Maximize Milling with these Best Practices
Tom Chastain has learned a thing or two about quality milling in his 29 years in the milling industry. Currently the milling product manager at Wirtgen America, Antioch, Tennessee, he regularly conducts application training on milling machines for dealers, contractors, unions and municipalities and has assisted with projects and conducted training on four continents. At CONEXPO-CON/AGG 2023, Chastain shared some of his advice for maximizing milling quality.
“The job of a milling crew is to give the paving crew a clean palette to paint a masterpiece on,” he said. With each passing year, Chastain said the standards for milling crews have risen. “With asphalt paving, they’re using big skis and more technology to get a smoother ride. With rolling, they’ve got intelligent compaction, oscillation, etc. to get a smoother ride. Now, the focus is on [milling to get a smoother ride].”
“The job of a milling crew is to give the paving crew a clean palette to paint a masterpiece on.”—Tom Chastain
Of course, achieving a smoother milled surface involves technological improvements, but Chastain said milling quality is mostly about getting the basics right. “I hear all the time, ‘We’ve been through basic milling training. We want advanced milling training,’” he said. “But advanced milling training is the basics.”
Here are eight best practices any milling crew should know based on Chastain’s nearly three decades of experience.
Know the details of the job.
Chastain shared a story from a project he was on in Missouri many years ago on I-55 southbound. “The milling crew was already there when I showed up to the job site,” he said. “I asked them where the machine was and they said they didn’t know. I asked where we’re going to start milling and they said they didn’t know. Eventually, the machine shows up and we start rolling down I-55 southbound. The DOT shows up an hour later and says, ‘What are you doing? You’re supposed to be going northbound.’”
His example stresses the importance of quality communication. “If you communicate with your team regarding the job, the better chances you have to achieve quality,” he said. Chastain stresses the importance of the information in the office making its way to the superintendent, foreman and milling crew. “But it doesn’t stop there.”
“We have to remember we’re making ½ of VELCRO®. The paver is coming in with the other half.”—Tom Chastain
“We also need our safety personnel—our flaggers and anyone else involved—to be on the same page,” Chastain said. He also recommends discussing with the truck drivers how you want them to maneuver in front of the machine, how to leave the job site, and the importance of operating safely in the work zone. “If we’re working for a prime on a project, we need to have an understanding of what we’re going to do on that project.”
“Everything we do starts with communication,” Chastain said.
Know the right machine for the job.
Choosing the right machine for the job is key to a project’s success and efficiency. “We’ve got to understand what machines we need for the project,” Chastain said.
“Maybe we think we can get away with a small machine because we think we’re just doing some patchwork,” he said. “So, you take out the compact machine, knock those out, then you find out you need to mill three lane miles. We can do it with that [compact] machine, but it’s not going to be the most time efficient.”
However, small milling machines are handy to have in one’s fleet. “The reason is they can flush cut around manhole covers,” he said. One issue he’s noticed with using a skid steer with a milling head attachment is that they are challenging to operate well. “That manhole cover often ends up looking like a great white shark’s been gnawing at it and we have to go in with a pickaxe or a jackhammer to clean it up. Having a small milling machine makes it that much easier.”
Know your milling modes.
These days, Chastain said most manufacturers offer milling machines with several operating modes. For example, eco mode for fuel efficiency and power mode when “you want everything the machine’s got,” he said.
Many machines also have a pattern mode for optimal milling quality, Chastain said. “With the way milling used to be, when we hit a hard section of asphalt the machine slows down because of the load against the machine drum, and that would tighten up our pattern,” he said. “With new machines, you can put them in pattern mode, tell the machine what drum you’ve got, what teeth you’re running, and what pattern quality you want it to maintain. If you hit that same hard section of asphalt, not only is that machine going to slow down but now the drum is going to slow down as well. That way, we keep a consistent pattern, curb to curb.”
Like 3D grade control, Chastain said, this is another way to “make it easier for inexperienced operators to get us to a higher quality of milling.”
“I hear all the time, ‘We’ve been through basic milling training. We want advanced milling training’. But advanced milling training is the basics.”—Tom Chastain
Know your drum types.
“You’ve got standard drums, then you have fine drums, then you have micro drums,” Chastain said. “A standard drum is made to mill to the machine’s maximum cutting depth. If the machine’s maximum cutting depth is 14 inches, it’s made to cut 14 inches. Fine or micro drums are for thin lifts or profiling.”
Sometimes, Chastain said, a milling crew will get the idea that they can use a fine drum to mill faster while maintaining pattern quality. For example, on a job in the Midwest, an operator planned to cut 7 inches with a fine textured drum. “He put the machine in the ground, one track pad rolled over, and it stalled the machine.”
Why? “Because the material couldn’t get out of the housing,” Chastain said. He explained that with a standard drum, there’s space between the material and the teeth. With a fine drum, “there’s not much space. If you pick that machine, you’re going to have to do a bunch of 1 ½-inch passes all day.”
Another thing to keep in mind if using a fine or micro drum is its implications on reclaimed asphalt pavement (RAP). “If we’re going to use a finer micro drum, we need to understand we’re going to lose 10 to 12% of that material,” Chastain said. “We’re turning it into dust because we hit it with so many teeth.”
However, the worst thing about a micro drum, according to Chastain? “Somebody’s got to change all 1,100 teeth on that drum.”
Know how your grade technology works.
Chastain said these days there is a wide variety of grade controls available for milling machines.
“We need to understand what grade controls we need,” he said. “There’s a big difference between copying the road and profiling it.”
“If you’re copying the road, basically what you see is what you get,” Chastain said. “If there’s a bump in the road, unless you’re really good at using your grade controls, there’s still going to be a bump in the road.” Although the road may look all right after milling and immediately behind the paver, Chastain said once it’s compacted, “you’re back to exactly what you had.”
When a crew is simply copying the profile of the road, Chastain said they may even risk making bumps worse if they’re milling at a high feet-per-minute rate. “If you’re milling faster, you didn’t do anything to that bump except lengthen the size of that bump because the hydraulics need enough time to react.”
“With profiling, like with a sonic averaging system, you can take a big bump and turn it into three smaller ones,” he said. “Is it perfectly flat? No. But it’s a whole lot smoother than it was before.” Depending on the road conditions, Chastain said a crew may need to cut the road two or three times with an averaging system to get it perfectly flat. “It all depends on that road.”
Although 3D technology has been a “big topic in dirt,” Chastain said, “it’s coming over to the asphalt side.” He said there are a variety of 3D systems out there. “The big thing now is doing road scans. They’ll drive the job, get their plotting, make a 3D plan, then plug the plan into the street and go.”
“[3D] takes the guesswork out of milling,” Chastain said, while achieving a quality milled surface. “This also helps less experienced operators to get acclimated to the machine quickly.”
Know your teeth options.
“[Manufacturers] make different teeth for different applications,” Chastain said. “When you have soft asphalt, it’s going to wear out the body, but not really the carbide. Hard asphalt is going to wear the carbide, but not really the body.”
In addition to carbide teeth, diamond teeth may be a good alternative depending on the application. “Say you’re in the middle of nowhere, diamond teeth can be the way to go because they’re rated for 400,000 tons,” he said. “You can get a whole season on a set of teeth.”
The higher-wear teeth aren’t the solution for every application. “Someone who’s been milling a while might think, ‘I don’t have to change teeth anymore? Yeah, I’ll do that’,” Chastain said. But you have to understand the application. “I once saw a guy put a diamond drum on a machine, it goes 50 yards and hits a manhole cover.” Chastain shared that the operator wasn’t experienced enough to know what to do. “The whole time, that manhole cover is inside the housing bouncing around.”
“Your standard carbide milling tooth is $7, whereas a diamond tooth is $275 a tooth,” Chastain said. “That mistake cost 38 teeth. You do the math.”
Know how to start—and how to stop.
“My philosophy of starting off is this: start slow, stop slow,” Chastain said.
“The hydraulics need time to react, so when we start off we have to go slow to let those rear tracks walk into the cut,” he said. Otherwise, the machine will put a dip in the road about 3 feet from where it started. “Once the tracks get into the cut, then ramp up speed.”
The same applies when it’s time to stop the machine. “With older machines, everybody used to think that it would leave a dip in the road because the machine settled when it came to a stop,” Chastain said. However, that isn’t the case.
Let’s take a closer look at how the teeth of the mill work. “Some people think that the teeth rotate while they’re in the cut,” Chastain said, but that’s not the case. “When the tooth is in the cut, it’s actually flexed. There’s pressure working against that tooth. Once the tooth releases from the cut, that’s where we’re actually getting the tooth rotation, and that’s what we’re wanting to see between that flex release and material flying around inside that housing, we want to get that tooth rotation.”
When the mill comes to a stop, it isn’t that the mill settles. “It’s actually that you didn’t have pressure working on that tooth, so it relaxed from the holding about ¼ inch,” he said. “Now, when good operators come to a stop, they bump their grade controls up about ¼ of an inch. When they get going again, they bump it back down.”
Safety First | “When you think about what’s inside that cutter housing, it’s chaos,” Chastain said. “We’re trying to control chaos. We’ve got chaos working for us. When you give chaos an opening, chaos can bite back.” He shared an example of a milling job on an interstate where the mill hit a buried manhole cover. “It got underneath the side plate, skipped like a frisbee and embedded into the guardrail,” Chastain said. “You need to put a bubble around that machine. You have to put safety first.”
Know how fast to mill.
“The three magic words to a milling crew are production, production, production,” Chastain said. “We’re production oriented. We’ve got to go, go, go.”
Is it possible to get quality and quantity? Chastain said it’s possible, but that he sees one of these “unicorn projects” about once a decade. “The last one I had was shutting down a major interstate outside New York City where we had six full lane machines, each with their own water trucks, and thirty trucks,” he said. “We went 2 ½ miles down and 2 ½ miles back without stopping.”
However, on a normal job where a milling crew might only be running half an hour out of every hour, going 30% faster in order to wait longer isn’t worth the potentially reduced quality, Chastain said. “Instead of milling at 150 feet per minute and then sitting there 40 minutes [instead of 30], slow down your feet per minute,” he said. “Work more out of an hour and get better quality while still getting your production. Unless we have trucks to burn, that should be sufficient.”
Milling at high speeds, Chastain said, can result in a number of issues. First, “You’re not actually cutting at the depth you set,” he said. “The machine starts to water ski a bit when you go that fast.”
Having a poor pattern also reduces the interlocking capability between the milled surface and the asphalt to be paved. “We have to remember we’re making ½ of VELCRO®,” he said. “The paver is coming in with the other half.”
Because milling at high speed can result in erratic milling depth, the tonnage may be off. Although Chastain said this might not matter much on a small project, it can have a significant impact on an interstate project.
Furthermore, poor milling quality is going to cause compaction numbers to fluctuate. “Say we’re doing a thin lift mill-and-fill at full speed,” he said. “The paver paves it, then the breakdown roller comes by and hits one of our high spots. We could break the aggregate. It’s not the roller’s fault. It’s our fault.”
Milling quickly can also put an unnecessary amount of wear on the machine. “These machines are beating themselves to death and we’re trying to put one foot in the grave by running them that hard,” Chastain said.
Lastly, milling at high speeds can cause issues with sizing material. “We’re trying to use that RAP material,” Chastain said. “The slower feet-per-minute we go, the tighter the pattern, the smaller the RAP material. The faster we go, the more coarse the pattern, the larger the RAP material. We have to screen that RAP, we may even have to crush it, too. So, we’ve added a process simply because we like to mill fast.”
Those chunks can also present a risk to safety. “It’s possible that a chunk could find its way toward live traffic,” Chastain said.
According to Chastain, 30 feet per minute is a tight pattern; 60 to 80 feet per minute is still a good pattern, “but you can see that the drum is no longer [pulling] that material toward the center.”
“When we get to 85 to 90 feet per minute, we’re starting to get a little coarse and stretching out that pattern,” he said. “When you get over 170 feet per minute…the machine’s actually walking faster than the drum is turning.”
He gives milling machine operators the following bit of homework: “If you’re used to running fast and you don’t believe me, slow down by 20 to 30 feet per minute and look at what happens to the pattern quality and the RAP going into the truck.”
“[Milling too quickly] equals lower quality, a waste of material and loss of money,” Chastain said. He reiterates that advanced milling best practices are, in fact, the basics. “Milling in a nutshell is this: common sense and simple math. I can teach simple math, but I cannot teach common sense.”