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

Fig. 207. Side Milling Cutters Mounted as a Heading or Straddle Mill.

Fig. 208. Interlocking Cutter with Four Teeth Cut Away.

Courtesy of Union Twist Drill Company, Athol, Massachusetts 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.

Fig. 209. Gang Cutter.

Fig. 210. Forms of Angle Cutters.

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.

Gang Mills

Cutters may be mounted in gangs of great variety and combination, a typical one being shown in Fig. 209. These cutters may be of any desired form, and can be made to produce a variety of shapes.

Angle Cutters

The so-called angle cutters, Fig. 210, are often employed in the manufacture of other milling cutters. When used in making spiral cutters, they must have an angle on both sides, the customary angles in such cases being 40 degrees, 43 degrees, 45 degrees, or 48 degrees on one side, and 12 degrees on the other. The common single-angle cutters vary from 40 degrees to 80 degrees, either right- or left-hand. Double-angle cutters, as shown in the center of the lower row, Fig. 210, can be had with either 45 degrees, 60 degrees, or 90 degrees included angle.

Inserted-Tooth Cutters

Only such cutters as are made from a single piece of tool steel have been so far considered. In large cutters, however, the cost of the steel becomes an important item, and there is the ever-present danger of losing a large amount of labor

Fig. 211. Cutter with Inserted Teeth.

Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts by breakage when hardening. To make an economical, serviceable cutter of large size, it is customary to use a cast-iron body with inserted tool steel teeth. There are several different methods of inserting and holding these teeth. Usually, when the inserted tooth is in the form of a blade, they are held by taper pins or screws, Fig. 211. These blades are renewable, the cast-iron body being used many times.

Fig. 212. Form of Inserted-Tooth Cutter Called Slabbing Cutter.

Another form of inserted-tooth cutter consists of round, hardened steel pins driven into holes in a cast-iron body. This cutter is also permanent in form, Fig. 212, as broken teeth cannot be replaced; and, when the teeth are worn almost down to the body, the whole cutter is thrown away.