December 2012 / Volume 64 / Issue 12|
Achieving reasonable wheel wear
By Dr. Jeffrey Badger
Dear Doc: I grind fine threads into hardened stainless steel with a fine-mesh aluminum-oxide wheel. The threads are 0.8mm deep in a 5mm-dia. ID. There are three threads per part. This operation just chews through the wheel, and I get only one part before I have to dress a 0.2mm depth in my 4mm-dia. wheel. What am I doing wrong?
The Doc Replies: Before jumping to conclusions about doing something wrong, let’s calculate the G-ratio, which is the volume of material ground divided by the volume of wheel worn away. Because the maximum wear probably occurs when grinding the bottom of the thread, let’s simplify and calculate the area of the material ground divided by the area of wheel lost—at the thread bottom.
The area of material ground equals depth removed times length removed. The thread depth is 0.8mm. The length of material removed is the circumference of one thread times the number of threads (p × 5mm × 3 threads = 47.1mm). That gives an area of 37.7mm2 (0.8mm × 47.1mm).
Now let’s look at the wheel lost from being worn away. You’re dressing a 0.2mm depth. Let’s assume 80 percent, or 0.16mm, of that is wear. The circumference of the wheel is 12.6 mm (p × 4mm). That gives an area lost on the wheel at the point on the thread bottom of 2.01mm2(0.16 mm × 12.6mm).
Inputting those two numbers gives a G-ratio of 18.8 (37.7 ÷ 2.01). That’s pretty good when grinding hardened stainless steel with a fine-mesh Al2O3 wheel. So you’re really not doing anything wrong. You’re just trying to cope with one challenging aspect of your grinding operation—the small wheel diameter.
A 4mm-dia. wheel just doesn’t have much abrasive around its circumference. Compared to a 400mm-dia. wheel, it has only 1 percent of the abrasive. It’s not going to mean more wheel wear, but it’s going to mean a much larger depth of wheel wear. And you’re just going to have to live with it.
There are ways to reduce that wheel wear, which is not considerable but the depth of wheel wear will be high because the diameter is so small. A harder-grade wheel, such as I to K; a larger grit size, as long as you can hold the corner; an oil-based coolant instead of a water-based one; and a higher wheel speed will all help.
You could switch to a pricier CBN wheel, which will wear less than an Al2O3 one, but wear will still be high. If the goal is to reduce dressing frequency, try it.
Dear Doc: I cylindrical grind IDs with small, bonded diamond wheels. Is it possible to true them in the machine?
The Doc Replies: Absolutely. Mount a vitrified-bond Al2O3 or silicon-carbide wheel that has a hardness grade of at least L onto an adapter. Then, mount that adapter in the chuck of your cylindrical grinder and true away. Take a DOC of around 0.001 " and quickly traverse across the wheel. Use an Al2O3 or SiC wheel with a grit size a few mesh sizes larger than the diamond grits. If it’s a resin- or metal-bond diamond wheel, be sure to stick it after truing.
Dear Doc: I grind tungsten-carbide IDs with 240-mesh electroplated diamond wheels. Initially, a few parts have poor finish and chatter marks before good parts are produced. How can I avoid 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, which 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. CTEAbout the Author: Dr. Jeffrey Badger is a grinding consultant. His Web site is www.TheGrindingDoc.com.
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