To deburr from the bottom, we use a four flute "double angle cutter" (this one from @HarveyTool), which goes down the center of the hole, then goes around in a circle and cuts the chamfer "facing up" around the bottom of the hole. It's kind of magical. /🧵
ALT A double-angle cutter; a cutting tool larger at the bottom than at the top, which can make 90 degree chamfer cuts facing up or down.