Broaches and broaching

Just how far back the knowledge of broaching dates is not known, though the great Leonardo Da Vinci, who was born in 1452 and died in 15 19, made numerous sketches in his note books, depicting the broach in practically the forms used today, but it is extremely doubtful if he or his immediate successors made use of them; it shows, nevertheless, how far ahead the gigantic genius of the Florentine saw, to outline tools and machines that are just being appreciated.

However early the broach may have been invented, it is only in recent years that it has shown signs of taking the place its merits deserve. As one of the numerous branches of mechanics which owes its principal development to the evolution of the automobile, broaching has grown enormously in application, and is used today to a considerable extent in almost every branch of mechanical industry, and so quietly has it spread that few realize its great importance.

It is to bring to the attention of those who would know more of broaching work and machinery as it exists at present that this book has been compiled, thereby placing in their hands data that will enable them to judge whether it is applicable to their particular class of work or not, and if it is, to give them whatever working directions and advice I have been able to gather.

Just what machine work may be classed as broaching and what may not, is, like so many other mechanical questions, not so easy to answer as it looks. It is common shop practice to class many shop jobs worked out with a single-point tool as broaching, whether the work is done on a shaper, punch press, keyseater, mandrel press or some other machine or device. However, no one ventures to call the work of a slotter broaching.

What then are the boundaries within which work may properly be classed as broached work? Everyone admits without argument, that holes finished to certain sizes or shapes by the use of tools having a number of teeth of increasing size is broaching, no matter whether the tools are pushed or pulled through.

Again, it is evident that a hole finished to size at one pass, by means of a single-point tool forced through by any means whatever, whether by the ram of a shaper, a punch press, a mandrel press or a hammer, is not broaching. It is either punching or drifting. Neither can the working out of a hole or recess, in a number of cuts, by means of a single-point tool held in a shaper, punch press or other machine, be called broaching. It is slotting.

A case that is not so clear, perhaps, is the work of a keyseater using a multiple-tooth cutter. Where the keyseat is finished at one pass with a tool having a number of teeth of increasing size, it is broaching. If it requires several passes to finish the keyseat with this kind of tool, it is still broaching, as the successive cuts simply take the place of the passage of several broaches of increasing size. However, if the tool has a number of teeth, and all are the same cutting length or size, and it is not backed by a taper strip to give the effect of increasing tooth sizes, it is a saw and not a broach. It might be well to state here, that the common file is also in the saw class.

Sometimes in cutting slots or certain shapes to a shoulder, it is impossible to use a single broach with a number of teeth, and it becomes necessary to employ several tools carried in a turret or otherwise, each successive cutter having the proper shape and the correct increase in size, to produce finally the finished hole. This also is broaching, as the different cutters represent the successive teeth of a single broach, which would be employed did not the shoulder prevent.

Almost innumerable examples might be cited, but these are sufficient, and taking the various factors into consideration, the following definition seems to cover the case: Broaching is the working out of holes or slots, or the machining of surfaces, by tools having a number of successive cutting teeth of increasing size, no matter whether these teeth are arranged singly or in multiple.


THE USES OF BROACHING TOOLS

Originally broaches were principally used to finish square holes, round holes and keyways. Then gradually they began to be used for splines, irregular shaped holes, internal gears and numerous other applications of internal finishing. Later external broaching was used for a variety of work, though so far it is used very little in comparison to internal broaching.

No other machine-shop operation has relatively extended so rapidly in use during the last few years as broaching. Only a little while ago, it was looked upon as a very special method of machining. It was not considered in the same class as the then common operations. But this has been changed with surprising rapidity.

The tremendous improvements made on broaching machines and broaching tools, are largely the direct result of many years of experience and effort on the part of a small group of men who will be given their proper share of credit farther along in this book. The improvements made have greatly cheapened the cost of production in broaching various kinds of work. It is remarkable how interesting the proposition has grown since it has become so specialized. Broaching is now at a point where it is considered indispensable on certain classes of work.

One great advantage of broaching as a means of finishing certain parts, over machining by any other method, is that the action of the broach itself serves as a clamping medium and often nothing else is needed to hold the work in place, it being slipped on the work bushing or broach shank in a loose manner according to the work to be done. The work then becomes fastened in its proper position as soon as the machine is started. This single item is of great importance in many cases where the chucking of an irregular piece, in order to finish it by boring or reaming, would be extremely difficult. In a great many cases the operation of broaching may be finished in the time ordinarily taken to chuck the piece. In the case of an ordinary keyway, for instance, no clamps or setting devices are needed other than a guide bushing and in many instances this is dispensed with. As a rule the keyway is finished in one operation and but 1 min. of time is consumed. Sometimes three or four pieces can be done in one operation, thus increasing the usual output that many times.


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