SYMMETRIC SYSTEMS
Symmetric System
Symmetric systems are typically used on overburden and can be used at any angle. The Symmetric systems’ key function is to insert the casing while drilling. This is to prevent loose materials from the overburden from caving into the hole behind the drill bit. The Symmetric system consists of a pilot bit, a symmetrical reamer (ring bit) with bayonet coupling, and the casing shoe. This complete system is attached to the DTH hammer and threaded drill rod.
The pilot bit and symmetrical reamer are locked together by the bayonet coupling, causing them to rotate simultaneously with the drill rod. The casing shoe is welded to the casing, keeping it in a static position. Together, they drill a hole large enough to allow the casing to be inserted into the hole. Once the desired depth is reached, the pilot bit is unlocked with a simple reverse movement of the drill rod, allowing the driller to retrieve the pilot bit, symmetrical reamer, and drill rods. The casing shoe is left behind in the hole permanently.
