The shape and the dimension of the base widening depend on the type of belling tool used: standard reamer or bucket reamer ‘bottom hinge’ which has a bell shape (a).
Franki uses the 'top hinge' underreaming model (belling bucket) or standard reamer (b), usually cut at 45 or 60 degree angle, with the maximum diameter of the underream being not more than three times the diameter of the shaft, see figure on the right.