Cold Planers

A cold planer, also called a cold milling machine, pavement profiler or road planer, is a self-propelled machine that removes the top layers of asphalt or concrete pavement with a rotating drum fitted with cutting tools. It grinds the old surface into small pieces, known as reclaimed asphalt pavement or millings, and loads them onto a truck with a conveyor. The result is a clean, even surface at the required depth and profile, ready for a new layer of asphalt.

Milling is the first step in most road resurfacing jobs. Instead of laying new asphalt on top of the old, which raises the road level and causes problems with kerbs, drainage, bridges and manhole covers, contractors remove the worn material and replace it with the same thickness. The millings can then be recycled into new asphalt mixes or used as base material, which makes the cold planer one of the key machines in modern road maintenance.

Brief History of Cold Planers

Before cold milling, worn asphalt was removed with heaters and scarifiers, or broken out with excavators and loaders. Hot milling machines, which softened the asphalt with gas burners before cutting it, were used in the early 1970s. Wirtgen, for example, built hot milling machines from 1971 for its own contracting fleet.

The oil crisis of the early 1970s raised the price of bitumen and made recycling old asphalt more attractive. In the United States, CMI introduced the Roto-Mill in the mid-1970s, often described as the first dedicated milling machine in the country. In Germany, Wirtgen moved to cold milling at the end of the decade by applying carbide-tipped round shank picks, a technology borrowed from mining, to a milling drum. Its first cold milling machine appeared in 1979.

During the 1980s the industry shifted from rear-loading to front-loading machines, which discharge the millings forward into a truck driving ahead of the planer. Replaceable toolholder systems, stronger engines, electronic levelling and dust control followed. Today cold planers range from small machines that fit on a trailer behind a pickup to large crawler machines with engines of well over 700 kW.

How a Cold Planer Works

Milling drum. The heart of the machine is a steel drum fitted with hundreds of cutting picks arranged in a spiral pattern. The drum rotates against the direction of travel, cutting the pavement and moving the material towards the centre, where it is picked up by the conveyor.
Cutting tools. Each pick has a tungsten carbide tip and sits in a toolholder welded or bolted to the drum. Picks rotate in their holders to wear evenly and are replaced regularly. Tool spacing determines the texture of the milled surface.
Levelling system. Sensors on both sides of the machine control the milling depth and cross slope. They can follow a string line, the existing surface, a reference ski or a laser, or work with a 3D positioning system.
Conveyor. A primary conveyor under the machine collects the millings and passes them to a slewing discharge conveyor, which loads the truck. Water sprays or vacuum systems reduce dust at the drum and along the conveyor.
Undercarriage. Large and mid-size machines run on four or three tracks, while many small machines use wheels. Hydraulic legs adjust the height of each track or wheel to set the depth.
Power. A diesel engine drives the drum mechanically through a belt drive or gearbox, while hydraulics power travel, steering, the conveyors and the levelling system.

Types of Cold Planers

Small Milling Machines

Compact machines with milling widths of about 0.35 to 1.3 metres, used for patch repairs, trenches for utilities, around manhole covers and in tight areas. Most are rear loaders or have no conveyor at all, and many run on wheels. Some small machines can mill flush to a kerb or wall on one side.

Compact Class Machines

Machines with widths of about 1 to 1.5 metres and front loading conveyors, used on urban streets, car parks and smaller resurfacing jobs. They combine manoeuvrability with enough output for full-lane work on short sections.

Large Milling Machines

Crawler machines with standard widths of about 2 to 2.2 metres, and optional drums up to about 4 metres, for highways and airport runways. They can remove a full lane of surfacing in a single pass and load trucks continuously.

Front and Rear Loaders

Front loaders discharge millings forward over the machine, so the truck drives ahead of the planer in the same lane. Rear loaders discharge backwards, which suits small machines and tight spaces.

Planer Attachments

Hydraulic cold planer attachments for skid steer loaders, compact track loaders and excavators are used for small repairs, edge work and trenching, where a dedicated machine would be too large.

Size Classes

Class	Typical milling width	Typical engine power	Typical use
Small	about 0.35 to 1.3 m	about 50 to 250 kW	Patching, utility cuts, edges
Compact	about 1 to 1.5 m	about 200 to 400 kW	Urban streets, car parks
Large	about 2 to 2.2 m (up to about 4 m)	about 400 to more than 800 kW	Highways, runways, full-depth removal

Figures are indicative.

Main Manufacturers

Wirtgen, part of the John Deere group, with a wide range from small to large machines.
Caterpillar, with its PM series cold planers and skid steer planer attachments.
Roadtec, part of Astec Industries, with large and mid-size milling machines.
Bomag, part of the Fayat group, with small and mid-size milling machines.
Dynapac, also part of the Fayat group, with compact milling machines.
XCMG and other Chinese manufacturers, with growing ranges for domestic and export markets.
Simex, among the attachment makers offering planers for loaders and excavators.

Main Uses of Cold Planers

Removing worn or damaged asphalt layers before resurfacing roads and motorways.
Correcting the profile and cross slope of a road to improve drainage and ride quality.
Removing asphalt completely down to the base during reconstruction.
Milling concrete pavements and bridge decks before overlays.
Cutting trenches for cables and pipes, and patching potholes.
Texturing slippery surfaces and removing road markings.
Producing reclaimed asphalt for recycling into new mixes.

Planning a Milling Job

Match width and power to the job. A large machine removes material quickly, but on short or narrow jobs a compact or small machine may be faster overall because it moves and turns more easily.
Plan the truck fleet. A large planer can fill a truck in a few minutes. The number of trucks, the distance to the asphalt plant or stockpile and traffic management decide whether the machine keeps working or waits.
Choose the drum. Standard drums suit most resurfacing. Fine and micro-fine milling drums, with closer tool spacing, leave a smoother surface for thin overlays or temporary traffic.
Check the tools. Worn picks raise fuel consumption, reduce output and leave an uneven surface. Carbide picks are checked at every shift, and polycrystalline diamond picks are used on some jobs for longer life.
Set up the levelling. A good reference and correctly adjusted sensors determine the quality of the finished surface, and therefore the quality of the new asphalt layer.

Safety

Cold planers work in live traffic lanes and with trucks moving just ahead of them, so traffic management and clear communication between planer and truck drivers are essential. The main hazards include people being struck by the machine or by trucks, contact with the rotating drum or conveyors, flying debris and damage to buried services, manhole frames or steel covers. Milling concrete and some aggregates produces respirable crystalline silica dust, and many markets now require water spray or vacuum dust control. Maintenance on the drum must only be carried out with the engine stopped and the drum secured. Cameras and proximity warning systems help drivers and ground staff keep track of each other.

Trends and Technology

Levelling systems are becoming more accurate, with 3D control that mills to a digital design model instead of a physical reference. Machines record milling area, volume and pick wear to support job reporting and maintenance planning. Automatic functions help the operator lift the drum over obstacles and resume at the correct depth. Engine management systems adapt power to the load to cut fuel use. Quick-change drums allow one machine to switch between widths and textures. Diamond cutting tools, improved dust extraction and battery-electric small planers for urban work are also part of the trend.

Key Terms

Milling drum: the rotating cylinder that carries the cutting tools.
Picks: the carbide-tipped cutting tools mounted in toolholders on the drum.
Millings / RAP: reclaimed asphalt pavement produced by the machine.
Front loader: a planer that discharges millings forward into a truck ahead.
Fine milling: milling with closely spaced tools for a smoother surface.
Levelling system: sensors and controls that keep the correct depth and slope.

Frequently Asked Questions

What is the difference between a cold planer and a milling machine? There is no difference. Cold planer, cold milling machine and pavement profiler are different names for the same type of machine.

How deep can a cold planer cut? Many large machines can mill to a depth of about 300 mm or more in a single pass, but most resurfacing jobs remove between a few centimetres and about 10 cm.

What happens to the milled asphalt? Millings are usually taken to an asphalt plant and recycled into new mixes, or used as base material, shoulder material or for temporary surfaces.

Why is it called cold milling? Because the pavement is cut at ambient temperature, without heating it first, as was done with the hot milling machines used before.