Impregnated Bit Sharpening Techniques
Where the matrix type and waterway configuration have been correctly selected for a drilling job, an impregnated core bit should drill steadily with an even wear pattern until the matrix layer is totally consumed. At this point, the circulating pump pressure will usually increase due to the fluid restriction caused by the diminished waterway depth on the bit face.
If the matrix type and/or waterway configuration is not exactly suited to the formation conditions, it may become necessary to sharpen the bit face in the event that the penetration rate becomes unacceptably low. A decrease in penetration rate is usually caused by the diamond crystals on the bit face becoming blunt. The metal bond matrix is probably too hard for the formation conditions and will not erode at a controlled rate to expose new layers of diamond crystals. If possible, change to another D-Series or HR-Series bit with a softer (higher number) matrix type and/or a bit with a lower face contact area that has a more open waterway configuration such as the ‘Turbo’ style.
If it is not possible to change the bit, it will be necessary to sharpen the bit face using one of the following methods. Bit sharpening techniques should only be used when absolutely necessary as they tend to have an adverse effect on overall bit life.
Method 1 – Bit Face Dressing (Preferred Method)
Remove the bit from the drill hole and sandblast the face with an abrasive media such as Aluminum-Oxide or Silicon-Carbide grit. If sandblasting equipment is not available on site, use a tapping action with a flat file to roughen the surface of the bit.
Method 2 – In-Hole Method
With the bit on the bottom of the drill hole and with the drill string rotating, reduce the rotational speed to about 50% to 75% of the normal rotational speed. Try to maintain a constant rate of penetration for a distance of 6 to 50 mm (0.25 to 2 inch) allowing the bit load to gradually increase and then suddenly decrease thus indicating that the bit face has sharpened and will now cut freely. IMPORTANT: It is imperative that at the instant this sharpening (bit load decrease) occurs, the applied bit load must be decreased and the rotational speed must be increased to their original values. Failure to make this adjustment will result in further stripping and could cause a premature failure of the bit crown.
In extremely hard formations with a high Silicate content, it may be necessary to reduce the fluid circulation rate below recommended values in order to maintain bit sharpness.
THE FLUID CIRCULATION MUST NEVER BE COMPLETELY CUT-OFF TO A BIT ROTATING UNDER LOAD AND IS NOT AN ACCEPTABLE METHOD OF SHARPENING ANY DIMATEC IMPREGNATED DIAMOND BIT.