"Topping" is short for "top hobbing", wherein the root of the hobbing cutter generates the tip of the gear teeth at the same time the flanks of the hobbing cutter generate the flanks of the gear teeth. One benefit is that the gear o.d. is then 'guaranteed' concentric with the pitch circle, and can be used for fixturing. Another is that there is no tip burr.
No, I didn't know that off the top of my head. I found it here:
Actually, I had heard the word before, but I remembered it a little differently. I once got a shipment of Helicon gears with burrs on the o.d.. They were expensive, so I complained about it, and was told they had missed a secondary 'topping' operation and should be returned for processing. The resulting gears were burr- free, and had a nice small uniform chamfer at the o.d. extending around the entire periphery, including the root, tip and flanks of every tooth, as if they'd been deburred, but not by hand.
So, there are two slightly different definitions for you.
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