They’re called many names: Milling machines, pavement profilers and asphalt cutters, among them. But regardless of the moniker, these highly specialized road-building machines with their faintly prehistoric looks fascinate both the general public and contractors alike. In fact, the History Channel will be dedicating an entire episode of its “Guts and Bolts” series to them this month.
Milling machines first appeared in the mid-1970s, when CMI Terex modified a fine-grade-dirt-cutting machine to handle the more aggressive task of grinding asphalt. Today the uninitiated can be forgiven for assuming recycling asphalt was the primary driver for creating milling machines. The reality, however, according to Larry Jack, vice president, sales and marketing, Terex Roadbuilding, is much more prosaic. “Recycling wasn’t the initial appeal,” he says. “The goal was better roads.”
As any asphalt contractor knows, simply laying down new asphalt over an existing paved road will only work for a short period of time. “Eventually any existing cracks or fractures in that pavement will manifest themselves through the mat you’re laying down,” Jack says. “Up to the mid-’70s, contractors weren’t able to efficiently correct the problems with the roads they were paving. Curbs were covered by successive paving jobs and there was no way for water to drain away. As more asphalt was laid down on bridges, weight became a factor. And ruts created by large trucks on roads couldn’t be adequately compacted and would quickly return after a paving job was complete. These were all serious safety issues, and we determined the best way to handle them was to mill off the existing asphalt and pave fresh.”
Very quickly, however, contractors realized asphalt could be cost-efficiently recycled. And asphalt-plant technology evolved to meet the demand. Milling really took off because rap material became a valuable commodity for paving and milling contractors.
Diverse machine choices allow contractors options for
specific milling applications
Milling machines in the 80- to 87-inch class are commonly called “half-lane” machines. Unlike many classes of construction machinery, milling machines in this class come in a variety of power options ranging from 500-horsepower models at the lower end of the class, up to 800-plus horsepower units at its upper end. “These models are popular because they can handle a wide array of applications,” says Don Lamb, national sales manager, Roadtec. “They can mill a typical highway lane in two passes – hence the term ‘half-lane machine.’ They are also easier to transport than models in the larger classes and are easy to maneuver in urban settings – they can turn around in a cul-de-sac or intersection without assistance.”
“There’s a lot of diversity in this class because milling contractors demand it,” explains Jeff Wiley, product manager, Wirtgen America. He says a typical milling machine cuts anywhere from zero to 12 inches deep in one pass. “But contractors need a selection of different sizes and varieties to match the productivity they want,” he says. If you plan to use a machine in city applications, you’re better off with one that cuts the same width but has more maneuverability so you can turn in a tight radius and cut around curbs. That would dictate a smaller, lower horsepower model. But some contactors want bigger, high-horsepower machines. Even though they cut the same width as the smaller units, you can take them out on an interstate highway and cut 3 or 4 inches at a higher production rate.
“Milling machine sizes tie directly into productivity,” Jack adds. “The high-horsepower models can cut asphalt and load a Class 8 dump truck in a couple of minutes. But getting enough trucks to the machine to equal that production rate may not be practical for contractors working in urban surroundings. So they’ll opt for a 500- or 600-horsepower unit in those applications.”
But if you’re milling airports, where jetliners are slamming down onto the mat and really compacting the asphalt, you need an 800-horsepower machine to mill that material – and of course, space isn’t as big an issue for you.
A majority of milling contractors in North America today are subcontractors who work ahead of a primary paving contractor (although a large number of paving contractors opt to incorporate their own milling machines into their operation). Given the wide array of applications they may be faced with, Jack says it makes sense for contractors to accurately pin down their applications and select machines accordingly. “If you’re a primary contractor and the bulk of your market is cutting 3 or 4 inches deep, you can probably get by with a 600-horsepower machine,” he says. “If you’re a subcontractor and you’re never sure what type of jobs you’ll be getting, you might opt to own one higher horsepower machine and several smaller models, depending on how big your operation is.”
Because they can be used in many different applications, milling machines come in a wide array of sizes and horsepower offerings.
Ground crews control the tempo and quality of milling jobs
“Solid project planning is an easy way to enhance your milling production,” says Bill Heitschmidt, product manager, Caterpillar Paving Products. “Train your crews to examine and understand the structures in the pavement they’ll be dealing with to save time and avoid machine damage. Remember many factors influence productivity, including the hardness and abrasiveness of the material to be milled, the presence of manholes, curbs or other structures in the pavement and restrictions on opening the pavement after milling.”
And remember safety is a critical element since many projects are performed under high traffic conditions. You may have to shift your work hours to avoid high traffic periods, possibly even milling at night.
Much like running a paver, Wiley believes the most important aspect of effective milling is properly staging trucks. “This is an area where machine size is again crucial,” he stresses. “A good crew knows how many trucks they need in front of their machine to keep it running continuously at the proper speed.”
For instance, it doesn’t make sense to have a high-horsepower machine but sit 30 minutes out of an hour waiting on trucks to catch up. “So before each job,” Wiley says, “ask yourself how far the trucks have to travel to dump the material, and how long it takes them to get back. This will go a long way to setting up a productive milling job.”
“Milling machines today are so powerful, there are many times when they can easily out-produce the pavers following them,” Jack notes. But it makes sense to stay close to the paver for other reasons. The trucks might fall behind or the asphalt plant breaks down and you’re stuck with a machine full of material and an exposed road.”
Wiley says another important productivity aspect to running a half-lane milling machine is the composition and deployment of its crew. “You don’t just run these machines with one operator,” he says. “In fact, I’d go so far to as say your most experienced milling crewmen need to be on the ground while the machine is running.”
Jack concurs. “A milling machine operator should hold the machine in a straight line, and swing the conveyor belt so it’s feeding the trucks. Any questions or decisions should be made by the foreman on the ground.”
According to Wiley, your ground crews should be monitoring the cutter pattern, checking for leaks and watching the conveyor system as the machine works. Finally, Wiley says they should observe the milling operation, make sure the machine is cutting to grade and make necessary adjustments to correct any deviation from the desired cut.
Be alert for any tooth that starts to go bad on the cutter. If you don’t replace a worn tooth quickly, it can wear into the holder and even the drum. This leads to excessive vibration, lost production due to a slower-running machine and streaks in the cutter pattern on the asphalt.
Milling machines today have high conveying capacity to quickly remove milled asphalt from the drum. Because of this, your ground crew needs to watch the conveyor, since belts can walk off to the side or material can build up and jam the system. Having an automatically reversing conveyor system can help prevent jams. “Spec’ing wide-width conveyors with tall cleating on the belts is helpful as far as getting material out of the machine,” Wiley adds.
Also make sure the machine has a properly adjusted moldboard. The moldboard is what scrapes up material inside the cutter house, cleaning up behind the machine and holding material in the house until it can be removed from the cutting drum. “If you have too much moldboard pressure, it can drag and slow the machine down,” Jack says. “Too little pressure, though, and you can leave a lot of milled asphalt in the road that has to be swept up and carried off – a production killer.”
Maintenance vital for longevity and productivity
Your crew should check and service the following systems on the machine at least once a day, according to Lamb:
· Check the water spray system
· Change teeth as required (with appropriate tools)
· Replace paddles as required
· Replace cutter housing liners as required
· Check the conveyor system and related components
· Perform engine and drivetrain checks
“Use extraction tools to remove teeth or air-assisted tools to change worn or damaged teeth,” Heitschmidt suggests. Most crews change cutting teeth at shift intervals or before or after the work day if the tool life matches the shift. This avoids machine stops during production to replace cutting teeth.
Remember milling machines kick up a lot of dust and work in a highly abrasive environment. “In addition to all prescribed maintenance checks,” Jack says, “I like to see a crew take a hose and simply wash their milling machine down at the end of the day. It’s a simple procedure that pays you huge dividends because it removes large amounts of gritty material that can accelerate component wear throughout the machine.”
And, Heitschmidt cautions, don’t forget to service the machine’s automatic guidance systems while you’re at it. “These crucial systems reestablish a road’s profile geometry, so it’s critical to maintain them to get the results you want,” Heitschmidt says, “Once you get a crew used the maintenance routine, you’ll see productivity increase and get a better final product too.”