Machine shop work - Turner

Comprehensive manual of approved shop methods including the construction and use of tools and machines, the details of their efficient operation, and a discussion of modern production methods.

Marvelous accomplishments in the mechanical world have become so common in this day and age that we scarcely realize the slow process of evolution which machine shop work has been undergoing during the last century. Barely a hundred years ago Watt, the father of the steam engine, had to be satisfied with engine cylinders which were three-eighths of an inch out of true because neither the machines nor the workmen could do better. Our present day machinist, on the other hand, must make his error two hundred times as small to meet the requirements. He also must deal with machines and mechanical problems which would have staggered his untried and unskilled brother of the previous century.

And it is not only the workman who has progressed in accuracy. There are myriads of machines which have been built to facilitate the manufacture of all parts that go to make up a mechanical device speed lathes, milling and stamping machines, die presses, and the jigs, tools, and dies which go with them and all of these contribute mightily to the accuracy and speed of manufacture. One of our best known automobiles is made with such precision from radiator cap to differential that the parts are shipped "knocked down" to distant points and are assembled with practically no fitting a truly marvelous performance and one which necessitates an exactness in the duplication of parts which would have been impossible even ten years ago.

The tale of this development of machine shop work is an interesting one and should appeal not only to the technically trained man who desires the best advice on the correct mechanical process to follow, but also to the man who wants to know how to "do things" or at least how things are done. It is the hope of the publishers that this book will satisfy a real demand and prove of sterling value in its field.

Milling Machine vs. Shaper and Planer. The operation known as milling differs so radically from the removal of metal by methods previously described, that it merits much more careful and lengthy discussion than has been devoted to the other methods. Owing, also, to its increasing importance and general use, it calls for a somewhat detailed discussion. While milling is coming rapidly into favor as a means of doing work formerly done on the shaper and planer, it does not follow that the shaper and planer are to be entirely abandoned. There has been a tendency to belittle the planer and shaper in favor of the milling machine. This tendency is not altogether warranted even by the rapid and economical method of milling. There is a large class of w T ork which can be done as accurately and in many cases as cheaply by means of a single-pointed tool such as is used in the planer and shaper.

Simple Milling Operations. The fundamental difference between planing and milling lies in the character of the tool employed. The planer uses a fixed single-pointed tool, with a reciprocating motion either of the tool or of the work. Milling is performed by the use of a rotating tool with several cutting points. This rotary multiple cutter is the basis of all milling operations; and, as the saw may be taken as a good example of such a cutter, so the work done by the circular saw in cutting metal may be said to be an example of milling, Fig. 198. The ordinary milling cutter is nothing more than a saw which has exceptionally broad teeth and in which the contour of the cutting blades is made to suit the work in hand.

It was but a step to make a saw wide enough to cover a considerable surface, or to have a thick saw with a suitably formed cutting edge. Several saws of different shapes and sizes can be mounted in a gang on an arbor, and perform operations which it would be hard to duplicate on the shaper or planer. Even in the present age of special machines for milling, a great deal of work of this character is still performed by the method indicated.

One of the great advantages of milling is the certainty of exact duplication a feature of prime importance in the manufacture of interchangeable work.

About the first machine built exclusively for milling was the so-called Lincoln miller, Fig. 199, which consists essentially of a bed carrying the equivalent of the headstock and tailstock of a lathe, with means for rotating the cutter arbor, which is carried directly by the headstock spindle, and steadied and supported by the tail-stock. There is also provided a table upon which the work can be fastened either directly or by means of a vise; and an automatic feed across the machine at right angles to, and below, the cutter arbor. This type of machine in various designs is much used in modern manufacturing.

Classification. As the type of cutter used determines, in a large measure, the design of the machine itself, it will be better at this point to take up a description of some of the different cutters, in order that the adaptation of the machine to the cutter may be clearly seen.

Plain Milling Cutters. Screw-slotting cutters, Fig. 203, and slitting saws, Fig. 204, are saws of a special type. The true milling cutter, Fig. 205, has a face much wider in proportion to its diameter than the common slitting saw. It is for the production of surfaces, rather than for a thin saw kerf in separating pieces of metal. These plain cutters are made in a large number of diameters and lengths, and are all designed for the generation of plane surfaces.

Spiral Cutters with Solid or Nicked Teeth. As we have seen in the case of reamers, heavy cuts can be taken more easily when the chip is broken up in small pieces; therefore, in milling cutters designed for roughing, it is customary to nick the teeth, Fig. 206, in such a way that the stock left by one tooth may be taken out by the following tooth. This makes the cutting easier. A plain cutter of any considerable length, with teeth formed by straight grooves, will not often make a smooth surface because of the varying pressure of the cutter as one tooth after another leaves the work. To avoid this springing tendency, cutters are made with spiral teeth, Fig. 201, either right or left-hand, so that there is practically a uniform distribution of pressure at all points during the cut.

Side Milling Cutters. When it is desired to mill the side of a piece, it is necessary that there should be teeth on the side of the cutter. Such cutters are usually made comparatively narrow and with teeth on both sides, as shown in Fig. 207. These side milling cutters are often sold in pairs. When mounted together, as in Fig. 207, they are often used to mill off both sides of a piece of work, as, for example, a bolt-head; and they are therefore called heading or straddle mills.

Interlocking Cutters. If two cutters of the same diameter are mounted together, it is difficult to mill a surface which will not show the line of separation of the cutters. This can be avoided by making the ends of the cutters, where they come together, of such a shape that they interlock one with the other. This feature of interlocking, Fig. 208, is especially valuable when cutting slots which must be of a definite width. An ordinary cutter will wear away by use or by grinding, and thus lose its correct size. The thickness of the interlocking cutters can be maintained, however, by means of very thin washers; and, owing to the interlocking of the cutters, no space will show between them.

Methods of Mounting Milling Cutters. The plain milling cutter is mounted on an arbor in a way very similar to that in which its spindle, prototype, the circular saw, is mounted.

Where the cutter teeth are formed integral with, or fastened to, the taper shank, as in the case of end mills, the shank, if it be of a proper size, is placed directly into the taper hole in the spindle. In many cases, however, the taper shank of the cutter is much too small to fit the spindle hole; and taper collets, Fig. 219, are used to bush down the spindle hole to the proper size. Of course, it is necessary that the axes of the outer and inner tapers should coincide; otherwise the cutter will not run true. In some cases it is necessary to use two collets, one within the other, before introducing the cutter shank.

End mills, having taper shanks, rely largely on the friction of the taper for holding in position-, although being driven by a tongue at the end of the shank. Therefore cutters of this description should not have a spiral in a direction which would tend to pull the cutter out. This is not a serious objection when using the cylindrical portion of the cutter; but when using the end of the cutter, it means that the teeth can have no rake, and must scrape rather than cut the work. In order to use a leading spiral on the cutter, the shank must be held positively in the spindle. This usually is accomplished by inserting in a threaded hole at the rear end of the shank, a rod which extends through the hollow spindle and brings up against a collar on the out- side. This can be set up solidly, and all danger of loosening-up of the cutter shank will be avoided.

When the cutter is small, as compared with the diameter of the spindle taper, a screw collet may be used, as the friction of the collet will be greater than the tendency of the leading spiral to move the cutter from the spindle. These screw collets are commonly made of machine steel, while the end mills are made from tool steel. The short, steep taper and threaded end are shorter than the long taper shank, resulting in a cheaper cutter.

One of the best means for holding small end mills with straight teeth is by the use of spring collets, Fig. 221, which can firmly grasp the straight shank of the cutter. When cutters are to be changed frequently, this is a particularly satisfactory method, although it will not answer for roughing cuts where cutters of large diameter are used, as the torque will be too great for the jaws of the collet to prevent turning.

An ordinary drill chuck can be held in the spindle by means of a taper shank, and furnish a means of holding straight-shank drills and other small straight-shank tools.

A very convenient method of holding certain tools consists in fitting a three jawed universal lathe-chuck to the threaded nose of the spindle, thus enabling straight-shank tools of large size to be held firmly and accurately. Cutters of any kind are rarely held in chucks on the milling machine, but a large number of other small tools can be held advantageously.