August 2012 / Volume 64 / Issue 8|
The real cause of burn
By Dr. Jeffrey Badger
Dear Doc: I creep-feed grind slots in hardened steel and am constantly battling burn. Some days I can crank up the feed rate and grind without burn, but other times burn occurs even at a low feed rate. Why?
The Doc Replies: My customers think “grinding faster” is the primary cause of burn. Therefore, I first have them test for burn and chart burn severity vs. specific material-removal rate (SMRR). The SMRR is calculated by the following equations, both of which give results in mm2/sec.
Courtesy of J. Badger
DOC (mm) × feed (mm/min.) ÷ 60, or 10.75 × DOC (in.) × feed (ipm).
If you keep all other grinding conditions constant, burn should increase as you increase the SMRR. However, other factors play an even more important role than SMRR, and if these aren’t chosen correctly you’ll get results like those shown in the figure above, taken from a company flute-grinding HSS, which shows the severity of burn from acid cooking. Acid cooking is a process whereby the workpiece is boiled in hydrochloric acid to show the severity of residual tensile stresses.
Why are the results all over the place? The biggest cause of burn is not an SMRR that’s too high. Instead, it’s caused by dull dressing, a timid grit-penetration depth (i.e., not grinding aggressively) or bad cooling when creep-feed grinding. If you have one—or all—of these conditions, you’ll burn the workpiece regardless of the SMRR.
The theoretical surface temperature vs. SMRR for creep-feed grinding at a 1mm depth and increasing feed rates can be calculated using “The Grinder’s Toolbox,” a program I use to predict grinding temperatures. For a given set of dressing and grinding conditions, temperatures rise with an increasing SMRR. That’s fine and reasonable, but if you have dull dressing, a timid grit-penetration depth and bad cooling, you’re going to experience burn at a much lower SMRR.
Let’s say your burn threshold is 600° C. If you dress aggressively and choose an aggressive grit-penetration depth, you can grind about as fast as you want and not burn. But if you don’t dress right or if your speeds and feeds are timid or your cooling is poor, be prepared for burn at low SMRRs.
Dear Doc: I ID grind tungsten carbide with 240-mesh electroplated diamond wheels. When I first use a wheel, the first few parts have a poor surface finish and chatter marks before good parts are produced. Is there a way around this?
The Doc Replies: I recently visited a company that had the same issue. To overcome it, the operator mounts the wheel and then uses a dial gage, a piece of wood and a mallet to tap the wheel as close to true as possible—within 0.001 " and preferably less. He then grinds several IDs undersize and allows the wheel to break in. This knocks off the high, rogue grits. Once the wheel is broken in, he reruns those parts to size while producing the others. CTE
About the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. He’ll be giving a Rollomatic-hosted grinding course about grinding tungsten carbide Nov. 6-8 in Mundelein, Ill., near Chicago.
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