Modern Machine Shop

DEC 2018

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Addressing Machine Vibration Modern Machine Shop 61 But in other cases—some involving other parts of the machine, some involving the natu- ral frequency of the system—a simple fix is not enough to sufficiently address the problem. In these cases, the very best response is to have the machine serviced, repairing whatever failing machine element is allowing vibration to affect the workpiece. But service takes time and it means taking the machine out of production. For shops that need to keep going for the short term, researchers have proven out a pro- cess for overcoming vibration's effects without compro- mising productivity and without stop- ping the machine for the time being. What follows is derived from a paper about a technique called "contact-length filtering" written by Saint-Gobain corporate applications engineers John Hagan and Mark Martin. By reducing the work feed rate while increasing the depth of cut, the effects of severe vibration can be eliminated without any net effect on overall productivity. Large Wheel-to-Work Ratio The aim of contact-length filtering is to get the wheel-to-work contact length very large relative to the wavelength of the surface affected by vibration. When the former is high enough rela- tive to the latter, the wheel effectively removes vibration-related peaks from the workpiece, smoothing out the surface even though vibra- tion is still occurring. When the depth of cut is increased by the same multiple that feed rate is decreased, material removal rate (and therefore productivity) can remain the same. The depth of cut controls contact length. Obviously, the contact length's increase needs to avoid unfavorable effects such as material burn, workpiece deflection and so on. The wheel's depth of cut is increased to a level that is heavy compared to standard cutting conditions but still avoids these ill effects. Meanwhile, the feed rate (or work speed) controls the wavelength of the vibration marks in the part. A slower work speed shortens the wavelength. Contact-length filtering begins to achieve a smooth surface when double the wheel-to-work contact length surpasses the wavelength of the chatter, or surface waviness. In other words, the condition required for chatter amplitude reduc- tion is... 2 x Contact Length ( l c )≥ Chatter Wavelength ( chatter) ...where the two figures above define Contact Length ( l c ) and Chatter Wavelength ( chatter). Contact Length = l c = a e = depth of cut d e = equivalent diameter For Surface and Creep-feed Grinding d e equals the wheel diameter + for OD grinding – for ID grinding d w = workpiece diameter d s = wheel diameter For Cylindrical Grinding d e = d s • d w d s ± d w l c a e • d e How to calculate the contact length for contact-length filtering. V w = Work Speed (ipm) f chatter = Chatter Frequency (cpm) High work speed results in longer Chatter Wavelength Lower work speed results in shorter Chatter Wavelength Chatter Wavelength (initial feed rate) Chatter Wavelength (reduced feed rate) Chatter Wavelength (λchatter) = V w ÷ f chatter How to calculate the chatter wavelength given frequency. In testing, researchers used a sensor mounted near the grinding machine's spindle to measure frequency precisely. CREEPING UP ON CREEP-FEED Increasing the depth far enough turns surface grinding into creep-feed grinding. Find out what that means: gbm.media/creepfeed

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