Practical Treatise on Milling and Milling Machines/Chapter 2
chapter ii
It has been previously stated that the foremost advantages attending the employment of the milling machine are, the production of a great variety of work, and the exact duplication of pieces at an economical cost. In order that these advantages may fully materialize, it is necessary that many requirements be fulfilled in the design and construction of the machine.
These requirements vary to a certain extent with the style and size of the machine; taken as a whole, however, they are materially the same. The machines must all be accurate, economical to operate, and durable. Hence, these may be said to constitute the general requirements of a milling machine. Those qualities upon which accuracy is chiefly dependent are thorough workmanship, especially in aligning the working parts, and sufficient rigidity. In order to be economical in operation, a milling machine must have ample ranges of spindle speeds and table feeds, and plenty of power, so as to adapt it to the many varieties of work. Further, its efficiency must be high, and its parts must be conveniently arranged to allow quick manipulation and ready adjustment. The third general requirement, durability, is, to a great extent, dependent upon the design and quality of materials that enter into the construction of a machine. It is also influenced by several of the already-mentioned points that are essential to accuracy and economy. To particularlize then, the requirements of a milling machine are thorough workmanship, correct alignment of all working parts, sufficient rigidity, wide ranges of speeds and feeds, ample power, high efficiency, durability, and convenience in design and operation.
Workmanship. It is stated above that the dependence of accuracy upon workmanship in the building of a milling machine is of greatest importance in connectoin with the alignments of the different working parts. Correct alignments are most essential because they establish exact positions of the various parts with relation to one another. Any error in alignments is transmitted from one part to another utnil it is finally communicated to the piece of work, where it is liable to be multiplied. If the work is of the coarser grade, or mere roughing cuts are being taken, a few thousandths of an inch over or under size do not matter; but when finishing a piece that must come within close limits of a pre-determined size, a very small error is often sufficient to seriously impair its quality.
All of the important alignments in milling machines are obtained by scraping, a process consisting of going over the different bearing surfaces by hand with a chisel-like tool, and removing the highest spots until a maximum number of bearing points is secured. Flat bearings are scraped to conform to master surface plates and straight edges, and the boxes of important cylindrical bearings are scraped to fit the revolving piece, which is ground. This work necessarily calls for much skill on the part of the workman, and the care with which scraping is performed largely influences the accuracy of the resultant bearings.
Principal Alignments of Milling Machines. Broadly speaking, the principal alignments of all milling machines are those of the spindle and table. They are, of course, affected by various minor alignments throughout the machine, but it is not essential to take up each of these in detail. The alignments of the table on horizontal spindle column and knee machines should be such that its upward and downward movements will be perpendicular to the spindle axis. Its longitudinal and transverse movements should be in horizontal planes, the longitudinal being parallel to the face of the column on plain machines, and on universal machines when the table is set at zero; and the transverse at right angles to the column.
On universal machines, the table should also swivel in a horizontal plane.
These alignments of the table and spindle of column and knee machines are typical, and it is easy to understand from then what the alignments of other types of milling machines should be.
While we have emphasized the importance of good workmanship in scraping bearing surfaces, in order to obtain accurate alignmnets, it must be understood that certain elements in design are largely responsible as to whether the alignments remain accurate or not. A bearing surface may be scraped ever so carefully, yet the lack of sufficient weight in the casting, or of ample proportions of the bearing surface itself, will quickly result in the alignments becoming inaccurate. Thus it is apparent that if alignments are to be permanent, the proportion of the different parts, including the bearing surfaces themselves, must be ample to easily support the weight brought upon them. The accuracy of alignments can be ascertained upon first operation of a machine, but their permanency can be determined only after a considerable period of service.
Rigidity. This requirement is of just as great importance to the success of a milling machine as correct alignments. Any machine tool must be rigid in order to produce accurate, well finished work;
the milling machine must be particularly so. It is not until within the past few years, however, that the real value of this essential has been fully appreciate. This is owing to the fact that up to that time the milling machine had not become so extensively used for manufacturing purposes. In this field it must be capable of not only producing accurate work of high quality, but of producing it rapidly. The more rapidly a machine is operated, the greater is its tendency to vibrate. This is further augmented by the use of cutters made from high speed steel, for they can be made to take unusually heavy cuts at fast speeds and coarse feeds. It is impossible to eliminate all vibrations from even the very best types of machine construction, but they may be reduced to a minimun, or, in orther words, to a point where they will not affect the accuracy of the work, if every part is so constructed that it is capable of resisting heavy stresses, and absorbing vibrations. Weight and well-proportioned construction are most necessary to overcome vibrations.
Knee of Brown & Sharp Milling Machine
illustrating the points mentioned oppositeThe essentials in the design and construction of the column and knee machine that serve well to illustrate the general points that conduce to rigidity in all machines, follow:
First, the base must be large and heavy enough to provide a firm foundation, and the walls of teh column must be thikc and strongly braced, in order to support rigidly the weight of the working parts and withstand the strains of operation. Especially is this true of the front wall, which forms the basis of support for the table. If this is not heavy enough and well braced, it will have a tendency to buckle under the heavy loads it is required to support, which will not only admit of vibrations, but also destroy the alignments of the machine. Another point in connection with this front wall, or vertical slide, is that it should be wide in proportion to the size of the machine, as the wider a flat bearing, the more stable it is.
All shafts should be of large enough diameter to resist bending and torsional stresses, and gears should be of ample size to give strength and good wearing qualities, and to transmit the requisite power to the spindle. Cylindrical bearings should be firmly supported, and the boxes should be as long as is consistent with a high degree of efficiency. Those of the spindle are most stable when mounted directly in the thick walls of the frame.
A heavy, well-braced construction is necessary in the knee in order to overcome all tendency to vibrate or sag under the load of the saddle and table during operation. It is also well, to have the back of the knee that fits the vertical column extended above teh top as this gives a larger bearing surface to resist sagging tendences and vibrations under heavy loads.
It has been found from experimenting that vibrations arising during operation are usually manifested first in the table, and are transmitted from there to other parts. One reason for this is the several joints between the table and column. It is impossible to eliminate all lost motion between the bearing surfaces, and still have the parts free to perform their different functions. But weight has much to do with the stability of the table, and in many cases vibrations have been practically overcome by simply adding more weight to this part. It is important, therefore, that both the table and saddle be of sufficiently heavy construction. Transverse braces, however, placed at frequent intervales on the under side of the table often produce the required rigidity without adding unduly to the weight. Efficient clamps on the flat bearings of the knee, saddle, and table also provide means of rigidly fastening any one or two of the table movements that may not be in use, thus eliminating vibrations. 
Showing Firm Support of Arbor on Heavy Job
Another point that influences largely the rigidity of the table is the size of the flat bearing surfaces in the saddle and on the knee. It is essential that the table bearing in the saddle be wide and sufficiently long to prevent too great an overhang when the table is at the ends of its traverse, and the top of the knee be of ample width to easily support the weight placed upon the table.
Other features which conduce to rigidity are: a large overhanging arm with a support the the outer end of the cutter arbor, and an intermediate bearing on the larger machines, also arm braces that firmly tie the overhanging arm and knee together.
Speeds and Feeds. It is rare that the conditions surrounding any two jobs on a milling machine are the same. Sometimes the work is of the heavies class to which the machine is adapted, requiring gangs of cutters operating at a comparitively fast speed and coarse feed; again it is of a lighter type, requiring only one cutter operating at a fast speed and fine feed. The shape of the piece sometimes demands that the cutter be fed through faster or slower than would ordinarily be done in milling a plain surface. Different materials cannot be milled at the same speeds and feeds. Cutters of large diameter are employed for some jobs, and to get the proper peripheral speed, they must be rotated at a slower rate than those of smaller diameter. A finishing cut with the same cutter is usually taken at a faster speed, and correspondingly lower rate of feed per revolution of spindle than the roughing cut, in order to obtain a smoother finish. All these, and many other conditions, make it necessary that a machine have a wide range of spindle speeds and table feeds. Furthermore, there must be many intermediate speeds and feeds between the highest and lowest in the ranges. In many cases it is also advantageous to have the speeds and feeds independent of one another, so that the spindle speed may be changed without disturbing the rate of table travel. This is possible in the constant speed driven machine, 
The cone drive machine is admirably adapted to all classes of work where it is not necessary to use combinations of extreme speeds and feeds. In these cases, however, it cannot fulfill the requirements. For instance, it is impossible to obtain a course enough feed for a cutter of very large diameter, because the feeding mechanism is invariably driven from the end of the spindle, and is subject to the speed variations of this part. Consequently, when a large cutter is being used, the spindle is usually driven at its slowest speed, and the fastest feed that is then available is not coarse enough. Likewise, a correct combination of speed and feed cannot be had for a small mill, as this should run at the fastest spindle speed, and, when it does, the finest feed obtainable is much too coarse. The majority of work, however, does not require such combinations, and when medium-sized mills are used and work of ordinary classes is done, the cone drive machine is very satisfactory.
Owing to the dependence of the feeds upon the spindle speeds in the cone drive machines, it is necessary to rate them as so much per revolution of the spindle. This requires that the feed being used be multiplied by the spindle speed, in order to obtain the rate of production in inches per minute—the most generally accepted standard.
With the constant speed type of drive any combination of spindle speed and table feed within the ranges of the machine can be obtained, and thus the large, medium, or small sizes of cutters can all be run at the most practical speeds and feeds. This is due to the fact that the spindle and feeding mechanisms are driven independently of each other from the same main shaft, which revolves at a constant velocity at all times. Feeds obtained in this manner can be rated directly in inches per minute, a point that in itself constitutes an important advantage.
On practically all of the Brown & Sharpe constant speed drive machines, sixteen changes of spindle speed, and at least sixteen different feeds are available, while some sizes have as many as twenty feeds. Their range varies slightly in the different sizes of machines, but is such in every case that the correct combination can be had for any cutter that is used.
Power. A milling machine must have ample power, or its use is exceedingly limited. This applies to all styles and sizes of machines, but more particularly to the larger ones that are used in commercial manufacturing, where an economical production means the taking of heavy cuts at fast speeds and coarse feeds.
In driving machine tools, the power delivered to a machine depends upon the diameters of the driving pulleys, and size and velocity of the belt. A wide belt running at a high velocity on pulleys of large and equal diameters develops the maximum power, and, as its speed and width are lessened, its pulling ability decreases correspondingly. Likewise, it transmits less power, as the pulley on the machine exceeds in diameter the pulley on the driving shaft, for, when the surface contact on the driver becomes smaller, the belt has a tendency to slip.
Hence, in the factor of power is found another important difference between the cone and constant speed drive machines, with the advantage in favor of the latter.
The cone drive machine is very suitable for light and medium work, of such as the majority of milling consists, but when it comes to driving a large cutter through a heavy cut at a slow spindle speed and coarse feed, the requisite amount of power is lacking. This is due to the belt being upon the smallest step of the driving pulley, where it runs at its slowest velocity, and has a small arc and surface of contact.
On constant speed drive machines, the pulley is of the same, or almost equal diameter to that on the overhead shaft, and runs at a constant high velocity, irrespective of the spindle speed. Furthermore, a wider belt can be employed than on cone drive machines. As a result, a maximum amount of power is delivered to the machine pulley, and is transmitted through heavy gearing to the spindle, under all conditions, thus fitting this style of machine particularly well to the heavier classes of work. Another advantage of this drive is its particular adaptation to the application of a motor. The constant speed type of motor, which is more economical, both in first cost and in the amount of power consumed, than the variable speed motor, can be employed. This is also the most simple and compact form of motor drive. When applied to Brown & Sharpe Machines, the motor is mounted on a bracket at the back of the column, where it is away from dust and chips of the table (see page 176). Furthermore, by placing it in this position the floor space occupied by the machine is not increased, as it is necessary to leave room behind the machine to allow the overhanging arm to be pushed back when not in use. Efficiency. Production costs are of vital importance to the shop owner, and no one factor influences them to a much greater extent than the efficiency of the different machines employed. Where this is low, the amount of power consumed for which there is no apparent return is higher than it should be, with the result that the cost of production is increased. It is essential, therefore, that a high degree of efficiency may be attained in the milling machine, so that a maximum amount of work may be produced for the power consumed.
In order to obtain the highest degree of efficiency in milling machine construction, it is necessary that the utmost care be taken in designing the different parts, selecting materials, and in the quality of workmanship in building.
All parts must be proportioned in accordance with the functions they perform. They should be heavy enough to resist any stress that would tend to cramp operating movements. For instance, cylindrical shafts should be large enough in diameter to eliminate bending tendency, for this will cramp them in the bearings, thus interfering with their free revolution. Care must be taken, however, that the different parts are not proportioned so heavy that they will be cumbersome and thus produce excessive friction, which is detrimental to efficiency. It is here that the selection of materials is of
Pointed Teeth of Hardened Change Gear
value, for often the weight of a part can be made lighter by the use of a material of greater strength.
The size of bearing surfaces is of especial importance to efficiency, as well as to permanent alignment and ridigity. It is between them that friction arises in operation, and in order to reduce this to a minimum, their proportions should be such that the parts may move freely under the heaviest load.
Correct alignments of bearing surfaces are as essential to efficiency as to accuracy, in order that the working parts may move freely. Any error in alignemnts tends to cramp or wedge the moving parts.
Simplicity of parts and the use of spur gearing as far as possible are also elements that contribute largely to high efficiency. Durability. The first cost of a milling machine, like any other modern machine tool, is comparatively great, and to make its employment economical, this cost must be spread over a long period of service—in other words, the machine must be durable. Strong design and the use of high quality materials throughout the machine are most essential to durability.
Thorough workmanship is also an important factor. Seemingly small details in construction should receive careful attention, for it is these that many times give rise to serious trouble. The fitting of different parts, and making of all alignments should be carefully done, and means should be provided for taking up wear at any points where it is apt to occur. In connection with the wearing qualities of different parts, the selection of materials is an important factor; parts that are subject to continuous usage, such as the change gears in constant speed drive machines, should be made of a hard material having good wearing qualities. In Brown & Sharpe machines, these gears are made of steel and are hardened.
Where change gears are being thrown into and out of mesh frequently by a tumbler arrangement, it is well to have the tops of the teeth pointed, and the ends of teeth in sliding gears chamfered. These features not only facilitate throwing the gears into mesh, but also reduce the danger of teeth becoming bruised or broken, which is apt to happen when gears with teeth of the ordinary shape are thrown into mesh.
Rigidity is as essential to durability as to accuracy, since the existence of vibrations causes very rapid wearing of parts. Hence, every part should be of stable enough construction to resist vibrations under all practical working conditions.
Beyond these points, and that of provision for lubricating all bearing, the matter of durability is more especially a question of the care devoted to the machine while in use. Its failure to be durable because of lack of proper care cannot be attributed to any faults in design or construction. The information given in the next chapter on the care of milling machines is very important to those who have charge of these machines.
Convenience. Much time is lost in operating a milling machine that is inconvenient in any way for the workman to handle: therefore, from the standpoints of economy and efficiency, convenience is a most desirable quality. To be convenient, a machine must be so designed
All Hand-Wheels, Operating Levers, etc., Located Where Workman Can Reach Them Handily
| A, | Transverse hand feed. | I, | Vertical movement clamp. |
| B, | Vertical hand feed. | J, | Feed reverse lever (all feeds). At right of knee on small machines. |
| C, | Longitudinal fine hand feed. | ||
| D, | Longitudinal automatic feed trip and reverse lever. | K, | Adjustable dials graduated to thousandths of an inch. |
| E, | Transverse automatic feed trip lever. | L, | Transverse and Vertical feed locking lever. |
| F, | Vertical automatic feed trip lever. | M, | Lever to disconnect table feed screw when using circular milling attachment. |
| G, | Longitudinal movement clamp. | ||
| H, | Transverse movement clamp. |
As the station of the operator is at the front of the machine, all controlling levers and hand-wheels for stopping and starting the machine and the different table movements should be within reach from this point.
The spindle speed and table feed changing levers of constant speed driven machines are placed on the left-hand side of the column by some builders, and on the right by others. This is more a matter of choice than anything else, the chief advantage being in having them conveniently grouped and so designed that the manner of operation is clear.
Arrangements for lubricating the various parts and making adjustments to compensate for wear should be such that these can be accomplished with a minimum loss of time.
Hand or Automatic Feed. It is essential that the table of all milling machines used for manufacturing purposes, with the exception of the very smallest of the plain type, be fitted with both hand and automatic feeds. In the case of this exception, the work done is of such a small character that the machine can be operated more rapidly by hand than it could be if an automatic feed were applied. By the use of automatic feeds, one operator is enabled to run several machines on the majority of commercial work.
Tool room machines, and those used for miscellaneous milling, should be fitted with both hand and automatic feeds, for, while much of the work requires careful feeding by hand, there are, nevertheless, many times when an automatic feed can be employed and the mechanic can devote his attention to some other detail of the job while a cut is being taken.
Power Fast Table Travel. On large machines it is necessary that the table be provided with a power fast travel in order that the minimum amount of time will be consumed in moving the work to and from the cutter.
Also a faster rate of travel is thereby provided than is possible by hand, and the operator is relieved of the laborious task of moving the heavy table and work many times a day. Both of these are points which materially influence increased production.
| There are Friction Clutch Levers at Both Sides of Machine for Convenience of Operator | 
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Operator Does Not Have to Go Around Table to Clamp Knee |
No Exertion to Run the Table Back or
Vertical Spindle Milling Machine