Upon the completion of the pilot bore, a cutting head known as a reamer is attached to the end of the steel rods. The required reamer dimension is used to accommodate the final pipeline product that is going to be pulled into the bore hole. A general rule of thumb for the final bore diameter is 1.5 times the Outside Diameter (OD) of the pipe to be installed.
When the reamer is attached on the exit side of the drill – known as a back ream – the reamer is pulled back whilst rotating through the existing pilot to enlarge the bore hole. In certain ground conditions, for example rock, a forward reaming process can be adopted. In this case the reamer is attached to the drill rods on the entry side and rotated to the rock face. For larger diameters, whether forward or backward reaming, the hole is increased in size in stages. These stages vary in size depending on the ground conditions. The diameters achieved by backward reaming are also much larger than those that can be achieved by forward reaming.
Types of reamers are:
- Stepped reamers – step by step pyramid-shaped, only suited for light soils.
- Conical/Fluted reamers – most common form of Horizontal Directional Drilling (HDD) reamer
- Blade-type reamers – light reamers for loosely packed soils; arch-shaped blades carry various cutting elements.
- Fly cutters – reamers with braces or spokes carrying an outside cutting crest for light, loose soils, also those sensitive to settling.
- Reamers with elements of a blade – for very light, non-bearing soils.
- Barrel reamers – barrel-shaped reamers, with conical closings, edged with chisels on the front conical surface; often armoured with protective welds; for producing wide upsizing diameters.
- Removable blade reamers – basic construction with canted, self-blocking removable plates with cutting edges; optimised to loosen and excavate most common types of soil.
- Hole openers – equipped with a bearing shaft that is protected by tungsten carbide elements (TCIs) and arranged on the round chisel in an axially symmetrical line-up, partially staggered.