December 2009 / Volume 61 / Issue 12|
Grinding burn reduction
By Dr. Jeffrey Badger
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.
Does this meet safety guidelines? Yes. ANSI B7.1 from the American National Standards Institute states that as the diameter gets smaller, you can increase the wheel rpm as long as the maximum surface speed—in sfm or m/sec.—established for the wheel when new is not exceeded.
Dear Doc: I flute grind tungsten-carbide tools with resin-bond diamond wheels. My wheel supplier is pushing hybrid-bond wheels. What’s your take on these wheels?
The Doc Replies: I have numerous customers flute grinding tungsten-carbide tools and have seen great results from hybrid-bond wheels. Upon further investigation, I discovered that those who hadn’t had success with them hadn’t done their due diligence in choosing the correct type of hybrid bond. Even within the family of “hybrid-bond fluting wheels” there are different bond formulations. Some formulations are for more aggressive grinding conditions with short contact lengths and some are geared toward less aggressive grinding with longer contact lengths.
You won’t be able to figure out which bond is which based on the wheel specification, so ask your wheel salesman to guide you. If he can’t, find someone who can recommend a specific hybrid-bond type/grade based on your contact length, tool size and spindle power. If the bond is too hard, your wheel will generate a lot of heat. If it’s too soft, you’ll eat up your wheel.
Finally, I’ve had hybrid-bond wheel salesmen tell me the aggressiveness of wheel conditioning, or “sticking,” should be based on the bond type. I haven’t seen this myself. My philosophy has always been to use a soft stick with small grit (50 to 75 percent of the diamond-grit size) and stick the wheel hard. But if salesmen can provide specific sticking guidelines for a particular wheel type, follow them. CTEAbout the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com. In January, he will conduct his “High Intensity Grinding Course” in Dublin, Ireland.
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