CUTTING SPEEDS AND FEEDS Cutting Speeds for Turning. - The cutting speed is governed principally by the hardness of the metal to be turned; the kind of steel of which the turning tool is made; the shape of the tool and its heat-treatment; the feed and depth of cut; the cooling medium used, if any; the power of the machine; and its design and condition. The table "Cutting Speeds and Feeds for Turning Tools" will be found useful in determining the most economical speeds for a given depth of cut and feed when turning hard, medium and soft steel or cast iron. These tables are based upon the results of the experiments conducted by Mr. F. W. Taylor, and it is assumed that a tool made of a good grade of high-speed steel, properly heattreated and correctly ground, is used. It will be noted that the cutting speed is much slower for cast iron than for steel. Cast iron is cut with less pressure or resistance than soft steel, but the slower speed required for cast iron is probably due to the fact that the pressure of the chip is concentrated closer to the cutting edge, combined with the fact that cast iron wears the tool faster than steel. The speeds given are higher than those ordinarily used, and, in many cases, a slower rate would be necessary to prevent chattering, or because of some other limiting condition. Ordinary machine steel is generally turned at a speed varying between 45 and 65 feet per minute. For ordinary gray cast iron, the speed usually varies from 40 to 50 feet per minute; for annealed tool steel, from 25 to 35 feet per minute; for soft yellow brass, from 150 to 200 feet per minute; for hard bronze, from 35 to 80 feet per minute, the speed depending upon the composition of the alloy. While these latter speeds correspond closely to general practice, they can be exceeded for many machining operations. The cutting speed is limited by the durability of the turning tool or the length of time it will turn effectively without re-grinding. The hardness of the metal being turned, combined with the quality of the tool, are the two factors, aside from the speed, which largely govern the time that the tool can be used before re-grinding is necessary. The experiments of Mr. Taylor led to the conclusion that, as a rule, it is not economical to use roughing tools at a speed so slow as to cause them to last more than 11⁄2 hour, without being re-ground; hence, the speeds given in the table previously referred to are based upon this length of time between grindings. Sometimes the work speed cannot be as high as the tool will permit, because of the chattering that often results when a machine is old or not massive enough to absorb the vibrations. The radius of the tool point or "nose" also affects the cutting speed and durability. In order to determine how the durability is affected by the nose radius, tests were made in cast iron and steel with tools having a cutting angle of 75 degrees. The radius of the tools was gradually increased without changing the speed, feed or depth of cut, and in all cases, the durability of the tools increased as the radius became larger. The United States Navy Department requires the following tests to be made on high-speed steel tools. The lathe tool, known as %-inch standard, must be able to take a cut 16 inch deep with 16 inch feed and a surface speed of 60 feet per minute, for 20 minutes, without re-grinding, on a steel forging of open-hearth steel having a minimum tensile strength of 80,000 pounds per square inch, elastic limit of 50,000 pounds per square inch, and elongation in 2 inches of 25 per cent. This steel forging is annealed prior to the test. Effect of Cooling Water. It has been estimated that the cutting speed, when using high-speed steel tools for turning steel, can be increased about 40 per cent by throwing a large stream of cooling water (supersaturated with soda to prevent rusting) directly upon the chip at the point where it is being removed by the tool. For ordinary carbon steel tools, the gain is about 25 per cent. The most satisfactory results are obtained from a stream of water falling at a rather slow velocity, but in large volume. The gain in cutting speed by the use of water or other fluid is practically the same for all qualities of steel from the softest to the hardest. Feed and Depth of Cut for Turning. - Ordinarily, coarser feeds and a greater depth of cut can be used for cast iron than for soft steel. In general, with a given depth of cut, metal can be removed more quickly by using a coarse feed and a slower speed, than by using a fine feed and a proportionate increase in speed. When the turning operation is simply to remove metal, the feed should be coarse and the cut as deep as practicable. Sometimes the cut must be comparatively light, either because the work is too fragile and springy to withstand the strain of a heavy cut, or because the lathe has not sufficient power. The effect of the feed and the depth of cut on the cutting speed is shown by the table "Cutting Speeds and Feeds for Turning Tools." CUTTING SPEED 20 40 60 80 100 120 Influence of Temperature on Durability of Tool. By testing various samples of carbon steel in a special testing machine, it has been found that carbon tool steels have a very low durability at a low cutting speed; that there is an increase of durability as the cutting speed increases; and that a maximum durability is obtained at a cutting speed of about 50 to 80 feet per minute. There is then a decline of durability to a very low value if the cutting speed is further increased. These general characteristics are common to all tool steels, whether of the carbon or highspeed steel type (tungsten or tungsten-vanadium varieties). All of these steels show, when the durability is recorded in diagrammatic form, a single- or doublepeaked curve, according to the heat-treatment they have received. All show a low durability at low cutting speed, this characteristic being especially marked in the case of some high-speed steels, which latter often retain their durability at very high speeds. One of these double-peaked curves is shown in the illustration. With this particular steel, the durability increased up to a cutting speed of 50 feet per minute; it then diminished until the cutting speed reached 70 feet per minute, when the durability again increased up to 80 feet per minute. Hence, the tool, in this instance, would work longer between grindings with a cutting speed of 80 feet per minute than with a speed of 70 feet per minute. The durability, however, might be affected by chattering, as the result of the higher speed. The changes in the durability of cutting tools are mainly caused by the changes in DURABILITY INCHES OF TUBE CUT 2 0 Diagram showing Relation between Cutting Speed and Durability, the latter measured by Length of Test Tube cut the temperature of the cutting edge, due to the heat generated at different cutting speeds. This heat theory has been confirmed by experiments showing that changes of durability corresponding to those which occur under varying cutting speeds can be produced by varying the temperature of the tool in other ways, while the cutting speed remains constant for instance, by varying the temperature of the water with which the tool is flooded, by varying the depth of the cut or by dispensing entirely with the cooling water. Many cases are known to have occurred in ordinary machine shop practice, where an increase in cutting speed has actually resulted in increased durability of the tool. Low durability at low cutting temperatures (for example, on finishing cuts) is a familiar characteristic of high-speed steels, and is most marked in tools which have been suitably hardened for very high temperature work. High-speed steel can be so hardened as to retain its durability at fairly low temperatures, and there are now on the market tungsten steels specially adapted for low temperature work, such as finishing heavy forgings; but all of the steels tested proved to be less durable if the cutting temperature was low enough. Rules for Calculating Cutting Speeds. To find the number of revolutions required for a given cutting speed, in feet per minute: Multiply the given cutting speed by 12, and divide the product by the circumference (in inches) of the turned part. To find the cutting speed in feet per minute for a given number of revolutions and diameter: Multiply the revolutions per minute by the circumference, and divide the product by 12. Expressing these rules as formulas: in which N = revolutions per minute; C = cutting speed in feet per minute; and D = diameter in inches. To find the time, in minutes, required to take one complete cut over a part to be turned, when the feed per revolution, the total length of the cut, and the number of revolutions per minute are given: Divide the total length of the cut by the product of the number of revolutions per minute and the feed per revolution. If L = total length of cut in inches; N = revolutions per minute; F = feed per revolution in inches; and T time required to take one complete cut, in minutes; then: = T= L Speeds and Feeds for Milling. - A general idea of the speeds that are feasible when using carbon-steel cutters can be obtained from the following figures, which represent the velocity (in feet per minute) at the circumference of the cutter: For roughing cuts in cast iron, 40 feet per minute; in machine steel, 60 feet per minute; in tool steel (annealed), 25 feet per minute; in brass, 75 feet per minute. For finishing cuts, the speeds vary from 50 to 55 feet for cast iron; from 75 to 80 feet for machine steel; from 30 to 35 feet for annealed tool steel; from 95 to 100 feet for brass. These figures represent a fair average for carbon-steel cutters; high-speed steel cutters of good quality can usually be operated at approximately twice the speeds given. The speed of a milling cutter, in revolutions per minute, may be obtained by means of the following formulas which give results representing ordinary shop practice: Let N revolutions per minute; D= diameter of cutter; then, = The rate of feed, aside from the cutter design, depends upon the width and depth of the cut, the kind of material being milled, the quality of finish desired, the rigidity of the work, the power of the machine, etc. As a general rule, a relatively low cutting speed and a heavy feed is used for roughing; for finishing, the speed is increased and the feed diminished. The data given in connection with the examples of milling shown by the tables "Speeds and Feeds for Milling" indicate, in a general way, what speeds and feeds are practicable for different classes of work, when using a rigid machine and modern cutters. Diam.. Inches Time Required for Cutting Tool to Travel 1 Inch, When the Feed is 20 Feet 25 Feet Surface Speeds in Feet per Minute 30 Feet 35 Feet 40 Feet 45 Feet 50 Feet 60 Feet Min. Sec. Min. Sec. Min. Sec. Min. Sec. Min. Sec. Min. Sec. Min. Sec. Min. Sec. |