OK, we busted 4 mandrel bolts and a finger yesterday, Here is how I fix these.
This method works well on mandrel through hole broken bolts.
Start off center punching the broken bolt as close to center as possible.
Make the part as level as possible, I used a stack of washers to achieve this.
Use plenty of oil, sharp bits, and clean often. Drill all the way through, and check it didn’t drift.
Step up the drill size till the hole will accept the tap.
Use the same drill / washer setup to “Manually” start the tap in a couple turns.
Finish up by hand tapping, One down three more to go.