Abstract: A percussion drill comprises a drill bit and a reciprocable hammer piston for imparting blows to the bit. Pressurized air is supplied to the piston for reciprocating the piston. Some of the air is by-passed to an exhaust passage and is exhausted ahead of the bit for removing cuttings. An indexable choke valve is disposed in the exhaust passage for regulating the amount of air which is exhausted. The choke valve comprises a portion of a valve carrier which carries a check valve at its rear end. The valve carrier is rotatable to produce an indexing of the choke valve. The valve carrier is insertable in a front-to-rear direction into a central opening of a rear sub and is secured to the rear sub by a pin which also holds the choke valve in selected positions of adjustment.

Abstract: In a percussion drilling tool, a bit is connected to a drill motor through a mating spline connection. External splines on the bit engage internal splines in the drill motor and have respective load bearing faces disposed in a plane extending radially outwardly from central axes. The splines also include a crest land, and a root land. When the splines are in an abutting centered position, there is a uniform radially measured outer clearance between the external spline crests and the internal spline roots, and a uniform radially measured inner clearance between the external spline roots and the internal spline crests. The outer clearance is less than the inner clearance to prevent abrasive damage below the root diameter of the bit resulting from axial relative movement so that the external spline crests will engage the associated internal spline roots while continually maintaining a spacing between the external spline roots and the internal spline crests.