The work done by a cutter is afflicted by the number of teeth, speed, and feed. The feed being directly proportions to the number of teeth, then, if the most economical speed is determined, the quantity of material removed depends upon the number of teeth in the cutter, the latter being designed with all due regard to easy production and re-grinding, the former taking into consideration primarily the stability of the machine, also the material being operated upon, its skin finish, cubical contents removed, class of cutter, and an other factor that may present itself to the operator. The following table gives the speed and feed for various operation upon different materials.

The speed for cast iron appears to be open to the greater amount of contention, and the author gives 45 feet per minute as that from personal experience works most satisfactorily; although from 10 to 80 feet per minute have been recorded The difficulty lies in the fact that a lubricant cannot be used in the ordinary way, because of choking the pumps; cons quently, with high speed, the temper of the cutter is soon destroyed, which is not econoinical.

Although the feed may be sometimes even more than the given above, the rate seldom exceeds 1/50 inch per revolution and frequently only one-half of that; and where gauge wo is done, more than the proverbial roughing and finishing cut are necessary.

The emery discs for grinding milling cutters are about 4 inches diameter and ¬, inch wide, and run from 4000 G000 feet, circumferential speed, per minute. It follows fro this speed that any inaccuracy due to eccentricity, uneven balance, or an untrue spindle, is immensely intensified; con-sequeutly, these points must be attended to. Also the be should be even and true; but above all, keep the wheels from glazing and free from moisture. Let them cut freely, other wise they will soon blue-burr; finally, let the finishing cut be fine, and then the last tooth will be of the same radius as the first. The machines themselves are of various designs and generally supplied by the makers of milling machines. They are adapted to suit a range of cutters, having tables with moveable headstoeks which take different lengths. The tables are arranged to move upon slides, either longitudinally or transversely, by hand or gear. The wheel is generally upon another headstock, which is capable of taking various angles. It is generally a fixture as far as traverse is concerned, and the cutter is brought to it. If it is necessary to use water on fine light work, the speed of the wheel should be reduced.

Although so much has been said about hardening as an accessory operation in the smithy, the importance of this point upon the milling cutter is so great, that no hesitation or explanation is necessary for these few remarks, and here again experimental data is of great assistance. The lowest temperature at which the steel will harden should be obtained, and then that limit never exceeded. Evenness of temperature, a soaking heat and freedom from oxidation, are points for attention, and rectangular chambers about 2 feet square have been used, heated by coke. The object to be hardened generally rested upon a thin plate and was heated by radiation, but now, ordinary lighting gas has received much attention for heating purposes, and many successful furnaces are upon the market. The oven furnace, introduced by the American Gas Furnace Company, fulfils admirably the points just enumerated, as the heating chamber is free from an oxidising flame. This apparatus is divided into two chambers by a firebrick slab, the lower one being the combustion chamber, and the upper one is for heating, already referred to, the heat being forced into the latter by passing around the sides of the slab, thorough a narrow interval or slot between the side walls and the edge of the slab. Lead baths, got to a red heat, have also been used with good results. Here oxidation is impossible, as the atmosphere is excluded, and this arrangement also gives the teeth a chance of obtaining the necessary temperature before the interior, resulting in a soft tenacious backing. For thin grooving and angle cutters, washers have been put on the body to obtain a similar result, this also being applicable in many instances.

The following are a few operations that can be done by milling, which perhaps do not adequately represent what the author would wish to convey, but will tend to show, and perhaps suggest to some, the extent to which milling may be applied as far as locomotive or any other work is concerned.

After the coupling and connecting rods, Figs. 227 and 229, p. 134, have been marked out, they are placed two at once upon the machine table for milling. This is performed by two cylindrical cutters, each 10 inches diameter and 11 inches long, on a 5-inch spindle attached to the cross slide, and supported by three brackets. Each rod is fixed at its ends by aid of a jacket, which is simply a box, attached for the occasion to the machine table. These boxes have adjusting screws through their ends and sides, so that the rods can be set to their centre lines by aid of the surface gauge in the first instance, but of course after one side has been milled it acts as a foundation for the opposite. These are parallel cuts and simple radii, which are formed by the horizontal traverse of the table and the radius of the cutter, but for irregular profiles advantage is taken of a former made to the exact dimensions required, fixed upon a stand at a convenient distance from the actual work, bearing against which is a cone, which is kept there by suitable weights, so that the cutter is able to take longitudinal and transverse feeds without impediment by fixings. This method is illustrated by Fig. 276, which shows the profiling arrangements for the joint end of the coupling rods.