A basic rod and disk are forged and machined to make a wheel axle, with the rod at a right angle to the disk. The other end of the rod is splined inside the differential housing. The disk is further machined and tapped to make a wheel hub. It was later found that cracks were occurring where the rod meets the disk. Supply the missing word: A ___________ geometry at the rod-disk interface would have prevented cracks from forming. Open to all.