Swivel or suffer the consequences

Dear Doc: We grind the ID and bottom face of hardened-steel open-top cylinders with a vertical-spindle grinder. The ID isn’t a problem, but the bottom face is killing us: corner breakdown, rubbing, weird patterns and burn. One operator said to swivel the wheel. Is this a good idea?

The Doc replies: It’s a great idea.

When cylindrical grinding, the general rule is: Curve on curve is good, curve on flat is good, flat on curve is good, and flat on flat is bad. You’re grinding two surfaces. The first is the inner-diameter surface, which is a curved wheel on a curved workpiece, so this isn’t a problem. But you’re also grinding the bottom surface, which is a flat wheel on a flat workpiece. And flat on flat almost always produces pain and suffering.

You might think that the material on the bottom of the cylinder is being removed by the bottom flat face of the wheel. But this isn’t the case. As the wheel slowly feeds down into the rotating bottom surface, all the material is being removed by a very thin section on the outer diameter of the wheel. How thin? You’re effectively doing cylindrical traverse grinding with a wheel width of 0.008 mm (0.0003″). This produces a huge grit penetration depth and huge corner breakdown.

What is that big bottom face of the wheel doing? Rubbing and only rubbing — and generating heat and weird spiral marks.