This section is from the "Modern Machine Shop Construction, Equipment, And Management" book, by Oscar E. Perrigo. Also see Amazon: Modern Machine Shop Construction, Equipment, And Management.
We may say, therefore, that it is more economical to transmit power to distances of over one hundred feet by electricity than by shafting and belting. Large machines that are not in nearly constant use should be motor driven, as power is only used when the machine is in actual operation, while in the use of line shafting it must be kept in order, and we must use the power necessary to drive it continuously, even if there is only one machine of the group in operation.
Where machines are driven from a long line shaft, it should be run at sufficient speed to permit the use of pulleys of moderate diameters. Slow running shafts require to be of comparatively large diameter and the pulleys much larger and heavier, consequently the friction is greater and the power must be increased in proportion. If a shaft 3 inches* in diameter is located in the center of the room, driving machines on both sides, its speed being 150 R. P. M. and requiring pulleys from 18 to 36 inches in diameter to drive the machines, it will be found much more economical to replace it by two shafts of 2 inches diameter, and running 300 R. P. M., on each side of the room and carrying pulleys from 9 to 18 inches in diameter. The aggregate weights of the two shafts and their pulleys will be much less than that of the one large shaft, the belts may be shorter, narrower, and lighter, and consequently the power much less.
* Regular sizes of shafting are " on the odd sixteenth," that is 21/1⅝, etc. The even inch is here given for convenience only.
The weights upon line shafts will be materially reduced by the use of the pressed steel pulleys. This will apply to all pulleys of ten inches and larger. A still further economy will result by driving each of these two shafts by a motor. This would also permit the stopping of either of them in case of an accident, without interfering with the other.
In cases where the factory is of several floors and the power transmitted by vertical belts, the driven pulleys should be of the friction clutch form for the purpose of throwing out any one shaft without stopping any of the others. All shafting of two inches or over should be provided with roller bearings, preference being given to those of the flexible type. An automatic system of lubrication for shafting has become a necessity and will be a great saving of power. The simple form of a reservoir beneath the bearing, from which the oil is drawn through a piece of felt, and returned by way of grooves at each end of the box, is an excellent and economical device, although there are many others equally as efficient.
The location and relative arrangement of machines is a matter of much importance. If the department contains a variety of quite small and medium sized machines, the smaller ones may be well arranged on benches along the walls where the light is always good. The bench may be very useful for holding stock and material in process of manufacture. This method will leave the central portion of the room for larger machines set up on the floor, and still leave ample passageways in a building of the usual width, say forty feet. This would allow, on each side, a bench two feet wide, another two feet for the operator, behind whom would be a passageway six feet wide, and still leave a 20-foot space in the center for larger machines, and a central space for a passageway, car track, etc.
Frequently machines set on wall benches may be driven from a very light shaft located only three or four feet above the bench, thus eliminating about 50 per cent of the lengths of the machine-driving belts, and still leaving them long enough for good service. Machines of the same general type and doing the same class of work, or consecutive operations upon it, should be grouped together, for convenience in handling the stock in its continuous progress.
The degree of efficiency of machines operating on cutting metals may be very materially increased by careful experiment and study of speeds, the best qualities of tool steel for the particular purpose, and the form of the tools. It is not good economy to force cutting speeds to the highest limit, even with the best high-speed steel. The point to be determined is the highest economical speed for the metal operated upon, the form of the piece being made, and the particular machine used for the work. And however much we may experiment in this matter we shall probably never arrive at any fixed rule of say so many feet per minute for machine steel, so many for cast iron, steel, brass, bronze, etc.
Much will depend on the design of the machine, the manner of holding the tool, as well as the method of holding the piece to be machined; the whole combining to give rigidity and prevent vibration, both laterally and in the direction of motion, as even the slightest vibration of tool or work will reduce the possible cutting speed.
Neither shall we arrive at any fixed angle of side clearance or top rake for a cutting tool, inasmuch as that it depends to a considerable extent on the form and rigidity of the machine used, the nature of the cut, etc.
The operations of milling, drilling, and tapping, as well as nearly all similar operations, will require the same observation and experiment to arrive at the best, most efficient, and economical speeds, in view of the individual conditions governing the work.
Where the work is heavy enough to warrant it, there should be shop tracks, let in flush with the floor level, upon which should run shop cars of such dimensions and weights as their loads may demand, and of such form as may be necessary to adapt them to the particular form and character of the stock and material to be handled. For this purpose they may be provided with racks, shelves, trays, boxes, crates, etc., as may be needed; these accessories being readily removable so that others may be substituted. In a factory of several floors, the elevators connecting them should be provided with similar tracks, so that cars may be conveniently run upon them and taken to any desired floor.