Up-grinding vs. down-grinding

Dear Doc: I took your course, and you said that in 99% of grinding operations there’s no significant difference between up-grinding and down-grinding. But I think that our case falls in that 1%. Due to centrifugal expansion, we have to cylindrical-plunge-grind our workpiece at the speed it runs in its actual application, which is 3,000 rpm. Our machine is set up to go unidirectional. Will that be a problem? Our workpiece is 200 mm in diameter, and our wheel is 450 mm in diameter running at 1,800 rpm.

The Doc replies: Yes, this would be one of the rare cases in which there will be a big difference between up-grinding (also called anti-directional) and down-grinding (also called unidirectional). Here’s why.

Imagine you’re a single grit on the wheel. As the wheel plunges into the workpiece, you will carve out a chip. If the wheel is moving fast, you will carve out a thin chip. If the wheel is moving slowly, you will carve out a thicker chip.

What we care about is how fast the wheel is moving relative to the workpiece. In the vast majority of cases, the wheel is moving so much faster than the workpiece (say, 500 times faster) that we just ignore the velocity of the workpiece. Theoretically, we are in error. But an error of 0.2% is trivial, and we ignore it.