Cutting speed: using our favorite constant.In working with machinists over the years I've noticed that if you stand in front of a machine tool and ask about speed, most people think "speed of the spindle spindle: see spinning. A rotating shaft in a disk drive. In a fixed disk, the platters are attached to the spindle. In a removable disk, the spindle remains in the drive. Laptops use spindle designations to indicate the number of built-in drives. " or rpm. The fact that the spindle is revolving slowly or quickly does not indicate the value of the cutting speed. In our CNC (Computerized Numerical Control) See numerical control. CNC - Collaborative Networked Communication training classes, it is my goal to have student think in terms of sfpm (surface feet per minute Surface feet per minute (SFM or SFPM) is a unit of velocity used in machining to identify the machinability ratings of a material. SFM is the velocity around the tool or the material measured in feet-per-minute as the spindle of a lathe turns. ). It is a challenge to get people to think in terms of sfpm and not rpm. We see many machinists who guess rpm values and keep on guessing until they get it working. Wouldn't it be better to just start out with the correct sfpm? What is sfpm and how is it calculated? Surface feet per minute is just that--the distance (measured in feet) traveled by the cutting tool each minute. This value is determined by the following formula. sfpm = (rpm x part or cutter cutter, small, one-masted sailing vessel, with a rig similar to that of a sloop except that it usually has a sliding bowsprit and a topmast. From 1800 to 1830 cutters were in service between England and France. diameter) + 3.82 You can re-arrange the formula and calculate spindle rpm. rpm = (sfpm x 3.82) + part or cutter diameter The cutting speed is dependant on Adj. 1. dependant on - determined by conditions or circumstances that follow; "arms sales contingent on the approval of congress" contingent on, contingent upon, dependant upon, dependent on, dependent upon, depending on, contingent the spindle speed and the part (or cutter) diameter. The other piece of the formula is the constant 3.82. Remember that the math constant 3.82 is derived from 12 divided by Pi ([pi]). Recall from school days that Pi ([pi]) is the mathematical constant A mathematical constant is a quantity, usually a real number or a complex number, that arises naturally in mathematics and does not change. Unlike physical constants, mathematical constants are defined independently of any physical measurement. that represents the ratio of a circle's circumference to its diameter. Pi is used to calculate circumference (formula: [pi] x diameter = circumference); so when we calculate the movement of a circular part or tool, we need Pi. The number 12 is used to convert the inch value of the part diameter into feet. Remember, we measure our parts in inches but use feet in cutting speed calculations. We encourage machinists to use the 3.82 constant. The above formula is the most popular, although there is another, which uses the reciprocal Bilateral; two-sided; mutual; interchanged. Reciprocal obligations are duties owed by one individual to another and vice versa. A reciprocal contract is one in which the parties enter into mutual agreements. of the 3.82 constant, a value of 0.262. rpm = sfpm + (part or cutter diameter x .262) Whichever formula used, calculating rpm from a recommended sfpm is a must in machining. Calculating rpm is easy once you have the sfpm--but where does the sfpm come from? Selecting an ideal sfpm is a combination of research and experience. Research a sfpm by reviewing tooling catalogs. Tool suppliers provide much information on their products, including recommended cutting speeds for various materials and cutting conditions. Be aware that these values are based on a short tool life and may be too aggressive for many machining applications. Keep in mind that some tools can be run too slowly. Problems can occur from running both too fast and too slow. Also review your own experiences or those of your colleagues or company. Each company deals differently with certain materials, uses different machine tools, workholding styles, and cutting tools. All of these can impact the successful selection of cutting speeds. Using all of this information you can select good working cutting speeds that should be productive. Once you've selected a sfpm and can calculate the rpm, use the CNC program to set the values chosen. When turning, boring, and grooving on a lathe lathe (lāth), machine tool for holding and turning metal, wood, plastic, or other material against a cutting tool to form a cylindrical product or part. It also drills, bores, polishes, grinds, makes threads, and performs other operations. we can program a G96 command that sets the cutting speed and allows the machine to calculate the spindle rpm. However, when using tools that are designed to cut on the tool point, such as center drills, spot drills, twist drills twist drill, n 1. a drill having one or two deep helical grooves extending from the point to the smooth portion of the shank. 2. a spiral bone bur. , insert drills, taps, and reamers, we must calculate a cutting speed for the program. This is the same situation for all tool types used on machining centers. For example, we know that a specific sfpm works well on a certain steel. We'll use 100sfpm for a HSS HSS Humanities and Social Sciences HSS High Speed Steel HSS Home Subscriber Server (3GPP) HSS Hospital for Special Surgery (New York, NY, USA) HSS Hospital for Special Surgery HSS History of Science Society drill and 28sfpm for tapping. These known values can be used in our program and the control calculates the rpm.
Example: S[100*3.82/.4] (spot drill)
S[100*3.82/.312] (twist drill)
S[28*3.82/.375] (tap)
Programming the spindle speed in this manner encourages everyone to think in terms of sfpm. Remember, using the correct sfpm results in better tool life, improved chip control, and more predictable production efficiency. Using the sfpm in your program for all tool types has got to beat the rpm guessing game. Steve Rose is a professional trainer and president of RTSI RTSI Radiotelevisione Svizzera di Lingua Italiana (Radio and Television of Switzerland) RTSI Real Time System Integration (National Instruments) RTSI Remote Trunking System Interface , Solon Solon, Athenian statesman Solon (sō`lən), c.639–c.559 B.C., Athenian statesman, lawgiver, and reformer. He was also a poet, and some of his patriotic verse in the Ionic dialect is extant. At some time (perhaps c.600 B.C. , OH. Rosaleen Rose offers Internet website development. They can be reached by phone at 440.542.3066; e-mail srose@rosetraining.com; or on the web at www.rosetraining.com. Copyright Rose Training Systems Inc., 2007 |
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