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.
Related Glossary Terms
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- cylindrical grinding
Grinding operation in which the workpiece is rotated around a fixed axis while the grinding wheel is fed into the outside surface in controlled relation to the axis of rotation. The workpiece is usually cylindrical, but it may be tapered or curvilinear in profile. See centerless grinding; grinding.
- depth of cut
depth of cut
Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.
Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grinding wheel
Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.
- surface grinding
Machining of a flat, angled or contoured surface by passing a workpiece beneath a grinding wheel in a plane parallel to the grinding wheel spindle. See grinding.