1. Selection of cemented carbide grades

The right selection of cemented carbide grades is one of the most important factors that guarantee the effective application of

cemented carbide. Attention should be paid to the following when the selection of the grades is done:

a.The properties of the rock formations, such as hardness, frictional characteristics, and their configuration, such as compactness,fissures, joints, caverns, etc. The basic principle is: the harder the formation of rocks to be drilled, the much tougher the mining inserts should be. They, however, should not be too soft, and that is to say their wear resistance should not be too low, since it will increase the consumption of cemented carbide and reduce the efficiency in the drilling as well; Cross drill bits should be used if the rock

formations are full of fissures without regards to the hardness of the rock formations and what types of drilling machines used.

b.The types of rock drilling machines, light or heavy types, etc.

c.Method of fragmenting the rocks formations (cutting, shearing, crushing, impacting, etc.);

d. Capacity of the equipment (e.g. low or high air pressure, etc.);

2.Selections of types of cemented carbide

K0 type mining inserts are for embedding chisel drill bits for side face milling and solid chisel drill bits with diameters of 25-49mm.

Some of the types are for embedding drill bits of big diameters for center milling operations. K048 and K049 are mostly used for drilling highly frictional rock formations.

K 1 type mining inserts are for embedding chisel and cross drill bits of various diameters.

The drilling efficiency of chisel drill bits is higher than that of cross drill bits. Cross drill bits can, however, withstand a high load.

Chisel drill bits are used for drilling homogeneous rock formations of various hardness and the impact load of the drilling machines shall be less than 6 kilograms ·m. It is advisable to use cross drill bits for drilling rock formations with high hardness.

3.Manufacturing of cemented carbide drill bits

a.Distance between cemented carbide inserts and the surface of the embedding cavity.

It will be 0.3-0.5mm on each side with compensating shims and 0.1-0.15mm without compensating shims. The use of compensating shims is to decrease the thermal stress during the welding of cemented carbide inserts. The shims can be manufactured with low-carbon steel wire nets or copper pad s with the same width and length of the cemented carbide inserts to be able to wrap the 3 sides of the inserts. Cemented carbide inserts without compensating shims are to be fixed in the desired position by seam melding.

b.Welding of cemented carbide inserts

(1)The drill bits are to be preheated until borax gets smelt and then welding flux is placed on the inserts and heated until the flux gets smelt.

(2)The inserts are immediately adjusted and placed in the correct position after the flux gets smelt and the inserts are moved back and forth towards the bottom for tight fixing.

(3)The welded drill bits are placed immediately into flake asbestos or dry lime powder for them to cooling down slowly.

(4)The best heating method is high-frequency heating with high efficiency, convenience and low cost, but with good quality. It prevails in a big production scale and in mines with favorable conditions. In addition, coke or oxygen-acetylene can be used for heating without any heat transmission from the drill body to the inserts.

c. Regrinding of drill bits

The regrinding of drill bits includes the drill body and inserts. The body is to be ground with grinding wheels of aluminum oxide of a grit size of 40-50, hardness of Z1-Z2 and a rotating speed of 25 m/sec. Carbide inserts are to be ground with grinding wheels of green

silicon carbide with a grit size of 40-50, hardness of R3-ZR1 and a rotating speed of 18-20m/sec. During the regrinding, the working edges of the drill bits should face the rotating direction of grinding wheels with left and right oscillation, making possible the regrinding of all the working edges.

Coolant is to be used continuously or no coolant is to be used at all, but intermittent cooling or dipping coolant is to be forbidden, as it will result in cracking in the inserts. Coolant can improve the quality of regrinding and reduce the wear of the grinding wheels.

When the inserts are to be reground, they should be slightly pressed against the grinding wheels. The pressure should be not too much,as otherwise it causes much wear of the grinding wheels and the edges of the inserts to break, resulting in local overheating of the inserts and cracks in the inserts.

During regrinding, the angle of the 2 edges of the inserts should be chamfered 0.5 x 45°and the working edges themselves should beblunted 0.3-0.5mm.

During the grinding of the working edges, attention should be paid to the symmetry of the edges and the center line of the drill bits, so

that there will be an even load on the working edges and a balanced drilling.

4. Applications of drill bits

a. Rock drilling

When drilling is started, it should be done with a little force with the air door slightly open and the air door will be left wide open when the drill bit gets in 2-3 centimeters to increase the pressure for normal operations.

The drill bits and the rods are to be replaced when they get in 1.5 meters of the shot holes. Normally 3 drilling rods are used with 3 drill bits replaced when drilling highly frictional rock formations. The diameters of the drill bits reduced 1-2 mm successively.

Deviations in the diameters of each group of drill bits should be minimized as much as possible. Drill bits with diameters differing much are to be used when the wear is significant in drilling highly frictional rock formations.

Each group of drill bits should get into the shot hole freely, since otherwise it will damage the inserts.

b. Repeated regrinding

What wears fastest is the edges of the outer peripheries of the drill bits during rock drilling operations. When the working edges are 5-6mm from the outer peripheries with a blunted width up to 3-3.5mm, or when inverted cones occur in the working portions, the drill bits have to be repeatedly reground. The repeated regrinding of the drill bits should keep the original geometries and they are to be checked with templates.

The reground drill bits should also be kept in a compatible package and the diameter of the drill bits should be checked making bright colored marks on the bits, making it easy for choosing the bits when they are to be used.In drilling soft rock formations, one drill bit can be used repeatedly until it reaches a status of bluntness that requires regrinding.

The blunted drill bits should be repaired in time in order to avoid the reduction of drilling efficiency and unreasonable wear of the carbide.

5. Other matters to pay attention to

Collision of the working edges of the drill bits with metals should be avoided during transportation and in operations.

When the bits are in operation, there should be no wanton knocking at the drill body. When the drill bits are to be replaced, anapparatus should be used for dismantling the bits. If a hammer is used to knock the drill body to dismantle the bits, it will damage the drill body resulting in the premature rejection of the bits.