By Steve Fiscor, Editor-in-Chief
Roof bolting is one of the more hazardous occupations underground. The roof bolting machine operators, or roof bolters, drill 4- to 8-ft holes in the roof underground. After they drill each hole, they insert a resin cartridge, spin the bolt through it and then hold it in place until the resin cures. In a way, it’s like installing the rebar after the concrete is poured. The process allows the miners to band three or four weak layers of rock together to form a solid beam.
Working at the active face near an unsupported top has its challenges. To add a little more pressure to the situation, the continuous miner section cannot advance until the roof is supported. Any process, equipment or tool that improves safety and productivity for roof bolters is of vital interest to the underground coal mining sector.
U.S. Synthetic Mining & Construction manufactures diamond bits for roof bolting applications. Miners are reporting a great deal of success using diamond bits. When compared with carbide-tipped bits, they last longer, drill faster and have a usable life that exceeds carbide on average by 100 times or more, depending on geology. While they are more expensive than carbide, the actual cost per foot of advance is dramatically reduced. “We focus on creating a premium diamond insert material for the drilling tool,” said Joe Memmott, manager for mining and construction tools for U.S. Synthetic. “Traditionally we work with the customer to create and craft a product that fits their requirements.”
Bill Brady introduced the gold roof bits in the early 2000s. In 2008 Brady sold his company to U.S. Synthetic, a subsidiary of ChampionX. U.S. Synthetic is a premier manufacturer of polycrystalline diamond products, operating more than 80 diamond presses at its 200,000 ft2 facility in Orem, Utah.
The Wasp and Viper bits, as an example, have been designed specifically for roofbolting applications. An available reamer attaches to the bit and cleans the hole so the drill steel doesn’t get hung up. Ideally, the roofbolters want the drill steel to drop out of the hole quickly and easily, and they do not want to fight to get it back into the hole.
The Viper is the workhorse of the product line. The Wasp was developed as a more cost conscious alternative. “Coal operators were looking for something that was a little less expensive than the Viper,” Memmott said. “So, we recently introduced a variation, the Wasp. The primary difference is with the diamond, which is not the hardest diamond we make, but hard enough for coal applications, with a slight change in geometry.”
The usable life of a diamond tipped bit far exceeds that of a comparable carbide bit. Some of the mines using the Wasp bit are seeing consumption rates of roughly 100:1 (carbides:diamonds), Memmott explained. “One of our coal customers recently said they were getting about 250:1,” Memmott said.
Comparing prices, a 1-in. carbide bit sells for around $3 and a 1-in. Wasp bits sell for $166. When a roof bolter is getting a minimum of 75 to 80 holes with one bit, Memmott explains, it pays for itself in addition to all the time saved changing bits and handling them underground.
“The Wasp has high abrasion resistance,” Memmott said. “The sharp edge of the diamond is maintained for a long time and drills consistently over a much longer period of time than carbide. The edges of carbide inserts wear quickly compared to diamond.”
Understanding the Impact of Pressure
The U.S. Synthetic bits are hard, but they are not indestructible. “If the roofbolters hit geology like cracks or laminations, where they are drilling through a soft zone and then hit a hard layer, the transition could damage the bit, especially if the bolter operator is applying a lot of pressure to the steel and moving quickly through the soft zone.
“When the bits hit something extremely hard at high rpm and high thrust, the diamond (like carbide) can crack or chip, and that’s an area that we’re working on right now to create a more crack-resistant diamond,” Memmott said. “We’ve started testing a new diamond composition with a couple of lines and we will be excited to bring that to the market soon.”
Memmott said that he and his team like to meet with miners, who are using diamond bits for the first time, and help them make the adjustment. “Oftentimes, we go underground and watch as they cycle through the first few holes,” Memmott said. “If we need to, we give feedback on pressures and speeds and provide recommendations. They usually have a ton of experience and can use their gut instinct to get to the right place. We learn from them as well.”
The Wasp maintains its sharpness and it doesn’t need to be pushed as hard. “We find that a lot of bolters are running 7,000-9,000 lb thrust and they are typically pushing the carbides as hard as they can,” Memmott said. “We use a load cell to measure the force as they push up against the roof. Roofbolting machines are capable of 10,000-11,000 lb thrust and sometimes that doesn’t quite register.”
Memmott said he likes to tell the roofbolters to picture two fully loaded Chevy Silverados sitting on the end of the drill steel. “That’s the load they are placing on the steel,” Memmott said. “That’s not meant to be trivial, we take safety very serious.”
With a diamond tool, U.S. Synthetic recommends drilling at pressures around 3,000 lb. The drill steel carries less load, and it is less likely to fail. Memmott has observed that roof bolters will attain better productivity levels at 3,000 lb and speeds around 500-600 rpm. “Since these tools are sharp, they will maintain a consistent penetration rate throughout the hole, and hole after hole,” Memmott said. “The edge of the carbide inserts wears much more rapidly than diamond. At first, the penetration rate is fast and then it slows as the edges wear. Meanwhile, the roof bolters are applying full force to maintain a steadily slowing penetration rate until the bit is replaced.
“With the Brady Bits, we are encouraging the roof bolters to let the edge do the work and not the pressure, and they will find the sweet spot,” Memmott said. “And even though they may not be pushing quite as hard, the operators will save time overall with bit and steel changes.”
Memmott said that roofbolters that have used the Wasp love them. “They love them so much, they tend to put them in their pockets and hold onto them for later use,” Memmott said. “We coach the continuous miner section coordinators and warehouse managers to ask them to show them the worn bit before they hand out a new one. It generally works quite well that way.”
U.S. Synthetic is passionate about the use of diamond for drilling, so much so that they bought a roofbolting machine to use in their lab. “We have a Fletcher bolter with large blocks of granite, sandstone and limestone and we perform tests,” Memmott said. “It’s not just that we ‘think’ this bit will work. We know it will. We test them on a bolter and it’s a full tool test. It’s instrumented up like crazy. We measure the thrust, torque, and penetration rates.”
Carbide bits also generate more dust. “When the edge of the carbide is rounded off, it’s doing less cutting and more rubbing,” Memmott said. “The bit produces more fi ne powder. With the sharp edge, the Wasp produces more cuttings and less dust. The cuttings are collected by the vacuum, and you have less dust passing through the filter media.” Diamond bits also generate less noise.
According to Memmott “The Wasp is premium product, but pays for itself over and over again. U.S. Synthetic provides quality materials and support. We do a number of manufacturing checks along the way to make sure that these bits are amazing.”
A customer did the math recently on one of their sections. “They spent $1,000 on seven of our bits and to drill the same distance they spent $3,500 on carbide,” Memmott said. “They provided the number of bits they used and measured the footage.”
Mines will save time and money with diamond bits. When used properly, they can drill quickly and consistently with less force.