Coolant questions

Dear Doc: We struggle with grinding burn. There’s talk of putting in a coolant refrigeration system. Will that fix the problem?

The Doc replies: Probably not. There are good reasons to install a refrigeration system, but eliminating grinding burn isn’t one.

Let’s assume you’re talking about rehardening burn, in which a workpiece becomes hotter than approximately 750° C (1,382° F), resulting in phase change in the steel, or “white layer.” A coolant at 20° C (68° F) sucks out only slightly more heat than coolant at 30° C (86° F) because the rate of convective heat transfer depends on the temperature differential between the coolant and the hot workpiece. Coolant at 20° C sucks heat out of a 750° C workpiece only 1.4% faster than coolant at 30° C [(750-20) / (750-30) = 1.014 = 1.4%]. In other words, if you burn with hot coolant, you’ll almost certainly burn with cold coolant.

The exception is if you experience burnout in creep-feed grinding. Here, you get large-scale coolant vaporization and lose almost all your cooling effect, causing a surge in grinding power and sudden, catastrophic burn. But this is very rare. In the vast majority of cases, you should fix your burn problem some other way, likely by dressing sharper (and using a smaller-grit wheel if surface finish becomes too rough), improving cooling velocity and aim (especially in creep-feed operations) or changing machine parameters.

Dear Doc: When we arrive on Monday mornings, we cycle machines without workpieces to warm up things. Some operators do it with coolant on, and some do it with coolant off. What’s your take?