Down-the-hole hammer drill bit assembly

A down-the-hole drilling assembly activated by fluid under pressure, said assembly comprising:

• a hammer including an elongate casing having an axially rearward end and an axially forward end;
• a fluid-powered piston arranged movably inside the casing;
• a top working chamber at a top side of the piston;
• a bottom working chamber at a bottom side of the piston;
• a plurality of fluid passages for feeding and discharging pressurized fluid into and out of the working chambers for generating reciprocating movement for the piston;
• a drill bit connectable to the axially forward end of the casing and provided with an anvil facing towards the piston for receiving impacts of the piston;
• and
• an annular drive sub surrounding at least a portion of an elongate shank and cooperating with the drill bit to form fluid passages extending length-wise along the shank, wherein the drive sub includes a concave arcuate groove extending circumferentially in the drive sub forming a chamber, the chamber being connected to channels arranged for collecting fluid from a hammer exhaust flow and redistributing a volume of the collected fluid from the channels to flushing holes arranged in the drill bit, the channels being formed between a plurality of splines of the drive sub and a plurality of splines of the drill bit, wherein the chamber is located on the top side of a head-shank transition area where feed force transmission occurs, wherein the concave arcuate groove forming the chamber has a minimum volume dimension which is constrained by the lesser of a cumulative flow area between the splines or the flow area of the flushing holes in order to permit even redistribution of flow from the hammer.