Hybrids, temperatures and chatter

Dear Doc: What’s your take on hybrid bond wheels when grinding carbide?

The Doc replies: I’m a fan, especially if you have a lot of material to remove quickly, such as in flute grinding of endmills. Keep in mind that when you try them out, you may need to increase your grit penetration depth (compared with resin-bonded wheels). How do you do that? In surface grinding, you increase the depth of cut, increase the feed rate or decrease the wheel speed. In cylindrical plunge grinding, you increase the plunge speed, increase the workpiece rpm or decrease the wheel speed. In cylindrical traverse grinding, you increase the DOC, increase the workpiece rpm (and probably simultaneously increase the traverse velocity so as not to change the overlap ratio) or decrease the wheel speed. Or you do some combination of these. I’ve found that if you use hybrid bond wheels, you can go longer without having to stick. Or if you’re really skilled, perhaps you never have to stick — or almost never.

Dear Doc: I attended your three-day class and learned that speeding up workpiece rpm reduces grinding temperatures in cylindrical plunge grinding. Is this also true for cylindrical traverse?

The Doc replies: No, absolutely not. In fact, it’s just the opposite: In cylindrical traverse grinding, increasing workpiece rpm increases workpiece temperature. How can that be? It’s because cylindrical plunge and cylindrical traverse are two different animals.

In cylindrical plunge, when you increase your workpiece rpm, you simultaneously decrease your effective DOC. (Effective DOC in mm = plunge speed in mm/min. divided by workpiece rpm.) That moves you from the slower, deeper regime to the faster, shallower regime. In cylindrical grinding, grinding faster and shallower reduces workpiece temperature.

In cylindrical traverse grinding, the effective depth is fixed. It doesn’t change as you increase workpiece rpm. Increasing your workpiece rpm simply increases your workpiece surface velocity (at the same DOC). That means larger temperatures in the (now smaller) cutting width of the wheel. In other words, you’re asking less of your wheel (a narrowed width) to do more of the work. And that increases temperatures.

Dear Doc: During crankshaft grinding, we get a weird pattern on our workpiece. We’re trying to figure out if it’s snakeskin chatter or fish scale chatter. How can we know?

The Doc replies: Easy. If the chatter goes away after dressing, it’s likely snakeskin chatter. If the chatter becomes worse after dressing, it’s likely fish scale chatter.

Snakeskin and fish scale have two completely different causes. Snakeskin chatter is caused by uneven wear around the wheel, which imparts a weird pattern into the workpiece. That uneven wheel wear is inevitable, but it does take time to develop. When you dress the wheel, you make it round again, and the weird snakeskin pattern disappears.

Fish scale chatter, on the other hand, is caused by intermittent contact between the dresser and the diamond during dressing. It can happen during stationary traverse dressing (when the diamond bounces up and down as it traverses), but it’s more common in rotary traverse dressing, especially when the dressing disc is newly mounted (and therefore eccentric). Here, the eccentric diamond disc makes intermittent contact with the grinding wheel as it traverses across the wheel, creating lobes on the wheel, with those lobes having a “phase shift” as we move axially down the wheel width. Those lobes then are imparted into the workpiece, giving the fish scale pattern.

Note that fish scale chatter doesn’t exist in rotary plunge dressing. With rotary plunge, intermittent contact may occur, but it doesn’t impart a fish scale pattern into the wheel. That is because it’s not traversing the wheel. Instead, it imparts a straight-line pattern into the wheel.

As the wheel wears away, the funky fish scale pattern in the wheel wears down, meaning less fish scale on the workpiece. Then, when you dress the wheel, it comes back. The solution is to get rid of that intermittent contact during dressing. Make that contact continuous — usually by removing dressing disc eccentricity or imbalance.