Circle-segment end mills - carbide end mills factory

Directing coolant underneath the cutting area and directly to the cutting area controls the temperature of the cut for longer tool life. Photo courtesy of Sandvik Coromant.

“With more machines capable of producing complete parts, faster metal removal rates go right to the bottom line.”

The higher the number of flutes per given diameter, the smaller the flute space on the end mill, Fiedler said. Depending on the material and its specific chip formation behavior, sufficient chip evacuation is critical and needs to be closely observed. In general, cut-off material moves down to the core and then breaks or rolls into chips, but it’s helpful to know how to read different materials and their tendencies. Steels up to 45 HRC, depending on the type of alloy, tend to roll and then break. Hardened steels are brittle and create thin chips. In general, stainless steels have less tendency to roll, but this is also heavily dependent on the alloy. Cast iron breaks into dust particles. Titanium tends to curl and fills up the available flute space quickly.

“Once these conditions are met, it is possible that not only high efficiency is achieved, but also that tool life and the life of the machine spindle are greatly extended,” said Hashizume. “In such an environment, it is less important to consider chip evacuation by enlarging the chip pocket of the end mill, but rather how to increase the number of flutes to increase the tool rigidity and feed rate to achieve high efficiency.”

Change any variable and the productivity of the turning operation and the life of the inserts are affected. The trick is finding the solution set that enables the insert to remove the metal while adding the most to the bottom line.

Don’t forget about the material you’re removing, said Clynch. “In theory there is not a limit, but you’ve got to have some place for the chip to form correctly,” he said. For normal, everyday materials like ISO P, ISO M, and high-temperature alloys there has to be a limit on the number of flutes. The rule of thumb is for every millimeter in diameter of a tool you get one flute, he said. For example, for tools with a ½" (12.7-mm) diameter, the maximum flute number to be effective is 12 and for tools with a 1" (25.4-mm) diameter the maximum flute number to be effective is 25. “For practicality that’s a good way to do it,” Clynch said.

What to do if chip formation doesn’t look right? “Unfortunately, there is not a simple answer to this. It depends on the application and material,” said Fiedler. In the case of chips changing their color, the coolant supply into the work zone needs to be improved. Vibration might be the cause of all this, so the tool and workpiece clamping need to be checked. Modifying the feed rates and axial DOC can also help. Curling and ruffling of chips often indicates feed rates are too high, so adjusting the feed rates can help, but it is important to maintain sufficient average chip thickness

Counter-balance chucks can help smooth out the cutting forces if there are balance issues to allow runs, but they aren’t a perfect answer, said Geisel, because the forces can be slightly different from part to part. “Something with a counterbalance weight might be good for one part but slightly off for the next.”

Edwin Tonne, training and technical specialist, Horn USA Inc., Franklin, Tenn., said the perceived application for multi-flute end mills is for semi-finishing and finishing workpieces. “But, actually, if the shop is willing to re-program the job multi-flutes can be used to rough and to pocket as well,” he said.

Choosing an insert designed for high material removal rates can be a good place to start the search for the sweet metal removal spot.

How can the use of multi-flute end mills lead to higher productivity if they take such small bites of metal? “Because the normal operation of the multi-flute is decreased radial engagement, let’s say less than 25 percent of the diameter, the arc of contact is smaller,” said Horn’s Tonne. This allows the use of two to three times the normal cutting speed range.

If coolant reduces the cut zone temperature too quickly, increasing the spindle speed, DOC, or both will add heat. Photo courtesy of Iscar.

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Q = Vc x Ap x fn x 12. That’s one multi-variable formula using the relationship of feed, speed, and depth of cut (DOC) to determine the rate of metal removal during the turning process. Vc is the speed in surface feet per minute (SFM); Ap is the DOC in inches; and fn is the feed in inches per revolution (IPR).

“The industry standard for insert life is determined by running an insert as fast as possible for 15 minutes without having it lose its edge. Let’s say you are using an insert for 180-Brinell steel. If you are running a harder material, you can run a little slower than the industry standard to reduce the heat and extend insert life. Or you can slow down if you are running a large part that needs more than 15 minutes of cut time,” said Winter.

There’s even more encouraging news for smaller shops when it comes to multi-flute end mills and CAM software. “They’re great tools for low power and smaller taper machines like 40-taper because with multi-flutes we’re taking deep axial cuts and balancing the radial forces by using the length of the tool for stability,” said Matt Clynch, national product specialist-milling, Iscar USA, Arlington, Texas. “With those smaller taper machines, if we take deep radial cuts with the large widths of cut it would start to bend and the assembly topples.”

Using these tools in an HEM/VoluMill environment (toolpath software from Celeritive Technologies, Moorpark, Calif.) will mitigate assembly disasters, he said. This approach can make the smaller tapered machines competitive with larger taper machines. “The metal removal rates we can achieve are very close if they aren’t in fact beating the standard way to rough material out on bigger taper machines like 50-taper and HSK A100,” Clynch said. As a result, a wider segment of the industry can be competitive because smaller taper machines are less expensive and easier to learn.

Don’t equate cycle time with tool life, though, said Clynch. Just because a machine ran six hours, that doesn’t mean it used six hours of tool life. The end mill may have only been engaged with the workpiece a fraction of that time due to the short arc of contact. “Pay close attention to this to make sure you are getting the maximum [life] out of your tools,” he said. “If not, you may be leaving money on the table!”

“In high-volume production there is always a balance,” said Geisel. “You need to look at the cost of running a machine tool versus the cost of an insert. Even using $60 an hour, a minimum amount of $1 a minute for machine time, every minute saved by improved material removal rates saves a lot of money. Let’s say a four-sided insert costs $10. That’s about $2.50 an edge. If I adjust the process formula to run faster and save two minutes, that brings the cost of each edge down to a few cents. Using additional inserts to gain that faster material removal rate can be a money-saver.

However, Clynch offers several cautions. When using this strategy, a machine tool’s acceleration/deceleration rates have to be higher because with the smaller moves the tool makes, the machine has to ramp up and down more to adjust the speed. The machine tool needs more memory for longer programs and it also needs enough “look-ahead,” or buffer space, to run smoothly. If the machine can’t read the code fast enough, it jerks, stalls or dwells trying to keep up, he said.

A good starting point for choosing the most effective insert based on surface footage, material type, and material hardness can be a consultation with the tooling supplier because of the continuous technology changes.“It seems like every couple weeks there is something new,” said Geisel. “The new tools are designed to last longer, run faster, work better, and reduce tooling costs, and ultimately help manufacturers make more money. A shop using the same tooling it has used for the last five years is probably out-of-date because of the technology advances, and chances are it is throwing away a lot of money.”

A part’s stability and strength need to be considered when increasing speeds. Photo courtesy of Sandvik Coromant.

John Winter, product manager for eastern U.S. at Sandvik Coromant, described how they approach working with the formula. “Typically, we optimize the DOC for an operation first. The second thing we consider is the feed rate. The third is the surface footage.

Since every material machines at its own temperature, the designs are specific to the material being processed. Insert geometry, chip former, grade, and coating work together to provide higher material removal rates for a longer period of time per insert.

“There are many modern machining techniques and strategies that really turn [that wisdom] on its head,” said Strauchen. Now, faster machines with more horsepower and faster, more precise spindles have made possible aggressive machining strategies like high-efficiency milling (HEM), also known as dynamic milling, and trochoidal milling.

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In contrast to the linear radial toolpath in conventional machining, trochoidal milling uses a spiral (or D-shaped) toolpath with a low radial DOC to reduce load and wear on the tool. Since trochoidal milling uses a tool to machine a slot wider than its cutting diameter, the same tool can be used to create slots of varying sizes. This can free up space in the tool carousel and save time on tool changeouts, depending on the requirements of the part.

“Dynamic milling is defined as a method that is done with a large axial depth of cut (DOC) and small radial DOC to reduce the engagement time of the cutting edge of an end mill, which reduces the force load on the tool and spindle and the generation of cutting heat, while increasing the amount of material removed,” said Tyler Hashizume, product engineer II, OSG USA Inc., St. Charles, Illinois.

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“In a typical, traditional approach, when machinists use end mills for more material [removal], more roughing, and if the tool is buried more, the fewer flutes they would use,” said Drew Strauchen, executive vice president at GWS Tool Group, Tavares, Fla. “With conventional wisdom, roughing uses two- or three-flute end mills and semi-finishing and finishing operations uses more flutes—four, five and beyond.”

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“For example, most people will machine titanium at around 150 SFM. With rapid-cooling pumps, you can machine that same titanium at 900 SFM, upping metal removal rate by 700 or 800 percent.”

Tonne agreed with Strauchen that modern CAM software is what helps make possible processes like trochoidal milling and high dynamic milling. “CAM software has gotten really good at high dynamic milling, where it’s managing the chip thickness,” he said. “So, you can use that in a roughing operation.”

“Some people believe that running at a high spindle speed with a moderate feed and DOC is the way to go. Others believe that running at a slower spindle speed with a more aggressive DOC is better. Which way is best? I don’t think there is a clear answer. When we run a job both ways, the cycle times come out reasonably close.”

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“Any kind of insert cutting for any kind of material will have a band, or zone, to cut in without running too hot or cold,” said Steve Geisel, turning/grip product manager at Iscar. “You can manipulate the speed, feed, and DOC by increasing one or decreasing others to stay within that zone to optimally affect the metal removal rate.

Tonne also agreed the software enables processes that lead to higher productivity. With trochoidal milling “you have some unproductive time in the cut but [the process] more than makes up for it because you can take a really big axial DOC, even with a small end mill,” he said.

“The surface footage has the most detrimental effect on an insert from the tool life standpoint. Extending the surface footage puts more heat into the insert, and it can wear out prematurely. Too little heat and you can get a built-up edge or chip the insert.”

Dynamic milling relies on the ability of CAD/CAM software to create a trochoidal milling program; a milling machine to read complicated trochoidal programs at high speed; and a machine that can rapidly move the spindle and table.

By eliminating the need to change out one mill for another and employing more cutting-edge techniques, today’s machinists can go faster, which leads to increased productivity.

These strategies change the way a machinist tackles a job, and it’s becoming popular for machinists to use multi-flute end mills—those with five or more flutes—to do both roughing and finishing, eliminating the need to fill up the tool carousel with an array of different end mills. These modern strategies mitigate the need to bury the tool into a part and any worries about getting chips clogged up in the flute gullets, which can lead to a broken end mill and the failure of the part in progress.

“Putting the coolant right at the cutting edge where most of the heat is generated and on the back side underneath the cutting area allows you to run at higher parameters,” said Winter. “You can increase your surface footage or feed it harder.”

End mills, traditionally made with two to four flutes, are used in one of the oldest mechanized machining processes—milling. Cutting-edge software, machine tools, novel strategies, ever-improving techniques and design updates in the tools themselves keeps milling useful in the 21st century. The machinist who masters the art of these metal eaters can save their shop time and money while producing superior parts.

By adding flutes, the machinist can decrease the feed per flute and still maintain the same feed compared to an end mill with a lower flute count. For example, for a four-flute end mill running 0.002" (0.005-cm) per flute, substitute a five-flute end mill and maintain the same feed with decreased pressure per flute. “So, you get a little more flexibility in your tool wear without decreasing your productivity,” Tonne said. “The linear feed rate can stay the same and the cycle time will stay the same but you’re decreasing the section each flute has to take.”

High-pressure coolant directed at the cutting edge can allow much faster metal removal rates. The higher the pressure the better, and directing the coolant to the cut zone where it can cool the cut faster handles the heat from increased speeds regardless of the system’s pounds per square inch rating.

“Very important is the amount of coolant that can be provided and ensuring that the coolant stream direction maximizes the chip evacuation out of the cutting zone,” said Bernd Fiedler, senior product manager-solid end milling, at Kennametal, Fuerth, Germany. “Sometimes high-pressured air can be a good option to remove chips out of the working area and prevent chip clogging, especially in pockets. ”

“High-speed tooling was primarily used in milling operations, but more and more tooling manufacturers are offering lathe tools specifically designed to improve metal removal rates. Those tools will have unique chip formers or edge preparations or a unique overall design,” said Geisel.

Geisel said, “Normal cooling on a lathe is done with a flood-type system or nozzles that are far away from the cutting edge. Switching to high-pressure coolant that gets close to the cutting edge—the cutting zone--is one of the biggest influences in helping manufacturers achieve higher removal rates. It rapidly cools the cutting zone so you can run faster. If you end up cooling too quickly and need to get some of the heat back to prolong the insert life, you can increase the spindle speed, the DOC, or a combination of the two.

Newer software and machining techniques can even help make an older machine perform like a shiny new model. “If they have machines even with moderate speed, a lot of times an older machine that has moderate capabilities—if it’s partnered with a new machining strategy—can still take advantage of modern high-efficiency machining and toolpath strategies,” said Strauchen. “The best way to figure it out is to bring specialists in that are savvy with modern programming techniques … and help customers maximize what they have.”

The question is how can a machinist seemingly defy the laws of physics and use the speed of a higher-flute tool without clogging it with chips and causing it to break? The answer is in new programming strategies. Today, CAD/CAM software, with sophisticated toolpath generation built in, allows programmers to generate more efficient toolpaths that are speedy but prevent the tool from getting into danger zones. The software’s approach is very specific, so the mill is never over-engaged with the part. Users can tell CAM software, “I don’t want to exceed this amount of tool engagement,” and the application will create the toolpath necessary to ensure the tool never gets engaged beyond the point he defined.

Is there a limit on the number of flutes for one end mill? The primary method of manufacturing end mills is grinding using automatic NC grinding machines, said OSG’s Hashizume. As long as they are manufactured using such machines, the capabilities of CAD/CAM applications and the grinding machines themselves (especially the size of the grinding wheel) impose limitations on the number of flutes it is possible to create. “The larger the OD of the end mill manufactured, the bigger the space that can be used for one cutting edge, and the more cutting edges that can be manufactured,” said Hashizume. The maximum number of flutes depends on the diameter of the tool, Tonne agreed.

The newer processes enabled by modern CAM software are a boon for machining hard materials that need to be machined with low radial engagement, which otherwise would risk breaking the end mill. For example, when machining steel greater than 50-60 HRC, a two-flute end mill probably will snap.

“But using a multi-flute end mill with high-speed techniques and low radial engagement you can mill a slot or any kind of feature in it,” Tonne said.

“There are several general indicators that chip formation is insufficient,” Fiedler said. “Chips are very curled or rippled, have no uniform edge, or they are deeply colored. For instance, when the side that rolls over the cutting edge is no longer shiny but shows color changes.”

With a higher number of flutes, though, chip formation and evacuation become concerns. Mitigate these concerns by adjusting radial engagement and table feeds to the application and target material; choosing the correct tool for a specific application; and selecting tools tailored for a high number of flutes—for example, those with a specific core design enabling bigger flute space toward the front end, or a design that optimizes chip formation.

Finding the right numbers for the metal removal formula is a balancing act to find the fastest cycle time and lowest production costs. It may take some trial and error to find the most profitable mix. Start by setting a baseline for the process. Then track, compare, and adjust.

Part size and configuration come into play when speed increases. “When you are talking about high metal removal rates, you are also talking about above-average tooling pressures,” said Geisel. “Stability and strength of the part and setup are extremely important. A large-diameter, stable part may be able to withstand the increased pressures, but a small-diameter or thin-wall part may not be able to handle the cutting forces.”

“All the CAM systems have different names for this type of programming,” he said. “HEM, VoluMill—there’s all sorts of them. If you really want to give it a blanket name it’d be ‘optimized roughing.’ These CAM systems have made it so easy, you just say this is my percentage of diameter width of cut and it does a lot of the back figuring for elevated surface footages and correcting of feed rates. It figures the tool path for you. It’s just made it so much easier for the small guy to be competitive.”

The DOC can also be increased. For example, the machinist could run a process using a 5/8" (1.6-cm) diameter, two-flute end mill on titanium 6AL-4V at 130 sfm using full slotting and 1× the diameter DOC for productivity of 1.49 in³/min. (24.4 cm3/min.). Doing the same process with an eight-flute end mill with 230 sfm and 0.019" (0.048-cm) radial engagement or width of cut and 2× the diameter depth of cut increases productivity to 1.57 in³/min. (25.7 cm3/min.). “So, the net gain on productivity is significant,” said Tonne.

From what he’s seen in the industry, 20 flutes on a 1.25" (3.18-cm) tool is the maximum. “With that many flutes, the radial engagement due to the limited usable flute volume is much less than 10 percent,” he said. “So, you start to diminish the practicality in most applications or limit your work to pure finishing and not much else.”