Keeping cool grinding stainless

The Doc Replies: Stainless is more difficult to grind than hardened steel primarily for two reasons: lower thermal conductivity and a nasty chemical reaction. First, stainless steel, such as 304 or 316, has about half the thermal conductivity of, say, 52100 steel. That means 52100 sucks away the heat generated during grinding more quickly, reducing surface temperatures. Based on the Jaeger heat-source model, having half the thermal conductivity results in grinding surface temperatures that are about 40 percent higher.

Second, the high chrome content—at least 10.5 percent—in stainless steel causes a thin passivation layer of chromium oxide (Cr₂O₃). The chemical formula for aluminum-oxide wheel grits is Al₂O₃. Those two molecules are mutually soluble, meaning they dissolve in one another, and that spells trouble. If you perform X-ray spectroscopy on a ground stainless steel workpiece, you’ll find aluminum is present. This means the stainless steel workpiece dissolved a little Al₂O₃ from the wheel. The result is loading at the wheel surface and excessive wheel wear.

What’s the solution? In terms of the reduced conductivity, you must reduce the feed rate by around 30 percent to achieve the same grinding surface temperature. There’s no getting around it.

In terms of minimizing the chemical reaction, improved cooling is the best approach. If you can get a thin layer of coolant between the grit and the workpiece, you’ll retard that chemical reaction. You can achieve this with a coolant velocity that’s close to the wheel surface velocity and by aiming the coolant directly at the wheel/workpiece interface.

Dear Doc: When creep-feed grinding tungsten-carbide endmills, some of our operators push the feed rate hard and others not at all. Can you provide some guidelines?

The Doc Replies: I don’t think in terms of feed rate because it’s just not very useful. Instead of feed rate, a better parameter is the specific material-removal rate, or the “Q-prime” value.

The formula is:

Q-prime (mm²/sec.) = DOC (mm) × feed rate (mm/sec.)

Or, if you’re an imperial guy:

Q-prime (mm²/sec.) = 10.8 × DOC (in.) × feed rate (ipm).

Over a few days, take a survey of the Q-prime values everybody is using. You’ll find they’re all over the place. Joe is grinding at a Q-prime of 3.2 mm²/sec., Frank is at 1.8 mm²/sec., José is at 7.2 mm²/sec. and Barney is at 8.4 mm²/sec. Pick a value—5.0 mm²/sec. is a respectable number when creep-feed grinding carbide—and bring everybody in line with that value. You’ll find your grinding to be more consistent. Once you do that, slowly start increasing the Q-prime value to reduce cycle times.

The next step is to find the optimal grit penetration depth and then keep that constant as you increase Q-prime values. This is a little more complicated, but something I teach at my Carbide Master-Grinder Clinic. CTE