Extending CBN wheel life: General Industry Coverage

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.