How to choose the drill bit for rotary drilling rigs?

1. Classification of common rotary drilling rig bits

Common rotary drilling bits include spiral drill bits, rotary buckets, barrel core drill bits, bottom expansion drill bits, impact drill bits, impact grab cone drill bits and hydraulic grab buckets. The following is a brief introduction of each category.

1.1 Spiral drill bits

(1) Conical: Double-headed double-screw, suitable for hard bedrock. Double-headed single-screw, suitable for weathered bedrock, pebbles, frozen soil, etc. The above drill bits are equipped with various pick teeth, and many types of drill bits can be derived through changes in tooth type, pitch, and helix angle.

(2) Straight spiral drill bits:

a. Bucket tooth straight spiral

Double-headed double-screw, suitable for sandy soil, small-diameter gravel layers with poor cementation; double-headed single-screw, suitable for sandy soil and soil layers; single-headed single-screw, suitable for large-diameter pebbles with poor cementation, clay soil and hard clay.

b. Pick tooth straight spiral: There are double spirals, triple spirals and quad spirals, suitable for hard bedrock or gravel.

1.2 Rotary Drilling Bucket

According to the teeth installed, it can be divided into pick-tooth drill bucket and bucket-tooth drill bucket; according to the number of bottom plates, it can be divided into double-layer bottom bucket and single-layer bottom bucket; according to the number of doors, it can be divided into double-door bucket and single-door bucket; according to the taper of the bucket, it can be divided into cone-barrel drill bucket and straight barrel drill bucket; according to the shape of the bottom plate, it can be divided into pot-bottom drill bucket and flat-bottom drill bucket. The above structural forms are combined with each other, and with the changes of whether there are vents and door opening mechanisms, dozens of rotary drilling buckets can be combined. Generally speaking, the double-layer bottom drill bucket is suitable for a wider range of strata, the single-layer bottom is only suitable for soil layers with strong viscosity, the double-door drill bucket is suitable for a wider range of strata, and the single-door drill bucket is only used for large-diameter pebbles and hard clay.

1.3 Barrel-type coring drill bits

There are two common types: pick-tooth barrel drill (suitable for medium-hard bedrock and gravel), and cone barrel drill (suitable for hard bedrock and large boulders). Among the two major types of core drill bits, there are those with and without coring devices, which mainly depends on the difficulty of coring. Because roller core drill bits are mainly used for hard rock drilling, and the annular area drilled is large, if conditions permit, reverse circulation drilling can be added to the drill bit to improve drilling efficiency.

1.4 Bottom expansion drill bits

On the basis of not increasing the pile diameter and the pile depth, in order to improve the bearing capacity of a single pile, the design department often achieves this by expanding the bottom pile. The bottom expansion of the rotary drilling rig can be constructed without any modification, and only the bottom expansion drill bit needs to be selected. The commonly used bottom expansion drill bits are mainly mechanical. This type of drill bit is relatively simple to use and maintain. There are upper opening and lower opening types. The opening mechanism is generally a four-link type, which is used for soil layers, strong winds, medium weathered strata and even hard bedrock. Since rotary drilling is non-circulating drilling, the slag can be cleaned with a slag bucket after the bottom expansion is completed.

1.5 Impact drill bit, impact grab cone drill bit

When drilling large-diameter pebbles, large boulders and hard bedrock, the use of impact drill bits and impact grab cone drill bits in combination with rotary drilling is particularly effective. This type of drill bit is used by hanging the auxiliary hook of the rotary drilling machine. Because it has an impact effect, the auxiliary hook is required to have a free rope release function for better results.

1.6 Hydraulic grab

The construction of continuous walls and anti-seepage walls is becoming more and more common. If the rotary drilling rig is slightly modified, it can be operated. The opening and closing of the hydraulic grab is driven by hydraulic pressure. There is only one oil cylinder on the hydraulic grab, so only two oil pipes and a control valve are needed.

2. Select drill bits according to geological conditions

The main function of rotary drilling rigs is to form holes and grooves on the surface. The working object is rock. Since the depth of the pile holes is shallow, the rock undergoes complex changes in structure, particle size, porosity, cementation, occurrence, and compressive strength after tectonic movement, mechanical and chemical effects of nature. In addition, my country has a vast territory, so the working object of rotary drilling rigs is particularly complex. In summary, there are the following categories.

According to lithology (common), it can be divided into mudstone, sandstone, limestone, granite, etc.; according to genesis, it can be divided into igneous rock, sedimentary rock, and metamorphic rock; according to mechanical properties, it can be divided into solid, plastic, and loose. So how to choose a drill bit according to the stratum conditions? The following classification is introduced.

(1) Clay: Use a single-layer bottom rotary drilling bucket. If the diameter is small, a two-piece bucket or a drill bucket with a soil discharge plate can be used.

(2) Silt, weakly viscous soil, sand, and pebble layers with poor cementation and small particle size can be equipped with a double-layer bottom drilling bucket.

(3) Hard clay: Use a rotary drilling bucket with a single soil inlet (single or double bottom is acceptable) or a bucket tooth straight screw.

(4) Frozen soil: For soil with low ice content, use bucket tooth straight screw drill bucket and rotary drilling bucket. For soil with high ice content, use a conical spiral drill bit. It should be noted that the spiral drill bit is effective for soil layers (except silt), but it must be used in the absence of groundwater to avoid suction and jamming.

(5) Well-cemented gravel and strongly weathered rock: A conical spiral drill bit and a rotary drilling bucket with a double bottom are required (single-port for larger particle size and double-port for smaller particle size)

(6) Medium-sized bedrock: Equipped with a rotary drilling bucket with a pick-tooth core drill bit-a conical spiral drill bit-double bottom, or a rotary drilling bucket with a pick-tooth straight spiral drill bit-double bottom.

(7) Slightly weathered bedrock: Equipped with a cone core drill bit, a conical spiral drill bit, and a double-bottom rotary drilling bucket, if the diameter is too large, a graded drilling process must be adopted.

3. Select drill bits according to the functions of the rotary drilling rig

If the rotary drilling rig has the function of continuous construction, a liquid grab bucket or a double-wheel milling machine can be selected for continuous construction. If the auxiliary winch has an automatic rope release function, an impact drill bit and an impact grab drill bit can be selected to drill large-diameter pebbles or boulders. If equipped with an air chamber and a faucet, gas-liquid drilling can be performed. If equipped with a dual power head or a tube rolling machine, casing drilling can be performed, which is an effective method to solve the problems of flowing sand, broken pebbles and gravel, and silt layers.

4. Select drill bits according to other factors

If the thickness of the sediment is strictly required or when expanding the bottom drilling, a special slag bucket is required. When drilling into the casing, the gap between the drill bit and the casing is small. In order to prevent the drill bit from hanging on the casing and reduce the suction effect, the drill bit should be equipped with a vent hole, a guide device, and the opening and closing device should be as close to the center of the drill bit as possible, preferably integrated with the drill and the hand. This special problem can also be solved by selecting a combined drill bit.