STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 811
INNER AREA
OUTER AREA
213 RADIUS 1/3 RADIUS
Figure 4-166. Schematics of cutters wear [55]. (Courtesy SPE.)
schematically in Figure 4-166. When grading a dull bit, the average amount of
wear in each area should be recorded. For example, in Figure 4-166 the five
cutters in the inner area would be graded a 2. This is calculated by averaging
the grades of the individual cutters in the inner area as follows: (4+3+2+1+0)/5=2.
Similarly, the grade of the outer area would be a 6. On an actual bit the same
procedure would be used. Note that for a core bit, the centerline in Figure 4-166
would be the core bit ID.
The third space is used to describe the primary dull characteristic of the worn
bit, i.e., the obvious physical change from its new condition. The dull characteristics
which apply to fixed cutter bits are listed in Figure 4-165.
The location of the primary dull characteristic is described in the fourth
space. There are six choices: cone, nose, taper, shoulder, gauge, and all areas.
Figure 4-167 shows four possible fixed cutter bit profiles with the different areas
labeled. It is recognized that there are profiles for which the exact boundaries
between areas are debatable and for which certain areas may not even exist.
Notice that in the bottom profile there is no taper area shown. However, using
Figure 4-167 as a guide, it should be possible to clearly define the different areas
on most profiles.
The fifth space will always be an “X” for fixed cutter bits, since there are no
bearings. This space can be used to distinguish dull grades for fixed cutter bits
from dull grades for roller bits.
The measure of the bit gauge is recorded in the sixth space. If the bit is still
in gauge, an “I” is used. Otherwise, the amount the bit is undergauge is noted
to the nearest + of an inch.
The seventh space is for the secondary dull characteristic of the bit, using
the same list of codes as was used for the primary dull characteristic. The reason