Achieving the best possible surface finish with face milling

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Positive lead, 45-degree face mills like this one leverage the chip thinning effect. This tends to improve tool life and surface finish, but does increase tool pressure over 90-degree shouldering-style cutters. IMAGE: Sandvik Coromant

By Kip Hanson

One of the first operations I learned in vocational school was how to fly cut a workpiece. That was back in 1978, when we sharpened high-speed steel (HSS) tool bits by hand and trammed in the heads on our knee mills with a dial indicator and gentle taps from a five-pound rubber mallet.

Times haven’t changed all that much. “I still see a lot of shops doing it this way,” says Sandvik Coromant’s Dan Tucker, Americas regional product manager, rotating indexable. “Yes, it’s old school, but it’s effective and shops are comfortable with it. Why change?”

He’s right. Fly cutters are easy to set up. There’s only one cutting edge to worry about, and even if the head isn’t perfectly square, it’s still possible to achieve a fairly smooth, flat surface. Even a teenage kid who forgets to lock the Y-axis in place can do it. 

The problem? It’s the “one cutting edge” just mentioned. Granted, there’s none of the hassle that comes with dialing in multiple inserts and wipers, nor concerns about toolholder balancing and squareness, but no one can dispute this basic fact: fly cutters are far less productive than the modern, indexable alternatives. 

For cast iron milling applications as shown here, dry cutting or MQL (minimum quantity lubrication) is often preferred over flood coolant. IMAGE: Kennametal

Facing facts
That said, there are a lot of these alternatives out there. Sandvik Coromant lists 14 families of face cutters and close to 900 unique SKUs on its website. Kennametal, whom you’ll hear from shortly, shows nearly 20 distinct face mill product lines, most with multiple variants. 

With all that choice, how do you know which one is best for your application? 

Tucker gives the frustratingly simple answer that all cutting tool people offer when asked this question. “It depends on the application.”

Fair enough. Face mill selection comes down to a handful of variables, he explains, starting with diameter. As with many things in life, more is often better, because the larger the cutter, the more material it removes per pass and the fewer passes are required. That said, there are exceptions. 

“If you have a 40-taper machine and are aiming to get the ultimate surface finish, you’re generally limited to around four inches (100 mm) as a maximum diameter,” says Tucker. “Get much beyond
that and the cutter edge will be too far away from the spindle gauge line and the cutter body’s going to flex too much.”

Shown here: Sandvik Coromant’s CoroMill 425 face mill in action. 
IMAGE: Sandvik Coromant

About face
The humungous face mills sporting dozens of inserts you see on cutting tool websites are impressive looking, but they’re also designed for specific applications like automotive engine blocks and cylinder heads. And they need an equally humungous CNC machining centre to drive them (or more likely in these examples, a transfer line). For the typical job shop, it’s a good idea to aim smaller. “It’s the machine that dictates how big a face mill you can use,” he says. 

Then there’s the workpiece material. For most metals, a sharp, PVD-coated insert is the first choice. TiAlN (Titanium Aluminum Nitride) and TiCN (Titanium Carbonitride) are two popular coating choices. For extended tool life in steels and cast irons, cubic boron nitride (CBN) is another option, provided it has the appropriate edge preparation to offset its brittleness. 

If the components just described are made of aluminum, however—especially the high silicon cast aluminum used by automakers—then polycrystalline diamond (PCD) is the way to go.
It will not only last far longer and produce a better finish than carbide, but run at speeds
many times higher. 

But as Tucker is quick to point out, there’s more to this equation. “CBN and PCD tools are not inexpensive, so proper training of your workforce is important. They need to learn and follow best practices, or you can end up damaging the tool. And for PCD, at least, one of these practices is the use of a clean, well-maintained cutting fluid. A water-soluble coolant mixed to at least 6% concentration is best, and if the workpiece requires drilling and tapping, a coolant with extreme pressure (EP) additives should be used.”

Although this image does not show it, many of Kennametal’s inserts have a number or dimple on each cutting edge. This feature allows the setup person to “clock” the inserts, minimizing variability and runout. IMAGE: Kennametal

Face value
Tony McClain is the regional product marketing manager for Indexable Milling – Americas at Kennametal. He seconds the machining advice just given, noting that other variables like tool geometry and cutting parameters also play a significant role in a shop’s ability to produce high-quality surface finishes, whatever the operation. 

“Consider depth of cut,” says McClain. “A lot of machinists think that leaving the bare minimum for the finishing cut is going to help, but it’s often just the opposite. For example, I got a call the other day from a customer who was struggling to get a good surface finish. I asked him a few questions and quickly found he was only leaving 0.003 inches of material for the final pass. It wasn’t even enough to get past the insert hone, so the cutter was just rubbing. The same goes for an insert’s chipbreaker and tool nose radius—everything is designed to work at a specific depth of cut and will be less effective if you don’t meet those criteria.”

Tool geometry is equally important. Using a face mill with a positive lead angle to leverage the chip-thinning effect is usually good advice, but only if the setup supports it. “By tipping that insert to 45 degrees or even 30 degrees, you can crank up the feedrate quite a bit, but you’re increasing tool pressure at the same time. You have to make sure the fixturing, machine tool, and workpiece can handle that without deflecting or creating vibration.”

Both experts suggested that toolholder balancing is crucial at elevated spindle speeds, and because perpendicularity and proper insert seating are similarly critical, high-quality holders and insert bodies are a must. Machinists should also use a dial indicator or, better yet, an offline tool presetter to check runout at the cutter face—some face mills offer adjustment capabilities to accommodate the need to “dial-in” insert height. 

This is especially true for cutters with a wiper insert (or inserts), which as the name suggests, “wipe” the workpiece surface, thus imparting a smooth finish free of toolmarks. “You should also look for cutters that use a wedge clamp rather than a screw, as these are more precise and easier to adjust insert height,” McClain adds. 

Face the consequences
Other success factors include the number of teeth in a face mill, affecting both material removal rate and surface finish. Fewer teeth allow for a higher chip load, which is beneficial for roughing operations. For finishing, more teeth and a lower chip load is usually more effective—as a rule of thumb, 5-8 teeth are generally recommended for better surface finishes, although this value varies depending on the tool diameter. 

Proper programming techniques are similarly essential. A stepover between 50 to 75% of the tool diameter usually delivers a smooth surface. Always climb mill, and always offset the cutter from centreline, positioning it to generate a thick-to-thin chip. Arcing into and out of the cut during roughing will help to reduce tool wear. And generally speaking, a slower feed rate and higher RPM tend to improve the surface finish.

Lastly, don’t be afraid to ask for help. Says McClain, “Assuming you buy your tools from a reputable manufacturer, there’s all kinds of application support out there and people should take advantage of it as much as possible, whatever they’re working on.” He laughs. “If I ever went back to running a shop again, I’d wear my Kennametal reps out. They’d be in there all the time.” SMT

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