Additional Design Features

CODE

FEATURE

CODE

FEATURE

A

Air application

N

B

0

C

Center jet

P

D

Deviation control

0

E

Extended jets

R

Reinforced welds

F

S

Standard steel teeth

G

Extra gauge/body protection T

H

u

I

V

J

Jet deflection

w

K

X

Chisel insert

L

Y

Conical insert

' M

Z

Other insert

Fixed Cutter Codes First code designations are

  1. D - natural diamond/matrix body
  2. M - polycrystalline diamond cutters/matrix body
  3. S - polycrystalline diamond cutters/steel body
  4. T - thermally stable polycrystalline diamond cutters/matrix body
  5. O - other

Second code designations are

  1. long taper/deep cone
  2. long taper/medium cone
  3. long taper/shallow or no cone (parabolic)
  4. medium taper/deep cone
  5. medium taper/medium cone
  6. medium taper/shallow or no cone(rounded)
  7. short taper/deep cone (inverted)
  8. short taper/medium cone
  9. short taper/shallow or no cone (flat)

Third code designations are

  1. bladed/changeable jets
  2. bladed/fixed ports
  3. bladed/open throat
  4. ribbed/changeable jets
  5. ribbed/fixed ports
  6. ribbed/open throat
  7. open face/changeable jets
  8. open face/fixed ports
  9. open face/open throat 10. alternative codes a. radial flow b. cross flow c. other

Fourth code designations are

  1. cutter size; large, medium, and small
  2. cutter density; light, medium, heavy

Drill Bit Characteristics

Roller cone drill bit characteristics of importance in regard to selection are:

  1. Teeth length and size
  2. Number of teeth (indexing)
  3. Cone offset
  4. Cone bearing type
  5. Heel row teeth configuration
  6. Gauge configuration
  7. Journal shaft angle (how flat or round the bottom hole pattern)

The teeth length and size and cone offset affect the drill bits penetration rate. The next three items on the list have a large affect on the bit life. The last item affects hole straightness.

Drilling Heel Row Bit

DRILL BIT SELECTION 526 MITCHELL Box 1492 Golden CO 80402

Rock Drilling Bit

Rock Failure Models

Two rock failure models are recognized. These are the tension and shear models and are illustrated in the following sketches. The tension model may be demonstrated by hitting a rock with a hammer. The events are (1) the tooth hits the rock and makes a powdered sphere below it. The tooth enters the powder and increases its pressure which pushes the sides of the rock apart causing it to crack in tension.

The shear model (also called the fslip-stick?) has the tooth scraping. Chips of rock are formed along shear planes. It has been argued that the shear type action is more efficient than the tension model because the chips are larger; ie, very little powder size chips are made.

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