Modern drilling practice

A treatise on the use of various types of single and multiple spindle drilling machines, including their application to standard and special operations, the relation of speeds and feeds to intensive production, and the different types of tools and fixtures utilized in progressive machine shops for increasing the range and efficiency of machines of this class.

During recent years, a number of important changes have been made in methods of drilling holes in the parts of manufactured products. Noteworthy among these are the great increase in the speed at which twist drills are driven, the application of various types of universal or special multiple-spindle drilling machines or auxiliary drill heads, and the successful employment of semi-automatic drilling machines. All of these developments in drilling practice have been introduced with the view of increasing rates of production; and, in preparing the subject matter of this book, the author's object has been the same. The methods discussed are those which have been thoroughly tried out under actual manufacturing conditions, so that their practicability has been conclusively demonstrated. As a result, men who are responsible for the selection of methods of per- forming machining operations can decide upon the application of any of these suggestions for handling their own work with complete assurance that the resulting benefits are likely to be as great as in the case of other plants where substantial economies have been effected.

Probably it is safe to say that there is no class of cutting tool used in machine shops which receives as little consideration as the twist drill. This is largely due to the fact that drills can be bought ready for use in practically any size. As a result, the mechanic is likely to assume that the twist drill manufacturer has produced tools which are not only ready for use, but which are capable of remaining in good condition with very little attention. As a matter of fact, the proper grinding of a twist drill is of the utmost importance, and unless the drill is ground to the proper shape, its cutting efficiency is certain to be very seriously impaired. Realizing the importance of proper drill grinding, a comprehensive discussion has been presented of the theoretical considerations which must be fulfilled in order to grind a drill and maintain the point of such a shape that it will have the same cutting efficiency as a new drill of the same size. Drills may be ground on either a special drill grinding machine or on an ordinary tool grinder, and information is given concerning the proper method of procedure in both cases.

All mechanics are familiar with the various types of drilling machines which are extensively used in machine shops. Bearing this fact in mind, it was felt that nothing beyond a brief description of the essential features of each type of machine would be of practical value. After this preliminary discussion of machine design, examples of good practice in operating each type of machine are illustrated and described. In this connection, complete information is given concerning the material, the size of holes being drilled, the speed and feed at which the operation is performed, and the rate of production which is obtained. The examples selected show operations which are conducted under conditions approximating maximum output and, as a result, should prove of value in suggesting conditions under which a new job may be successfully handled. No attempt has been made to take up the subject of jigs and fixtures beyond explaining certain fundamental points in their design and the essential features of equipments used in performing the particular operations which are described. The reason is that this subject has been considered of sufficient importance to warrant its treatment in a separate volume in which a full discussion is presented of various principles of jig and fixture design.

In those shops where the most remarkable improvement has been made in rates of production secured on drilling machines, the increase has been largely due to constantly higher speeds and the rates of feed which are employed in the performance of drilling operations, but more particularly as a result of increasing the speed. There are many noteworthy advantages secured through drilling at high speed, and these will receive detailed consideration in Chapter IV on "Speeds and Feeds for Drilling." The remarkable increase in the speed at which drilling operations are performed has created a condition which was formerly unimportant, i.e., in regard to the relation between the time actually consumed in the performance of a drilling operation and the time required for setting up the work. Obviously, increasing the speed and rate of feed cuts down the time required to drill a hole, and, therefore, jigs and work-holding fixtures must be so constructed that the setting-up time does not become the limiting factor in performing the drilling operation. The steps which must be taken to secure this result will vary according to the depth of the hole, and consequently according to the time required to complete the drilling operation. In any case, the two chief points which require consideration are to design jigs and fixtures with clamping devices which may be quickly operated, so that the minimum amount of time is required to secure the work in place ready to be drilled, or to provide jigs or fixtures of the so-called indexing type, so that the operator may be setting up a piece in the fixture while the machine is engaged in drilling work held in other sections of the fixture. This point will receive detailed consideration in connection with a description of examples of drilling machine equipment which will be illustrated and described.

Types of Drilling Machines. Drilling machines which find the most general application in American manufacturing plants may be roughly divided into three general classes, as follows:

1. Vertical drilling machines.
2. Radial drilling machines.
3. Multiple-spindle drilling machines.

Each of these general classes is capable of further subdivision, so that drilling machines are finally classified under the following headings:

1. Vertical or "upright " drilling machines.
2. Vertical sensitive drilling machines.
3. Vertical high-duty drilling machines.
4. Radial drilling machines.
5. Multiple-spindle drilling machines of straight-line type.
6. Multiple-spindle drilling machines of cluster type.
7. Automatic drilling machines.
8. Turret-type drilling machines.

In addition to the eight preceding types of machines, a great deal of useful work is done by special machines built to meet the requirements of individual cases. Such machines are generally of the multiple-spindle type, but they are especially designed for specific classes of work.

Vertical or Upright Drilling Machines. The vertical or upright machine is the most commonly used type of "drill press" employed in the machine shop. It is usually equipped with power feed, and a tapping attachment is often provided, which may be engaged to provide for handling work in which holes have to be tapped.

The term "sensitive" is applied to those types of light drilling machines which are equipped with hand feed, so that the operator is able to judge the amount of feed pressure with which the drill is being driven into the work. These machines are usually adapted for drills from the smallest sizes up to from f to J inch in diameter. They are used on a great variety of work, and for handling small parts in quick-acting jigs or fixtures they are capable of giving very satisfactory results. One advantage of the hand feed is that an experienced operator may use his judgment in releasing the feed pressure, if he finds that the drill has struck a hard spot in the work. This is the means of saving the breaking of drills. Machines of this type are now being built for operation at speeds which were unheard of a few years ago. For instance, some types of sensitive drilling machines are built for operation at speeds ranging from 10,000 to 15,000 revolutions per minute.

Vertical High-duty Drilling Machines. As their name im- plies, high-duty drilling machines are adapted for the performance of heavy work, and they are commonly employed for using a range of drill sizes running from the maximum capacity of sensitive drilling machines up to the largest sizes in which drills are made. In addition to the performance of drilling operations, high-duty drilling machines are used for a great variety of other classes of work, including such operations as hollow-milling, spot-facing, facing, counter boring, threading, tapping, etc. In general, machines of this character may be employed to advantage wherever it is desired to use a rotating tool on stationary work under conditions where heavy cuts are to be taken. To meet the requirements of such severe service, the high-duty drilling machine is equipped with power-driven feed, and the rates of feed are commonly much greater than that employed on sensitive drilling machines, while the speed at which the drill is operated is correspondingly reduced, owing to the greater diameter of the drill. There are various forms of mechanisms used on these machines, but in all cases provision is made for obtaining any of a range of speed and feed changes suitable for the work on which the machine is engaged.

Radial Drilling Machines. On the familiar type of radial drilling machine the spindle head is carried on an arm, which may be swung around the column of the machine, and the spindle head may also be moved back and forth along the arm. This combination of movements makes it possible to locate the spindle of a radial drilling machine at any desired point over work which comes within this range of movement. Radial drilling machines are commonly classified according to the length of arm. Radial drilling machines are generally employed for handling those classes of work where there are a number of holes to be drilled and where the work is either too heavy or too large to be conveniently set up on multiple-spindle drilling machines. Multiple-spindle Drilling Machines. A great many parts that have to be drilled require holes of different diameters, and other operations, such as counter boring, reaming, or counter-sinking, are frequently necessary. When work of this class is done in a machine having one spindle, considerable time is wasted in removing one drill and replacing it with a different size or with some other kind of tool. For this reason, drilling machines having several spindles are often used when the work requires a number of successive operations. The advantage of the multiple spindle or "gang" type as applied to work of the class mentioned is that all the different tools necessary can be inserted in the various spindles, and the drilling is done by passing the work from one spindle to the next.

Drilling machines of the multiple-spindle type are also commonly used for drilling a number of holes simultaneously. The arrangement of these machines is varied considerably to suit different kinds of work, but they may be divided into two general classes; namely, those having spindles which remain in the same plane but can be adjusted for varying the center-to center distance, and those having spindles which can be grouped in a circular, square, or irregular formation. The first class referred to is used for drilling rows of bolt or rivet holes in steel plates, etc., and the second type is adapted to the drilling of cylinder flanges, valve flanges, or similar work.

Automatic Drilling Machines. For drilling holes in small parts, and particularly in those cases where the diameter and depth of the holes are not great, profitable use may often be made of automatic drilling machines. These are built in various types, which 'will be illustrated and described, but in each case the aim is to provide means of keeping the drilling spindle or spindles constantly employed while the operator is removing drilled pieces from the work-holding fixtures and loading fresh blanks into these fixtures, so that the parts may be drilled when they have been carried around under the drilling spindles.


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Modern drilling practice