Final Thing To Do Before Grinding

Dear Doc: There’s debate in the shop whether we should balance the wheel before grinding or dress the wheel before grinding. What’s your take?

The Doc replies: Textbook grinding says that the final thing you should do just before grinding is (a) true/dress the wheel; and (b) to do so at the exact same wheel RPM you plan to use during the grinding operation. Balancing should be done BEFORE truing/dressing.

Here’s why. Balancing your wheel doesn’t actually balance your wheel. It just makes your wheel less unbalanced. When a wheel is unbalanced, the centrifugal force from the imbalance causes the wheel to pull away from the Z-axis of rotation. It causes the wheel to oscillate up and down in the Y-axis.

That oscillation creates a problem: The wheel bangs against the workpiece in the Y-axis. We fix that by truing/dressing the wheel. Now the wheel is not round, it’s egg-shaped. The wheel still has the imbalance (it always has some imbalance), but we’ve corrected the wheel geometry for that imbalance by making the wheel egg-shaped in a way that exactly corrects for the imbalance. Now, when we go to grind, the spindle still oscillates up and down in the Y-axis due to the imbalance, but the wheel has a slightly larger radius on the light side and a slightly smaller radius on the heavy side. That gives us perfect conformity between the workpiece and the wheel in the Y-axis.

What if we balance the wheel after dressing/truing? We reduce the imbalance. But the wheel is still egg-shaped for the previous imbalance condition. It’s egg-shaped in the wrong way. The result? The wheel bangs against the workpiece (in the Y-axis) until that wrong egg shape is worn away.

However, in 95% of grinding operations, none of this matters. It’s all academic. We live with the banging because it’s small. And then it goes away.

But in a few grinding operations, it does matter. An example: Grinding of small-diameter tools. Let’s take a look at what happens here, and let’s add in an additional
factor — the eccentricity added when we move the wheel from the truing spindle to the grinding spindle, even when the wheel stays on the same adaptor.