It's not just about the cutter. As a matter of fact, it's not just about the spindle speed or machine tool, either. Selecting the most appropriate tool to maximize high speed milling applications requires a balanced approach that takes many considerations into account. Chief among them are workpiece material, part configuration, types and sizes of cuts required, machine rigidity, torque availability, toolholder balance, and safety.
High speed milling in itself is but a means to an end. Tim Marshall, product manager, and T.J. Long, standard milling product engineer, at Kennametal Inc. (Latrobe, Pennsylvania) prefer the perspective of maximum metal removal. "Machines can offer up to 30,000 rpm and higher in spindle speed, but, to take advantage of today's high speed milling tools, the machine tool's horsepower/torque curve needs to be considered," Mr. Marshall explains. "Combining the machine tool's peak operating range with current technology of high speed milling tools allows you to deliver maximum metal removal rate. Maximum metal removal is where the money is. Anyone considering high speed milling needs to understand this."
Such a statement holds equally true for high-volume, high-production operations or in producing small- or single-lot sizes. In high-volume operations, saving seconds can translate into sizeable cost reductions. In small-lot or one-off production, such as in the die/mold businesses, improved surface finishes and true 90-degree cuts can obviate the need for subsequent grinding or polishing operations, enhancing quick turnaround or just-in-time delivery.
One of the first considerations a user needs to take into account is the surface speed range for the tools in the selected workpiece material. "Hitting 20,000 rpm on a 5-inch cutter is vastly different than on a 1-inch cutter," Mr. Marshall points out. The tooling suppliers' speed recommendations must be followed to assure proper tool life (sfm = cutter diameter x rpm x 0.262), adds Mr. Long.
Balance can also make a difference in high speed milling performance and results. "As a rule of thumb, it's good to be aware of balance considerations anywhere above 8,000 rpm," Mr. Long explains. That's because balanced toolholders make a demonstrable difference in improving surface finish and in the wear characteristics that affect tool life. Some tools, such as slotting cutters, are not balanceable by design. Others, such as end mills with integral shanks, are. For example, Kennametal's Mill1 Max, its newest tooling for high speed, high-feed aluminum milling, are pre-balanced to a specification of G2.5 at 10,000 rpm in integral shank (monoblock) tools, and they are balanceable to higher specifications, if required.
"With higher balancing specifications, every time an insert is changed, the tool may need to be rebalanced," Mr. Long advises. "If shops don't have access to balancing equipment of their own, a supplier like Kennametal can do it. For the highest security at high spindle speeds, users should replace insert screws every time they index an insert and make sure they're tightened to the correct torque specification."
Safety is also a paramount consideration, particularly with high speed machining. "Anything coming loose at even 8,000 rpm is a projectile," Mr. Long says. "Essentially, any windows, doors or shrouds on a machine tool need to be bulletproof."
Selecting tooling for optimizing high speed milling applications equates to a decision to maximize a company's milling productivity and reduce manufacturing costs. Speeds (sfm) and feeds make a big difference, as does achieving maximum metal removal (MMR). Additionally, as with many production decisions, there is a guiding formula, according to Mr. Long: MMR equals the number of inserts multiplied by axial depth of cut times radial width of cut times chip load times rpm.
"Operating the machine at the rpm where you can achieve the highest MRR provides users the greatest benefits," says Mr. Marshall. "Calculate the results and use a machine tool's highest horsepower/torque rating, and you'll be on your way to achieving the higher feed rates and maximum efficiencies tooling such as Mill1 Max offers."