft., for narrow and wide workings 80 cuts, or 555 sq. ft.
When handled by expert men, and with not too hard coal, machines can make about 30 to 35 cuts a day in from nine to ten hours, making it necessary to prepare at least four rooms for each to work in.
With the exception of one type, all the rotary coal-cutters used in America are fastened down in proper position at the face of the coal to be undercut. They then make a cut in the coal to a certain depth, and of a width depending on that of the cutting device. The latter• is then withdrawn, and the whole machine moved sidewise, and placed in position to make another cut adjoining the former. The time consumed in shifting the machines averages about 11 min. To reduce this lost time as much as possible, it is advisable to undercut as many square feet as possible with one setting of the machine. There is, however, no advantage in making the cut deeper than the vein is high—that is, in a 5-ft. vein the ent would ho 5 ft. deep. us otherwise the coal will not " shoot " down properly and tumble over. If the coal simply settles down in its former place, it is in a worse condition for mining than if it had not been undercut. Neither is it advisable to make the machines longer than required for• the 6-ft. cut, as they would become too unwieldy. It is necessary to make the cut as wide as possible. so as to reduce the number of times the machine has to be shifted to cut the coal in a room of a certain width.
Handling llachin13.—Coal-cutters aro generally handled by two men only, and for this reason it is necessary to reduce the weight of the machines as much as possible. It must also be borne in mind that they are not only handled very roughly, but have to do very hard work, being at times forced through coal containing stn•all streaks of sulphur. or other impurities, harder by far than the coal itself. Should these foreign substances occur very frequently in the " bearing-in seam "—that is, in that part of the coal in which the undercut is to be made— the reciprocating cool-cutters, of course, would be the proper machines to use. If, however, only small streaks of sulphur occur, the rotary coal-cutters are generally forced through them.
The main feature of a successful coal-cutter is great strength. To show that this is of far greater importance than lightness, the record is given of the time required to shift a 3,000-lb. machine. 36 seconds being the average time in six tests to shift the machine from one position to another. This, of course, is exceptionally quick, and it is not to be expected that men
would be able to keep it up all day. This machine is probably the heaviest on the market, the motor alone on it weighing about 1,700 It is hardly reas,mable to expect that the machine can be shifted in less than a minute and a half as average for a day, no matter how light it is made, and this is being easily accom plished by expert men with utichines having the abnormal weights given above.
To convey the machines from room to room they are mouuted on small trucks and hauled by mules or horses from one place to the other. These trucks are generally provided with a suitable winch and chain, by means of which the machines can be readily loaded. The average time to do this is about 2 min. 45 sec.; the average time to unload the coal-cutter is 2 min. 35 sec.; and to get the machine ready for the cut will take 3 min. A quick record for this work is 1 min. 45 sec. to load. 1 min 30 sec. to unload. 1 min. 26 sec. to set and get ready for the cut, The time required to move the machine may be estimated as from 40 to 50 sec. for each room between the one cut and the one to be cut, although it may take all the way from 10 min. to an hour before a mule can be secured for this work. A truck so constructed that it can bs operated by electricity in mines using the latter for power purposes is, therefore, very desirable.
Reciprocating sdcond style of machine used in America is the recipro cating coal-cutter. This is not capable of quite as rapid work as the rotary cutter. It has, however, some features which make it well adapted to certain kinds of coal and certain con ditions. It has already been said that when the quantity of sulphur or similar substances is not too great in the bearing the seam of the coal, the rotary cutter can he need. Should sulphur occur in large quantities. and in the shape of what is called -sulphur balls." or "nigger-heads." it will be necessary to use reciprocating cutters. Another reason for using the latter machine in preference to the former in small veins can be found in the following: In certain districts the miners are paid for the amount of lump coal mined. The small sizes of coal which pass through the screens having bars from 11 to 11 in. apart—namely, nut. pea coal, and slack—are clear profit to the operator. In these districts the royalties on the coal are also paid by the amount of lump coal mined. Whenever the small grades of coal. there fore, have a good market, it may be to the advantage of the operator to get out as much of these sizes as possible; and this can lie done by means of the punch ing or reciprocating cutter, All the coal coining out of the cut made by the rotary machine is in the form of fine slack, and is not marketable; that coining out of the cut made by the punching machine is generally in the shape of nut or pea coal. It is also necessary to make the height of the cut with the latter machines higher than that made by the rotary machine, to enable the tool to enter it and to undercut the coal to the proper depth. We present various improved forms of drills and coal-cutters.