Determining actual wheel costs

Dear Doc: Previously, our company consumed about $80,000 of Norton grinding wheels annually. After the purchasing manager pushed for management to authorize a changeover to wheels made in China, which cost about half the price, we recently switched. The Chinese wheels seem OK. In spite of this, I’m afraid the wheels will eventually increase overall costs. Are my fears ill-founded?

The Doc Replies: So begins a new adventure. From what I’ve seen at other companies, the following is how it’ll play out.

If, like most, your company didn’t do any real testing, such as measuring grinding power and wheel wear, but just stuck a Chinese wheel on and found, “Yeah, it works OK,” you really don’t know how the Chinese wheels are performing. If you did conduct tests, you might’ve found that the Chinese wheel wore about the same amount and generated about the same grinding power as the previously applied wheel.

But quality is not the only issue. Consistency is key. That Chinese trial wheel might’ve been fine. However, after consuming numerous wheels, you might find some are good and some aren’t so good. Eventually, overall wheel wear probably will be slightly higher, which requires more dressing.

Over time, you’ll probably find that grinding power is (off and on) a little higher, and you must decrease your feed rates. A year later you’ll be operating at a slightly reduced level.

If you do the math, you’ll find wheel-consumption cost is not the killer—it’s cycle time. If you’re running 20 machines for two 8-hour shifts a day, 5 days a week for 48 weeks a year, that’s 76,800 hours. Let’s say cycle time increases 5 percent. That’s 3,840 extra hours a year. If each operator conservatively costs $30 an hour, that’s an extra $115,200, more than the Chinese wheels initially saved you.

Dear Doc: Is there a way to measure temperatures when grinding?

The Doc Replies: A lot of academics have tried, but even under controlled laboratory conditions with test pieces manufactured with thermocouples mounted inside, measuring temperatures is just not feasible.

The maximum temperature is at the wheel/workpiece interface and tapers off quickly—within 0.001 " from the surface. Therefore, you have to grind through the thermocouple to get an accurate measurement. Then, because of the insulating properties of the thermocouple, it never measures the maximum temperature. There’s also the issue of aligning the thermocouple with the hot spot, where temperatures are highest, which is difficult. 

In addition, using thermal imaging to measure temperature is problematic. Seeing the hot spot is difficult enough when grinding dry, and it’s even more difficult when coolant is being applied.

Dear Doc: I switched from aluminum-oxide to CBN wheels. I grind with the same speeds and feeds but spindle power consumption is 10 percent higher with CBN. Am I at a higher risk of burning parts?

The Doc Replies: Probably not. When you grind with Al2O3, about 80 percent of the rubbing heat enters the workpiece, and the rest enters the grit. With CBN, about 40 percent of the heat enters the workpiece and 60 percent goes into the grit. So the high thermal conductivity of the CBN grit means it sucks in a large portion of the heat. In addition to its hardness, that’s one of the key benefits of CBN.