Defining good operational practices
By Jeff Winke
The asphalt milling machine—also known as a cold planer, pavement planer, pavement recycler, or roto-mill—is a construction machine used to remove bituminous pavement or asphalt concrete from roadways and other pavements. The result is a somewhat rough, but even surface, that can immediately be opened to traffic.
The milled surface is accomplished by bringing a rotating mandrel or “head” into contact with the pavement at an exact depth or slope. The mandrel has hundreds of hardened spikes or teeth on its surface, which bite and cut away at the pavement’s surface.
“Make sure the milling teeth are appropriate for the pavement,” stated Bill Bethards, OEM sales manager, RoadHog, Inc., Brownsburg, Indiana. “Many factors can affect tooth life, such as depth of cut, aggregate type, remaining oil content and pavement density.”
Bethards continued, “Make sure the teeth, and their holders, are in good condition. Examine tooth wear early in the job, to determine tooth life characteristics, and check regularly during production. Make sure the teeth rotate freely in their holders, so that they can self-sharpen. Frozen teeth will result in flat spots and ultimately wear to the point of damaging the holders.”
The material that is removed during milling is normally fed by conveyor into a dump truck or semitrailer, but can be left in place or windrowed to be removed or recycled later. A water spray system provides cooling for the mandrel, as well as dust management.
In the 1970’s, Galion Iron Works, based in Galion, Ohio, manufactured the first production milling machines, which were called Galions. These first units resembled motor graders in shape and size. The difference was there was a 760 mm-wide (30-inch) milling head where the scraper blade would normally be. The cutter drum was set into action by a large hydraulic pump.
Asphalt milling is the process of grinding up asphalt that can then be recycled. The process came about because many streets were getting layered higher and higher as new surfaces were added, thus reducing the curb height and creating roadway drainage problems.
Early milling machines were simply a mining mandrel attached to a mobile undercarriage. They were designed to remove a layer of old concrete or asphalt so that a new layer could be applied to a better-quality base than resurfacing over the old road surface.
From the start, the emphasis for milling machines was to place more power to the cutting drum, which is needed to remove more material. Thus, the cutter head itself and the cutting teeth designs became critical. The cutting teeth would dull fairly quickly and needed frequent replacement. The replacement process could cause enough downtime to greatly detract from the initial efficiency of the milling process itself. So, manufacturers worked on designs for quicker replacement, as well as increased durability of the cutting teeth. Different sized cutting drums were offered so that machines could mill at different widths.
“Knowing your milling speed along with drum speed are the two keys to ensure the best performance,” said Austin Miller, milling and paving business development for BOMAG Americas, Ridgeway, South Carolina. “On thin lifts you want a higher ground speed because it is normally a mill and fill job and the paver will be on your tail. With a higher ground speed you will need a faster drum speed to maintain a quality pattern. On thicker or full depth milling it is just the opposite, you will slow the ground speed down along with the drum speed so you do not burn up the cutting tools.”
Milling machines feature conveyor systems to collect the material during the milling process, thus reducing the labor of picking the material up from the roadway. Newer machines require two or more people to operate safely and efficiently. The operator stands on the deck of the machine and controls most of the machine’s functions, while a worker on the ground controls the depth of the cut and keeps an eye out for obstructions in the roadway, such as manholes and/or water valves.
Today’s machines are bigger and more technologically advanced. They are designed to handle any asphalt aggregates in use today. Depending on the depth of the cut, some of the larger machines can cut close to 15,000 square yards (13,000 m²) a day, at 75 feet per minute.
“Understanding the newer technologies and how to use them to achieve performance and quality is important,” stated Tom Chastain, milling product manager, Wirtgen America, Antioch, Tennessee. “Operating systems that allow the operators to simply tell the machine through the display what they are trying to achieve. Whether this be efficiency, performance, or pattern. Let the machine do the heavy lifting.
“Even with machine technology, it still takes quality operators to achieve quality results. Every milling job is not simply cut out the material. There needs to be some finesse on certain jobs. Quality crews and the latest technologies allow these quality operators to get an even higher quality they have achieved with less work.”
In addition to faster speed, added precision to the milling process has become important. The innovation of controls and automation has brought greater precision for controlling slope, depth, and speed.
There is no question that the technological advancements made in microelectronics have benefited milling machines. Electronics designed to improve performance, include electronic sensors and a built-in cross slope. A pair of sensors can read a variety of references from 12 to 55 inches directly below the bottom of the sensor. Each sensor can be calibrated and adjusted from the ground level or at the operator’s console.
The position of the rotor in relation to the grade reference can be constantly displayed on the central controller. Changes to the elevation controls include the addition of a raise/lower switch that is used when milling around obstacles. In many cases, the electronic control module monitors and regulates the performance of major machine systems, including speed, steering, rotor drive, and other functions. If a problem occurs, a warning is issued.
Along with the electronic features that keep milling machines on track are the advances being made in the cutting end of the machines. The ability to change cutting drums quickly to achieve multiple cutting widths with the same machine is a benefit to contractors who may only need one machine to accomplish multiple job requirements.
Today’s milling machines also reduce the time required to change the all-important cutting teeth. Early machines had the teeth welded on, so tooth replacement required a fair amount of downtime as each had to be re-welded to the drum. Now, teeth are held in variously designed bolt-on housings that permit faster changing. For example, some machines come with an air compressor as standard equipment allowing the use of pneumatic tools for quicker tooth changes.
“Proper drum maintenance is key to ensuring the quality of the milling surface,” Miller said. “If you run your bits too long you will lose production. If you change them too soon you lose the life of the cutting tool. There is always a breakeven point to accomplish production and not waste cutting tools, if you can find that trade off you will have the best of production and quality of product produced.”
At the end of the day or shift, the experts all agree that a thorough inspection of the milling machine should be performed to uncover maintenance needs.
“End of shift inspections will help ensure that you are prepared for the next shift,” stated Miller. “Just shutting down and walking away and hoping everything is ok for the next shift is normally a disaster. Just because it was working fine when you shut down does not mean that there is not a problem just around the corner that could have been prevented.”
Employing good operational practices dictate that milling operations will run smooth and yield the quality results a contractor expects.
Jeff Winke is a business and construction writer based in Milwaukee, Wisconsin. He can be reached through firstname.lastname@example.org