PRIOR TO DRILLING
Clean Air Delivery - Pipework, Hoses and Drill String
A down hole hammer relies on a supply of clean lubricated air to operate reliably. The hammer piston is a close tolerance fit with the case bore and cycles at 900 to 2500 blows per minute ( 15 - 42 blows/second ). Failure to ensure that a clean lubricated air supply is being provided may result in overheating, material pick-up, seizure or failure. Hose ends may be accidentally contaminated prior to connection and drill strings may be contaminated during storage. Even new hoses and drill string parts could be contaminated from the factory. See Contamination for more information.
All hoses, piping and drill string components should be plumbed to the compressors prior to connecting the hammer. Then, air should be blown through the system to reduce the possibilities of contaminants entering the hammer.
Once the air supply system is flushed the hammer can be connected.
GENERAL OPERATING PRINCIPLES
Regardless of the bit type being used (conventional, reverse circulation, horizontal or under reaming), always follow the following sequence for proper operation of the down hole hammers.
1. Turning on the Air - With the bit "off bottom" in the "stop action mode", turn on the air . Air will blow straight through the hammer without cycling the piston.
NOTE: Failure to turn the air on prior to lowering the hammer on bottom or into a wet hole could result in plugging the bit exhaust holes or contaminants entering the bottom of the hammer. The same air that initiates the percussive action of the hammer, exhausts through holes in the bit face to clean the hole.
2. Starting Rotation - Start rotating slowly. Please note that a general rule of thumb for rotation speed is as follows. See Rotation Speed for more information.
1/2 the penetration rate in feet per hour
1.6 the penetration rate in meters per hour
Contrary to conventional rotary drilling, increasing the rotation speed will not necessarily increase the penetration rate. The primary reasons for rotation is to index the carbide inserts to fresh unbroken rock between impact and to keep the hammer joints tightened. Excessive rotation speed will only generate premature carbide insert and bit body wear.
NOTE: The hammer should always be rotated in a clockwise direction. Operating the hammer without rotation or in a counter clockwise direction could cause the hammer joints to become loose and may result in loss of the hammer or damage to the threaded connections.
3. Lowering the Bit - Lower the bit to "bottom". The bit will move into the work position and the hammer will begin to operate . Adjust the weight on bit to attain smooth operation and optimum performance from the hammer. The general rule of thumb for a starting weight on bit is as follows:
500 pounds per inch of bit diameter
9 kg per mm of bit diameter
In a wet hole or unconsolidated material, it will be necessary to feed the hammer slowly to sufficiently clean the hole as the hammer is advanced. Care should be taken to limit the power developed by the hammer when drilling in these conditions. This is done by the operator controlling the feed.
When the edge of a boulder is encountered and rotation becomes erratic or stalls, raise the drill enough to clear the edge of the boulder and slowly feed until the edge is drilled off. Repeat the process until the rotation becomes smooth.
When a solid formation is encountered, the rotation will stabilize and the hammer will sound strong and smooth. Adjust the rotation speed and weight on bit accordingly. Stay alert to the possibility that the solid formation may be a large boulder with loose material below. As soon as the hammer sound changes or the rotation torque fluctuates, lift the hammer and bit off bottom. Keep the rotation on and lower carefully to continue drilling.
It is good practice to monitor the hole flushing. It may be necessary to lift the hammer off bottom periodically to flush the hole until all the cuttings have been removed. Maintain rotation during this procedure.
Upon completion of the hole, lift the bit off bottom and flush the hole for several minutes to ensure no suspended material falls back on top of the hammer and drill bit. Maintain rotation during this procedure. It may be necessary, in difficult drilling conditions, to continue flushing the hole and rotating until the bit is at ground level.
NOTE: If a hole cannot be completed during a shift, remove the drill string from the hole. Holes can fill with water and foreign material overnight and contaminate the hammer.
4. Drilling with Water - While drilling, water injection serves two general purposes:
In moist formations, mud rings develop and hinder hole cleaning. These mud rings can become severe enough to cause hammers and bits to become stuck in the hole. Water injection will cause the cuttings to become liquefied and reduce the possibilities of mud rings.
Water injection also aids in keeping the internal hammer components cool.
5. Lubrication - Down hole hammers require a continuous flow of rock drill oil during operation. Failure to supply a continuous flow of rock drill oil for only a short period of time will cause unrepairable damage. Heat cracks formed in the surface of the piston during this period propagate through continued usage and often initiate a failure at an unrelated time. See lubrication for proper type and amount of required rock drill oil.
6. Bit Inspection - Sharpening
Carbide inserts should be sharpened when the buttons exhibit flats equal to 1/3 the diameter of the insert.
When the wear flat reaches 1/2 the diameter of the insert they are subjected to severe radial forces.
Although carbide inserts can withstand extreme longitudinal loading, they are easily fractured when subjected to severe radial loading. In addition, when the carbide inserts are severely worn drilling speed is reduced dramatically.
7. Recognizing Carbide Failure - Drilling After Carbide Insert Failure
After the bit has experienced carbide insert failures, further drilling allows the steel matrix to be eroded quickly. It is not particularly evident when only one insert is broken. However, when more than one insert in succession are broken, the steel matrix erodes rapidly.
Once the matrix of the bit body is worn away, that area of the bit can no longer be repaired. If possible, carbide insert failure should be reported immediately and the bit scheduled for repair. Continued usage after carbide failure may damage the bit to a point where it needs to be replaced.
8. Hammer Storage
If a hammer is to be stored or unused for any period of time the following procedure should be followed:
While connected to the drill string, blow dry lubricated air through the hammer for a period of ten (10) minutes. This will remove any water that has been used in the drilling process and lubricate the internal hammer.
Remove the hammer from the drill string and cap both ends.
Store the hammer in a horizontal position.
Prior to usage, pour rock drill oil into the backhead to insure immediate lubrication. See lubrication for proper type and amount of required rock drill oil.