This is a heavy duty holesaw mandrel. The shank on this is 7/16 inch and requires a 1/2 inch drill chuck. My drill has a 3/8 inch chuck.
I had to make several dozen 3 inch holes in our soffets (also spelled 'soffit') for ventilation louvers. I ground the shank flats down freehand so the mandrel would fit my drill. It worked, but it wobbled terribly.
(The image is from Amazon.com.)
Step 1: Build It Back Up
I decided to restore my mandrel, but sized for my drill chuck and on center so it does not wobble. The first step was to build the shank up again by welding. A wire feed welder makes this much easier than with a stick welder. (I did not take photos while I was actually doing this project, so this is a recreated photo.)