WATER JET CONFERENCE - Waterjet Technology Association

NAME: George Savanick
COMPANY: Bureau of Mines
QUESTION TO: MIKE HOOD
DISCUSSION
QUESTION: "There appears to be an inconsistancy between your cumulative curve and
your 'sawtooth' force distribution curves. The cumulative curves indicate that cutting with
the water jet augmented bit is no more efficient than cutting with bits along but your
'sawtooth' curves of water jet augmented bits are less accentuated than those of
unaugmented bits. Would you comment on this apparent contradiction?"
ANSWER: "The cumulative curve that you refer to is the particle size analysis plot of
cumulative weight percent finer than a given size fraction vs. particle size (Figure 1
below). You are correct in your observation in that the slope of these curves is a measure
of the efficiency of the rock breakage process and, as can be seen from Figure 1, the
efficiency of this process is the same for both dry and water jet assisted cuts.
The force:time or force:displacement measurements (Figure 2a and b) on the
other hand, as you correctly point out, indicate that water jet assisted cutting is more
efficient. The area under these force:displacement curves is a measure of the energy
expended during the cutting process. Thus, the smaller this area, the more efficient the
cutting operation.
The reason for this apparent contridiction lies in the energy expended by the jets.
This component of energy is accounted for in Figure 1 but is not included in Figures 2a
and 2bo Unfortunately, because most laboratory tests are conducted at slow cutting
speeds, where the fraction of the energy exerted by the jets is large in comparison to the
mechanical energy exerted by the bit, it is not possible to qualify meaningful specific
energy figures for the over-all cutting process. However, field trials that have been
conducted using water jet assistance have shown that the specific energy is decreased
when water jets are used (Hood, 1978; Tomlin, 1982). That is, the efficiency of the
overall cutting process is increased by the action of the jets. The reason for this is that the
rate of rock excavation is increased by more than the increase in the energy required for
the jets.
REFERENCES
HOOD, M. (1978) "A Study of Methods to Improve the Performance of Drag Bits used
to cut Hard Rock"; Ph.D. Thesis, Department of Mining, Eng., Univ. of the
Witwatersrand, South Africa.
TOMLIN, M. (1982) "Field Trials with a 10,000 psi Prototype System"; Proceedings of
Seminar for Water Jet Assisted Roadheaders for Rock Excavation, sponsored by U. S.
DOR/U.K.NCB, Pittsburgh, PA, May 26-27, pp. C1-C11.
398