Dull Grading
Dull Iadc Drilling" href="/iadc/bit-classification-and-grading.html">bit grading is the determination of the degree of material wear relative to tooth structure, bearing wear, and gage area wear.
The American Association of Drilling Contractors (AAODC) established the first dull grading standard in 1961. This was the origin of the tooth - bearing - gage (TBG) format for reporting bit wear. The AAODC bit grading method, adopted in 1963, used a 1-8 scale to describe teeth and bearing wear. In this method, an 8 represented teeth totally worn flat, broken, or lost; and bearings that were either worn out or locked. This system remained essentially unchanged through 1985. In March of 1985, the International Association of Drilling Contractors (IADC) Drill Bits Subcommittee of the IADC Drilling Technology Committee agreed that dull bit gradings are an important part of bit records. In 1987 the IADC adopted an improved Rock Bit Dull Grading System. Minor changes were adopted in 1992. Most drillers have adopted the IADC Grading System. The IADC System provides a better description of a dull roller cone bit. The IADC System also provides a code to describe why a bit was pulled or a run was terminated.
The IADC Dull Grading System can be applied to all types of roller cone bits. Bits with steel teeth or bits with tungsten carbide inserts can all be described with this system. Following is a description of the dull grading system as it applies to roller cone bits.
This graphic shows the eight column system now used for dull grading.
Column 1 (I-Inner) is used to report the condition of the teeth not touching the wall of the hole (Inner).
Column 2 (O-Outer) is used to report the condition of the teeth that touch the wall of the hole. These teeth are also called the gage row. (Gage condition is important to good bit performance.)
In columns 1 and 2, a linear scale from 0 through 8 is used to describe the condition of the cutting structure as follows:
- Figure 2 Steel Tooth Grading 1 - 8
- 0 - no loss of cutting structure
- 8 - total loss of cutting structure
CuHIng Structura—Tungsten Carbide
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Figure 3 Tungsten Carbide Insert Grading
Example: A bit missing half of the inserts on the inner rows of the bit due to loss or breakage with the remaining teeth on the inner rows having a fifty percent reduction in height due to wear, should be graded 6 in column 1. If the inserts on the outer rows of the bit were all intact but were reduced by wear to half of their original height, the proper grade for column 2 would be 4.
3.Column 3 (D - Dull Characteristic - Cutting Structure) uses a two - letter code to indicate the major dull characteristic of the cutting structure. Note that this column uses only cutting structure related codes.
The IADC Dull Grading Chart lists the two - letter codes for the dull characteristics to be used in this column. It is important to use only the standard abbreviations.
4.Column 4 (L-Location) uses a letter or number code to indicate the location on the face of the bit where the cutting structure dulling characteristic occurs. The IADC Dull Grading chart lists the codes to be used for describing locations on roller cone bits. Location is defined as follows:
- G - Gage row - those cutting elements that touch the hole wall
- N - Nose row - the center most cutting element(s) of the bit
- M - Middle row - cutting elements between the nose and the gage
- A - All - all rows
Cone numbers are identified as follows:
The number one cone contains the center most cutting element; the spear-point on a steel tooth cone and the nose insert on a tungsten carbide insert cone. On certain TCI designs two cones may have a nose insert. In this case the nose insert that is offset is on the number 1 cone.
Cones two and three follow in a clockwise orientation as viewed looking down at the cutting structure with the bit sitting on the pin.
Column 5 (B-Bearing/Seals) uses a letter or a number code, depending on bearing types, to indicate the bearing condition of roller cone bits.
For non-sealed bearing roller cone bits, a linear scale from 0 - 8 is used to indicate the amount of bearing life that has been used. A zero (0) indicates that no bearing life has been used (a new bearing) and an 8 indicates that all of the bearing life has been used (locked or lost).
For sealed bearing (journal or roller) bits, a letter code is used to indicate the condition of the seal. An "E" indicates an effective seal and an "F" indicates a failed seal. An "N" indicating "not able to grade" has been added to allow reporting when the seal / bearing condition cannot be determined.
Column 6 (G-Gage) is used to report on the gage of the bit. The letter "I" (IN) indicates no gage reduction. If the bit does have a reduction in gage it is to be recorded in 1/16ths of an inch The Two Thirds Rule is correct for roller cone bits.
- Figure 4 Two Thirds Rule for Measuring Gage
The Two Thirds Rule, as used for roller cone bits, requires that the gage ring be pulled so that it contacts two of the cones at their outermost points. Then the distance between the outermost point of the third cone and the gage ring is multiplied by 2/3's and rounded to the nearest 1/16th of an inch to give the correct diameter reduction.
Column 7 (O -Other Dull Characteristics) is used to report any dulling characteristic of the bit, in addition to the cutting structure dulling characteristic listed in column 3 (D). Use the same standard two-letter codes as used in column 3. Note that this column is not restricted to only cutting structure dulling characteristics.
Column 8 (R-Reason Pulled) is used to report the reason for terminating the bit run.
The IADC Dull Grading Chart lists the two-letter or three-letter codes to be used in this column.
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