Handbook of small tools

In the following pages the author has endeavored to present an original and, as far as possible, complete treatise on the design and construction of small cutting tools, such as threading tools, taps, dies, milling cutters of all classes, reamers, drills, counterbores, hollow mills, etc. The material has been prepared with special regard to the requirements of the tool-maker, tool draftsman, foreman, inspector, and superintendent, for specific information relating to tools of the class mentioned. The immediate reason for the placing of this book on the market is the lack of definite data on this class of work in existing treatises on shop practice, and the book has been written to supply a distinct demand in this direction. The author also wishes to emphasize the fact that the information given is authentic, and that the book places on record the most modem practice in tool manufacture, the experience gained by him during several years connection with one of the foremost tool-making firms in the country, the Pratt & Whitney Company, being the basis of the treatise.

In arranging the material, a great deal of space has been devoted to tables, formulas, and general data, giving the tool-maker and the designer of tools specific working figures; and while methods and processes have not been neglected, the author's personal experience has been that the demand of the tool-making trade is for directions what to do rather than how to do it. An effort has been made to prepare the material for this book so as to give specifically, in plain figures, in tables, and in formulas, the desired information. While the book is of a practical character, and intended for the use of practical men, theoretical considerations have Hot been overlooked, and formulas and deductions of formulas are included where ever considered advisable. Those who have no interest in the deduction or use of formulas will find the results sought for directly in the tables, without calculations.

The portion of Chapter II devoted to change gearing for the lathe has been prepared with the intention of presenting this matter in as simple a manner as possible, in order to meet the requirements of those whose knowledge of mathematics is limited; hence the rather extended and elementary treatment of this subject.

CHAPTER I - SCREW-THREAD SYSTEMS

Introductory
 

Notwithstanding all that has been written about standard screw-thread systems, data which completely cover all the recognized standards are very scattered, and it is often necessary to search for information in many various handbooks and works of reference. For this reason we will of necessity, before entering upon the subject of taps and tap-making, devote our attention to the different kinds and systems of thread in common use. While a great many more systems than we will review in the following have been proposed from time to time, only those which are mentioned below have been officially recognized by mechanical men, or gained prestige by means of universal use and adoption. It will be found that the list given embraces all standards, whether in use principally in the United States, in Great Britain, or on the European continent. Anyone having to do with tool-making, and, of course, tap-making in particular, must be equally familiar with the systems abroad as with those of this country, because the trade relations between the United States and Great Britain and the continent make it necessary to produce a great number of tools in this country, made in accordance with the systems in vogue in the country where the tools are to be used. The recognized British standards are also used to a great extent by machine builders in this country, and even the number of American manufacturers who introduce what is termed the French and International standards in their establishments is steadily growing. To question the advisability of such a course is not within the limitations of this treatise, but the fact is referred to merely in order to point out the universal use of all the standard systems of screw threads, and to call attention to the necessity of a complete record of the peculiarities of each system.


Standard Systems

The most common systems which will be treated in detail in the following pages are:

-    The United States standard thread,
-    The sharp V-thread,
-    The Whitworth standard thread,
-    The British standard fine screw thread,
-    The British Association standard thread,
-    The Briggs standard pipe thread,
-    The Whitworth standard thread for gas and water piping,
-    The square thread,
-    The Acme thread, and finally
-    The French and International standard threads.

Comparison between Usual Methods. - There are two common ways of producing screw threads, cutting the threads in a lathe or cutting them by means of dies. The first method, and the one with which we will deal here, is the one used whenever any greater degree of accuracy of pitch and diameter is desired. By special methods, and by extreme care in making the dies as well as cutting the thread, screws within close limits of accuracy may be produced by means of dies; but for cutting the threads of taps, where any original error or imperfection would be duplicated in all the pieces of work afterward threaded by the tap, the only desirable method is the cutting of the thread in a lathe. All screws of any considerable length must also be cut in this manner, as accuracy in lead cannot be insured unless the accuracy of a tested lead screw is duplicated in the piece threaded.

Examples have been pointed out where, in using dies for thread cutting, the inaccuracy of ordinary commercial dies in the pitch has been so great as to cut a thread which, if continued for a foot in length, would have had an error of one-eighth inch in the lead. If the thread is cut with dies by hand there is also a chance for error in the starting of the die. The thread may not be true with the axis of the work, for although most dies intended for use by hand are either themselves provided with a guide or mounted so that the piece to be threaded enters a guide before reaching the die, this guide does not always fit the piece closely enough to start the die perfectly true. In all these respects lathe threading is superior, and can- not be too strongly recommended in all cases where a  thread of good qualities is required.

Cutting Screws without the Aid of a Lead Screw. - Because the lead of a screw being cut always depends upon the lead of a thread that has been previously cut, any incorrectness in the master thread (as in a lathe, in the thread of the lead screw) will be reproduced in the screw. For ordinary purposes, the errors in the lead of lead screws of lathes of good manufacture are insignificant, but occasions arise when these errors must be taken into consideration. In order to avoid the duplication of errors of this character, Messrs. de Fries & Co., Diisseldorf, Germany, have designed a new screw-cutting lathe, working on the principle of producing a thread independently of a previously cut lead screw. The lathe employed for this purpose is of common design, the feature of extraordinary interest being the arrangement for feeding the carriage; a flexible steel band is used for this purpose instead of the lead screw. This band is located centrally between the two ways of the bed, and one end of the band is fastened to the front end of the carriage, while the other end extends under the head-stock and is fastened to a drum, turned accurately to a definite diameter. When this drum is revolving, the steel band is wound up on it, and thus feeds the carriage. The drum, of course, must be large enough so that the steel band when winding up does not reach fully one complete turn around the drum, because if it reached more than one turn around, the band in winding up on itself would be wound up on a larger diameter than that of the drum, thus causing incorrect results. The drum is driven from the cone pulley by means of a worm and worm wheel.

For the return, another steel band is fastened to the rear end of the carriage, this band extending to the rear end of the lathe and running over an idle pulley. This lathe is not used for cutting the whole screw from start to finish, but simply for finishing the thread. The arrangement is by its construction too weak to stand up for the heavy cuts necessary for rough threading. The thread is therefore cut in an ordinary screw-cutting lathe, somewhat over size, and then placed in this special lathe mentioned and there finished. It is claimed that by this machine it is possible to cut the most correct thread as yet produced for commercial purposes.

Cutting Threads in the Thread Milling Machine. - A method of producing threads which has been but lately brought into more general use is the milling of the thread in special thread milling machines, which, while embodying the principles of a lathe, are provided with a cutter head in place of the lathe tool-post, and a cutter, driven from the countershaft in place of the ordinary tool. As this method contains all the principles which insure accuracy in thread-cutting in a lathe, equally perfect threads will result from milling. The cutting of threads in a thread milling machine is also more economical, at least when fairly long threads are to be cut. The thread is milled to its full depth at once, and as the center of the cutter is always at the same height as the center of the work, there is no risk of improper setting of the tool. The only objection that could be advanced is that the cutter head is tilted to the angle of helix of the thread, and consequently, if the same cutter is used for all diameters with the same number of threads per inch, the thread form will be slightly inaccurate, owing to the different angles to which the cutter head is tilted. For all ordinary angles of helix, that is, for all diameters provided with a pro- portionate pitch, this inaccuracy, however, is so small as to command no consideration.

Method of Rolling Threads, - Some manufacturers of taps finish the thread by a process named rolling. The tap is first rough threaded, and afterward passed through a set of three rollers, mounted in a kind of a chuck.

These rollers are provided with circular grooves of the same shape as the thread, and in order to insure the correct lead, each roller must be cut with its grooves one-third of the pitch in advance of the next preceding roller. All the rollers are mounted in the same horizontal plane if the tap passes through them vertically, or in the same vertical plane if the tap passes through them in a horizontal direction. What has been said in regard to rolling threads may be better understood by referring to Fig. 13, where the outline of a chuck with three rollers is shown.