Overhanging wheels, making your wheel ‘act harder’

Dear Doc: We cylindrical-traverse grind. There’s disagreement in the shop about whether to traverse off the workpiece and then downfeed, or to downfeed while still on the workpiece. What’s your take?

The Doc replies: I’m mostly a fan of downfeeding on the workpiece. Here’s why.

As you traverse-grind across the workpiece, you have two forces pushing up on the wheel: the grinding force and the coolant hydroplaning force. Those forces cause deflection. When you sparkout, that grinding force reduces to almost zero. But the coolant hydroplaning force doesn’t reduce. It’s still causing deflection. That hydroplaning force isn’t small, and so the deflection isn’t small.

Let’s say that your coolant is causing 5 µm (0.0002″) of deflection. When the wheel traverses off the workpiece, that deflection goes from 5 µm to 4 µm to 3 µm to 2 µm to 1 µm to 0 µm. Now you’ve got a 5 µm taper on the end of your workpiece. And sparkout isn’t going to fix that taper.

If you traverse only, say, 1 mm of your 50-mm-wide wheel off the workpiece, you’re not going to get that taper. This is especially true in cylindrical-ID grinding, where spindles are notoriously unstiff.

Be careful. When you downfeed while still on the workpiece, what are you doing? You’re cylindrical-plunge grinding. I’ll often ask operators: “What is your downfeed speed?” They reply: “ten microns.” I say: “That’s the downfeed, not the downfeed speed. What’s the downfeed speed?” Often, they don’t know. It’s buried somewhere in the G-code. If the wheel just drops that 10 µm, you’re crashing into your workpiece. And that means high wheel wear, burn, etc. Ease into the workpiece, just like in cylindrical-plunge grinding.

Dear Doc: An old-timer grinder at our shop used to say: “1,000 surface feet per minute equals one wheel grade.” He’s now long gone. What did he mean?