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DTH Drilling Tools and Application
Jul 27, 2018

This article focuses on three areas: downhole impactor, drill bit and drill pipe size selection, downhole drill bit type selection, and practical applications.

DTH Hammer, DTH bit, DTH pipe(DTH rod) size selection

The size of the downhole hammer depends primarily on the borehole diameter and rock type. For blastholes, the borehole diameter of the downhole hole is from 89mm to 252mm. The aperture smaller than 89mm is mainly selected from the top hammer type, and the multi-purpose rotary drilling method is larger than 252mm aperture.

In general, the smallest aperture that can be applied to a DTH Hammer is its nominal size, meaning that the minimum aperture for a 4-inch impactor is a 4-inch aperture. Generally speaking, in this case, there is enough annular space between the impactor and the hole wall between the drill pipe and the hole wall for slag discharge.

The maximum size of the matching drill bit is 1 inch for the DTH Hammer size. For example, the largest drill size for a 4-inch impactor is 5 inches.

The closer the outer diameter of the DTH pipe to the outer diameter of the Hammer, the better the slagging can be achieved and the possibility of stuck drilling is also reduced.

From the processing technology of the drill pipe (drill rod), the surface finish and dimensional accuracy of the cold drawn pipe are superior to those of the hot rolled pipe. Good surface finish means that the surface of the steel pipe is not easy to peel, and the metal scrap caused by the peeling will reduce the service life of the impactor. In addition, the thread of the drill pipe and the main body connecting part are better if friction welding is used, and the strength of the drill pipe can be increased. At the same time, the heat treatment of the thread portion is well performed, the reliability and strength of the thread can be increased, and the connecting rod can be further improved. Smooth, improve work efficiency and total piercing speed.

Choose the right DTH drill bit

Let's take a look at the drill bit. The main body of the drill bit is usually machined from a cemented carbide machine and then heat treated to a specified hardness to provide sufficient fatigue resistance to the surface compressive stress and then embedded with carbide drill teeth.

The convex, pointed tooth design allows for the fastest perforation efficiency and is best suited for medium soft rock with low grindstone.


For hard rock with high grindstone, a flat drill bit can increase the life of the drill bit. If the outer edge of the ball teeth is large enough, the drill bit life can be improved by repeatedly grinding the drill bit, reducing the cost of use. In addition, the concave ball teeth can also be applied to such hard rock with high grindstone.


As shown in the figure below, the concave drill bit is more suitable for joint development, and the medium hard rock with many cracks can effectively reduce the probability of hole deflection, which was also mentioned in the previous article series.


Practical application

Drilling in open pit mines and quarries often requires a more durable downhole impactor, as quarries are often long-term jobs, and it is not cost-effective to replace impactors frequently. There are some better DTH impactors that can be repaired many times before being completely replaced, such as changing the direction of the outer tube of the impactor.


Drilling of building stone (marble, etc.) is more important for hole straightness. In boreholes larger than 89 mm, the downhole type is usually better than the top hammer type.


Exploration drilling needs to work in relatively remote and poorly constructed sites. Therefore, the requirements for the down-the-hole impactor are simple design, high reliability, and high-pressure drilling.


Reverse cycle impact drilling is also more common, and this sampling method is relatively cost-effective for diamond core drilling. The reverse cycle impactor uses the same process as a conventional down-hole impactor, but uses a reverse circulation drill pipe. High-pressure air is blown through the gap between the inner and outer walls of the drill pipe, and then the drill cuttings are discharged from the inner wall of the drill pipe. Then collect the cuttings with a dust bag as shown below.


In actual use, an important factor to consider always is the rig. An experienced driller can reduce the chance of impactor failure and improve the life of the impactor by effectively adjusting the drilling parameters.


There are many types of impactors on the market, both cheap and high-end. But measuring the value of the impactor is not only about its materials and design itself, but more importantly, it is important to consider the balance between efficiency and rock cost.

For example, an impactor is cheap and durable, but the fuel consumption is high, and the result is that rock rice costs are high, which is not a good impactor. Another impactor is expensive, but it is also very efficient, and the cost per rock is low, so this is a good impactor

Of course, as a user, it is also important to consider whether the impactor manufacturer can provide sufficient technical support and usage guidance. This is also very important, which can effectively help our users to improve drilling efficiency and reduce the cost of use.

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