Published 3/16/2009

Insert Grade Reduces Tool Wear In Demanding Cut

When shops experience insert wear, they often turn to their trusted cutting tool suppliers for a solution. In this case, however, Seco Tools not only supplied one customer with a new grade, but also conducted an extensive investigation into the shop’s processes to help it use that grade as effectively as possible.

Cutting Tools

The flanges are machined on this Doosan 240 CNC lathe at 300 sfpm and 0.01 ipr.

Due to the rectangular shape of the flanges shown here, the shop had to take severely interrupted cuts during turning operations. Seco’s Duratomic TM4000 grade has helped the manufacturer resist the resulting wear and edge breakdown.

Seco supplied Main Manufacturing with its TM4000-grade inserts equipped with the MF4 chipbreaker, which is designed to provide consistent chip formation and breakage.

Providing the best possible product is important to stay ahead in today’s competitive marketplace, but many suppliers also view the ability to enhance clients’ profitability through value-added customer service as an essential component for success. While Seco Tools is no exception to his philosophy, the cutting tool manufacturer says it recently went above and beyond the norm to address insert failure experienced by one customer on a troublesome stainless steel application. In the end, the supplier’s detective work enabled that customer to reduce costs and machining time on the application by 85 percent and 52 percent, respectively.

The customer, Main Manufacturing, is a family-owned business that started in 1959 as a manufacturer of hydraulic presses. During the 1970s, the company began to transition away from building presses, choosing instead to focus on hydraulic flanges and associated components. In 1999, the company moved to a new, 48,000-square-foot facility in Grand Blanc, Michigan, where it currently houses 200 machines. It produces more than 15,000 parts, half of which are specials or non-cataloged requests. Typical part runs range from 5 to 500 pieces.

Flanges are a critical part of any hydraulic system. The company’s flanges are used for a variety of equipment built to withstand pressures ranging from 3,000 to 8,000 psi, including oil rigs, stamping presses, wave pools, lubricating systems and theme park effects. One primary goal is to turn around flange orders within 3 days (including 1 day for processing). To achieve this, operators set up machines in less than 15 minutes and run eight to 10 production jobs per day.

Main has been using Seco cutting tools for more than 10 years, mostly for drilling and turning applications. This particular job involves a high-pressure flange machined from 304 stainless steel on a Doosan 240 CNC lathe. The flange begins as a 2.25-inch by 1.5-inch rectangular plate, which is rough-faced down to a hub in the center of the part at 300 sfpm and 0.01 ipr. The company originally machined the flanges with three 0.1-inch passes and a final pass at a 0.05-inch cutting depth. The interrupted cut caused significant wear and edge chipping on the cutter, limiting tool life to approximately 1.5 parts per insert.

Seco was originally called in to supply tools and expertise for milling a ring groove into the flange. However, the cutting tool manufacturer was in search of a demanding interrupted cut application to test out its new Duratomic turning grades for stainless steel, particularly the TM4000 grade. Seco sales and application specialists Joel Henige and Todd Miller asked Main if they could give the new grade a try.

"Cutting a square in a turning application is a real challenge for inserts," Mr. Henige says. "An interrupted cut can cause failure in even the toughest multi-layer coating, but we felt confident that the Duratomic coating could improve Main’s performance."

Duratomic coatings improve toughness and wear resistance by altering the crystal structure at the atomic level, the company says. Through a combination of the coating and a graded substrate, the TM4000 grade is designed to maximize toughness in the cutting zone without compromising the strength of the base material. According to the manufacturer, lab tests demonstrate that this construction substantially improves resistance to cratering and edge breakdown.

The initial decision was to run the TM4000 at the same parameters as the tool previously used for the flange application. To everyone’s surprise, however, it performed no better than the other insert. Believing that the grade was capable of higher machining parameters, the applications specialists increased the cutting speed to 650 sfpm and incorporated the MF4 chipbreaker.

The MF4 is designed to provide consistent chip formation and breakage in stainless steel turning and other ISO materials. According to Seco, its main advantage is a positive cutting angle that significantly reduces cutting forces to allow higher speeds and increased productivity. With the MF4 and new cutting parameters, the team tripled tool life.

However, this was far from the results the team had anticipated. Mr. Miller and Mr. Henige examined the inserts under a magnifying loop, but couldn’t find any problems. At this point, the two decided to take a hard look at the tool under a microscope that provides 60 times the optical power of the loop. "I have to say that we seldom see this kind of investigative service from our suppliers," says Terry Bettinger, Main Manufacturing’s plant supervisor.

The microscope revealed edge chipping that had eluded Mr. Miller and Mr. Henige with the magnifying loop. They discovered that the chip flow was eroding the back of the land on the MF4. By examining the wear pattern, the two also noted that a larger nose radius would help increase edge strength. They recommended increasing the feed rate to 0.016 ipr using a 0.0468-inch radius. Cutting speed remained at 650 sfpm, as the team hadn’t discovered any other heat-related problems.

"We were pleased," says Main Manufacturing machinist Craig Bendle. "Productivity doubled relative to our original setup, and tool life jumped to seven parts per corner—seven times better than we were getting before."

But Mr. Henige and Mr. Miller weren’t yet done with their analysis. Another pass under the microscope revealed that insert failure was now being caused by thermal cracking. Large differences in temperature between the cutting edge and the insert can cause cracks that run perpendicular to the cutting edge, Mr. Henige explains. These temperature fluctuations are common in interrupted cutting applications, which tend to generate high heat during the cut.

To further improve the process, the team decided to run the application dry because coolant, which was water in this case, can exacerbate thermal cracking. As a result, tool life doubled again to 15 points per corner, and any worn edges were caused only by normal abrasive flank wear. With the Seco teams’ recommendations, Main went from removing 3.6 cubic inches per minute to 12.483. With in-cut time reduced by half, part output increased from 17 pieces per hour to 54 pieces per hour.

"We run this job quite frequently and total about 3,000 flanges per year," says Bob Mackey, general manager at Main Manufacturing. "But what we truly find exciting is that we can take what we’ve learned from this experience and apply TM4000 to hundreds and maybe even thousands of similar jobs that we run."

Understanding Insert Failure

Given today’s emphasis on cost control, company’s are continuously challenged to increase the throughput of parts that are often difficult to machine. One part of achieving this goal is reaching the highest percentage of predictable tool usage. This enables manufacturers to schedule their insert indexing at optimal intervals, maintain part dimensional accuracies, and reduce wear and tear to their machines.

Understanding the factors contributing to insert wear can go a long way in obtaining optimal cutting tool performance. When manufacturers understand the aspects of insert failure modes—the causes, what to look for and when to expect it—they can take corrective actions to reduce overall insert wear.

Failure modes can be divided into eight distinct, yet often overlapping areas: normal flank wear, cratering, built up edge (BUE), chipping, thermal mechanical failure, edge deformation, notching and mechanical fracture. These common failure modes are outlined below, along with possible remedies suggested by Seco Tools.

Flank Wear

All inserts wear out eventually, and this “normal” flank wear is the most desirable failure mode because it is predictable. Normal flank wear can be expected in all materials, and an insert will fail due to normal wear if it doesn’t fail from something else first.

However, rapid flank wear, although it looks the same, happens much more quickly. This can occur hen hard inclusions of carbide or work-hardened material in the workpiece cut into the insert and/or when small pieces of coating break off and cut into the insert. The cobalt eventually wears out of the matrix, and when the carbide grains no longer have sufficient adhesion, they break off. Signs of rapid flank wear include relatively uniform abrasion along the cutting edge; metal from the workpiece smearing over the cutting edge; and when the insert’s undercoating shows through the top coating.

Corrective Actions:

reduce cutting speeds
Use a more wear resistant, harder or coated carbide grade

Cratering

Cratering is caused by a combination of diffusion, decomposition and abrasive wear. The heat from workpiece chips decomposes the tungsten carbide grains in the substrate, and carbon leeches into the chips (diffusion). This wears a crater on the top of the insert that can eventually grow large enough to cause the flank to chip or rapidly wear. Cratering often occurs when machining iron or titanium-based alloys. Even with no visible craters, improved chipbreaking may indicate the presence of this failure mode.

Corrective actions:

Use a coated grade (preferably a coating such as TiAlN, which has thick layers of aluminum oxide).
Select a freer cutting geometry to reduce heat.
Reduce speeds and feeds.

Built-Up Edge (BUE)

Built-up edge, or BUE, occurs when the workpiece material is pressure-welded to the cutting edge. This is often a result of chemical affinity, high pressure and high temperatures in the cutting zone. Eventually, the built-up edge breaks off and takes pieces of the insert with it.

This failure mode is commonly experienced with gummy materials, low speeds, threading/drilling operations, high-temperature alloys, stainless steel and nonferrous materials. Signs of BUE include shiny material on the top or flank of the insert edge and/or erratic changes in part size or finish.

Corrective Actions:

Use an insert coating, especially nitride coatings.
Increase speeds and feeds.
Select inserts with sharper cutting edge geometries and/or smoother surfaces.
Increase coolant concentration.

Chipping

Chipping emanates from mechanical instability and is often the result of vibration in the workpiece or spindle. Hard inclusions in the surface of the material being cut and interrupted cuts can also result in local stress concentrations that contribute to chipping. Chipping commonly occurs with powdered metal materials and/or non-rigid setups resulting from bad bearings, worn spindles or other issues.

Corrective Actions:

Ensure that setups are rigid.
Minimize deflection.
Select tougher insert grades and/or stronger cutting edge geometries.
Reduce feed rates.
Use honed inserts.
Implement the same strategies used to avoid built-up edge (above).

Thermal Mechanical Failure

A combination of thermal cycling (when the insert temperature changes rapidly), thermal load (temperature differences between warm and cold zones) and mechanical shock can cause thermal mechanical failure. Stress cracks form along the insert edge, eventually causing sections of carbide to pull out and appear to chip.

The presence of multiple cracks running perpendicular to the cutting edge is telltale sign of this failure mode. It is most often experienced during milling and interrupted-cut turning, operations with intermittent coolant flow or during facing operations on large numbers of parts.

Corrective Actions:

Select tougher insert grades.
Reduce speeds and feeds.
Use a freer cutting geometry to reduce heat.

Edge Deformation

Two factors can contribute to edge deformation. The first is thermal overloading, which occurs when excessive heat causes the carbide binder (cobalt) to soften. The second is mechanical overloading, which occurs when the pressure of the insert against the workpiece causes the insert to deform or sag at the tip. Edge deformation is common with high-heat operations, high speeds and feeds, and the use of hard steels, work-hardened surfaces and high-temperature alloys.

Corrective Actions:

Reduce feeds and speeds.
Use an insert with a larger nose radius.
Select a coated carbide grade or a harder, more wear-resistant grade with a lower cobalt content.
Use a freer cutting geometry.

Notching

Notching is caused by differences in hardness or abrasiveness within the workpiece. It commonly occurs with forged or cast workpieces in which the surface is harder or more abrasive than material deeper in the cut. Local stress concentration can also cause notching. As a result of compressive stress along the cutting edge and lack of stress behind the cutting edge, inserts can be particularly stressed at the depth-of-cut line. Impact of any sort can create a notch. Notching is common in materials with surface scale or oxidation, work-hardened materials and cast or irregular surfaces.

Corrective Actions:

Reduce feeds.
Vary cutting depth on multiple passes.
Select a tool with a larger lead angle, tougher insert grades or a chipbreaker designed for high feed rates.
Increase cutting speed if machining a high-temperature alloy (though this can increase flank wear).
Increase the hone in the depth-of-cut area.
Prevent built-up edge.

Mechanical Fracture

Excessive wear of any type can cause mechanical fracture and overload. When the mechanical load is great enough, the insert breaks during the first moments of a cut.

Corrective Actions:

Correct for all failure mechanisms besides normal flank wear.
Reduce feeds and/or cutting depths.
Verify setup rigidity.
Use thicker inserts.
Select inserts with tougher grades, more secure cutting edges and/or chipbreaker geometries for high feed rates.