This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
As to the kind of lubricant to be used, it will vary with the kind of metal to be machined and its condition. Cast iron will require no lubricant. In fact it is probable that any kind of a lubricant would be a detriment rather than a help when turning cast iron. The same may be said of ordinary yellow brass castings and the usual kinds of sheet brass, brass rods and tubes. But for bronze, and similar alloys containing a considerable portion of copper, it is always advisable to use a lubricant, and, if very hard and tough, oil is the proper lubricant. This is also true of the turning of wrought iron, malleable iron and steel, or steel castings.
As to the kind of oil most appropriate, it is well known that lard oil leads all others. On account of its high price, this oil is often replaced by a mixture of lard and other animal oils or fish oil. Mineral oil should not be used, as it fails to prevent the heating of the work and the tool. Neither should a mixture of animal and mineral oils be made for such a purpose.
For reasons of economy certain soapy mixtures are sold for these purposes. These are mixed with water to a consistency to flow freely, and often answer the purpose nearly as well as oil. They are more convenient and cleanly to use around the machine.
While it is convenient to purchase these compounds, a good one is easily made by boiling for half an hour or more one half pound of sal soda, one pint lard oil, one pint soft soap, and water sufficient to make twenty quarts. The soda should first be dissolved in the water, and the oil and soap added successively while the mixture is hot. Should the mixture prove too thick to run freely from a drip can, or to pass through a lubricating pump, hot water should be added until the desired consistency is obtained.
Any purchased preparation of this kind that has a tendency to rust the cast iron parts of the machine should be rejected, as it contains either acid or an excess of soda, and sometimes, even potash, all of which will be detrimental to the work and the machine as well as to the efficiency of the operations being performed. Trouble will also be experienced with the pumps and pipes from becoming clogged by the undissolved portions of the compound.
As to the means used for applying the lubricant, the first and most simple is a small, round bristle brush. This will answer well enough for short jobs and for small parts, but for larger work is rather a tedious process, requiring the constant attention of the operator, and thus limiting him to the work of a single machine. Oil is the lubricant usually applied with a brush.
The gravity feed comes next in order. This is simply a "drip can," which is supported by a rod attached to the rear of the carriage or compound rest. This can, holding a quart or more, is provided with a bent tube having a faucet, or stop-cock, attached at or near the bottom. It is kept filled by the operator pouring from a tray under the work such portions of the lubricant as drip off the work.
As a constant stream of lubricant is always desirable, however large or small it may need to be, a small pump is resorted to. A small tank is located under the machine or near it, from which the pump draws its supply of the lubricant and forces it up through a jointed or flexible pipe to the tool. Its flow is regulated by a stopcock as in the gravity feed.
The tank is usually made of cast iron, and is divided into two parts by a vertical plate reaching up to within two or three inches of the top of the tank. The lubricant, as it flows from the tool, carries with it many fine chips which flow into one of these compartments, where the chips fall to the bottom while the lubricant fills the compartment, flows over the vertical plate and into the other compartment where the clear liquid is drawn off by the pump. This method is an improvement over the perforated metal plate or the wire gauze strainer.
Usually these pumps and tanks may be purchased independently of a machine and attached in any manner desired. As the pumps are usually of the rotary type, they may be driven from a small pulley on the countershaft of the lathe if no special provision for them has been made on the machine itself.
While these lubricating devices are usually more appropriate for a turret lathe or similar machine than for an ordinary engine lathe, yet the class of work and the kind of material to be machined will be the deciding factor more often than the type of machine.
It will be often desirable to know the power which is being consumed in operating a lathe on certain work for which data is required. For most purposes this can be sufficiently approximated by calculating the power of the lathe from the width of the belt and its speed in feet per minute.
For such purposes it is usual among mechanical engineers to consider that a one-inch belt traveling a thousand feet per minute will transmit one horse-power. This will give us a key to the entire calculation.
For instance, if we have a piece of work 6 inches in diameter, we know that for every revolution it will move through a distance equal to its circumference, that is, 18.85 inches. If the cutting speed is 30 feet, or 360 inches, we can easily calculate that it must make 19.6 revolutions per minute. If the back gear ratio of the lathe is 12, and we are using the back gears, the cone must make 12 times as many revolutions as the piece of work, or 235.2 revolutions per minute. If the step of the cone on which the belt is running is 19 inches, it will be practically 60 inches circumference, or 5 feet, and therefore the belt speed will be 1176.6 feet per minute, or 1.176 horse-power for every inch in width of belt. Now, assuming that the belt is 4 inches wide, we shall be using 4.7 horse-power, if we force the cut up to the full capacity of the belt to drive it.
This calculation is for single belts. A double belt is expected to transmit double the power.