measure and cut a piece of threaded bar that will pentrate approx three quarters the length of the handle plus an extra two inch longer that will fit into the shank
set the handle in a vice with the bottom diameter facing upwards, drill a central hole with a drill bit the same size as the threaded bar, to a depth of approx three quarters the length of the handle as measured previously but being carefull not to drill too deeply
Clean out the hole and try the bar for fit, if you are satisfied coat the threads with epoxy glue and push the rod gently into the hole, wiping off any excess glue that is pushed out, as this will interfere with the final joint and it is easier to remove when the glue is wet.
When the glue has set, it can be fitted to the shank.
In the same way the handle was held for drilling, the shank must also be held in the vice but with something wrapped around the bark for protection, an old rubber car mat comes in handy for this, or anything else you have lying around that will protect the bark from the vice but also keeping it firmly in place.
Drill a central hole in the shank to a depth of the remainder of the threaded bar sticking out of the handle, taking care not to overdrill.
Clean out the hole and try the handle for fit, make good any imperfections, so the joint sits square and the diameters are even, take your time over this part and keep trying the handle back against the shank until you are happy that the joint is true.
When you are happy with the fit coat the threads sparingly in epoxy glue and push gently into the shank, cleaning off any excess glue immediately before it sets.
cover the joint to protect it as previous and hold the joint lightly together in a vice but enough pressure to keep it together until dry.