Successful milling requires understanding how chips will be formed, and it’s safe to say there’s little that’s “conventional” about the process these days.
It’s the rotary cutter position when milling that determines chip formation, and as cutting tools suppliers will readily advise, you should almost always aim for thick chips on entry and thin chips on exit to ensure a stable milling process. In short, the golden rule when milling using today’s sophisticated CNC machines is “thick to thin” to ensure the lowest chip thickness when exiting a cut.
In the milling machining process the difference in direction between the cutter rotation and the workpiece feed define whether you are using a conventional milling strategy (also known as up milling) or a climb milling strategy (also known as down milling). For this article we spoke with two industry experts, Dan Tucker, regional product manager Americas with Sandvik Coromant and Evan Duncanson, milling support specialist with Emuge-Franken USA, to understand the basics of each strategy, evaluate the applications for which each is best, the instances where it makes sense to use both, and the one instance where it’s best to leave your “conventional” thinking behind and “climb” up to the better strategy.
Conventional milling
As its description suggests, conventional milling has been used for a longer period of time compared to climb milling. The concept of conventional milling dates back to the early days of milling machine development and has been a traditional approach in machining processes for many decades. In conventional milling, the cutting tool rotates against the direction of the workpiece feed. Chip thickness starts at zero and increases toward the end of the cut. The cutting edge has to be forced into the cut, creating a rubbing or burnishing effect due to friction, high temperatures and, often times, contact with a work-hardened surface caused by the preceding edge.
Climb milling
Climb milling on the other hand came into prominence with the advent of CNC machining technology. Climb milling involves the cutting tool rotating in the same direction as the workpiece feed. The chip thickness decreases from the start of the cut, gradually reaching zero at the end of the cut.
Climb milling’s enhanced performance when it comes to impact on heat generation, tool life, and power consumption has made it the preferred method across the industry.
“With climb milling you start with a very thick chip and you exit thin. You do create some heat on the entry but you maintain the heat at a constant and then it dissipates as you exit the cut. You are not pushing the work piece away from the cutter, you are pulling the piece into the cutter,” explains Tucker. “With conventional milling the insert starts with absolutely no chip thickness. It has to be forced into the cut so not only is it creating immediate heat generation but also immediate pressure in an opposing direction. You get much higher cutting forces with conventional milling, pushing the cutter and workpiece away from each other, so they are opposing each other.”
Although it’s dependent on the material being worked on, there could be as much as 50% more heat generation when using conventional milling compared to climb milling, according to Tucker.
“When you get into exotic materials like titanium, Inconel, and some stainless steels that get work hardened really easy, this is going to be even more so with the heat propagation that you’re going to see. It’s going to cause a problem at a much quicker rate for conventional milling,” Tucker adds.
Don’t let the big torque spike at the start of a climb milling session when the tool is hitting solid material and initially creating a thick chip fool you, says Duncanson. Having the cutting tool rotating in the same direction as the workpiece feed and chip thickness decreasing from the start of the cut, benefits tool life.
“In general, when using the right feeds, speeds and tooling, climb milling will provide much better tool life. It’s a more efficient way of making a chip and the tool’s cutting edge hits and shears the material.
Climb milling also performs better with regards to power consumption. Sure, there’s that initial spike in power consumption with climb milling but Duncanson says it’s a more energy efficient strategy because the spike is just that —a spike. The power spike is short lived and power consumption goes down as the chip size decreases. Conventional milling in contrast is a slow climb in power consumption as the chip size starts small but gets larger and the tool edge gets duller. Duncanson says when it’s averaged out that slow climb from conventional milling will be using more power.
“Power consumption between climb milling and conventional milling will look like a bell curve. Initially it will be fairly comparable but as the heat increases and the tool edge starts to diminish with conventional milling, it’s going to increase a lot more than climb milling,” Tucker adds.
When is conventional milling still a good idea?
With climb milling presenting so many clear advantages, why would a shop ever consider conventional milling? Because there are some instances where conventional milling is the better option. Both Tucker and Duncanson point to a few. One such situation, according to Tucker, is when edge milling and the edge of the material is cut out with a torch, laser cut or even waterjet cut, and the edge of the material is extremely hard compared to the parent material behind it.
“Here up milling allows the cutter, especially when you are edging, to get underneath into the parent material first and exit out of the hardened material. That’s where conventional milling is a problem solver. In situations like that where you are trying to climb mill you are taking a nice fresh edge and you are going right into the hardened material, causing premature insert failure,” Tucker says.
Another situation where conventional milling is advantageous, he adds, is with helical interpolation where it allows for better chip evacuation as you are helical boring down a particular type of application.
“It can be a problem solver whereas climb milling with helical interpolation, depending on the density of your cutter, could be trapping the chips as you’re going down,” Tucker says.
Duncanson provides the scenario of milling high density foam (think of a toolbox that has cutouts for all the tools.) It may not be a common job for most shops, but some shops do take on foam milling, and they would be better served using conventional milling. If you’re trying to use climb milling, the tool will hit that material very hard, pulling the foam into the cutter and ripping it. So you need that pushing effect of conventional milling to keep the material from ripping into the cutter, Duncanson explains.
Another scenario includes Emuge-Franken’s niche Cut and Form End Mill. It’s a six-flute end mill including three cutting flutes and three burnishing flutes. “The burnishing flute on this tool is right behind the cutting flute and is of a slightly larger diameter. The Cut and Form end mill takes the material and pushes it over and polishes it to a mirror finish,” Duncanson explains. “The tool is designed for non-ferrous materials –aluminum, brass and copper. In aluminum you will want to climb mill, but with brass and copper you need to conventional mill because if you try to climb mill it will sand the tool down.”
Combining conventional and climb milling
With some easy-to-machine materials such as aluminum, brass or plastics, it doesn’t much matter if you climb or conventional mill. In such situations it’s common to combine both processes, says Duncanson.
“For example, when milling out a one-inch wide slot using a ¼” tool with a trochoidal milling strategy, both climb mill and conventional mill can be used, zig zagging back and forth. You would use both approaches because it’s faster,” Duncanson says. “Think of making a trochoidal slot — if you just do climb milling, you are going to mill in a climb direction and then you have to bring the tool back to the other side of the slot to mill again. Following a zig zag pattern, the tool is in the cut for more of the time and you don’t have that air time where you have to work the tool back into the cut which may negatively affect surface finish.”
When milling aluminum by using a zig zag pattern, you will see a really nice finish, he adds. You might see some machining marks, but the finish will be nice and shiny on the climb milling side. On the conventional side, the finish may be a bit more dull and have some smearing because the chips are actually rewelding to the material.
When conventional milling is the only option
Then there are the instances where shops have no choice but to use conventional milling because they’re limited by their machine deficiencies.
Duncanson explains that conventional milling can be a solution when faced with a weaker machine which suffers from backlash and can’t handle the initial torque spike of climb milling.
“With conventional milling the forces are lower due to starting on the inside of the cut, which is the thin side of the chip, so the chip is peeled away from the material, ending with the thicker side of the chip. Conventional milling is a bit more of a stable process because it’s a slow buildup of forces instead of a spike and then backing down,” he explains.
Of course, one could ask, wouldn’t shops be better off investing in a better quality machine capable of handling the torque spike of climb milling given the stated advantages of the process?
“Absolutely,” is Tucker’s quick reply before conceding that some job shops have older machines they don’t want to give up on.
“We do that type of analysis as part of what we call a productivity improvement program. We go into customer facilities when they don’t want to invest in new machines or when they’re looking to justify investing in new machines. That’s one of the first things we look at,” Tucker says. “You have a really poor cutter path here. Why are you doing that? By hanging on to weaker machines and resorting to conventional milling you are not taking advantage of 50-60% productivity increases. Calculate that over a year and how many parts you run, and you could justify two or three new machines.”
Perhaps such shops would benefit from leaving their “conventional” thinking behind and undertaking the “climb” to a more efficient process. All puns intended. SMT
