b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Ask the Grinding DocCOOLANT QUESTIONSD ear Doc: We struggle with grinding burn. Theresthe growing, twisting and contorting reach a steady-talk of putting in a coolant refrigeration system.state position.Will that fix the problem? Lets throw out some ballpark estimates. Say theres The Doc replies: Probably not. There are good1 m (3.3\') of steel linkage stuff connecting your reasons to install a refrigeration system, but eliminatingworkpiece and wheel. During your Monday morning grinding burn isnt one. dry cycle, if all that steel heats up by a modest 2C, the Lets assume youre talking about rehardeningmeter will expand by 0.026 mm (0.001") (dimensional burn, in which a workpiece becomes hotter thanincrease = original dimensioncoefficient of thermal approximately 750 C (1,382 F), resulting in phaseexpansion for steeltemperature increase = 1,000 mm change in the steel, or white layer. A coolant at 1310 -6mm/mm/C 2 C = 0.026 mm).20 C (68 F) sucks out only slightly more heat than coolant at 30 C (86 F) because the rate of convectiveWhen you cycle without coolant, heat transfer depends on the temperature differential between the coolant and the hot workpiece. Coolantalmost all the heat ends up in the at 20 C sucks heat out of a 750 C workpiece only 1.4%metal of the grinding machine, faster than coolant at 30 C [(750-20) / (750-30) = 1.014 = 1.4%]. In other words, if you burn with hot coolant,increasing the temperature. And youll almost certainly burn with cold coolant. hot things like to expand.Theexceptionisifyouexperienceburnout in creep-feed grinding. Here, you get large-scale coolant vaporization and lose almost all your coolingSo far, no problem. You dress your wheel, grind a effect, causing a surge in grinding power and sudden,part, find the size, make adjustments and start your catastrophic burn. But this is very rare. In the vastlong batch run. But now youre running with coolant majority of cases, you should fix your burn problemon, which starts sucking out heat from everywhere. The some other way, likely by dressing sharper (and using atemperature increase is no longer 2 C but 1 C. The smaller-grit wheel if surface finish becomes too rough),linkage stuff gradually will shrink by 0.013 mm (0.0005"), improving cooling velocity and aim (especially in creep- bringing your wheel closer to your workpiece. Taken on feed operations) or changing machine parameters. diameter, thats a 0.026 mm decrease. Can you live with Dear Doc: When we arrive on Monday mornings, wethat? It depends on your tolerances.cycle machines without workpieces to warm up things.This is all a gross simplification, but you get the idea. Some operators do it with coolant on, and some do itSo, I say cycle with coolant on. Its not that a colder with coolant off. Whats your take? machine is any better. Its that we want the temperature The Doc replies: Im a fan of doing it with coolantto stay more constant. In fact, thats a main discussion on. Your grinding machine is a giant heat generationpoint when speaking with my customers about whether system,andallthatheateventuallyhastogothey need a refrigeration system. The question is not somewhere. When you cycle without coolant, almostwhether they need lower temperaturesits whether all the heat ends up in the metal of the grindingthey need more consistent temperatures.CTEmachine, increasing the temperature. And hot things like to expand.Your workpiece base ultimately connects to yourabout the author grinding wheel through a series of contacts: toolholder, headstock, machine base, columns, bearing assembly,Jeffrey Badger, Ph.D., is an inde-spindle and wheel. All those parts expand at differentpendent grinding expert who now is giving his grinding course online. rates, causing a growing, twisting and contortingFor more information, visit www.grinding machine. That really isnt an issue as long asTheGrindingDoc.com.16SEPTEMBER 2020GrindingDoc.indd 16 8/14/20 3:24 PM'