Grinding burn reduction

Dear Doc: I’m getting burn when cylindrical OD grinding. My wheel supplier recommended a more porous wheel to improve cooling. What’s your opinion?

The Doc Replies: Don’t bother. When cylindrical grinding, the coolant removes about 5 percent of the heat (compared to up to 70 percent when creep-feed grinding). A more porous wheel might get that number up to 7 percent, meaning an extra 2 percent of the heat is not going to the workpiece. That’s not much of an improvement. Focus your energies on the grinding and dressing parameters. That’s where I’ve had the most success reducing grinding burn when cylindrical grinding.

Dear Doc: Is there a hard-and-fast rule for calculating wheel rpm as the diameter becomes smaller?

The Doc Replies: There’s a simple answer that also meets safety guidelines. If you begin at the wheel’s full diameter running at the rpm allowed to achieve the maximum surface speed, then as the diameter decreases with wheel wear and dressing you can increase the rpm using the following equation:

New rpm = original rpm × original wheel diameter ÷ current wheel diameter

Therefore, if the maximum rpm for a new wheel is 3,000 rpm and the wheel diameter starts at 18 ” and goes down to 15 “, then when the wheel is at 15 ” you can run it at 3,600 rpm (3,000 × 18 ÷ 15 = 3,600). That’ll provide the same surface footage as you started with and the wheel will not “act softer” or “act harder.”

However, you now have 17 percent less abrasive to do the work (3 “÷18 “), so expect about 20 percent (18 “÷15 “) more radial wheel wear.