A dental drill defining a bore diameter and a drill stop sleeve which is transparent or translucent, thereby improving the view of the drilled hole formed by the drilling during the operation. The watertight door includes a cover, a water absorbing ring, and a water blocking ring. The conduit structure has a frustro-conical surface and a seal groove. A modified flute profile, compensating for flute pitch variations between the large diameter bonding roll and small diameter corrugating roll, assures uniform flute-to-flute engagement in the corrugating nip.