b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Ask the Grinding DocFINDING THE SOURCE OF CHATTER D ear Doc: We ID-grind steel bearing ringsworkpiece revolution = chatter frequency in Hz60 / with a CBN wheel running at 8,500 rpm andworkpiece rpm, or 14060 / 120 = 70.]a workpiece running at 120 rpm. We get about 70Since your wheel rpm and potential natural frequency chatter marks around the inner circumference of theare very close, how can we know if the chatter is ring. Im trying to figure out if the chatter is comingcoming from an out-of-balance/out-of-true wheel or from an untrue or imbalanced wheel or just basic, self- the self-excited natural frequency? Simply slow down excited chatter. Is my thinking right on this? And isthe wheel rpm and see if the chatter marks become there a way to tell? farther apart (with fewer chatters per revolution). If they The Doc replies: Your thinking is right, and theresdo, that points toward out-of-balance or out-of-true a simple test to see where the chatter is comingchatter. If they stay the same, that points toward self-from. First, something is happening at a frequency ofexcited chatter.140 chatters per second, and we have to figure outWhy is that? With self-excited chatter, the wheel/spindle (or workpiece/toolholder) likes to bounce up and down at its natural frequency, and thats independent of wheel speed. If the spindle likes to bounce up and down at 140 bounces per second, the spindle will do that at all rpm: 8,500 rpm, 7,500 rpm, 9,500 rpm, etc.But if its out-of-balance or out-of-true chatter, slowing down the wheel rpm will give a lower chatter frequency and hence a bigger distance between chatter marks. (7,500 rpm = 125 revolutions per second, which now gives 62.5 chatters per revolution instead of 70.) [12560 / 120 = 62.5.]Thats a quick test to point you in the right direction of the cause of chatter. Maybe you immediately can A shop seeks answers about inner circumference chatter marks onpinpoint it, or maybe youll need a vibrations expert bearing rings. with multiple accelerometers and four days of complex computersimulation.Thatsbecausechatteris what that something is. [Chatter frequency in Hz =notoriously complex. Frederick Winslow Taylor(1856-number of chatter marks per workpiece revolution 1915), the godfather of production engineering, said workpiece rpm / 60 = 70120 / 60 = 140.] in 1906, Chatter is the most obscure and delicate of Chatter has many possible sources, but lets startall problems facing the machinist. Its still true today. with the two most likely: 1) an out-of-balance or out- Chatter is tough.CTEof-true wheel or 2) self-excited chatterthat is, the wheel/spindle bouncing up and down at its natural frequency or the workpiece/toolholder bouncing upabout the author and down at its natural frequency. An out-of-balance or out-of-true wheel chatters at the wheel rpm, soJeffrey Badger, Ph.D., is an inde-8,500 rpm = 142 revolutions per second. If one of thosependent grinding consultant. Once COVID-19 is over, hell resume his natural frequencies is about 140 Hz and the workpiecethree-day High Intensity Grinding is running at 120 rpm, that will give around 70 chattersCourse. For more information, visit on the workpiece. [Number of chatter marks perwww.TheGrindingDoc.com.22JULY 2020GrindingDoc.indd 22 6/17/20 2:18 PM'