Roughing First: New Strategies for Blisk Machining

John Giraldo led the development of the CoroMill M5C331, a disc-style cutter shaped to follow the 3D profile between blades. Rather than roughing in steps or plunges, the tool engages continuously, removing maximum material in a single multiaxis motion. The tool can be modified by diameter, width, number of teeth, radius of arc and pitch. Photo Credits: All images provided by Sandvik Coromant

Blisk machining is a testing ground for process control. While it’s often the finishing operations that receive the most attention, it may be the roughing operations of integrally bladed rotors that matter most. Why? Because this is where the biggest gains or failures are to be found, especially in materials like titanium or nickel-based superalloys.

I recently sat for an in-depth conversation with John Giraldo, aerospace engineering manager for Sandvik Coromant in the Americas, where he serves as one of the foremost technical experts on blisk machining and spent more than a decade as a specialist in aerospace and machining projects. Aerospace shops understand the need to leave precise stock for semi-finishing and finishing operations, Giraldo says, yet they often rely on roughing strategies that add time, consume tools unnecessarily or introduce risk.

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“If the roughing isn’t optimized, you can be wasting time, burning through end mills or even creating distortion that sabotages the rest of the process,” he says.

During our conversation, as well as in a recent webinar for Modern Machine Shop, Giraldo detailed four roughing strategies tested on both Inconel and titanium blisks: full slotting, high-feed side milling, plunge milling and curve slotting. Each method carries different trade-offs in terms of cycle time, tool life, chip evacuation and process security. The goal isn’t to declare a single winner, but to help shops make smarter, more aggressive choices without sacrificing control.

Four Approaches to Roughing

This image shows the CoroMill Plura Gannet solid carbide end mill stepover application for plunge milling, and the forward and side stepover limits when applying the tool.

Roughing is about bulk material removal, but the strategy used for blisk production makes a dramatic difference. “There’s no single best method,” Giraldo says. “The blade geometry, machine dynamics, CAM software, and even how many blend points you’re allowed to use all factor in.” The four approaches we’ll examine here are full slotting, high-feed side milling, plunge milling and curve slotting.

Full slotting is best used when geometry or CAM limitations prevent more advanced strategies. The method uses large radial engagement, medium depth and lower feed rates to carve material between blades. The upside of full slotting is simplicity (due to straightforward tool paths that require minimal programming) but the downside is high thermal load, risk of tool breakage and, often, shorter tool life. Giraldo points to a Sandvik test on Inconel 718 where a solid end mill wore out after just 28 passes at 10-mm depth, versus 39 passes at 7.5 mm. “Full slotting is the least secure method,” he says.

When paired with CAM strategies like adaptive or dynamic toolpaths, high-feed side milling uses a small radial step-over and full flute engagement at high feed rates to improve efficiency and reduce cutting forces. 

High-feed side milling, when paired with CAM strategies like adaptive or dynamic tool paths, takes the opposite approach. This method uses a small radial step-over and full flute engagement at high feed rates to improve efficiency and reduce cutting forces.

Since the strategy reduces chip load spikes, spreads heat across the tool and promotes better chip evacuation, Giraldo says that tool life can be extended significantly. One trial he ran showed more than 225 minutes longer life in titanium while still removing material faster than slotting. The trial also showed that high-feed milling reduced Inconel cycle time by two-thirds in a head-to-head comparison.

In plunge milling, the cutter engages axially in a multi-axis pattern, stepping down through the material and creating a scalloped floor and walls. These scalloped sections require semi-finishing tools to smooth over, but plunge milling is more stable than slotting when reach and rigidity are factors.

Plunge milling is a strategy best described by its name: roughing out material via straight vertical plunges. In blisk machining it is especially useful when access is limited and tool overhangs are long, such as at the lower hub region of a blisk. Using a tool like Sandvik’s Gannet plunger, the cutter engages axially in a multi-axis pattern, stepping down through the material and creating a scalloped floor and walls as shown here. While these scalloped sections require semi-finishing tools to smooth over, plunge milling is more stable than slotting when reach and rigidity are factors. Plunge cutters are available in solid carbide and exchangeable-tip varieties that enable tailored overhangs for each level of the blade.

Curve slotting is the newest technique of the four. The technique uses a disc-style cutter that is shaped to follow the 3D profile between blades, and rather than roughing in steps or plunges, the tool engages continuously and removes maximum material in a single multiaxis motion. Giraldo led the development of the company’s M5C331 cutter specifically for this purpose, using tests that involved machining 17 Inconel slots — each in 13 minutes — using just one edge of each insert. With further optimization provided by Vericut Force software, he says that time was cut in half.

Choosing the Right Method

When choosing a roughing strategy, think back to the geometric factors that will influence your decision: blade pitch, height, curvature and allowable blend points. For example, “If the part has a tight curvature between blades, a disc-style cutter might not fit,” Giraldo explains. “But if there’s room, curve slotting becomes an extremely efficient and cost-effective solution.”

Other factors to consider include:

Machine capability: High-feed milling works well on lighter-duty five-axis machines. Curve slotting, on the other hand, may require more rigid equipment with sufficient torque and clearance.

Tool reach and assembly: Long overhangs that are necessary for deep cuts can introduce vibration. To combat this, Giraldo recommends modular assemblies with specially tuned mass dampers, exchangeable heads or shrink-fit holders depending on the level being cut.

Fixturing: Roughing strategies that require side access like plunge or curve slotting may require custom workholding to avoid interference between spindle housing and blade walls.

CAM software: High-feed milling and curve slotting require tool paths that account for air cuts, tilt transitions and chip-load variation. Giraldo’s tests for Sandvik used Vericut to optimize feed rates based on force and chip thickness predictions, extending tool life and reducing air time.

“Most CAM systems output a constant feed rate, regardless of where the cutter is in the material,” Giraldo says. “But that doesn’t reflect reality. The chip load changes as the tool enters and exits, especially with a disc cutter. Vericut lets us optimize around that, speeding up in air, slowing down during engagement and keeping chip thickness consistent.”

Where to Begin

Overhauling your blisk machining process takes time, but Giraldo says the place to start is clear: “Roughing is where you have the most flexibility and the fewest regulatory constraints. You’re not inspecting an airfoil at this stage. So that’s where the opportunity lies.”

For shops still using slotting as the default, the first step might be revisiting CAM strategies and tooling choices. From there, newer methods like curve slotting can be evaluated based on part geometry, machine capability and economic payoff.

“Even a five-year-old tool path might be holding you back,” Giraldo says. “Today’s roughing strategies are faster, more secure and more controllable if you know how to apply them.”