Determining burn depth

Dear Doc: We surface grind to a total depth of 0.1 ". Some of the operators burn the workpiece material during the several roughing passes and rely on the finishing pass to remove the burn. Is this safe and reliable in terms of guaranteeing a burn-free part? And, if so, is there a way to estimate burn depth.

The Doc Replies: “Safe” is a relative term. If you’re grinding turbine blades for jet engines, for everyone’s sake, please don’t use this approach. But I have seen companies get away with it when grinding cheap, hardened-steel drill bits.

Estimating burn depth is tricky. It depends on the thermal conductivity and specific heat capacity of the workpiece material, the maximum surface temperature reached and, most importantly, the time the wheel is in the grinding zone. If the time in the grinding zone is short, such as when grinding with a fast table speed, the heat has a brief time to penetrate into the workpiece, and the temperature-vs.-depth gradient is steep (see figure above). If the time in the grinding zone is long, such as when creep-feed grinding, the heat has more time to penetrate the workpiece, and the temperature-vs.-depth gradient is not steep.

Think about passing your hand over a lit candle. If you slowly move your hand, you’ll burn it. But if you pass your hand quickly—even in a back-and- forth manner where your hand is in constant contact with the flame—you won’t burn your hand. This is because the heat has less time to enter a particular point on your hand before the flame touches another point.

Courtesy of J. Badger

The burn depth strongly depends on the amount of time the grinding wheel is in the contact zone.

If you grind hardened steel and assume a maximum surface temperature of 1,200° C (2,200° F)—a worst-case scenario—you obtain the curves shown in the figure. All have the same specific material-removal rate (Q ') of 5mm2, but the speeds and feeds vary depending on whether the grinding is fast and shallow, or slow and deep. At the same surface temperature, grinding slow and deep will create a deeper burn.

The equation for time in the grinding zone when surface grinding is: 

(√DOC in in. × √wheel diameter in in.) ÷ table speed in in./sec. 

Let’s say burn begins at 600° C, which is near the tempering temperature in some hardened steels. If you grind at a 0.010mm DOC and a table speed of 500 mm/sec., for a very high surface temperature of 1,200° C, the burn depth will be about 0.4mm. But if you creep-feed grind with a 1mm DOC at 5 mm/sec., the burn depth will be off the charts—several millimeters deep—for the same surface temperature of 1,200° C.

These are ballpark figures because different materials vary drastically in terms of thermal properties and other factors, such as actual contact length vs. theoretical contact length. So take them with a grain of salt. But they do give you an idea of how grinding slow and deep vs. shallow and fast impacts burn depth.