Reamers and pocket knives are both cutting tools with about as many sizes, shapes and descriptive names for one as the other. Both utilize a variety of cutting edges to suit the job at hand. If you tried using your pocket knife with your eyes closed you would have a near-perfect similarity.
Blades, barrels, fly cutters, combinations, hole openers, swabs and packers all describe tools generally referred to as reamers. The differences in reamers are, for the most part, a function of the ground conditions or geology to be cut and to a lesser extent the size of the hole needed. For example, a large hole might dictate the use of a fly cutter and a packer while a smaller diameter bore in the same geology could require a barrel reamer.
As every underground contractor knows, each kind of reamer can be described as applicable to a specific ground condition, particularly “good ground,” “bad ground” and solid rock. The cutting of the hole is the primary concern.
In good ground, such as clay, the shape of the hole is maintained by the stiffness of the soil. In the sand and gravel under a river bottom, the reamer must cut and open the hole, packing bentonite into the formation to stabilize the hole. Rock requires its own specialized cutting tools. Some basic terminology can apply to all bore hole cutting tools.
The blade reamer is a simple and basic tool for the driller. As its name implies, it can be as simple as three or four blades welded onto a 2 to 3 ft (610 to 914-mm) sub (drill pipe). The blades can be tapered or stepped so as to progressively cut the hole to a larger diameter as it is pulled into the formation.
Blade reamers cut through firmer soils (good ground) mixing the cutting with the drilling solutions for ease of removal. Cutting teeth are often welded to the blades for more aggressive cutting action. The resulting slurry can then flow past the blades of the reamer to the product annulus (or drill pipe annulus in case of a pre-reaming operation).
In diameters in excess of 12 to 14 in. (305 to 356 mm), blade reamers usually have a 2 to 4 in. (51 to 102-mm) band or ring around the rear or the largest diameter portion of the tool. The ring centralizes and allows smoother rotation of the reamer by creating a wheel at its back end. The blade reamer is the most efficient tool for cutting and mixing soils with the drilling fluid and is usually built in diameters from 4 to 24 in. (102 to 610 mm).
Another common reamer used by directional drillers is the barrel reamer, which might look like a harbor buoy laid on its side with some cutting teeth attached. The barrel reamer, unlike the blade reamer with its open paddles, has a solid round body. Its bullet-shaped nose is laced with rows of replaceable cutting teeth that allow the tool to cut and open the hole to the diameter of its solid body. The reamer’s cutting teeth overcut the tool body, allowing the slurry to flow past the reamer. Small diameter reamers and packers may have flutes or canals cut into the body of the reamer to allow fluid passage through rather than over the body.
The fly cutter is another type of reamer built by deleting the front 80 or 90 percent of a blade reamer, using only the rear section that has a band or ring wrapping it. The band may only be 6 to 8 in. (147 to 196 mm) in width for reamers 12 to 44 in. (305 to 1078 mm) in diameter.
A fly cutter may look like a wagon wheel with spokes in the center rather than blades and cutting teeth closely spaced around the cutter band. The spokes provide structural integrity for the reamer and a pathway for the drilling fluid to reach the outer diameter of the hole. Nozzles are often installed in the spokes so the driller may control the actual volume and pressure of the fluid flowing through the hole.
If the skinny fly cutter was stretched to look like a 30 or 55 gallon (113.5 or 208-liter) drum and spokes placed in the front and rear, a combination fly cutter/barrel reamer is made. Because the cuttings must flow through this drum, past both sets of spokes, the cleaning of the tool as it cuts is critical. Combination reamers are generally only used by the mid-size and large rig operations doing large diameter crossings.
Reaming a hole in solid rock is a different matter from the good ground and bad ground scenarios above. The rock hole opener typically has replaceable wheel-type cutters placed around a tool body. In some cases, a used oil field tool is refurbished and put to work horizontally. However, more and more tools are currently being built specifically for horizontal directional drilling.
The cutters themselves are usually a mill-tooth or deep V-shaped tooth design for soft to medium rock formation or have nub-like tungsten carbide inserts (TCI) commonly called “button bits” for hard rock formations. The newer non-oil-field designs, like most conventional reamers, come in 2-in. (51 mm) size increments. While backreaming rock, a reamer centralizer may be used to hold the drill pipe and reamer concentric with the bore hole.
The contractor can usually rent the tool body and purchase the cutters. It is critical to know the rock to be cut, select the proper cutter, know the life of or expected footage from a set and calculate the number of cutters required to complete a crossing.
Perhaps the most significant aspect of the rock hole opener is the torque and rotary speed required of the drill rig. The harder the rock, the more critical these become. A 26-in. (637 -mm) hole opener in very hard rock would like as much as 25,000 ft-lbs (33,900 Nm) of rotary torque while turning at as many as 75 rpm for optimum cutting.
Whether in hard rock, good ground or bad, the proper selection and use of reamers affects the speed, ease and financial success of completing a crossing. It has been said that a contractor “drills for show and pulls back for dough.”