What makes for maximum metal removal?

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By Staff Writer

Is your current milling strategy considered high-speed machining, high-feed machining, or something else entirely? And who cares what you call it, provided it gets the job done in the shortest time possible without breaking cutters or yanking the part out of the vise?

The term high-speed machining (HSM) is nothing new. In fact, it predates commercial CNC machine tools by many decades. Machining legend points to Dr. Carl Salomon as its discoverer, who found that as cutting speeds reached a certain point, temperatures in the cutting zone began to decrease. He achieved a whopping 16,500 surface metres per minute (54,200 surface feet/minute) in aluminum, and in 1931, went on to patent his work. History remembers him for the Salomon Curve that illustrates this phenomenon. 

It’s all about the chip thinning

Curves to the contrary, Brian MacNeil, milling products and application specialist at Sandvik Coromant in Canada, explains that HSM also utilizes a radial engagement that’s usually less than one-fourth of the cutter diameter while maximizing the axial (Z-axis) depth of cut. Because of this, 90-degree indexable and solid carbide end mills intended for this programming strategy generally have greater length-to-diameter ratios than general-purpose tools. 

High-feed milling (HFM), on the other hand, applies a shallower axial depth of cut and significantly greater radial engagement. It typically uses specially designed cutters with a lead angle between 10 to 25 degrees. Both processes rely on the chip thinning effect to support increased feed rates and, depending on the application and amount of heat generated during the cut, higher spindle speeds as well. 

When used properly, he says, both processes reduce heat in the cutting zone compared to “traditional” (that is, outdated) milling processes. This benefits tool life or allows higher spindle speeds (and therefore feed rates) and sometimes both. Either way, however, the feed rate must be high enough to achieve the manufacturer’s recommended chip thickness. “Indexable tool geometries are designed to work within a specific chip thickness range,” says MacNeil. “If you go below that, the insert will rub rather than cut and create even more heat. Feed too fast, though, and chipping or breakage can occur.”

MacNeil ticked off several additional considerations. For example, the low lead angle tools used with HFM tend to drive cutting forces up into the spindle, decreasing radial deflection in lighter-duty machines. This also allows longer tool overhangs, which can be advantageous in deep pockets and mold cavities, but might cause problems in components with thin floors, such as landing gear beams. 

With the solid carbide tools used in HSM, look for asymmetrical chip splitters, conical cores for greater stiffness, variable helix angles, and a sharp, PVD-coated edge, he adds. Avoid chip recutting in tougher materials like Inconel and austenitic stainless steels by using generous amounts of flood coolant, or better yet, HPC (high-pressure coolant). And always use a high-quality, well-maintained hydraulic, heat shrink, or mechanical milling chuck to prevent tool pull-out and minimize run-out.

What’s in a name?

Mitchell Parker of LMT Onsrud has similar advice. A process improvement and applications manager, he echoes what MacNeil says about shops underfeeding or overfeeding their cutting tools. “As long as they’re engaging less than 50% of the tool diameter, shops should always perform chip thinning calculations when programming a job, no matter what milling approach or end mill they’re using. If not, it will definitely affect tool life.”

Parker also suggests that most “book values” are conservative, and that when machinists run into trouble, the first thing most of them try is to back off on feeds and speeds. They then blame the cutting tool for poor performance, when in reality, they should re-evaluate their approach. “It’s critical to apply the correct machining parameters based on the number of flutes, the programming approach, depth of cut, and of course the material,” he says. “You have to look at the whole picture, but I can tell you that doing so will often increase tool life and productivity by 40% or even 50%.”

One word of warning, though: always pay attention to the cycle time when increasing feed rates. Older machine tools and those not designed for high-performance toolpaths will certainly accept whatever feed rate you plug into them, but that doesn’t mean they can achieve it. “If the job is programmed for 50 inches a minute and I speed it up to 80 inches a minute and it still takes the same amount of time to cut the part, then the machine’s not capable of running at the higher feed within that part configuration.” 

This diagram illustrates some of the machining applications possible with the CoroMill MH20 indexable cutter from Sandvik Coromant. 

And what about the toolpaths? Sandvik Coromant’s MacNeil notes that “all current CAM systems have excellent tool path options for HFM and HSM” but adds that there are still many considerations. “Always try to avoid burying end mills on inside corners by choosing a tool whose radius is no more than 70% that of the corner radius. And don’t forget the machining basics like arcing or angling into and out of the workpiece to reduce shock to the cutter, and never use a 50% radial width of cut, as it’s sure to create vibration.”

Jesse Trinque works for one of those CAM providers. A sales and applications engineer for Mastercam developer CNC Software, he agrees on all counts, but describes another milling approach in vogue these days called high-efficiency milling, or HEM. “A lot of people use the two terms interchangeably, but I like to think of it as turbocharged high-speed machining. That’s because HEM leverages the latest in carbide technology, machine tools, and controls to take metal removal to the next level.”

Bring on the insanity

That said, HEM might look very different on one CNC machining centre vs. another. A geared head milling machine with generous torque at low speeds allows the programmer to use a much larger diameter tool than one with linear axis drives and a 30,000-rpm spindle. Trinque suggests that HEM is possible in each of these scenarios, but that the appropriate cutting tool, toolpaths, and machining parameters must be used to be successful. 

“The next step beyond HEM is what we refer to as power milling,” he says. “It applies the same chip thinning principles as seen in high-feed and high-speed toolpaths, but with tools that are capable of nearly full-width machining, where you’re using 80 to 90% stepover and trochoidal motions to cut slots and pockets. The result is absolutely insane material removal rates.”

Here again, the correct cutting tool and machining conditions must be used, otherwise you might be looking at broken tools and flying workpieces. But according to Trinque, when everything is dialed in, power milling provides results “unachievable with any other method.” SMT

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