Achieving reasonable wheel wear: General Industry Coverage

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.