The Evolution of Milling Machines

Today, milling machines are recognized as an integral part of the process for achieving quality pavement performance standards. Take a look at the journey these units have taken over the last 25 years.

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Milling machines, also known as cold planers, complete the same task as they did 25 years ago, but they have seen several innovations 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."

Cutter heads and tooth patterns have changed over the years. "Different tooth patterns were developed to allow an increase in the milling speed, enhance the gradation of material, and improve the surface left behind, so that cars can drive over the milled surfaces," says Phillips. "Cutter drives have been refined to allow an increased amount of horsepower to be delivered to the cutter head, allowing the milling machines to cut faster and deeper."

The first milling machines would cut to depths of approximately 2 to 3 inches. As time went by, these depths increased and reached 12 inches. Some of today's machines offer a 15-inch cutting depth and 15-inch plunge cut, allowing it to be used for full depth milling applications.

Horsepower has increased significantly over the years from original 375-hp machines to those pushing 1,000 hp – which are capable of milling the entire width of a lane. "More power is required to effectively cut the material from the road than what was required for trimming/leveling aggregate by the Autograde trimmers," says Phillips.

With the introduction of sophisticated electronic control for track drive systems, cold planer engines can run more efficiently as well.  "Because the load control via optimum engine speed has been taken away from the operator and is now directed by electronic control modules, today’s cold planers use available horsepower much more efficiently and, consequently, production is high without resorting to higher horsepower, fuel-guzzling engines," says Humphrey. "What this means in terms of machine operation is that the operator is free to steer and focus on truck loading while the electronic control regulates machine speed automatically."

Weight has always been a double-edge sword with milling machines. The machine needs the weight to keep the drum in the cut and maintain tractive effort, however, the machine still has to be light enough to transport to the jobsite.

Versatility rules

Today's contractors expect more versatility out of their milling machines and manufacturers are delivering.  "Variable widths are available with segmented cutter housings to allow contractors to bolt on extensions to their cutter housing to change widths," says Baker. "Some mills are bi-directional, meaning they can operate in either direction. This allows the machine to downcut with the cutter drum, which provides better sizing and also to provide a rear load conveyor, both are very useful for cold-in-place recycling applications."

Three- and four-track machines are now offered to fit contractor need and preference. Track design has changed over the years to deliver more traction when working in deep or tough cuts. Steel tracks with polyurethane pads have helped to increase tractive effort of these machines.

Milling machines are just plain smarter today, says Wiley, which leads to better overall machine performance. "They have computerized engine load controls, and control management systems for easier operation," he explains. "Performance is linked directly to better reliability, and improved fuel consumption is part of it. The machines' tighter turning radii allow tighter turns. Their conveyor slewing or 'swing' capabilities make loading faster and easier. Their much quieter sound emissions make for a better working environment."

More efficient higher horsepower engines, improved cutter bit technology, improvement of conveyor belts and other components have reduced the operating cost of the machines and therefore made milling a widely used application as well, says Baker. 

"With oil prices climbing and asphalt prices following, we have seen much more interest in cold-in-place recycling," he says. "Additive systems have been developed to mix additives such as emulsions, slurries and expanded asphalt into the existing roadway to rejuvenate and recycle the material in place.  This is a very cost effective solution for agencies and a useful tool in their toolbox."

The future

What do our sources see when they peer into their crystal balls? Machine designs will continue to evolve to meet the need of the contractor. Reduction of operating noise levels will continue to be an important issue with these machines, especially when working in urban areas. Cutter designs will continue to evolve to keep the traffic flowing on milled roads during the milling process.

Machine designs will continue to change linked to better efficiency, cleaner operation, and much lower owner/operator cost, says Wiley. "They will be quieter, safer, easier to operate and more ergonomically logical. The new modern designs will provide better visibility for the crew, be easier to transport, and have the operational flexibility to allow the contractor to have an all-in-one package to address the different milling applications he will encounter every day."

The silica dust issue (see box, "Silica Dust Reduction Technologies") will also bring about some design changes. "With the completion of the silica dust trials (last summer), machine designs will change to reduce the amount of dust generated by the milling process," says Phillips. "Look for more vacuum systems to be offered on future machine designs."

What else is on the horizon? More processing of material, improvements in gradation of RAP and weighing of material, says Baker. "At Roadtec, we have already started installing weigh bridges on some machines to ensure proper truck loading, if we can ensure that the RAP is properly sized then we can improve the cycle time for the RAP to go back into the mix and lower the cost of recycling."

Humphrey with Caterpillar sees 3D coming to milling machines in the near future. "These machines will be under three-dimensional control," he explains. "All aspects of pavement removal - depth, profile, elevation - will be regulated by integrated satellite control.  Operators will be technicians and the cold planers will be more autonomous.  The entire jobsite will be connected.  The cold planing operation will feed data into the paving operation that will feed data in the compaction train. There will be no communication gaps.  Roads will last longer and ride quality will improve due to the contribution of ultra-modern cold planers."