Practical Treatise on Milling and Milling Machines/Chapter 1
chapter i
Classification of Milling Machines
The existing types of milling machines are so numerous, and their designs merge into one another to such an extent, that it is very difficult to classify them definitely. But, taken as a whole, they may be said to consist of two distinct groups, those adapted to a variety of work, and those restricted to the performance of a single operation, such as gear cutting, bolt head milling, thread milling, etc. While this latter group embraces some valuable and interesting machines, the class of work done is of a more or less special character, and little can be learned from it of the general process of milling. For this reason, and also from the fact that it would be practically impossible to treat of every type in the limited space of this book, the first group alone will be considered. The machines of this group are classified in a variety of ways by different writers. We prefer to divide them, according to general appearance and design, into three classes, comprising the column and knee type, manufacturing type, and planer type. Such a classification brings out the characteristics of the different machines, and their relation to one another.
Column and Knee Milling Machines
An illustration of a representative example of the column and knee type of milling machine is shown on the opposite page. This machine is the most recent of the three types named, having been in existence about fifty years. The rapid strides, however, that have been made within the past few years in the process of milling are largely due to its versatility and convenience. Even with the most expert cutter making, milling could never have obtained its important position in the field of machinery and tool manufacture had it not been for the column and knee type of construction.
The name, column knee, is derived from the high, column-like design of the main casting, and the likeness of the bracket which supports the table to a knee or angle iron. The knee is adjustable on the column so that the table can be set at different heights to accommodate work of varying size. It can also be fed upward, thus enabling vertical cuts to be taken. Provision is made for movement of the table horizontally in two directions: one, longitudinally, at right angles to the axis of the spindle; and the other, transversely, parallel to the axis of the spindle. The combination of these three movements is found only in the column and knee machine, and it is due to the advantages derived from this construction that the machine is superior to the manufacturing or planer type for general milling purposes.
Several more illustrations of column and knee machines are shown on succeeding pages of this chapter, where a further classification is given.
Manufacturing Milling Machine
This type of milling machine is shown in the illustration on the opposite page. It is a development of one of the earliest forms that was built particularly for use in the manufacture of small parts of firearms, and has since been successfully adopted for machining parts of sewing machines, typewriters and other machines and tools. The advantages it offers for this class of work are due to the stiff construction and convenience with which it can be operated. These make possible an exceptionally large production of first quality work—factors of great importance in commercial manufacturing.
There are many minor variations of this type of milling machine, but the general features are similar in all. in that shown on the opposite page, the spindle is supported in bearings located in an adjustable head that can be raised and lowered. The capacity of the machine is rather limited as regards work of widely varying heights. Furthermore, there is no transverse table feed, the only movement transversely being obtained by a slight adjustment of the spindle. These, however, cannot be considered disadvantages, as provision for work of widely varying heights is not required, because all work done is of comparatively small dimensions, and there is seldom any necessity for a transverse table movement.
The longitudinal movement of the table is at right angles to the axis of the spindle. This movement is accomplished either automatically or by hand by means of a rack and pinion on the under side of the table. The pinion is driven from the spindle through a train of change gears and a worm and wheel when the automatic feed is in action.
A larger and improved style of manufacturing machine is shown on page 88. It embodies all features of the machine illustrated 
Milling Machine of Manufacturing Type
Planer Milling Machine
The planer milling machine is designed for the heaviest classes of slab and gang milling. It bears a marked resemblance to the planer, from which it derives its name. The spindle is mounted in bearings carried in a vertically adjustable slide similar to that of a planer, and the table is in a corresponding position This brief reference will enable one to easily distinguish these machines. And, as the class of work performed is identical in character, only heavier than that done on the column and knee type of machine, the same principles are involved.
Returning to the column and knee type, we can subdivide it into three classes, known as Plain, Universal, and Vertical Spindle Machines.* In the first two the spindle is supported in horizontal bearings that are fixed in the main casting of the machine instead of being adjustable vertically, as in hte case of both manufacturing and planer types of machines. This is one of the points where the column and knee machine is radically different from either of the other types. As we have already explained, vertical adjustment in this type is obtained by the movement of the knee upon the column. In the vertical spindle machine, the spindle is supported invertical bearings, verticle adjustment being obtained by the movement of both the knee and the spindle.
Plain Milling Machine. The word plain when applied to any milling machine is used to designate one in which the longitudinal travel of the table is fixed at right angles to the spindle. Both manufacturing and planer types are therefore essentially plain milling machines.
An illustration of a plain milling machine of the column and knee type is shown on page 19. In this machine, the table has the three movements: longitudinally, transversely, and vertically, that have already been mentioned. Some machines have both power and hand feeds for all three of the movements; others have longitudinal and transverse movements so controlled and the vertical is operated by hand; or the longitudinal movement alone is operated both by power and by hand, and the transverse and vertical movements are made only by hand. Lead screws are used for operating all of the table movements in many of the smaller sizes and all of the larger machines, but in some of the smaller ones a rack and pinion are employed for the longitudinal movement. The smallest sizes of machines have no power feeds at all, and are called hand milling machines. (See illustration on page 46.) In these, the table and knee are moved by means of racks and pinions operated by levers. They are convenient for manufacturing purposes on some classes of small work, as they can be operated very rapidly.
It is the practice in the classes of work to which the medium and larger sizes of plain milling machines are adapted to take heavy cuts at fast speeds and coarse feeds. The rigid construction of the machine enables this to be successfully done, and it is in this ability that the chief value of the plain machine is found.
Universal Milling Machine. The Universal milling machine is justly regarded by many to be the most important machine tool employed today; for with it much of the work of the planer and shaper—heretofore considered indispensable machines in every shop—can be done with an appreciable saving of time. Spur, bevel and spiral gears, twist drills, and all kinds of straight and taper milling can also be economically produced.
It was first patented February 21st, 1865, by Mr. J. R. Brown, of the firm of J. R. Brown & Sharpe, who designed it for the purpose of milling the grooves in twist drills, but adopted it shortly after for producing small spirals used in the manufacture of sewing machiens. (An illustration of the original universal milling machine is shown on page 6.)
The cuts on pages 10 and 44 are representative of modern universal milling machines. This style of machine is essentially the same in construction as the plain milling machine, and the table has the same movements. But, in addition, the table swivels upon the saddle and can be set at an angle to the spindle in a horizontal plane. Also, it is fitted with a mechanism known as a spiral head, for use in spiral milling and indexing to obtain any required spacing on the periphery of work. The introduction of the swivel renders the table a little less stable than that of the plain machine, though in common practice heavy cuts are taken. It is apparent, however, that the offices of the two machines are in a way distinc. A universal machine is the better for general shop purposes, but where continuous heavy milling of straight cuts is to be done the plain machine is preferable. 
Not all vertical spindle machines are of the column and knee type. There are several styles that have no provision for vertical adjustment of the table. Also some vertical spindle machines have two spindles instead of one, but these are more generally known as profiling machines.
But the combination of the vertical spindle and column and knee constructions has given the mechanical world an exceptionally valuable machine tool. With it, all of the advantages of the vertical spindle, together with those of the column and knee, are acquired. A modern example of this style is shown on the cut on the opposite page. A further convenience of this machine is found in the spindle head, which is adjustable vertically, and can be fed by power, thus enabling drilling to be conveniently done. With the adjustable spindle head and column and knee construction, it is apparent that work of a wide range of heights can be accommodated. Another syle of vertical spindle machine, where the spindle is driven by a belt, is shown on page 36.
Different methods of Driving Milling Machines
Milling machines of the column and knee and manufacturing types are either cone driven or gear driven. The latter class is more commonly referred to as the "constant speed drive."
Cone Drive. In cone driven milling machines, the belt runs directly from a stepped or cone pulley on the countershaft to one of like design fastened, either directly to, or mounted on a sleeve on the machine spindle. In one case the spindle is driven directly and only speeds that are obtained by shifting the driving belt on the pulley steps are available; while in the other an additional series of speeds is procured by the employment of back gears. The cut on page 10 is of the latter type, and the back gears referred to are enclosed at the front of the column, where they are rigidly mounted closely together to overcome torsion and cutter chatter. The feeding mechanism is driven from the rear end of the spindle by a chain and sprockets, and is subject ot the speed variations of the spindle.
When the cone method of drive is employed for vertical spindle milling machines, the belt usually leads from the cone pulley on the countershaft to one on a shaft at the back of the machine. Power is transmitted thence to the spindle on the lighter machines, by means of a quarter-turn belt. An application of this method of drive is shown in the illustration on page 36. The heavier machines are fitted with bevel gears, and a vertical shaft from which the spindle is driven by a chain and sprockets.
Constant Speed Drive. The invention of the gear type of drive, or, as it is better known, the "constant speed drive," is, without doubt, the most valuable improvement in design brought out in many years. It is the result of a demand for a machine in which the feeds would be entirely independent of the spindle speeds, and all speeds and feeds would be self-contained, thus doing away with complicated overhead works, or permitting the machine to be driven by a constant speed motor. More power and greater convenience in changing speeds and feeds were also important factors leading to the development of this type of drive.
The introduction of high speed steel marked a new era in cutter manufacturing, and brought about conditions that necessitated machines of higher efficiency. This added impetus to the already growing interest in a machine offering possibilities such as those of the constant speed drive, and, early in 1904, the Brown & Sharpe Mfg. Company placed the first constant speed drive machine upon the market. From the beginning, it was conceded an important improvement, especially for the larger sizes of heavy service machines, where an abundance of power is required, and this has led to its becoming almost universally adopted by milling machine manufacturers. Several examples of constant speed drive machines are shown in this treatise, notably those illustrated on pages 16, 19, and 44.
The general features of this drive are as follows: the belt delivers power to the driving pulley that runs loose on a sleeve on the main shaft of the machine. By means of a friction clutch on the main shaft, operated by levers at each side of the column, power is transmitted from the driving pulley to a train of hardened gears 
The mechanism of constant speed drive vertical spindle machines is essentially like that outlined above, except that a pair of bevel gears and vertical shaft are introduced to transmit power to the spindle head, from whence it is communicated to the spindle itself by spur gearing.
The feed changing mechanism is driven from the main shaft by means of a chain and sprockets in all constant speed drive machines. Hence it is completely separated from the spindle drive, in so far as its speeds are concerned, permitting the full range of feeds to be available for every spindle speed. Such an arrangement also permits the table feeds to be rated directly in inches per minute, which is an advantage in that it enables the production of a machine to be ascertained at a glance.