Optimizing Tool Life: The Effect of Lead Angles on Turning Operations

Tired of burning through diamond-shaped cutting tools for lead angle work? We explore the problems created by lead angles—and we talk to cutting-tool manufacturers on the best ways to optimize for your needs.

For general-purpose turning work, CNMG-style, 80-degree diamonds have long been the insert of choice. They’re widely available, reasonably priced and provide good tool life in a wide variety of materials. They also allow the machinist to face and turn the workpiece with the same tool, simplifying setups and reducing time lost to tool changes. Because of this double duty, 80-degree diamonds free up at least one tool station, an important consideration with crowded lathe turrets.

The Status Quo: Negative Lead Angle Affects Cutting Tool Life

The challenge comes when workpiece materials fall into the “not much fun to cut” category. Nickel or cobalt-based heat-resistant superalloys such as Inconel 718 and cobalt-chromium (CoCr), Ti-6Al-4V titanium, most tool steels, and 300-series or PH stainless steels can create a host of problems for machinists, including built-up edge (BUE), excessive flank wear and especially depth-of-cut notching.

Unfortunately, these problems are somewhat self-inflicted. By using an 80-degree diamond insert, the lathe machinist is cutting with a negative 5-degree lead angle tool, which is bad for tool life—assuming the usual “face and turn” style tool holder is used. Whether milling or turning, switching to a positive lead angle not only thins the chip, but also spreads cutting forces across a greater length of carbide, which reduces or, in some cases, eliminates the failure modes. 

“Tools with negative lead angles are the most commonly applied of all turning tools,” says Aaron Schade, program manager for the Knowledge Center Americas at Kennametal. “That doesn’t mean they’re the best, only that that’s what everybody’s using. Because you have clearance in both directions, a negative lead angle makes the tool more versatile—you can face the part and then turn up to a shoulder, whereas a tool with a positive lead angle means you’ll need to come back with a second tool for these operations.”

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Round and Round: The Value of Round Cutting Tool Inserts

Although the benefits are difficult to quantify, switching to a tool with a positive lead angle can offer significant improvements in tool life. And when tool life is better, productivity goes up, either because there’s less downtime or because feed rates can be significantly increased.

“We recommend round inserts wherever possible, primarily because they’re the most cost-effective solution for customers who want to remove large quantities of material in the shortest time possible or where tool life is problematic,” says Aaron-Michael Eller, product manager for ISO turning and advanced materials at Seco Tools. “On a straight turn using a light depth of cut, for example, you can easily get a dozen or more indexes per insert and feed it like crazy.”

This is why Kennametal, Seco Tools, Sandvik Coromant and all turning tool manufacturers offer tool holders that accept round as well as various rhomboid-shaped inserts with 15-degree, 45-degree and 75-degree lead angles. All offer the opportunity to dramatically improve a shop’s turning operations.