Mechanical excavators were introduced for soft rock cutting at the time of the industrial revolution. The early machines date back to mid 1800’s and were developed for rapid excavation of tunnels and to increase production in the mining environment. The early radial or drag type tools were very primitive and made of hardened steel and they wore out quickly. The radial tool was normally designed to have a sharp edge and thus cuts through the rock easier and consumes less energy. This coupled with the fact that the early machines did not have sufficient power compared to the modern machines, promoted widespread use of the radial picks.
The next step in the evolution of a tool was to have a tool which could be flipped over in order to use both cutting ends of the tool. The chore of transporting bits to the face was basically cut in half by using both ends of the tools which was a welcome relief for the operators.
In early 1900’s Tungsten Carbide was invented and proved to be very hard while having the toughness required for resisting impact loading. This product was immediately adopted for use in manufacturing of rock cutting tools.
Carbide tipped rock cutting tools soon dominated the market in every level of rock cutting and fragmentation from drilling small holes to lacing of the large excavators. The tools used in mechanical rock cutting were typically wedge shaped and were used to cut very soft rock types due to shortness of life in medium to hard rock formations. Several varieties of tools were produced and used over the years and numerous researchers looked into the effects of parameters such as tip angle, attack angle, width of the bit, spacing and penetration of cuts, etc. Also, many different designs were developed and tested and tools with positive or negative rake angle were developed and used on different machines. Figure 1 shows a typical drag bit.
The development of double edged or flipping tool was based on the concept of using a bit more than once by using different edges. This led to development of the conical shaped picks which could rotate and act as self sharpening. The first generation of these picks were cylindrical with coned tips and were known as “Pencil” bits. This was followed by development of “Plumbob” bits in early 1960’s which provided more support for the axial forces closer to the tip. Square Shoulder bits followed Plumbob bits with enlarged collar to increase the load bearing surface of the tool and to protect the front end of the holder from steel wash. In late 1970’s the new generation of conical bits were introduced to the market. This bit was comprised of a conical carbide tip held within a jacket of hardened steel.
The main advantage of the conical or point attack picks is the ability of the tool to rotate in the bit holder and hence wear uniformly. For a while these bits enjoyed a reputation of self-sharpening, which was never the case and it just implied that they wear uniformly by the rotation of the bit and thus the cone shaped tip could maintain its shape and performance for a longer time. These bits received widespread acceptance in the industry and became the tool of choice in harder formations. Over the years different cone shapes were designed and tested in the field and larger tips with stronger jackets were developed for cutting even harder rocks. Figure 2. shows some typical conical or point attack picks. Also, the roundness of the bit body allowed for reduction in machining and manufacturing costs for conical tools which coupled with longer life span means reduced bit cost in excavation.
The most recent development in design and manufacturing of the conical picks is the introduction of the sleeve around the bit. This allows for improved rotation of the pick, increased life of the pick and its holder, adopting different bits on the same holder, and many other enhancements in operation. Also, the application of the sleeve allows for better distribution of the stresses at the surface of contact between the sleeve and the holder to avoid deformation of the bit blocks. This eliminates or at least reduces the need for time consuming and costly repair and replacement of the bit blocks on the cutterhead. Figure 3 shows the cross section of a typical block, sleeve, and the conical pick.
The radial tool, also called “drag bit”, generally has a rectangular shank which indicates that the tool is not rotating in the tool holder. The shank is held in the tool holder during operation by one of many different means. Some of these are locking to the rear of the shank and some are locking to the sides of the shank. There are some types locking over the top of the shank, through the shank, to the front of the shank and even with threads to the bottom of the shank. The radial tool normally has a shoulder which ensures that the tool is well supported to the face of the tool holder. Because of the combination of rectangular shank and shoulder for tool support, these tools normally are made of a forging where both features easily can be incorporated.
The tip of the tool is generally flat with relief angles on the side. The reason for this is to minimize the friction and resistance to tool through the material to be cut. Low friction and resistance will reduce cutting forces, power consumption, vibration, and dust levels. The cutting tip on most radial tools normally has a sharp cutting edge. The edge design can be described with a few features, which determines how it will cut the material. Figure 4 shows the cutting action, and the related nomenclature for radial bits.
The rake angle describes the angle between the front of the tip and the shank. A positive rake will ensure a more aggressive cut but will also normally make the tip weaker. A positive rake of 5-10 degrees can be used to cut soft formations at higher rates. A neutral or zero rake angle is the most common for various conditions. A negative rake angle will normally keep the tip in compression during cut and thus will have a reduced risk of fracturing. This design may have a tendency to cut slower but last longer.
The clearance angle is the angle between the front of the tip and the edge of the tip. The clearance angle ensures that the tip will not be in contact with the material to be cut all the time. That would lead to loss of energy, excessive heat, and reduced life of the tool. The rake and the clearance angle are somewhat dependent on each other. A negative rake angle would normally require a smaller clearance angle and a positive rake angle needs higher clearance angle. In addition to the angles, the dimension and the shape of the carbide are important factors to consider. The front face of the radial bit is normally chevron (V shape) and has either two flat surfaces or curved surfaces. The curved surfaces have the advantage of making the insert stronger. This tip is used in harder cutting conditions even if the flat design is more common. Figure 5 shows the tip geometry for radial bits.
Point Attack Pick (Conical)
The basic styles of point attack picks are shown in Figure 2. They have rounded shanks and are mounted in a circular hole on the holder allowing for rotation and thus normally experience even wear during use. The round carbide tip penetrates the material being cut during a linear or rotational motion. The tool is held in place at certain attack angle to ensure the proper rotation for self sharpening effect and minimized the cutting force. The tool holder may also sit at a certain angle with the line of cutting or skew angle which promotes rotation of the cutting tool. Figure 6 shows the cutting action of a conical pick.
For each basic style of conical picks, there are numerous other features that may be available. Some of these are stepped shanks for special holders, various locking mechanisms, washers of various kinds in combination with the tool, hard facing at the tool tip, fins on the tool head for better penetration, slim and thick head profiles, etc. The picks are held in the bit block or the sleeve by using a snap ring in the back or spring clip. Figure 7 shows some variations of the pick mounting mechanism.
The Pencil picks are hardly used on modern machines. They may still be used on low powered machines. It does have a slim profile to reduce cutting forces and power consumption. With the Plumbob pick, the support surface has moved closer to the tip. This pick has better capability to withstand higher cutting forces. The main drawback with this tool is that it does not protect the front face of the holder. This can lead to increased wear of the front and outside surfaces of the holder which will increase the cost of maintenance of the cutterhead.
The most commonly used conical pick is the square shoulder design. The cutting force is supported closer to the tip. The main difference from the Plumbob is the shoulder, which has the capability to distribute the axial load over a larger surface compared and reduce the risk of pinching the surface or wearing the contact surface.
The efficiency and the cutting forces on these bits depend on the shape of the carbide tip. The geometric parameters of the tip include the size of the carbide, which is the diameter of the insert or the cap, the cone angle and the shape of the body. The larger the carbide, the harder the rock it can cut. This provides for higher forces typically required to cut harder formations. The tip angle has a major impact on the cutting ability and efficiency. Sharper tips indicated by the smaller cone angles are good for cutting softer less abrasive formations. They can transfer limited energy and forces to the rock and they are more prone to wear due to the sharp tip than do dull tips with bigger cone angle. Obviously, smaller carbides with sharp tips are used in soft rock applications such as gypsum, trona, coal, and salt.
For the heavy duty tools used on CM and road header machines, cutting hard and abrasive formations such as sandstone, limestone and similar sedimentary rock, the diameter of the carbide is bigger. It sometimes is up to 40mm (1.6”) diameter and the tip angles are up to 100 degrees with fairly rounded tips. These tools see forces as high as 4 -5 tons per bit and 35-38mm (-1.5”) shank diameter is standard to provide stiffness and structural strength for these forces.
The attack angle of the conical tools plays a major role in the cutting efficiency and tool life. The best attack angle for the conical tool is along the resultant of normal force and drag force. In other words, when the resultant force is along the axis of the tool. The tip is always compressed against the body and the body against the holder. This minimizes the bending moments and tension along the tool. In softer rocks, the depth of penetration is typically higher and therefore, the drag forces are higher than those experienced in harder rock. This means that the attack angle should be reduced. The lower cone angles of the smaller conical tools allow for lower attack angles (45-48 deg) while maintaining a clearance angle between the back of the tool and the rock. In harder rock the opposite is true and lower drag forces lead to the use of higher attack angles (up to 52 deg). This is also consistent with bigger tip angles used on heavy duty conical tools.