One size fits all: Turning Performance

Fifteen years ago I wrote an article for CTE called “Lowering your grades.” It argued that shops could save money by reducing insert inventory. The premise was that the productivity gains realized through use of the “perfect” insert for any given material would be eaten up by the setup time needed to change that insert and make the necessary feed and speed adjustments, program tweaks, tool registration and test cuts.

That argument still carries weight with some metalworking professionals. Shops that turn a range of materials can find themselves adrift in a

sea of cutting tool choices. Many struggle with bloated tooling inventories, insert obsolescence and cutting tools that get used for a single job, then spend eternity collecting dust in a forgotten corner of the toolcrib.

Courtesy of Walter USA

The good news is that insert costs haven’t changed much since then. Due to improved manufacturing techniques, an 80° diamond that sold for $10 in the Reagan era is just a few dollars more today. And because cutting tools have improved, that insert will run longer than its predecessor from the 1980s, making it more likely you’ll hang on to it once that 10-piece Inconel job is done.

My previous article used the example of a popular toolmaker with 54 grades of carbide. Today, that company offers twice as many grades and an even larger variety of shapes, edge preparations, nose radii, chipbreakers and through-coolant options. Of course, choice is a wonderful thing, but some might say: “Stop the madness.”

Not so Fast

Chris Wills wouldn’t be one of them. The turning products specialist for toolmaker Mitsubishi Materials U.S.A. Corp., Fountain Valley, Calif., said it’s always important to optimize cutting tool and machine performance. “Whether it’s a small job shop or a large company, American manufacturers are competing in a global market. Sacrificing production to improve changeover time is not a very smart choice.”

While the expense of stocking inserts to cover every material contingency might seem prohibitive, cutting tools account, on average, for only 3 percent of the total production price of most machined parts, Wills said. While that percentage is small, it is an area that the accounting department tends to focus on, because inventory and carrying costs are easier to quantify than the nitty-gritty of the machining process.

Productivity is far more important, according to Wills. He argues that using the best insert available reduces cycle time and scrap and that the few extra minutes of setup time incurred as a result is negligible. However, operators must know which insert to use and how to apply it. Mitsubishi Materials counters the growing complexity of the manufacturing environment by offering training classes for its distributors and customers, including operators.

Courtesy of EXSYS Tool

A cutter tooled with general-purpose inserts can be used for a variety of milling operations on a mill/turn center.

“Quite often we find operators have never been educated on the different grades of carbide and how to optimize the machining process,” Wills said. “It’s important to show them the difference between PVD- and CVD-coated inserts, what chipbreaker and insert geometry to use and how to identify tool wear, among other factors.”

Mitsubishi Materials stresses the need to create reports whenever a new insert or other cutting tool is tested. Wills said shops might think they’re saving money by going with a general-purpose grade but it can cost more in lost productivity. It’s only by analyzing part output, tool life and insert cost that the best decision can be made. “We’ve seen situations where a shop buys a toolholder and gets the inserts for free,” he said. “With a test report, we’ve proven that had they purchased an optimized insert instead, they would have saved money.”

There’s more to this debate than carbide. Optimized cutting calls for high-performance toolholders as well. Toolholder providers are constantly improving their wares, and sticking with the status quo or a one-size-fits-all approach is a recipe for mediocrity or worse. That’s because, as a cutting tool’s performance increases, so must its support systems. Insert pockets carry heavier loads, clamps must securely prevent movement, and the toolholder body itself may need an upgrade.

Therefore, manufacturers that invest in the right tooling are better off in the long run. “Shops need to decide whether they’d rather be more streamlined or be stuck in the hole of constantly replacing perishable tools,” said Scott Leitch, product support manager for EXSYS Tool Inc., San Antonio, Fla. “By choosing the right insert and the toolholder to go with it, shops will produce more parts in less time and, ultimately, save money on their production costs.”

EXSYS markets quick-change toolholders. For shops concerned about the additional setup time incurred when swapping a turret full of steel-grade inserts for ones geared towards cast iron, quick-change is the answer, Leitch said. With a properly implemented quick-change strategy, one that utilizes tool life management and offline presetting, tool touch-off is avoided. This eliminates much of the argument against material-specific insert optimization.

“When somebody invests in quick-change systems, they should go into it with the understanding that there will be tooling in the crib that won’t be used on a regular basis,” he said. “The upside is always having that one particular tool ready to do a specific job and cut parts without downtime or the loss of performance seen when using general-purposegrade carbide.”

General Purpose Gets a Boost

Kurt Ludeking, turning product manager for toolmaker Walter USA LLC, Waukesha, Wis., said there’s a line that shops draw between using material-specific and general-purpose inserts, and grade advancements have pushed the line in favor of the general-purpose ones.

“A lot of machine shops see everything,” he said. “One minute they’re cutting low-carbon steel, next it’s 17-4, after that they’re cutting Inconel 625 or some other material they’ve never even heard of. They can’t waste a lot of time figuring out how to run this stuff, especially with small lot sizes.”

According to Ludeking, the answer is often a general-purpose carbide grade. This is particularly true for stainless steel and high-temperature alloys, both of which can be effectively cut with a CVD-coated carbide grade that straddles the fence between hardness and toughness. Someone who spends his days cutting Inconel and 316 stainless might say CVD-coated tools simply aren’t sharp or lubricious enough to cut these sticky materials. Ludeking would have agreed with that person 10 years ago—but not today.

Courtesy of Walter USA

A general-purpose CVD-coated insert used in a boring application offers balance between toughness and wear resistance.

“What’s changed is on the CVD side,” he said. “We’re able to tune the structure of the aluminum-oxide crystals so that we can achieve better heat and wear resistance than in previous generations of CVD. We’ve also developed better post-coating treatments.” One example is a blasting process that creates compressive stresses in the coating surface to improve edge toughness and hold a sharper edge. “This is something we really couldn’t do until the last few years.”