In 1987 we purchased our first Air Core system. From the first job we had major problems with drilling clays, we couldn’t drill more than 200mm without the bit and innertubes blocking up. We removed the end surfaces of the bit face with a gas torch and noted an immediate improvement in penetration. However, the innertube still continually blocked up with the clays. The onsite solution was to coat the internal surfaces of the bottom 40mm of the innertube with brass braising, reducing the internal diameter, which immediately reduced innertube blockages.
We realised from the first air core rod drilled that;
1. Clay drilling requires the hole sample to be swagged to fit into the innertube for recovery to the surface as clay core. Swagging was required for clay, not drilling, (kerfing).Swagging maintains the volume, but reduces the diameter while increasing the length of sample recovered; much like rolling plasticine under your hand reduces the diameter and increases the length.
This process is the only efficient method to drill plastic materials.
2. A sizing swagging ring was required in the bottom of the innertube to allow the sample of clay to float without touching the innertube anywhere apart from the bottom 40mm.
Without a sizing ring the more clay core that is in the innertube, the greater the contact area and stiction until the plastic modules of the clay is exceeded and the core becomes jammed tightly in the innertube, which requires a rod pull and the clay to be drilled out of the innertubes.
With the Bostech system this doesn’t happen. All that is required is a momentary pull back which snaps the core, which when passed the sizing ring allows the core to be recovered to the surface.
Bostech Aircore bits are maximised for versatility, reliability and production. Bostech can drill almost all weathered ground types using one standard bit type. This is possible using combinations of; bit face, air pressure, venturi assistance, different innertube configurations, (high pressure and low pressure) and combinations of trumpet lengths.