Fitting the cutter
Note: Figures and pdf version still required.
There are two kinds of cutting device that are frequently used with the SBP:
a) A rock roller bit
b) A flat bladed drag bit.
Both are carried in a similar manner and use the same drill string. What is described now is appropriate for a flat blade cutter, but the procedure also applies to the rock roller bit.
See Figure 5-1 for an assembly drawing of the lower end of the instrument.
Steps for fitting the cutter
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The cutting assembly can be removed as one unit from the pressuremeter by removing the three cutting shoe retaining bolts. Having done so the cutting edge, the cutting edge carrier, the bottom bearing and the cutter can be removed as one piece together with the cutter extension rod and the first 1 metre length of cutter drive rod.
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To make the assembly manageable, separate the extension rod and the first 1 metre of cutter drive rod using wrenches.
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Note that there are two drive rod retaining bolts which prevent the drill string from sliding through the bottom bearing. Remove both these bolts. They can be accessed through the holes in the shoe edge carrier where the bolts were removed in step [1].
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Now remove the cutter drive rod extension and the flat bladed cutter by sliding the rod through the bottom bearing.
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The cutter can be removed from the rod by using wrenches. Do not lose the plastic and metal spacers.
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The bottom bearing pushes out of the shoe edge carrier.
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The shoe edge can be removed from the carrier if it needs to be replaced. To do this, note that they are joined by a fine LEFT hand thread. The shoe edge is not very thick and is easily distorted by applying excessive force to separate the two parts. The carrier is a more substantial piece of metal and this may be held in a vice. Try gently tapping the pieces with a soft headed hammer. It may be necessary to use the small chain wrench to break the joint, but do not apply large force.
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It is normally possible to resharpen a blunted shoe edge, and also to cut it back if damage has occurred to the leading edge.
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Clean all parts for re-assembly, making sure passages are not blocked.
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The cutting edge, either refurbished or a new one, is fitted to the shoe edge carrier. The threads must be clean, and smear the mating area with a little copper loaded grease. Screw the two pieces together (noting the left hand thread) and lightly tighten - firmly hand tight is enough.
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If the spacer rings are ok then they may be re-fitted. It is unlikely that the metal spacer piece is damaged, but if the plastic spacer has been pressed against the bottom bearing then this may have swelled.
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Check the bottom bearing. If it is not distorted then it may be used again, but check also that there is not too much play when the cutter extension rod passes through it.
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Screw the cutter to the cutter extension rod. Put on the metal spacer, and then the plastic spacer. Pass the assembly through the combined cutting shoe pieces.
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Now slide the bottom bearing down the rod and locate it in place in the shoe edge carrier.
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Now prevent the bottom bearing from coming out of its location, push the cutter as far back into the cutting shoe as it will go. Measure the distance B in Figure 5-1.
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Decide if this is ok (suggested settings are shown in the table below). Maybe it is too far back, in which case the cutting shoe edge can be changed for one of a smaller size (there are three lengths, from 45mm to 65mm); alternatively another metal spacer can be added so that the cutter sits further forward.
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When happy with the arrangement, replace the drive rod retaining bolts.
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Screw the 1 metre length of cutter drive rod back into place (checking that it is not blocked). Tighten with wrenches.
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Look up the inside of the instrument to ensure that it largely free of mud. If ok, then slide the complete cutter assembly back into the instrument, and secure with the three cutting shoe retaining bolts.
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Finally, measure and record the distance between the leading edge and the tip of the cutter when the cutter is both fully forward and fully back.
Suggested Cutter Settings
This table lists some suggested settings for the measured distance B in Figure 5-1
|
Material |
Distance B in Figure 5-1 |
|---|---|
|
Soft clay, loose sand |
20 millimetres back |
|
Firm to Stiff clay |
10 - 5 millimetres back |
|
Stiff to Very Stiff Clay |
5 - 0 millimetres back |
|
Dense Sand |
10 - 5 millimetres back |
Notes on rock roller bits
A rock roller bit can be used in place of a cutter in ALL the above materials, using a similar setting. There are two aspects to consider:
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The tendency to clog the wheels of the rock roller in clay
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The water passage, which points directly down on the rock roller and so could excavate a passage, resulting in unexpected disturbance.
The flat bladed cutter design sweeps very closely to the sides of the cutting shoe, a feature which was not available on earlier designs of cutter; although at the present time no great experience is available with this design it is expected that in clean homogeneous material this ought to work very successfully. If clay stones or the occasional very hard layer are expected then the rock roller bit is the preferred choice.