Milling machines, also known as cold planers, complete the same task as they did 25 years ago, but they have seen several innovations over the years and are continuing to evolve. It was actually two events in the mid-1970s that brought about the creation of the modern milling machine.
According to the Basic Asphalt Recycling Manual from the Asphalt Recycling and Reclaiming Association (ARRA), the petroleum crisis of the early 1970s and the development and introduction in 1975 of large scale cold planing equipment, complete with easily replaceable tungsten carbide milling tools, were the catalyst for a renewed interest in asphalt recycling.
“At the time (in the 1970s), recycling asphalt was relatively disjointed and inefficient, and the oil embargo of the 1970s created a demand for a more efficient way to recycle asphalt,” says John Phillips, senior project engineer for Terex Roadbuilding. “Incorporating recycled asphalt in the mix meant less liquid binder was required, lowering the mix production costs and the required amount of virgin aggregate. The milling machine solved many economic problems associated with limited oil supplies and high material costs of the time, much the same as it can do with today’s prices.”
Since then, milling machine manufacturers have been proactive in the development of the technology of these units which have advanced exponentially in the last 25 years.
Technology helps increase a quality finish
“Twenty-five years ago, the focus of the cold planing crew was production — plain and simple,” says Terry Humphrey, Caterpillar training consultant. “Get the tons in the trucks and get out of the way of the paving crew. Today, the cold planing crew is more connected to the paving crew in terms of the quality of the new asphalt surface.
“There is much more emphasis on quality control during the cold planing step,” Humphrey continues. “Has the cold planer created the correct profile so the paving crew does not have to use slope control? Has the cold planer made a significant improvement in smoothness? These changes came about as a result of pay factors associated with ride quality of the finished product.”
Another technology that helps with quality issues is automated controls and sensors. “[On older milling machines,] the grade control technology was mainly hydraulic sensors, which can tend to drift,” says Jeff Wiley, senior vice president of Wirtgen America Inc. “Today’s electronic sensors are very accurate within the specification tolerances of asphalt paving.”
Eric Baker, marketing manager with Roadtec Inc. concurs. “Automated grade and slope controls have greatly improved the quality of the work and have made milling an opportunity to improve the smoothness of the road. We can now average and do grade corrections.”
Higher-production machines evolve
A quality finish is important for today’s milling machines, but higher production hasn’t been forgotten. According to sources, today’s units have become even more reliable and productive. The base components and features of the original machines — cutters, conveyors, engines and machine weight — are still the required components of today’s machines. However, these components have evolved over time.
With the emphasis on increasing production, cold planer design evolved from “rear loading” to “front loading” machines in order to facilitate truck loading and truck maneuvering.
“The first milling machines were built with a rear discharge load-out conveyor,” says Phillips. “While this was the most efficient way to remove material from the cutter housing, it created issues with trucking and traffic flow problems. Machines with front discharge conveyors were introduced in the 1980s, which made it easier for the truck to move with the milling machine and in and out of traffic.”