Thread cutting methods

The universal use of the screw thread, either as a fastening device or for transmitting motion from one part of a machine to another, combined with the numerous conditions under which it is used, has led to the development of many different kinds of tools and machines for forming screw threads. These tools vary from the hand-operated tap or die to the specialized semiautomatic and fully automatic machines capable of threading duplicate parts with considerable rapidity. While much has been published on thread-cutting practice, most of this material has been of a miscellaneous character, and there is evidently need for a general treatise covering the various machines and auxiliary equipment used for this important branch of machine shop work.

The purpose of this treatise is not only to describe all the important types of tools, machines, and methods which have been developed for producing external and internal screw threads, but to show the relation between the different processes. If the screw thread problem were merely to determine how to cut a thread of the required size, pitch, and accuracy, it would, in most cases, be relatively simple, but like other manufacturing problems, it is usually complicated by the fact that the work must be done on a commercial basis. The various methods of forming screw threads are familiar to practically all machinists and machine shop foremen, but comparatively few know to what general class of work each method is best adapted. While it is often difficult to decide this question, an understanding of the reasons why different types of threading tools and machines were originated and the kind of work for which they are generally used will at least simplify the problem greatly. A special effort has been made to include in this book all the information possible pertaining either to the adaptability of various types of tools and machines or to their relative merits as applied to different classes of work.

This treatise deals with different standard forms of screw threads; general thread-cutting practice in engine lathes or other machines using a single-point tool; the special auxiliary thread-cutting mechanisms and attachments applied to engine lathes or other machines; the design, operation, and application of various classes of threading dies and taps; standard and special threading machines; taping machines and attachments; the causes of defective and inaccurate screw threads; thread milling machines and their use; and the production of screw threads by the rolling process. It was neither possible nor desirable to illustrate and describe every different design of machine or tool used in connection with external and internal thread-cutting operations, although many variations in design have been featured whenever there seemed to be a good reason for so doing.

The formation of accurate and interchangeable screw threads on a commercial or manufacturing basis has been a difficult proposition in many machine shops. The extent of the difficulties encountered in connection with this work may depend largely upon the meaning of that indefinite term "accurate" and also upon the length of time that is allowed for the screw-cutting operation. On many classes of threading work the degree of accuracy, combined with the speed of production demanded, makes screw cutting a troublesome operation, and even when the conditions are not unusually severe, the production of smooth, correctly formed threads requires tools that are properly made and applied. While some equipment is incapable of producing good screw threads, regardless of the time that might be expended (either because it was poorly constructed to begin with or is in bad condition), many notable improvements have been made in screw-thread forming tools and methods, and in this general review of the subject the more important types of tools or machines for producing screw threads will be described. In this book, those features that have a direct bearing on the accuracy of screw threads and the speed of production will be considered, as well as the relative merits of different types of tools or machines and the general classes of work for which they are adapted.

General Methods of Forming Screw Threads - The two general methods of forming screw threads may be defined as the cutting method and the rolling or displacement method. Some of the various modifications of these two general methods will be outlined. The cutting methods as applied to external threads are briefly as follows:

1. By taking a number of successive cuts with a single-point tool that is traversed along the part to be threaded at a rate per revolution of the work depending upon the lead of the thread. (Common method of cutting screw threads in the engine lathe.)

2. By taking successive cuts with a multiple-point tool or chaser of the type used to some extent in conjunction with the engine lathe and on lathes of the Fox or monitor types.

3. By using a tool of the die class, which usually has four or more multiple-point cutting edges or chasers and generally finishes the thread in one cut or passage of the tool.

4. By a single rotating milling cutter, which forms the thread groove as either the cutter or the work is traversed anally at a rate depending upon the thread lead.

5. By a multiple rotating milling cutter which completes a thread in approximately one revolution of the work.

When screw threads are produced by the rolling or displacement method, there are two general processes:

1. By rolling the blank in contact with a revolvable disk or roll, the periphery of which has either a single thread or a multiple thread corresponding in pitch to the thread required.

2. By rolling the blank in contact with flat dies having parallel ridges which are spaced in accordance with the required pitch and which form the screw thread.

Internal screw threads, or those in holes, may or may not be produced by the same general method that is applied to external work. There are three commercial methods of importance, namely:

1. By the use of a single-point traversing tool in the engine lathe or a multiple-point chaser in some cases.

2. By means of a tap which, in machine tapping, usually finishes the thread in one cut or passage of the tool.

3. By a rotating milling cutter of either the single or the multiple type.

Controlling Motion of Thread-forming Tool - The general methods of controlling the motion of a thread-forming tool relative to the work, for generating thread grooves of helical or "spiral" form, vary according to the type of tool or machine used for the screw-cutting operation. Considering, first, the action of the tool while cutting a thread groove, there are three general methods of securing the necessary motion:

1. The tool-holder or carriage (or the work in some cases) is traversed by a lead-screw driven by gearing of such a ratio that the relative motion of the tool per revolution of the work is equal to the lead of the screw thread required.

2. The motion is obtained by the direct action of a lead-screw, which is not driven through gearing and has a lead equal to that required on the part to be threaded.

3. The tool when in the form of a tap or die may be self-propelling or self-leading, the tool moving each revolution a distance equal to the thread lead as the result of its screwing action, caused by the location of the cutting teeth along a helical path.

After a thread-cutting tool has completed a cut, it is usually necessary to return it to the starting point either for the purpose of taking another cut (as when using a single-point tool) or for removing the tool from the work (as when withdrawing a tap or die from the threaded part). This return movement may be obtained by the following methods:

1. By a hand or power traversing movement of the tool carriage or holder in the direction of the axis of the work, after withdrawing the cutting tool or tools for clearing the screw thread.

2. By reversing the rotation of the work- or tool-spindle for backing a non-collapsing tap out of a hole or for unscrewing a non-opening die from the threaded part.

3. By running a tap or die at a slower speed than the work while cutting the thread and then at a faster speed for unscrewing the thread-cutting tool.

When a thread is being formed by the rolling process, the tool, if of circular shape, simply moves at right angles to the axis of the work for bringing the roll into contact with the part to be threaded and for removing it after the thread has been formed. In the case of flat thread-rolling dies, the blank to be threaded is rolled between a stationary die and a movable die having a reciprocating motion.

Selection of Thread-Forming Method. - One of the most important questions relating to the formation of screw threads is the particular method that should be employed for a certain class of work. What is the most rapid method of performing the screw-cutting operation without sacrificing the necessary accuracy? That is the question that has often puzzled shop foremen and superintendents and one that is not answered in the same way by those who have studied the problem of screw thread production. Variations of practice in different shops are often due to the fact that the equipment at hand is used not because it is considered the best, but because there is no choice. In other words, the amount of money appropriated for new tools is often the real reason why different types of tools are used for the same general classes of work. While each class or type of threading-forming tool or machine has a definite field, in some cases these fields overly, and then judgment, experience and records of past performance must be utilized in deciding what method is to be employed.

Among the important factors that affect the thread-forming methods may be mentioned the diameter of the screw, pitch of thread, degree of accuracy necessary, number of parts to be threaded, location of thread, material of which parts are made, and the relation of threading operation to other work which may precede or follow it. No inflexible rule can be given as a guide, and the different factors mentioned may be combined in various ways, thus changing the problem partly or entirely. Other conditions peculiar to any one job may also have a decided effect on the exact method of procedure. This matter of selecting equipment for the production of screw threads involves not only a study of the different classes of work for which various types of tools are adapted, but a careful investigation into the performance of different makes of tools of the same class or type. The difference in the quality of tools of the same class accounts for many variations of opinion. One man contends that a certain type of tap or die is incapable of accurate work, but, if the truth were known, the general type may be entirely satisfactory when properly made or applied. This is also true of thread-forming tools of other classes. In dealing with the various types of thread-forming equipment, the kind of work and the particular advantages of each type will be enumerated in as specific a manner as seems practicable. Unfortunately, however, it is not possible to deal with this question of selecting thread-forming tools in such a definite way that instructions may be followed to the letter and without the assistance of experience and judgment.