Neftegaz.RU #3-17 ENG

EQUIPMENT
FIG. 1. а) shear with compression, = 15°; b) shear, = 20°; c) shear and traction, = 25°
Therefore cutting force shall be
determined by the equation (2), and
will increase along with Rs growth
and shear angle decrease:
FIG. 2. RDT screw cut trajectory inclination angles (for bits of various diameters)
(2)
where:
mm 2 ;
– area of bluntness,
– thickness of the cut rock layer,
mm;
– radius of the cutting element
installation, mm;
– cut angle, degrees;
– shear angle, degrees;
– coefficient of cutting elements
friction on rock.
Operational capability of any
cutting-type RDT shall be
determined by reliability obtained in
the process of rocks crushing with
small radiuses at RDT axis where
screw cut trajectory inclination
angles (fig. 2) have the largest
value and are determined by the
ratio
FIG. 3. Diagrams of wear of the drilling tools cutting blade reinforced by TC8 (on the left) and
PCD (on the right) inserts
where: – depth of RDT
penetration per one rotation, mm/
rot;
– radius of the cutting element
installation, mm.
Cutting elements touchdown on
rear edge leads to breakages
caused by the effect of forces along
the rear edge.
Quite often the rear angle value is
increased in order to reduce growth
of the hard-alloy tools bluntness
area. The Figure 3 shows diagrams
of wear of the drilling tools cutting
blade reinforced by TC8 (a) and
PCD (b) inserts.
Bluntness area growth peculiar
for PCD inserts occurs only due to
increase of the cutting edge length
when the width along diamond layer
remains constant, where F 1 = F 2
= F 3 . Since PCD inserts diamond
layer wear coefficient by 50 or more
times exceeds tungsten-cobalt
base material wear coefficient, it
leads to higher PCD wear along
the rear edge and to the rear angle
formation. Thus PCD self-sharpens.
Therefore, based on the
calculations made by the equation
2, it becomes evident that it is
reasonable to apply smaller values
of the rear angle for PCD with the
front angle and the rear angle
being interrelated constructively.
This will lead to decrease of the
cutting angle and decrease of
the negative front angle , and
consequently to reduction of the
cutting force F cut .
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