Solve Poor Milling Patterns
BY Sandy Lender
To achieve a proper milled surface prior to opening a roadway to traffic, or prior to performing maintenance or preservation treatment(s), the milling machine operator must remain cognizant of tooth wear or damage.
If the pattern behind the milling machine is irregular, you could have a tooth—also called a tool or a bit, depending on what region you’re in—wearing incorrectly or even missing. Consultants agree that a veteran operator will be able to feel a change in the machine’s vibration when a tooth loses a tip or experiences some other problem, but new operators may have to rely on the power of observation to see a change in the pattern behind the mill to know when something’s going wrong.
“It’s not common for a tool to fail,” Aaron Scarfia said. He’s the sales manager of the construction tools Americas division for Sandvik Rock Tools—Cutting Division, Bristol, Virginia. What’s more likely is a tool will be broken if the machine encounters a manhole cover or other obstruction.
No matter how the damage happens, one tooth affects its entire row. “If you lose a tip, you’ll see a chain reaction,” Scarfia said. “The tooth behind the failed tooth will hit [the pavement] unevenly or roughly,” Scarfia explained. Not only will the operator feel this rough impact, he will be able to see it in the pattern behind the mill.
The photo at above shows uneven ridges, which indicates the milling contractor was using a drum with teeth that didn’t have the same gauge height.
Consultant John Ball, who is the proprietor of Top Quality Paving & Training, Manchester, New Hampshire, has preached the importance of the daily walk-around before a shift begins, and that comes into play here. If the milling machine operator performs a visual check of the equipment before the job begins, he’ll be able to see wear or a difference in gauge height between teeth on the cutting drum.
Scarfia explained that replacing the worn tooth will save the rest of the row from damage in addition to giving the machine its best chance to mill properly. The drum at left is in good condition.
Also check the water system. Both Scarfia and Ball spoke to the importance of the water spray for keeping teeth cooled and cleaned. “If the water doesn’t spray properly and you get one dry spot, you’ll get the same problem [of uneven tooth strikes],” Scarfia said.
During operation, be sure to lift the cutter drum when approaching utility caps and other structures in the pavement. Keep teeth from striking obstructions to keep them from damage.
The photos above shows a state inspector performing a sand displacement test on a micro-mill pattern. The ridge height is uniform.