Tool analyses-what a broken tool tell us:
★Striking end/foot valve
Problem: foot valve broken
Check that the foot valve is correctly located in its housing and that the protrusion length is correct.
After fitting the foot valve into the bit shank, its protrusion from the end of the shank mustbe checked. Too much or too little protrusion will seriously affect the performance of the hammer. After the foot valve has been pressed into its seat and protrusion is within the specified limits. Do not put more pressure on the foot valve, since this could result in damage.
Removing the foot valve: the worn or damage foot valve is removed by cutting it with a hacksaw blade or knife, and then prising it out of the bit shank with the aid of a screwdriver. Heating the foot valve to 50-70 degree Celsius can make removal easier.
Fitting a new foot valve: the new foot valve should be pressed into the seat in the flushing hole in the bit shank using a special assemble tool that guarantees guidance of the foot vale into the seat, and ensure that the amount of protrusion is correct. For easier installation, the temperature of the plastic foot valve should be 20-60 degree Celsius( it can be heated in the water, or on the compressor). Before fitting the foot valve into the bit shank, coat the part of the valve that is pressed into the drill bit with rubber glue( or a similar substance). The rubber glue will act as a lubricant during fitting. And as a fixative thereafter.If rubber glue is not available, use silicone grease or some other similar lubricant.
NOTE: do not use a hammer to seat the foot valve. Heavy blow can damage the foot valve or cause it to locate incorrectly so that it is struck by the impact piston during drilling. Use some kind of hydraulic press to press it gently but firmly on to its seat in the bit shank.
Problem: crack on the striking end
A broken striking end is caused generally by a badly worn piston, chuck, bottom bush or retaining ring.
Ensure that the foot valve is not broken. This is generally caused by a worn piston or cylinder bore causing misalignment, lack of lubrication causing pick-up or corrosion by water.
Problem: striking end damage
Inspect the striking end for damage or corrosion, generally caused by lack of lubrication or foreign particles caught between the piston and striking end.
★Drill bit head
Problem: body wash
In highly abrasive conditions, the metal may wear more quickly than the carbide inserts. This is known as ‘body wash’ and in order to minimize carbide breakages, the carbide should be ground so that it does not protrude more than 9mm(0.35”)above the metal surface.
Problem: unevenly worn
If the head is unevenly worn, the most likely causes are incorrect regrinding, bent drill tubes or drill string not being centralized in the drill rig table.
Problem: face cracks
Excessive feed force can cause cracks on the face of the drill bit which can lead to large pieces of metal breaking off.
Problem: breakage of bit face
Caused by excessive feed force .
Problem: deformation of flushing holes
Re-drilling caused by poor hole cleaning and also high abrasion can result in deformation around the edges of exhaust holes and flushing grooves. These should be ground to ensure a clean obstruction free passage for exhaustion air.
The depth of flushing grooves must be monitored as abrasion and/or high RPM wear away the drill bit body. The grooves must be regularly ground to ensure that drilling debris can adequately escape.
Problem: completely worn bit head
Still use the drill bit after it has exceeded the longest lifespan.
Problem: button ‘pop-out’
High levels of body wash and energy produced by the piston striking the drill bit can cause button to be hammered out of the bit body. This is known as button ‘pop-out’. Button ‘pop-out’ can also occur even in the drill bits with lilt body wash, where drilling in very soft conditions is being carried out and constant flushing is necessary because the drill bit face
is not in constantly contact with the bedrock.
Problem: bit head cut off
Excessive torque made it easy to break off when the head diameter is much larger than the hammer diameter. For example;3-inch drill bit ,head diameter greater than 105mm;4-inch drill bit, head diameter greater than 130mm; 5-inch drill bit, head diameter greater than 165mm; 6-inch drill bit, head diameter greater than 203mm; 8-inch drill bit, head diameter greater than 300mm; 10-inch drill bit, head diameter greater than 350mm.
Tungsten carbide wear/breakage
Problem: broken inserts
Broken inserts are generally caused overdrilling the drill bit before reconditioning, reaming the hole, using excessive feed force or where there are existing tungsten carbide or other metal particles in the hole. A DTH button bit can continue to drill with one or more broken buttons, but at a reduced penetration rate. Broken buttons should be ground off to prevent additional button particles breaking off and damaging other button.
★Drill bit shank
Problem: heavy spline wear
Regularly inspect the drill bit splines and remove any sharp edges or burrs with a file or emery stone. Heavy spline wear is usually caused by a badly worn chuck, excessive thrust, very high rpm or application of high torque in very difficult drilling conditions.
Problem: shank cut off across the retaining ring
Shank breakage across the shank retaining ring diameter is generally caused by the lack of lubrication or worn bit retaining rings, bit guide bush or bottom spacer.
Problem: shank breakage across the spline
Shank breakage across the splines is generally caused by the using a worn chuck,
Excessive torque or incorrect use of oversize drill bits.
★Piston and external cylinder
Problem: piston breakage
Problem: external cylinder worn
When you find the wall thickness of external cylinder is less than 4mm means you need to change a new one. Otherwise easy cause fracture.