Take a Bite Out of Milling Costs

Vibration, excessive heat and silicon dust can bring a $300,000+ milling machine to a grinding halt. With downtime averaging several thousand dollars a day, diligent preventive maintenance and training are wise investments.

The cutter head is subject to the most wear. "Depending upon cutting conditions and the type of machine, tools can last [from] just a few hours up to several days," says Thomas Chudowski, product business manager, global road construction, Kennametal.

"Normally, increased wear is due to aggressive operation without making adjustments to conform to the jobsite and material conditions," adds Philip Taraschi, product specialist, BOMAG Americas Inc.

Watch your speed/depth

Operating speed drives much of the wear. "The speed of operation is critical in calculating wear to the cutter bits, holders and drum of the machine, [as well as] the life you get out of track pads and conveyor belts," says Dennis Munks, The Sollami Company. "Running at maximum speeds reduces the life of every component mentioned and increases vibration, which leads to other costly repairs. Running the machine at excessive speeds is usually the main cause for uneven tool wear."

Heat generation due to excessive speed deteriorates carbide tips. "When you lose the protective carbide, the tooth becomes dull and blunt," says Jeff Wiley, Wirtgen. "That creates vibration and slows the machine down as much as 40%."

In addition, material needs a chance to evacuate. "Massive wear on tools can be caused by not getting cut material out of the housing quickly enough as a result of the machine's cutting depth and/or advance speed," says Chudowski. Material gets re-circulated, causing wear on all components (tools, holder system and even the housing). "Also, water cannot get to the tool, negatively affecting rotation, as well as tool wear."

Inspect often

The cutter head requires constant attention. "Start by checking hourly and adjust the interval based on the wear you are observing," suggests Eric Baker, marketing manager, Roadtec. "Usually, there is a gap in trucks when this can be done."

Visually inspect the cutter teeth and spot-change any that need to be replaced. "Deep cuts in hard asphalt require more frequent inspections, while shallow cuts in soft or deteriorated asphalt will require less," Baker notes.

Taraschi also advocates frequent inspections, along with keeping water systems in good operating condition. "Inspections and water are inexpensive and essential tools, whereas labor and downtime are costly," he points out. If abrasive material wears into the holders, the expense and labor costs increase.

Why tools fail

According to Chudowski, typical tool failures and their causes include:

• Carbide breakage - This can occur when tools are mechanically overloaded (hitting hard objects such as drainage covers or steel reinforcements), or due to thermal overload (excessive heat resulting from insufficient water supplied to the tool).
• Lack of rotation - This can be caused by too much dirt in the holder bore (possibly due to a lack of water supply to the tool) or a worn tool holder.
• Body wear/steel wash - This can be caused by high machine speed in soft (often abrasive) conditions. Reducing machine speed or using a different tool design could help solve the issue.

Poor tool rotation quickly destroys teeth. "When the tooth stops rotating, it creates a flat side," says Wiley. "When it flat sides, it gets hot and fails."

The type of material being milled can impact tool rotation."Material with high asphalt cement (AC) content can cause tools to stop rotating," says Taraschi. "In some cases, it can be corrected by adding low caustic to the water system.

"If tools in a certain position continue to prematurely wear, check impact and skew angle positions," he advises. "If tools become mismatched to the point that the newer tools are wearing faster, clean up the drum with all new tools."