Page 34 - Diamond Products Catalog

Quick Tip
• WOB is too low if the bit polishes and torque drops. Results in low life and penetration rate.
• WOB is too high if added weight does not increase penetration rate. Results in constant sharpening and rapid wear.
Quick Tip
• If the head stalls under normal operation, reduce the speed to produce more torque and maintain steady rotation.
• In broken ground, cut RPM in 1⁄2 and apply sufficient weight on bit to reach 1 to 2 ipm (3 to 5 cpm).
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© Copyright 2014 Boart Longyear. All rights reserved.
OPERATING PARAMETERS
This section of the manual is intended to provide guidelines to make the most of Boart LongyearTM bits. A condensed, convenient reference for
the selection of suitable drilling guidelines for impregnated bits is provided in the Impregnated Bit Drilling Guidlines following this section.
Rotational Speed
Rotational speed has a direct influence on the penetration rate of bits. If the speed is doubled for a given depth of a cut, the penetration rate also doubles. For this reason, rotational speed
is typically set at a fixed value based on the size of the tools being used. If the rig has sufficient torque, the speed can be increased to get higher penetration rates. Conversely, if the head stalls under normal operation, then the speed should be reduced to produce more torque and maintain steady rotation. The exception to this rule is drilling in extremely broken and hard ground. Broken ground is free to move so instead
of cutting, the rock is removed by a grinding action. Under these conditions (three body wear), diamond bit wear is extremely rapid and rotational speed should be cut to half RPM and weight on bit sufficient to reach 1 to 2 ipm (3 to 5 cpm).
Weight (Force) on Bit
Weight on bit (WOB) is the main method of controlling bit life and penetration rate. The bit weights shown on the chart after this section indicate the range of weights considered normal for a given size of core barrel. If very high bit loads are required to cut the rock, select a higher series bit to prevent increased hole deviation, excessive core barrel, and rod wear, and even down-hole failures. Increasing the bit series normally results in lower bit weights required
for the same penetration rates. If weight on bit
is too low, both the penetration rate and torque drops, resulting in low life and productivity.
This condition is referred to as polishing and describes the phenomenon of diamonds wearing faster than the matrix until the face is flat, leaving no diamonds protruding from the face. Polishing
is caused by the friction and heat accumulating in the diamond as it rubs rather than cutting the rock. If weight on bit is too high, characterized by very little or no increase in penetration rate for additional weight, the result is constant sharpening and rapid wear.
Sharpening of the bit results from burying the diamond in the formation and creating contact between the matrix and formation. The friction on the matrix removes it and exposes new diamonds. Sharpening is beneficial when bit penetration has dropped due to wear on the diamonds but if left in a sharpening mode the
bit is consumed very rapidly. As rods are added, force to maintain constant weight on bit will change, so weight on bit must be monitored to ensure consistent bit performance at all depths. The weight of the rods and the down force developed by the hydraulic cylinders provide the force exerted on the diamond bit. Unfortunately, drill rig controls do not display weight on bit but instead display feed cylinder pressure.
The weight on bit or force can easily be found by the following "off bottom" method:
1.Suspend the rods with the hydraulic system in the lowering position.
2.With the drilling motor running at a drill rotation, note the reading on the cylinder feed pressure gauge. This is approximately equal to the hydraulic pump pressure plus the weight of the rods in terms of hydraulic pressure. This is called the “off bottom” pressure.
3.Rotate the rods and feed them down by opening the restrictor valve. As the bit touches bottom, part of the weight of the rods is supported on the bit. This action
is indicated by a decrease in the gauge reading. The difference in the gauge readings is the pressure applied to the bit.
4. The pressure applied to the bit multiplied by the area of the hydraulic cylinders gives the force or weight on bit.