There are no hard and fast rules that will properly govern a majority of cases of the continually varying conditions of milling cutters, machines, and the material to be machined. In any case, much must be left to the judgment of the foreman and the operator. Prominent among the conditions that tend to vary the cutting speed are the following:
The cutter may be newly ground, keen, and sharp; or it may have been considerably dulled by use. While not dull enough to require grinding, it will not be safe to run it up to the speed of a sharp cutter. The teeth may be worn thin from long use and re-grinding, and not strong enough to stand the strain of maximum speed. The cutter may be of such a form-as a double-angle cutter-that the teeth will not bear the strain of full speed.
The machine may be well designed and built, and free from vibration; or it may be directly the reverse, a fast speed producing so much chattering as to spoil both work and cutter. The arbor may be large and stiff, or small and slender. In one case, a fast speed may be maintained; and in the other, both work and cutter would suffer. The driving gearing may be well designed and its teeth fit accurately with no backlash; or it may be poorly designed and made, or much worn, and cause much chattering on a fast speed. There are many other similar conditions.
The material may be of varying degrees of hardness and toughness, and of a great variety of forms. Some iron castings will be more severe on a cutter than tool steel would be. The scale on cast metal is very hard to cut through, and dulls the teeth of a cutter quickly. The varying hardness of steel, from that ordinarily found in the bar to that properly annealed, is great. The amount of carbon in steel is always a varying condition for which it is difficult to formulate rules. Therefore it is only possible to give rules that will meet a fair average of conditions.
In order to accommodate different sizes of cutters, maintain a uniform cutting speed, and also allow for difference in hardness of the material being worked, it is necessary that the milling machine should be supplied with several speeds. In the ordinary miller we usually have a four-step cone with back gears, which gives eight speeds with a single overhead belt. The countershafts for these machines are of the friction type, and are supplied with two driving pulleys driving in the same direction, but at different speeds, giving a total, including the back gears, of sixteen speeds for each machine.
Form of Cutter as Affecting Its Speed and Feed, A slitting cutter (practically a saw) may be run much fasten than one of broad face.
A cutter of small diameter will cut faster than a large one, as the arc of action is much less.
Angle cutters must be run at lower relative speeds so as not to break off the slender points of the teeth.
The speed may sometimes be profitably increased without changing the rate of feed. Again, the speed should be decreased according to the conditions of the work.
There is no direct and constant ratio between speed and feed. Conditions may vary either one without changing the other.
A roughing cut will often work better with a moderate speed and a coarse feed. The smoothness of the work is not so important as taking off the surplus stock. With a finishing cut, the conditions are reversed and a fine feed is necessary.
Cutters with inserted blades will not usually stand as high a speed as solid cutters, particularly when the blades have a large cutting surface. This condition is emphasized when cutting rather hard and tough material.
If there is a comparatively small space for chips between the teeth of the cutter, a light cut must be taken, or a slower feed used, so that the chips will not clog the cutter. Speed Used on Particular Work or Material. The speed used on any particular work depends, as before stated, on the diameter of the cutter and the character of the work. Thus, with carbon steel cutters, the cutting speed will be 30 to 60 feet per minute. With highspeed steel cutters, double these speeds may be maintained if the drive of the machine is strong enough to pull the cut. When using very small cutters, the machine itself will not usually give a speed which is high enough to suit the diameter of the cutter. For such work, a high-speed attachment, Fig. 233, is furnished, by which the small, light cutters may be driven at a suitable rate.
Of equal importance with the correct speed for the cutter, is the maximum feed or table speed, which is reckoned in inches per minute.
A more logical method of designating the feeds, and one which has been adopted by several makers, is to give the advance of the table in thousandths of an inch for every turn of the spindle.
Based upon the use of the ordinary carbon steel cutters, the
Brown and Sharpe Manufacturing Company have prepared the following statements regarding the speed of cutters:
Fig. 233. High-Speed Attachment for Milling Machine.
It is impossible to give definite rules for the speed and feed of mills. The judgment of the foreman or man in charge of the machine should determine what is best in each instance.
As usually the highest possible speed and feed are desirable, it pays to increase them both until it is seen that something will break or burn, and then