Drilling practice and jig design

A treatise covering comprehensively drilling and tapping operations, and the design of drill jigs and fixtures for interchangeable manufacture.

In the past, the whole subject of machine shop practice has often been dealt with in a single volume. Today, the new developments in all the branches of the trade would make it impossible to treat the subject comprehensively in so limited a space; instead, a volume is required to adequately cover the practice of each type of machine tool. This book on "Drilling Practice and Jig Design," therefore, deals exclusively with the processes used for producing drilled holes, and the machines, tools, jigs and fixtures employed. The subject of tapping and tapping attachments, so closely allied to drill-press work, has also been adequately covered. Especial attention has been given to the design of jigs and fixtures, because these appliances are indispensable in rapid and accurate work, and are used to a greater extent in connection with drilling operations than in any other branch of machine shop practice. Hence, the book will prove of great value not only to operators of drilling machines, but to tool designers and toolmakers, and to those responsible for the efficiency and the output of the drilling department. The step-by-step method of explaining the principles of jig design has been used, because in this way the subject can be handled in a manner at once clear and comprehensive. Instead of illustrating and describing a miscellaneous collection of jigs and fixtures, the subject has been analyzed, the principles explained, and the details of these devices dealt with as units. This is the only method by which a comprehensive study of the subject can be made.

In preparing this treatise, much of the material published on this subject in Machinery's Reference Books has been used, but many amplifications and additions have been made where- ever necessary. For the material contained, the Editors are, therefore, largely indebted to a great number of men who have furnished practical information to Machinery, and while it would not be feasible to give credit to each individual contributor, it should be mentioned that the framework upon which the chapters on jig design have been built consists of the articles prepared by Mr. Einar Morin, a recognized tool expert and production engineer. To all other writers whose material has appeared in Machinery, and which is now used in this book, the Editors hereby express their appreciation.

Rapid drilling in a shop is very necessary. It is, of course, important from the standpoint of economy in drilling alone, but the main feature to be considered is the fact that when work is sent to the drills, it b generally in steady progress of manufacture, and, under ordinary conditions, some of the assemblers will be found waiting for the drilling operations to be finished. The object of this chapter is to analyze the elements that enter into the problem of rapid drilling and point out some of the usual defects in methods and processes.

Selection of Drilling Machines. - The starting point for minimizing drilling costs naturally will begin with a discussion of the machine tool. There are several types of drilling machines, each adapted to a certain class of work. Sensitive drill presses having from one to four spindles are convenient for light work. The spindles have vertical movements by means of hand levers and are balanced by coiled springs. The table is counterbalanced by a weight and may be swung around as well as raised and lowered. These machines drill holes up to ^ inch in diameter. Besides having the advantage of high speed, the drive, which is some form of friction drive, lessens the danger of breaking drills.

The box column, high-duty type of drill press, having a table of the knee type, is the best form for comparatively small work that requires extreme accuracy and is drilled without the use of drill jigs. On account of the great rigidity of this type of drill, it is particularly well adapted for the making of jigs and fixtures. For this work, however, better results will be obtained by substituting a compound table, so that the work remains clamped to the table until all the drilling or boring operations are completed.

Gang drills, which consist of several stationary overhanging arms bolted to a common base and having a common work-table, are economical where several operations have to be performed on one hole. One operator usually runs the gang.

Radial drills are most convenient for handling heavy work, where a number of holes have to be drilled at different points. This type of drilling machine has a wider range of usefulness than any other, but, from the very nature of its design, it is imperative that the column and arm be made unusually stiff. The amount of work that can be turned out from a modem, high-duty radial drill, when driven by a variable speed motor of ample power and using the new flat-twisted drills, is astounding. A well-built, modern radial drill meets all general requirements for accurate drilling, tapping, and reaming, but when extreme accuracy is required, and when the machine is used for boring, jigs arc needed to guide the took. Universal radial drills are very handy for large work having holes to be drilled on an angle; but for angular work that can be drilled by clamping it to the swivel shelf of a box table, or where angular work is only occasionally met with, the plain radial drill is to be preferred.

Radial drills are sometimes placed in gangs and driven from a single shaft, a very good arrangement for long, heavy work. A very efficient and inexpensive horizontal drill for drilling, tapping, reaming, and boring consists of a radial drill head mounted on a vertical column. This column should travel in suitable guides on a base having a surface for supporting the work.

Adjustable multiple-spindle drills are designed for drilling, simultaneously, a number of holes in groups of square, circular, or other geometrical forms. The spindle heads are provided with vertical adjustment on the arms, to compensate for variation in length of drills or to enable drilling to be done at different levels, the amount of this adjustment varying according to the diameter of the spindles. In setting the spindles, the work jig is bolted to the table, the drills are inserted in the jig holes, and the arms are adjusted to bring the spindles into proper alignment and are then clamped. These machines are often used without the aid of drill jigs by clamping the work against suitable stops on the table. There are three standard types of multiple-spindle drilling machines:

1. The smaller sizes have the head fixed on the column and are provided with the knee type of table. The table is counter-balanced and its movements are controlled by adjustable hand and foot levers. This type is furnished with a No. i or No. 2 Morse taper hole in the spindles.

2. A heavier type consists of a work base having a column bolted to it. The head slides on this column and has a quick traverse by hand and power. A box table is furnished to hold small work. This type generally takes a No. 3 Morse shank.

3. The largest size of multiple-spindle drills have a head sliding between two columns. The work-table is mounted on . wheels and is arranged to run on a track for bringing the work under the drills. This type is usually motor-driven and has No. 4 Morse taper holes.

Any of these types may be furnished with a square, rectangular, or circular head. The spindles are all driven through universal couplings from a central gear. Power feed, automatic and positive stops are features common to all.

The value of multiple-spindle drills for duplicate work manufactured in large quantities can hardly be overestimated; but for work made in lots of twelve pieces or less, it is often quite a problem to decide just what class of work this machine will handle more economically than the radial drill. This is because the time required to set the spindles of a multiple drill to a complicated lay-out is an appreciable percentage of the total time required to complete the job. The lay-out often involves two or more settings of the work or the spindles, and this nullifying condition also enters into the problem. Provision should be made for tapping on these machines, as several holes can be tapped simultaneously.

General Requirements of Drilling Machines. - Accurate tests have demonstrated that the average drilling machine should have fully double the driving power contemplated in its design.

This is a point that must be considered carefully, and especially in the case of multiple-spindle drills. A proper increase in driving power means heavier motors, wider pulleys, stronger gearing, and larger bearings. Immediately there follows the question of rigidity. The fact that ample driving power and rigidity are the vital features in a drilling machine cannot be repeated too often. The fullest possibilities of economy attending the use of the new high-speed steel, fiat-twisted drills cannot be realized unless these provisions are made.


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