Advantages of SPM Overburden Drilling System:
1. The drilling system comes with an eccentric bit design which ensures less material cost as compared with concentric design, and reduces oil consumption of air compressor.
2. Eccentric designed bit rotates outward to drill a larger diameter hole than the casing. This enables easy drilling and helps to avoid cave-ins and wash-outs.
3. Constructed from imported, premium quality steel, the eccentric overburden drilling system has a three-wing reamer bit that is easily retracted into the casing for easier prevention of drill sticking.

Technologies
1. Casing
SPM casings boast high impact strength and exceptional portability. To meet the requirements for deeper depths, our casings have gone through unique treatments that ensure considerable increase of durability and rigidity (Yield strength: ≥515Mpa, and Tensile strength: ≥810Mpa). Smooth inner surface of casing makes it convenient for eccentric drilling tool to smoothly pass through.

2. Drill String
Our overburden drilling system employs geological drill pipe which features simple structure, high strength and outstanding performance in removal of rock dust and flakes.

3. Hammer
SPM DTH hammer with eccentric bit can produce sufficient impact energy to enable the drill string and casing to be lowered in the hole simultaneously, and at the same time ensure an extremely low breakage rate of casing.

4. Special Process
Larger diameter casing and smaller diameter drill string and hammer ensure easy drilling and retracting. For instance, it uses φ146 casing with φ108 drill string, φ168 casing with φ127 drill string and φ178 casing with φ146 drill string (Unit: mm).

Notes:
1) When start drilling, you should use the minimum propulsive force and impact force to enable the drilling tool to stably drill into the rock. The propulsive force should be reduced in a timely fashion as the weight of hammer and drill string increases.
2) Normally, the eccentric bit of the hammer rotates at a speed of 15-45rpm. Reasonable rotation speed ensures faster penetration, less energy consumption and longer service life of drill tools. When drilling in very hard rock formations, the rotation speed should be reduced to avoid fast wear of drill tools.

Working Steps
1. The overburden drilling system is composed of DTH hammer, eccentric designed reamer and pilot bit, guide device and casing. The eccentric bit rotates outward to drill a larger diameter hole than casing so as to enable the drill string and casing to be lowered in the hole simultaneously. Rock dust is blown clear out of the hole through the casing.
2. After drilling to the designated depth, the reamer bit is retracted by slightly reversing the rotation or by applying tension on the bit. The reamer bit will retract into the casing for easy removal of the drill string and hammer.
3. Pulling the casing up and meanwhile filling concrete or other things under the hole.
4. Using DTH drill bit to continuously drill the hold to required depth