February 2009 / Volume 61 / Issue 2|
Extending CBN wheel life
By Dr. Jeffrey Badger
Dear Doc: I OD plunge grind forms into cylindrical parts. I switched to CBN and am very happy with the results. However, CBN wheel life isn’t as long as I’d like it to be. Any advice?
The Doc Replies: First, are you using a water-based coolant or oil? With CBN, using oil instead of a water-based coolant provides much longer wheel life. And I’m not talking 20 percent longer; I’m talking 10 to 50 times longer wheel life. So if your machine is enclosed and has fire-suppression equipment, switch to oil.
If you’re already using oil or can’t make the switch, you can adjust the plunge speed and workpiece rpm.
Performing a simple calculation shows OD grinding is analogous to surface grinding. Divide plunge speed in ipm by workpiece rpm. That provides the actual DOC in inches per workpiece revolution, which is analogous to DOC in surface grinding.
Next, multiply workpiece rpm by 3.14 times the workpiece diameter. That equals workpiece velocity in ipm, just like in surface grinding.
Also as in surface grinding, take slow, deep cuts to reduce wheel wear. This technique generates a longer arc of cut and smaller forces on individual grits than fast, shallow cuts. For example, plunging at 0.1 ipm on a 1 "-dia. workpiece rotating at 20 rpm equals a DOC of 0.005 " (0.1 ÷ 20) and a workpiece velocity of 63 ipm (3.14 × 1 " × 20). That’s a slow, deep cut.
Now, let’s say you keep the same plunge speed but increase work rpm to 200. That’s a DOC of 0.0005 " (0.1 ÷ 200) and a workpiece speed of 628 ipm (3.14 × 1 " × 200). That’s a fast, shallow cut, which accelerates wheel wear.
Controlling burn and chatter is a different ballgame. In OD grinding, fast, shallow cuts almost always produce less burn than slow, deep grinding.
Chatter, on the other hand, is anybody’s guess. Slow, deep cuts can be conducive to avoiding chatter, but they can also cause chatter because of higher normal forces and wheel blunting. To avoid chatter, the best approach is to try it and see.
Dear Doc: I form-grind different radii on our parts, ranging from 0.004 " to 0.02 ". I use a 150-mesh wheel and have trouble getting a clean radius on the smaller sizes. Why?
The Doc Replies: I spent lots of time beating my head against the wall after visiting companies that insist on a one-size-fits-all mentality for grit sizes, especially those grinding threads and gears.
It’s a fact that to grind a small radius into a part you need a small grit.
Here’s a rough-and-ready calculation to put you in the ballpark in terms of grit size. Divide the number 1.0 by the radius (in inches) you’re trying to achieve. So, if you need a 0.004 " radius, use a 250-mesh size (1 ÷ 0.004 "). For a 0.01 " radius, use a 100-mesh size (1 ÷ 0.01 "). If you’re using a single-point diamond dresser, a little coarser is OK; if you’re crush dressing, you’ll need a finer grit.
Although you’re able to grind a 0.004 " radius with a 150-mesh wheel, you pay a heavy price. Check the dressing on your machine, and I guarantee your operator is dressing that wheel quite timidly. If he’s using a single-point diamond or a diamond disk, he’s traversing very slowly. If he’s using a plunge roll, he’s plunging very slowly and probably counter-directionally. This “splits the grits,” creating a dull, closed wheel. He’s able to produce the radius—just barely—but the dull wheel is generating gobs of heat, burn and chatter.
Stick on a fine-grit wheel, dress like hell and open the wheel. You’ll get a nice, clean radius, no burn and no chatter. And I won’t have to beat my head against the wall. CTEAbout the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. E-mail grinding questions to him at firstname.lastname@example.org. The Doc’s International Hard Facts HSS Grinding Course takes place March 25 to 27 in Paris. Details are at www.HSSGrinding.com.
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