Hollow chisels are devices that can be used with a drill press or dedicated mortising machine to produce square-cornered mortises that eliminate the need to round over tenons.
The hollow chisel consists of a very sharp, thin-walled, square bit with an auger in the centre. The auger closely resembles a drill bit.
While the auger bores out most of the waste material, the square bit takes care of squaring up the cut by shearing the wood around the auger. The auger then channels the sawdust it creates up through the hollow centre of the square bit.
Using the Hollow Chisel
It is important to have even pressure on all four sides of the hollow chisel as it slices through the wood. For that reason, it's best not to overlap cuts.
The hollow chisel is not nearly as loud as cutting mortises with a router and is quicker than creating mortises by hand.
The square bit must be kept very sharp. Any wood chips that clog up in the centre of the hollow chisel can cause the bit to overheat.