A DTH concave face drill bit is a button bit style drill bit used in Down the Hole (DTH) drilling systems, where a downhole hammer drives the bit directly at the rock face. The concave face refers to the shape of the face of the bit: instead of being flat or convex, the face has a recessed, dish or bowl shaped (concave) depression. The concave design provides certain drilling benefits for example, it tends to improve hole straightness and flushing of cuttings. In typical use, the bit is attached to a shank or coupling (DHD, QL, SD, BR series, etc.) which connects to the DTH hammer. The hammer delivers percussive blows and rotation; the bit breaks rock ahead of it and the flush air (or air + water) carries the cuttings out. The concave face bit is one of several types (flat face, convex face) chosen depending on rock conditions.
Design:
- Concave face geometry: The face includes a recessed area (dish or bowl shape) rather than being completely flat. This geometry offers better centering of the bit in the hole and tends to reduce deviation (wandering) of the hole.
- Button insert arrangement: The face carries carbide buttons (often spherical, semi-ballistic or ballistic) that do the actual rock cutting. The bit also has gauge buttons around the outer periphery to maintain hole diameter and provide support.
- Body and shank design: The bit body is made from high strength alloy steel (often NiCrMo or 23CrNi3Mo) and is heat treated. The shank interface (DHD, QL, etc) ensures mounting in the hammer. The DTH hammer strikes the piston which directly transfers energy into the bit body with minimal energy loss.
- Flushing holes / grooves: The face and body include flushing holes and grooves that allow air or air + water to flow through the face, remove cuttings and maintain bit cooling/cleaning. The concave design helps optimize flushing.
- Hole straightness / deviation control: Because the concave shape centers the bit and reduces side load, it helps produce straighter holes, especially in medium to hard or homogeneous rock.
Applications: The concave face DTH bit is suited for specific drilling conditions and has advantages in certain applications:
- Rock conditions: Best suited for medium hard to hard rock formations that are fairly homogeneous (not extremely abrasive) and where hole deviation is a concern.
- Straight hole drilling: Where the drilling program demands accurate, straight holes (for blasting, anchors, or grouting) the concave face helps maintain alignment.
- Drilling in fissured or broken ground: Some guides say the concave face is suitable for unconsolidated or broken rock with good flushing capacity.
- Down the Hole drilling rigs: Used in DTH drilling rigs for mining, quarrying, water well, geothermal, foundation drilling. The bit attaches to a DTH hammer and down the hole drill string.
- Hole diameters: The concave face variant is available across a range of hole diameters
