b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Ask the Grinding DocREDUCE REGENERATIVE CHATTER D ear Doc: We cylindrical OD- regenerative chatter, which worksthe natural frequency.grind steel shafts. Our machinemuch the same. Over time, theDoes it work? The answer is sort has options for varying the wheelbouncing up and down of the wheelof and sometimes. It seems to and/or workpiece rpm to reduce(at its natural frequency) causes un- work better on roll grinding.chatter risk. How does this work? even wheel wear, creating lobes onBut there are other benefits. First, The Doc replies: In the first op- the grinding wheel. This is also anit allows you to avoid the dreaded tion, the machine varies the work- excitation sourceat exactly theinteger values (wheel rpm/work-piece rpm up and down a few per- natural frequencythat exacer- piece rpm = 8.0000 or 9.0000 in-cent (say, sinusoidally from 160 tobates the chatter. These lobes takestead of 8.5972042), which cause 200 rpm instead of a constant 180longer to develop (minutes or evenworkpiece lobing. Second, it allows rpm) over a short period (usu-ally a few seconds) to reduce workpiece regenerative chat-ter. Its hard to explain why in less than 500 words, but Ill try. The wheel spindle likes to bounce up and down at its natural frequencysay, 180 bounces per second. (See the figure.) This creates 60 lobes on the workpiece (180 bounces/second3 revolu-tions/second). In subsequent workpiece revolutions, these lobescausethegrinding depth of cut to vary, creating a varying force pushing up on the spindle. This is an excita-tion force, which causes the wheel spindle to bounce up and down even more, creating even bigger lobes. To make thingshours instead of seconds). Here, weyou to avoid running the wheel or worse, because these lobes werevary the wheel speed (again +/-10%workpiece at some other natural created at the natural frequency,but over a longer period, like everyfrequency (say, of the workpiece or they now excite the wheel spindle attwo minutes) to vary the distance be- machine base).the natural frequency, causing chat- tween wheel lobes and make themIf its an easy modification, do it. ter to become out of control. obliterate themselves. Theyre stillBut dont expect miracles. Expect By varying the workpiece rpm, wethere but not as high, and theyremodest improvements. But modest vary the distance between lobes.not exciting the system at exactlyimprovements can add up.CTETheyre still there, but because the distance doesnt excite things at exactly the natural frequency, theabout the author situation isnt exacerbatedandJeffrey Badger, Ph.D., is an independent grinding consultant workpiece regenerative chatter iscurrently offering his one-day CovidMania Zoom Courses reduced. Hopefully. online. For more information, visit www.TheGrindingDoc.The second option is for wheelcom.14DECEMBER 2020 AskGrindingDoc.indd 14 11/15/20 10:02 AM'