Planing or cold milling is a machine cutting process that removes construction materials
in individual layers, whilst simultaneously granulating them into a reusable secondary
aggregate.
The milling operation is undertaken by machines ranging in size from 2.2m wide and
over 40 tonnes in weight to 350mm wide andunder 3 tonnes in weight.
At the heart of every cold milling machine is a milling drum fitted with point-attack
cutting tools. While the machine travels in a forward direction, the milling drum
rotates, granulating the pavement material with its cutting tools. The milling drum
rotates against the direction of travel, so that the pavement material is removed
from the bottom towards the top.
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A fully automated levelling system ensures that the correct milling depth is maintained.
In most applications, the milled material is removed from the milling chamber and
loaded on trucks via a one-stage or two-stage loading conveyor.
Milling drum assembly
Point-attack cutting tools are fitted on a mechanically driven milling drum in the
shape of a helix. They remove the pavement material while the milling drum rotates,
granulating it at the same time. Ejectors on the milling drum facilitate transfer
of the milled material to the conveyor system.
The milling drum is sprayed with water during the milling operation, cooling the
cutting tools. The milling drum works in a fully enclosed drum housing.
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The only opening in the drum housing is an outlet for transferring the milled material to the conveyor system.
In all front-loading machines, a gradation control beam presses down on the surface
to be milled immediately in front of the milling drum. It prevents large slabs from
breaking out of the pavement layer.
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Levelling system
Milling the pavement material at precisely the required depth is crucial in the
removal of pavement layers in construction, the cold milling machines’ levelling
system ensures that the specified milling depth is maintained.
The levelling system continuously matches the actual milling depth with the pre-selected
target values. The actual milling depth is measured by optical or mechanical sensors
scanning a reference surface. A slope sensor combined with a height regulator can
also be used if the milled surface needs to have a particular cross slope.
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Material loading system
A material loading system conveys the milled material from the milling drum to the
waiting truck. This system comprises one or two loading conveyors, depending on
the machine model.
In small milling machines with working widths of up to 1 m, the conveyor is located
at the rear of the machine. It can be adjusted in height and slewed to either side,
enabling the machine operator to flexibly adapt to job site conditions and to fill
trucks or wheel loader buckets right to the top.
All of the larger milling machines are front loaders and have a two-stage conveyor
system. The first conveyor (primary conveyor) accepts the milled material from the
milling chamber. The second conveyor (discharge conveyor) transports it to waiting
trucks in a continuous operation. The discharge conveyor can be adjusted in height
and slewed to either side. The variable belt speed enables the machine operator
to change the discharge parabola, and to fill trucks right to the top. |
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Key Benefits
Cold milling machines are capable of removing nearly any type of material in layers
and at precisely the specified depth, granulating the material simultaneously.
The milled material produced by the cold milling operation is a valuable raw material
that can be utilised for the production of new asphalt mixes in hot or cold mixing
plants. Cold milling machines can therefore play a key role in the economically
efficient recycling of road construction materials.
Cold milling machines are capable of working on narrow construction sites, with
traffic flowing uninterrupted on the adjacent lane. In this way, traffic disruptions
are kept to a minimum.
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A new pavement can be installed immedialey following the milling operation this
shortens construction times and increases the overall economic efficiency.
The milled material is also eminently suitable for reuse in trench construction,
owing to its excellent compaction properties and bearing capacity.
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