Knowing the cutter postion

Note: Figures and PDF version still required.

Refer to Figure 10-1 for a view of a cutter in its shoe.

There is a range of possible movement of the cutter in the shoe:

  1. The fully forward position is set by the two M5 bolts which stop the cutter falling through the bottom bearing.

  2. The fully back position is set by the spacers which sit between the cutter and the bottom bearing.

The difference between a. and b. must be known before the instrument is placed in the hole.

The RW rods which carry the cutter are so heavy that after about 6 metres it is impossible to move them without some mechanical assistance. In order to know the position of the cutter some ingenuity is required:-

  1. On lowering down the borehole it can be assumed that the cutter is sitting fully forward. In this position it is in front of the leading edge of the cutting shoe.

  2. As soon as the foot of the instrument comes into contact with the bottom of the borehole the inner drill string is pushed back – this is seen on the surface as the inner rods rising out of the top of the casing.

  3. The hydraulic rams can be raised or lowered to allow the full movement of the drill string to be observed. Once pushed back there may be some material in the shoe preventing the drill string from moving – rotating the drill string with a small wrench will overcome this difficulty.

  4. Having established that the drill string is moving over its full range, set it so that the cutter is fully back. This may well be the position in which drilling is to be carried out.

  5. If not, the rams can be raised by a few millimetres to put the cutter in the intended position.

All that keeps the drill string in this position is the two clamp bolts on the cutter drive unit rod clamp. When drilling two tendencies are sometimes seen:-

  1. The cutter drills itself into the material so that over a period of time it gradually moves forward.

  2. Alternatively, if set forward, the strength of the material is such that it is gradually forced to its fully back position.

As a consequence it is important that when a new length of rod is being added that the cutter position be checked.

Pressuremeter references: 
SBP