When the first milling machines made their debuts in the late '60s and early '70s, they were hailed as a major innovation in road reconstruction. The earliest milling machines were simply a mining mandrel attached to a mobile undercarriage. They were designed to efficiently remove a layer of old concrete or asphalt so that a new layer could be applied to a higher quality base than if the resurfacing simply went over the old surface.
An added benefit of the milling process was the creation of sized material that could be recycled. Recycling in all of its forms was becoming popular, and removed road material was no exception. As a result, DOTs across the nation were attracted to the milling machine because of what it could do. Some versions even evolved into machines that could remove the road surface, mix it with fresh binder and lay it down again in a continuous process.
From the start, the emphasis when developing a milling machine was power to the cutting drum, which was needed to remove more and more material. Additional emphasis was placed on the cutter head itself and the cutting teeth that really did the job. The cutting teeth would dull fairly quickly and needed replacement frequently. The replacement process could cause enough downtime to greatly detract from the initial efficiency of the milling process itself. So manufacturers worked to hasten the replacement process as well as the durability of the cutting teeth. Later, different sized cutting drums would be offered so that machines could mill at different widths.
Next to come were controls that added precision to the milling process—controls that could handle slope, depth and speed. Now much of that has been automated.
Power, durability and precision have been combined into a mix that makes today's milling machines reflect the years of development that have taken place. A closer look at what manufacturers now offer proves the point.
The most dramatic advances in milling machines might be the controls. Most construction machines now reflect the technological advancements made in microelectronics, and milling machines are no exception. For example, both of Caterpillar's milling machines, the PM-465 and the newer PM-565B, are equipped with electronics designed to improve performance. The PM-565B, a half-lane machine, features two electronic sensors and a built-in cross slope. The 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 is constantly displayed on each sensor and on the central controller. Changes to the elevation controls include the addition of an all raise/all lower switch that is used when milling around obstacles. As with other Cat equipment, 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.
The PM-465, designed for urban applications but capable of high-production work, has a simplified grade and slope control system that provides three operating menus. The system electronically controls the depth of cut to within 0.125 inches. Like the PM-565B, the PM-465 electronically regulates speed, steering and rotor drive, plus the self-diagnostic setup.
Wirtgen also employs several technological advancements to keep its milling machines precise. Leading the list is the Tempomat cruise control. Integrated into the machine's control system, the Tempomat feature stores the speed being traveled and restores that speed at a push of a button as would be the case when changing trucks. Further, an automatic leveling system ensures that a constant milling depth is maintained. A reference area is scanned by sensors registering any changes in depth of cut, with the machine quickly compensating without overshooting. Another feature is the Wirtgen Information and Diagnostic System, WIDIS 32 for short. It constantly monitors the status of the engine and hydraulic system, visually and audibly informing the operator if anything is amiss. A monitor displays current operating data.
Along with the electronic features that keep milling machines on track are the advances being made in the cutting end of the machines. Foremost among these is the ability to change cutting drums fairly quickly to achieve multiple cutting widths with the same machine. The payoff is that a contractor needs only one machine to accomplish multiple job requirements. Again, Wirtgen has come up with a system for doing so that it calls FCS, which stands for Flexible Cutter System. It is designed so that milling drums can be changed in as little as two to three hours, says Wirtgen.
Regardless of the machine's working width, the zero edge is always located on the right-hand side of the machine. Currently, eight drum widths are available, ranging from 300 to 2,000 millimeters. Regardless of width, all drums cut to a depth of 30 centimeters, about one foot.
Another part of the FCS system is the two-part scraper plate, which has a right-hand section that is located behind the drum and seals the working area while the left-hand part runs over the existing pavement during milling work. The scraper blade is adjusted to different working widths depending on the drum width being used. The system is designed in such a way that the two parts of the scraper blade can be lifted hydraulically independent of each other, controlling the amount of material being loaded.
Roadtec adds flexibility to its machines by manufacturing and assembling the milling tractors and cutters separately, allowing the contractor to choose from several cutter patterns and widths when ordering equipment. The cutter assembly is designed for quick changing of cutting pattern styles and widths. The company also offers a drum rental program for small or short duration projects.
Roadtec machines also have the ability to mill in two different directions to accomplish different results. Traditional milling is accomplished in an up-cut direction. But Roadtec's offerings also can down-cut. According to the company, down-cutting is best used to control slabbing, permit pulverizing and mixing, and is an effective way to remove material over wet base. It also permits rear loading. The down-cutting feature can be used to easily convert Roadtec milling machines into cold-in-place recycle machines.
To reduce the amount of material contained within the housing and left in the trench, CMI Terex has an inward slant at the bottom of the front and rear cutting housing doors. This reduces the amount of cleanup required and helps keep the cutting operation working at peak performance.
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 rewelded to the drum. Now, teeth are held in variously designed bolt-on housings that permit faster changing.
| Web Resources | ||
| Caterpillar www.cat.com |
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| CMI Terex www.cmiterex.com |
Dynapac www.dynapac.com |
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| Roadtec www.roadtec.com |
Wirtgen www.wirtgengroup.com |
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