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.)
Step 2: Replace the Pilot Bit Temporarily
I needed a way to put the mandrel into a chuck so I could spin it on center. The pilot bit is 1/4 inch. I loosened the set screw that holds it in place and inserted a piece of 1/4 inch rod in its place.
Step 3: Chucked in the Drill
I put the 1/4 inch rod into my drill chuck. The mandrel now spins in the drill on center without any wobble. Of course, you cannot use a holesaw on it like this.
Step 4: Grind Until Round and on Center
I held the drill so the mandrel made light contact with a spinning grindstone. Hold it so contact with the grindstone does not cause the drill to bump, but the grindstone slowly and smoothly takes away high spots and the shank of the mandrel is round and on center. Experiment with the speed of the drill to find the speed at which you can hold the drill most smoothly. If there are low spots and voids fill them with a little additional welding and grind again until round and on center.
Step 5: Making Three Precision Flats
I needed a jig to grind the flats on the shank. I began with two identical scraps of 3/8 inch plywood. The rod replacing the pilot bit is 1/4 inch in diameter. I drilled a 1/4 inch hole through both pieces of plywood. The hole is very near to the edge of the plywood pieces.
Step 6: One Larger Hole
I ground my shank down to 3/8 inch in diameter. I drilled one of the 1/4 inch holes from the last step out to 3/8 inch.
Step 7: Mount the Mandrel in the Plywood
Mount the mandrel in the two plywood pieces as shown. This simple support holds the mandrel parallel to the table surface below.
Step 8: Grind Flats
My grinding wheel is mounted on my radial arm saw. That means I can slide my mandrel holding jig under the grinding wheel. It also means I can lower the grinding wheel incrementally. I ground all three flats to just a bit less depth than I wanted so the last "cuts" could be very light. This was to insure that each of the final cuts would not be affected to any degree by part of the grinding wheel wearing away during heavier cutting on previous flats. The hex nut on the mandrel made a handy guide for rotating the mandrel 120 degrees between the three flats. To grind I simply slid the mandrel and its supports laterally toward the grinding wheel.
Step 9: Done Grinding
Here you can see the shank of my mandrel after I finished grinding. The three flats are equal. The round portion of the shank is on the mandrel's center. The mandrel no longer wobbles in my drill chuck. Yes, its diameter and strength is slightly diminished, but it will be fine if I do not force the holesaw when I use it in the future. I did a precision job without the usual machine tools needed.