DTH Operation

The only practical way to drill a hole through hard rock formations in the ground requires breaking rock into chips and then removing the chips from the hole.

For many centuries, chips were generated by lifting a hard object and letting it fall. Chips were then gathered in a bail periodically and lifted to the surface. Holes several thousand feet were drilled by this slow, laborious procedure.
Hollow drill pipe provided a much better system. As cuttings were created, a circulating fluid could carry them to the surface. Except for the time required to add strings of drill pipe or change bits, drilling was continuous. The fluid was pumped down the drill pipe, through the ‘eyes’ of the bit where chips were picked up, and then forced to the surface in the annular space between the pipe and wall of the drilled hole. Hollow drill pipe is used with both tri-cone bits in rotary drilling rigs and percussion bits in either out-of-hole or down-the-hole drilling rigs.

Down the hole hammers combine features of old cable-tool drilling – impacting a bit against the rock being drilled, with modern rotation. The combination is especially advantageous in medium to extremely hard formations such as granite, schist, gneiss, magnetite, limestone, dolomite, sandstone, basalt, quartzite and hematite. With sufficient air, holes are cleaned continuously; almost no energy is wasted chewing or grinding the rock chips produced.

THE HAMMER has essentially only one moving part – the piston. Air to move it up and down is directed by ports covered or uncovered during piston motion.

Air enters the hammer through a bore in the backhead, opens a rugged check-valve, and flows through holes in the valve chest into the control tube. Channels carry the air from here down into a port where it continues flowing down air passages in the piston to finally reach the lower chamber. Pressure created by the air influx forces the piston to move upward.

Upward motion continues until the piston pulls off the stationary foot valve; this allows air flow from the lower chamber through the bit and into the borehole area being drilled. Immediately after the lower chamber is exhausted, the top of the piston recess aligns with the port to allow air flow through the piston’s top outer passages into the upper chamber. This chamber has been sealed by the piston passing over the end of the control tube . The air pressure then decelerates piston movement quickly but smoothly, and piston motion reverses before the piston strikes the control tube. As downward motion starts, and just prior to the piston striking the bit, the upper chamber begins to exhaust.

Upon impact, the cycle is immediately repeated. The bit moves freely in the chuck splines so that full impact force is transmitted to bit cutters to produce formation chips. When the hammer is lifted off the bottom, the bit drops to an extended position with it’s top shoulder resting on a retainer ring. The piston rests on top of the bit and does not oscillate.

This “at rest” action is provided by full air volume being directed through the piston bore. In this way, the driller can blow water from the hole and accelerate periodic borehole cleaning as and when necessary.
Most hammers can use Bits with various head profiles including concave, flat face, drop centre, double gauge, step gauge or “Hole Opener”. These bits have long life, are very tough, and can be resharpened and remanufactured.

To remove cutting effectively between the drill pipe and hole wall the annular air velocity should be 3000ft/min or more, depending on the density of the rock and the size of the cuttings. Annular air velocity is determined by the compressor air output, bit size and drill pipe size. Decreasing the bit size and/or increasing the drill pipe size for a given hammer will increase the annular air velocity. The drill pipe size should not be larger than the outer diameter of the hammer.

To calculate the annular velocity, use the formula :

Q = Air volume ( CFM )
DH = Hole Diameter ( inches)
DP = Drill Pipe diameter (inches)

An air system should provide about 20% more free air at the hammer than required to operate the system. As just mentioned, this additional air is needed to create sufficient annular air velocity for proper removal of cuttings.

Wear rates for the tungsten carbide buttons in percussion bits are directly proportional to rotation speed. Lower speeds will decrease sharpening intervals and increase bit life but should be used only if penetration rates are satisfactory. Normally, harder and more abrasive materials such as granite require slower rotation speeds than softer and less abrasive materials such as shales and limestones. This occurs because increased rotary speed affects penetration rates very little in hard materials, but provides a definite ROP increase in softer materials. However, higher speeds should be used carefully because harder bands or layers may be encountered along with cracks and crevices.

Optimum rotation speeds depend on the type of formation, penetration rate, type of bit, and the weight used. Ideally, a percussion bit should penetrate 3/8” (96mm) per revolution of the pipe.

Using the following guide will help determine initial rotation speed : set RPM to about 1/2 the penetration rate in feet-per-hour.(Approximately 1.6 times penetration rate in meters – per – hour) For example : in a formation where the drilling rate is established by prior experience to be 40 feet – per – hour (12.2 m/hr or 8 in/min), the recommended rotation RPM is 20. This figure is obtained by :

(1.6 x 12.2 = 19.5 – rounded to 20 r.p.m.)

Wear rates for the tungsten carbide buttons in percussion bits are directly proportional to rotation speed. Lower speeds will decrease sharpening intervals and increase bit life but should be used only if penetration rates are satisfactory. Normally, harder and more abrasive materials such as granite require slower rotation speeds than softer and less abrasive materials such as shales and limestones. This occurs because increased rotary speed affects penetration rates very little in hard materials, but provides a definite ROP increase in softer materials. However, higher speeds should be used carefully because harder bands or layers may be encountered along with cracks and crevices.

Optimum rotation speeds depend on the type of formation, penetration rate, type of bit, and the weight used. Ideally, a percussion bit should penetrate 3/8” (96mm) per revolution of the pipe.

Using the following guide will help determine initial rotation speed : set RPM to about 1/2 the penetration rate in feet-per-hour.(Approximately 1.6 times penetration rate in meters – per – hour) For example : in a formation where the drilling rate is established by prior experience to be 40 feet – per – hour (12.2 m/hr or 8 in/min), the recommended rotation RPM is 20. This figure is obtained by :

Weight applied to the hammer, after the bit comes into contact with the formation, must be sufficient to keep the bit closed or shouldered in the hammer. Minimum weight can be calculated theoretically by multiplying tool piston area by air pressure. In most cases, however, more weight is needed for maximum drilling efficiency. How much additional weight depends on the type of bit and formation, this is normally established by the driller. Usually, it will be less than 3600 pounds (1630 kg) on the bit when drilling 6 1/2” (165mm) holes.
Another quick “Rule – of – Thumb” is to say, weight on bit can be calculated at 500 lbs per inch diameter of the bit. i.e. 6 1/2” x 500 lbs = 3250 lbs.
If correct weight on the bit is not maintained, these problems may result :

  1. Drop in penetration rate.
  2. Damage to bit shank causing premature shank failure.
  3. Damage to the driver sub splines and bit rotationing ring.
  4. Erratic hammer behavior causing damage to carbide, bit face and driver sub

Correct lubrication during drilling operations is extremely important. Inadequate lubrication is a major cause of hammer wear and failure.

Correct lubrication means proper lubricant and proper lubricating equipment. It is recommended that a positive – feed type lubricator is used to inject oil into the air line. A positive displacement pump of this type is better than venture types which have small oil reservoirs and reduce effective air pressure to the tool.

Recommended Minimum Oil Requirements.

4” — 1 ltr per hour
6” — 1.5 ltr per hour
8” — 3 ltr per hour
10” — 3.5 ltr per hour
12” — 4.5 ltr per hour
15” — 5 ltr per hour
18” — 5.5 ltr per hour

Only rock drill oil should be used. It has high film strength, good adhesion, stable viscosity, and high flash point. On the following page are some typical lubrication values.

We recommend that a litre of oil is poured into each new or uncoated drill pipe as and when put into use.

It is essential to sharpen bits properly. Unfortunately many in the field have been taught improper procedures. A bit needs resharpening when flats on the gauge buttons reach about 1/8” (3 – 4mm) diameter. These tungsten carbide buttons wear in such a pattern as to produce perfect circles whose centre is the actual rotation centre of the tool. In other words, the tool ma rotate slightly off its design centre as evidenced by flats being slightly larger on one side of the bit.

On the market today there are several types of buttoning machines available ranging from hand held grinders through to fully automatic robotic arms all of which do an excellent job of resharpening buttons.