teeth. If they are undercut so as to give the cutting-edge toprake, as in a lathe-tool, it makes a weak tooth liable to break easily, but adds to the efficiency of heavy ones.

There is far more danger of getting too many teeth than too few into a cutter.

If the cutter is small in diameter so that it will become too thin if the teeth are deep, take the first cut through at the proper depth and then mill around again after revolving the work so as to bring the proper angle.

MOST VITAL POINT IN MILLING-MACHINE

PRACTICE.

The most vital point in milling-machine practice is that cutters of whatever design be kept sharp. A dull cutter is like any other tool that is dull-its efficiency is greatly reduced, the work produced is inferior, and the cutter wears rapidly away.

The same principle applies to the cutting-edge of the millingcutter as to any other cutting-tool for metal. If too little clearance is ground it will not cut well, and if too much, it will chatter; about three degrees will generally give good results.

SPEEDS AND FEEDS FOR MILLING-CUTTERS.

A subject upon which too much cannot be written nor thought given is that of proper speeds and feeds for millingcutters. Often the question is asked: "What rule is there for determining the proper speeds of cutters." When a direct answer is not given to this question, the questioner is always dissatisfied and usually discouraged. Of course there is no "hard and fast" rule for determining the proper feeds and speeds of cutters, and in this book one cannot be given. The texture and hardness of the material to be machined determines the surface speed in each case. Thus, for cast-iron, a speed of forty feet per minute may be safely taken as a good basis when taking heavy roughing cuts, while for light finishing cuts on the same material, (after the scale has been removed) fifty feet per minute is not too fast. When working steel twenty feet per minute is

not too fast, and for brass sixty feet per minute is a good basis for determining the correct cutting speeds for these metals.

Although the hardness and texture of the material worked upon is the chief factor to be considered when determining milling speeds, the nature of the cut and the shape are also very important factors. Thus, for instance, a large slitting-saw can be run about twice as fast as a large surface-cutter when working on the same material.

Now, with regard to the rate of feeds for milling, the most advanced practice is to take a roughing-cut with the fastest feeds the machine will pull; that is, provided the cutter is relatively as strong in comparison as the machine in which it is used. If the nature of the work requires a cutter of such a form as to be comparatively weak, it is often better economy to break an occasional cutter than to allow the machine to work at a slow rate of speed.

When running a cutter at a slow rate of speed and advancing it at a fast rate of feed on cast-iron, compressed air, delivered to the cutter with sufficient force to clear away all chips as fast as

FIG. 267.

they are produced, will prolong the life of the cutter, even when the fastest feeds are fed against it. When working steel, a stream of oil on the cutter will have the same effect, providing the oil is delivered under pressure sufficient to wash away the chips entirely from the cutter.

In regard to "burning" cutters, or drawing the temper while working them, it must be understood that this will not happen through too fast a feed, but it is always to be traced to too high speeds. Thus, when both speed and feed are up to the maximum, the actual rate of table travel per minute can be further

increased by reducing the speed of the cutter and increasing the feed rate.

When taking finishing cuts, the rate of speed depends upon the quality and degree of finish required. Here it may be stated that experiments have determined that 0.030 per revolu tion of a 34-inch cutter when surface-milling leaves a good finish, and in machine work will leave a surface that will require little scraping to make a good bearing.

Fig. 267 shows a collection of forming cutters.

To succeed with milling-cutters they should be made right, hardened properly, sharpened regularly, and speeded and fed properly.

SUGGESTIONS FOR MILLING.

Experience in the use of milling-cutters will teach anyone that unnecessary expense and annoyance may be avoided by frequent and proper grinding of milling-cutters. A dull mill will not do good work and wears away very rapidly. At the first appearance of dullness, use your cutter-grinder, it will save your cutters, your time, and your patience, and will enable the cutters to do their best and most rapid work.

In order to preserve the correct shape of formed corners, grind the teeth radially.

No definite rule can be given for speed or feed of cutters, but the usual tendency in all classes of work, except for finishing cuts, is for slow speeds and coarse feeds.

For cutting wrought-iron or steel use lard, oil, or some one of the usual compounds manufactured for this purpose.

Small mills on horizontal millers will cut better and faster than larger mills; they also cost less and will last longer.

Wherever possible use a mill that is wider than the cut to be

taken.

CHAPTER XVI.

The Hardening and Tempering of Milling-Cutters.

HARDENING.

ALTHOUGH the quality of steel used for milling-cutters is of great importance the proper hardening of it is equally so. It is a fact that bad steel well treated will make better cutters than good steel poorly treated. The hardeners of such tools cannot complain of a lack of literature, as treatises and articles on the subject are continually appearing. However, practice alone can teach the details and refinements of the most interesting process in the making of milling-cutters.

In the following, methods are put forward for the proper hardening of milling cutters which are the result of experience, and while they are not necessarily the best, it is claimed that they have brought success when used.

It pays to spend time on filling blind holes, sharp internal angles, etc., with clay. In many cases asbestos should be used with wire over a weak place, or over a part which must be kept soft. The furnaces should be in a partially darkened room from which direct sunshine is excluded.

Though I have never found any disadvantage in using cold water for quenching, it is quite reasonable to suppose that water containing a considerable amount of air dissolved in it may not cool the cutter so uniformly as it would do if the air had been expelled, and therefore boiled water is to be preferred.

After machining, tools should have a few days rest before hardening. If they must be hardened immediately, they should be annealed first, but care must be taken to prevent a tendency for the surface to become decarbonized. To accomplish this, an excess of charcoal should be kept near the cutters in the furnace to maintain a reducing atmosphere.

HEATING.

It is not only necessary that the cutter should be at the right heat, and at a uniform heat, when plunged, but it must have reached that heat gradually and uniformly. If the heat be applied gradually, the cutter may be made hotter than the correct temperature, and yet not crack. If a crack appear under these circumstances, it will probably go through the cutter. If a cutter, after being heated too rapidly, or allowed to get much too hot, be carefully brought to the right temperature in the furnace and then plunged, the teeth may clink off. They are certain to do so if it be not nearly uniform in temperature at the time of plunging. In case of a mistake in heating, a cutter should be allowed to cool out, and heated fresh.

PLUNGING.

A thin cutter

The manner of plunging is worth attention. should be in a vertical plane when it enters the water. If it were plunged horizontally, one side would be cooled before the other, and would cause the cutter to warp. A cutter with a long hole should be plunged into the bath with the hole vertical, to allow the water to circulate freely. Cutters with large recesses should be plunged with the recess uppermost-to allow the steam to escape. The object generally is, in the first place, to cool symmetrical parts simultaneously; and, secondly, to let the water have free access to every part without delay. Thus a long thin reamer should obviously be dipped endwise, in order that all the flutes may cool simultaneously, notwithstanding the fact that the water would come into contact with every part in a shorter time if it were dipped horizontally.

Cutters need not be cooled right out in the water. They may be removed as soon as they are so far chilled that the temper color would barely show if they were polished immediately. Cutters of a few pounds weight may be lifted from the water as soon as the teeth are chilled. In a few minutes the heat from the inside begins to reheat the teeth, and just before the color shows they must be plunged again for a second or two. This