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Accurate blast-hole drilling tool

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article image The Alpha 330 from Sandvik based on a new design principle.

ALPHA 330, a new tool system developed by Sandvik Mining & Construction offers a 30 to 80% increase in rod life, more accurate collaring and straighter blast-holes than traditional tool systems.

The Alpha 330 was designed to use 20kW drifters to drill small-diameter holes in order to raise productivity while keeping the bit diameter small, at 45mm.

Conventional tools have R32 thread at the front of drifter rods for 45mm bits. However, these bits are typically not strong enough for use with 20kW machines, especially when rock obliquely resists the drill bit, causing the rod to bend.

On conventional tools with R32 threads, breakages occur typically at the gooseneck between the full rod-section and the R32 thread. With Alpha 330, the connection between the rod and bit is more robust, reducing the incidence of such breakages. This makes the tool more economic to use.

The Alpha 330 system has a slightly larger thread than conventional tools, at R33. The main reason for this increase was to eliminate the risk of users confusing the Alpha 330 with conventional R32, since Alpha 330 is based on a new, high-strength bit-connection concept.

The Alpha 330 thread was designed to minimise the effects of the reactive bending forces that cause the drill bit to deflect under the influence of feed force, impact power and rotation when collaring holes into oblique surfaces on the average drift or tunnel face, or when drilling into cracks or oblique changes in rock resistance inside the hole.

Analyses indicated that the best way to increase the resistance of a drifter rod's weakest point, the thread at the front of the rod, is to make the thread as short as possible.

Shortening the thread moves the weakest point of the rod forward and reduces the leverage of reactive bending forces on the threaded section. With a smaller amount of force acting on the weakest point, the front of the rod is less susceptible to bending and breakage.

The Alpha 330 system has R33 thread, giving it a 14% larger cross-section than R32, making it stronger than R32.

The rod in the Alpha 330 is of the Hex 35 section, with a T38 thread at the shank end.

The Alpha 330 also has more accurate collaring than R32 tools and is able to drill straighter holes that give better fragmentation and improved profile-control, further reducing overall drifting and tunnelling costs.

Sandvik also introduced a new guiding function between the rod and bit-skirt behind the thread. The snug interface improves the rigidity of the joint. After the bit skirt, the rod cross-section increases to Hex-35 quickly and smoothly.

The Alpha 330 does not have a gooseneck, improving its resistance to bending and sandblasting by drill cuttings. A bit skirt covers the thread in the Alpha 330 connection, protecting the thread from sandblasting and reducing the risk of corrosion-fatigue which is common in rock drilling tools.

The service life of Alpha 330 is between 30 to 80% longer than that of R32 and it offers a higher penetration rate. The Alpha 330 also produces straight, clean, accurately placed blast holes.

The Alpha 330 is available in rod lengths of 3.1m, 3.7m, 4.3m and 4.9m rods, with bit diameters from 43mm to 51mm.

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