Big, Dull Grits Bad, And Other Tips

Dear Doc: I attended your grinding course, where we all chanted The Grinder’s Mantra: “Big and dull bad, small and sharp good. Big and dull bad, small and sharp good.” But isn’t it true that large grits can remove material more quickly than small grits?

The Doc replies: Yes, all things being equal, large grits can remove material more quickly than small grits. But things aren’t always equal. The big-dull-bad/small-sharp-good mantra reminds grinders to avoid dressing big grits (say, 60 mesh) dull in order to get a better surface finish – instead of using smaller grits (say, 120 mesh) and dressing them sharp. A dull 60-mesh wheel will give you the same surface finish as a sharp 120-mesh wheel. But a 60-mesh dull wheel will give you high heat generation and exponentially high normal forces. It’s difficult to achieve high removal rates with high heat generation and high normal forces.

What about 60-mesh sharp versus 120-mesh sharp? For sure, you’ll be able to remove material more quickly with less wheel wear with the 60-mesh-sharp wheel. But 60-mesh dull versus 120-mesh sharp? You’ll be able to remove material more quickly with the 120-mesh sharp wheel.

Overhead dresser pros and cons

Dear Doc: We grind nickel alloys in the aerospace industry. Our surface grinder has both an overhead plunge-roll dresser and a table-mounted plunge-roll dresser. I want our machine operators to use the overhead dresser because, well, because we paid for it, and because it’s quicker. But they insist on using the table-mounted dresser. Why?

The Doc replies: Let’s take the pros and cons of overhead dressers.

First, the cons:

1. Overhead dressers are not near as stiff as table-mounted dressers. That big, heavy diamond roll spindle assembly is mounted on a giant cantilever sticking out over your wheel. It’s just not going to be as stiff as a table-mounted dresser. And lack of stiffness means deflection, lower natural frequencies (which are bad) and high chatter risk during dressing.
2. The Grinder’s Mantra “Dress and grind at the same spot” applies. In general, you want to dress your wheel at the same point where you grind with it. Doing this makes your wheel egg-shaped in just the right way, which corrects for imbalance (even slight imbalance), weird “2N” oscillations inside the bearings, and other anomalies. Dressing at 180° doesn’t correct anomalies. In fact, it can double them.

Now, the pros: