Speeds and Feeds. Speeds and feeds are of extreme importance when considered in connection with the life and efficiency of a cutter and volume of output. Little can be said, however, in the matter of general rules to follow in determining correct speeds and feeds, owing to the different conditions that exist in different shops, and, in fact, in the same shop, where one set of rules will not always hold on like jobs. The amount of power and rigidity in different machines, kind of material, width and depth of cut, quality of finish required, and many other factors, all enter into the question, and prevent the establishing of any definite rules. Sometimes the speed must be reduced, yet the feed not changed, and vice versa; again both speed and feed must be reduced or increased, as the case may be. Often the rate of feed depends almost wholly upon the degree of accuracy and quality of finish required. In general, work of a delicate character, requiring an accurate finish, demands light cuts and fine feeds, and work of a heavy character, where the principal object is to remove metal rapidly, requires deep cuts and coarse feeds. On work that permits of heavy roughing cuts, the finishing cuts should usually be light. The feed, inasmuch as it governs the output of work, is of greater importance than the speed of a cutter, and it is generally a safe rule to follow, that the speed should be as fast as the cutter will stand, and the feed as coarse as is consistent with good work. Much must be left to the judgment of the operator as to the correct speed and feed to use for the work in hand, and many cases will require repeated experiments before the best results are obtained. When any difficulty is encountered in obtaining the right combination of speed and feed, it is well to seek the advice of the foreman in charge of the job, or that of a widely experienced milling machine operator.
The following surface speeds will serve to give an idea, or basis, to work from. They may be varied slightly to suit the requirements of the work in hand. Using carbon steel cutters: For brass, 80 feet to 100 feet per minute; for cast iron, 40 feet to 60 feet per minute; for machinery steel, 30 feet to 40 feet per minute; and for annealed tool steel, 20 feet to 30 feet per minute, have been found satisfactory. With high speed steel cutters for the same materials, the following speeds are advocated: For brass, 150 feet to 200 feet per minute; for cast iron, 80 feet to 100. feet per minute; for machinery steel, 80 feet to 100 feet per minute; and for annealed tool steel, 60 feet to 80 feet per minute.
