BIT BODY METAL FAILURE

CAUSE: Poor button hole.

 

SOLUTION: See your bit manufacturer. 

 

When the interference between the button insert and the button hole is excessive, fatigue cracking around the base of the button hole can be encountered.  This type of body metal failure is particularly encountered when the manufacturing process of gun drilling is used to produce the button hole.  A gun drilled hole can usually be detected by the up turned bottom of the hole.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

BIT BODY METAL FAILURE

 

CAUSE: Excessive Button Interference

SOLUTION: See your bit manufacturer

 

Button hole cracking can also be encountered in holes drilled by other manufacturing methods if great caution is not given to proper button installation methods.  This is particularly true when thru hardened body material is employed.

 

 

 

 

Note: This particular example shows a bit that has extremely worn carbides and is overrun.  Regrinding at more frequent intervals will help improve drilling efficiency and reduce loads on carbides and bit body.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

CRACKED BIT FACE

PROBLEM: Poor Design.

 

SOLUTION: See your Bit Manufacturer.

 

The use of proper button spacing is extremely important in the design of thu hardened bit in order to prevent face cracking in the body material.  This type of cracking is generally characterised by cracks propagating from te buttons in the gage row.  The growth of these cracks will eventually cause the complete loss of the face bit, as shown below.

 

Correct button spacing is extremely important in quality bit designs.  Buttons placed too close will result in face cracking of the body material.  Remember, the carbides on the gauge converge on the inner row at the bottom of the socket.  The gap at the bottom of the hole may be much less than you are able to see at the surface of the bit body.

 

 


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

 

Carbide Breakage

 

CAUSE: Excessive Body Wear.

 

SOLUTION: Use through-hardened Bit or Bits with Tree flushing holes.

 

Excessive body metal wear can be encountered in drilling softer formations containing Pyrite.  In these applications, the body metal wears preferentially to the carbide inserts.  Eventually, the metal in the bit head will wear away sufficiently to cause the inserts to protrude excessively from the face metal.  The lack of support will result in the bits failing by button breakage.  In this drilling application the use of a thru-hardened bit body will be advantageous.  In addition, such bit design innovations as a convex bit face and three flushing holes help improve flushing and reduce the amount of body metal erosion encountered in these drilling applications.


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

THERMAL SHOCKED CARBIDES

 

CAUSE:  Loss of button Protrusion/Clearance.

 

SOLUTION: Re-sharpen Carbides and remove surrounding face width to restore original button protrusion.

 

In very hard and abrasive formations where the grain size of the rock is very small, excessive carbide wear can be encountered.  In these applications, the carbide materials wear preferentially to the body metal.  As the buttons flatten and the protrusion is lost, the bit will begin to squeal in the hole and the rate of penetration will be drastically reduced.  Attempts at sharpening the button inserts without removal of the body metal in the face of the bit (in order to restore the original amount of button protrusion) will not improve penetration rates.

 

Continued drilling will result in heating of the face of the buttons due to friction and resulting in thermal shock cracking.  Finally, total failure of the buttons in the gage row will be encountered.



Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

SNAKE SKIN CRACKING

 

CAUSE: Micro cracks.

 

SOLUTION: Sharpen at more frequent intervals/ Use softer grade carbide.

 

Similar in appearance to thermal shock cracking, but encountered in non abrasive drilling applications such as limestone, a snake skin wear pattern can be encountered on the face of the gage row buttons.  In these drilling applications, the surface of the cemented carbide begins to suffer from fatigue, with evidence of mirco cracks that resemble snakes skin, particularly at the corners of the inserts.  Snake skin is not always visible to the naked eye and a magnifying glass must be used.  At first sign of snake skin, the button must be ground, otherwise the micro-cracks will penetrate more deeply into the insert eventually causing chunks of cemented carbide to loosen and drop out.  In certain types of rocks, no visible wear occurs to the carbide insert at all, nevertheless, the button must be sharpened to remove the snake skin.

 

Failure to remove the snake skin will result in total button failure evidenced by a multifaceted appearance of the fractured surface.  Like thermal shock cracking, continued drilling will result in total failure of the buttons in the gauge row.  Harder gauge of carbides are more susceptible to snake skin cracking.  With a softer grade of carbide, cracks may be worn away at the same speed at which they develop.

 

Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

CUSP-TYPE CARBIDE FAILURE

 

CAUSE: Overloading of Carbide.

 

SOLUTION: Increase Rotation Speed, Reduce Weight on bit and/or use softer or a DP Grade.

 

Button breakage due to stress overloading generally results in a cusp-type (forming a peak) of fracture.  The first phase in this type of carbide failure is usually indicated by breaking away of the trailing edge of the button insert.  The progression of this type of failure with the breaking away of the leading edge of the button insert produces the cusp-type fracture.  The cusp-type of button failure is the result of stress overloading and is generally encountered in applications were the rotation speed is too slow causing excessive wear on the leading edge of the button.  Stress overloading resulting from the worn non-symmetrical top surface of the button causes the fracturing of the trailing edge of the button resulting in the cusp appearance of the final fracture surface.  This type of failure can be addressed by increasing the rotation speed, reducing the hold down pressure or using a carbide insert with greater toughness properties. 


Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

CARBIDE BROKEN FLUSH WITH BODY METAL

 

CAUSE: Shear Loading.

 

SOLUTION: Sharpen Carbides at more frequent intervals.

 

The carbide button is designed in such a manner to take compressive forces, but is relatively weak in shear.

 

 

 

As the button begins to wear, it develops a flat spot.  As the flat spot enlarges, continued drilling changes the load from compression or vertical loading to horizontal or shear loading.  Button breakage due to shear loading is one of the most common types of button failure.

 

 

 

This side loading or pinching of the carbide in the hole will ultimately result in carbide breakage.  Regrinding carbides will restore the original carbide shape and ensure correct loading of the carbide.  Not only is carbide life improved but also efficiency and penetration rate.

BROKEN CARBIDE

 

CAUSE: Badly Machined Button Hole.

SOLUTION: See your Bit Manufacturer.

 

Button breakage occurring significantly below the plane of the body metal is an indication of a tapered or distorted button hole or surface irregularities on the surface of the carbide insert.

 

 

 

 

Reference:

Trouble shooting manual

Driltech Mission, LLC

Sandvik

BUTTON FAILURE

 

CAUSE: Carbide Wear.

 

SOLUTION: Re-sharpen Carbide and/or grind body metal around carbide.

 

Button carbides should be sharpened if snake skin cracking is observed, a drop in penetration rate occurs or a flat approximately one third the diameter of the button develops.  If the flat spot has developed to over one half the diameter of the button, sharpening has been delayed too long.  When thermal shock cracking is encountered, the button must be sharpened immediately and the protrusion of the button above the face of the bit restored or failure is imminent.

 

 

 

When inserts looks like this with a flat spot no greater than one-third of the button diameter, it should be sharpened.

 

 

 

When the insert looks like this with a flat spot greater than one-half of the button diameter, sharpening has been delayed too long.

 

 

 

 

When the insert looks like this, immediately sharpen or failure is imminent.

 

 

 

 

Body metal on the gauge must be removed to ensure sufficient clearance between the bit body and the carbides.