With the increasing requirements for finishing, a new cubic boron nitride formula can provide a stronger alternative to cemented carbide. #Basic

Although cemented carbide inserts have proven performance in almost all types of CNC machining, aerospace alloy finishing provides a particularly good opportunity to start exploring alternatives. In a recent test, a new type of cubic boron nitride (CBN) precision turning inserts have been three times faster, their service life has been three times longer, and the carbide materials in the titanium alloy 6AL-4V have been removed by nine times. Times, all of them are on the same cutting edge. The researchers wrote: "Consistent chip control and long-lasting cutting edges make it an ideal alternative to the current stable finishing processes that use cemented carbide."

The main difference between this CBN and other blades is not what it contains, but what it lacks: the binder that holds the sintered material together. Instead, the nanoparticles directly fuse with each other to form an almost solid, continuous cutting surface. This structure can take full advantage of the extremely high hardness and thermal conductivity of CBN in a material known for work hardening.

Turning this titanium tube provides a consistent baseline for blade comparison. Image source: OMIC

The higher temperature, higher pressure sintering process that makes the binder-free structure possible is not only of great significance to the machining performance. Cobalt is a key component in the "cement" that combines cemented carbide tungsten carbide. It is becoming increasingly rare and expensive. The same is true for tungsten, niobium and other elements of these blades. There is no need to use these materials in cutting tools, and there is no need to pull them out of the earth, thereby protecting precious resources and benefiting the environment and people directly affected by mining.

The extent of these broader benefits depends on the extent to which performance advantages and economies of scale promote the expansion of binderless CBN (and possibly other types of binderless blades) to new applications and materials. At the same time, the new NCB100 binder-free CBN product provides a ready-made alternative to jobs that would otherwise require redundant tools and/or consider blade replacement, wear offset entry, and tooling during long automated machining cycles. Risk of damage.

Compared with cemented carbide, binder-free CBN runs 3 times faster, removes 9 times material, and has 3 times longer service life on a single cutting edge.

At the request of the blade developer Sumitomo Electric Carbide, Inc., a precision car test was conducted at the Oregon Manufacturing Innovation Center (OMIC), a non-profit cooperative research organization near Portland. OMIC processed three NCB100 binderless CBN turning inserts with AC51015S carbide grade, otherwise Sumitomo would recommend it for this application. As expected, the cemented carbide performed well in repeated tests, showing 0.00208 inches of tip wear after 45 minutes of cutting. "No abnormalities were noticed in these tests," OMIC wrote in a report on the tests. "Over time, the blade shows uniform and predictable wear, creating a good baseline for comparison."

Two CBN inserts (one with a low rake angle and one with a high rake angle) experienced a similar degree of tip wear after 45 minutes (both CBN geometries were 0.0024 inches). At this time, the carbide inserts need to be indexed to maintain chip control. However, both CBN inserts continued to produce short, tightly curled chips. The third adhesive-free CBN blade has a medium rake angle and better performance, with only half the wear (0.0012 inches) at 45 minutes.

In 400 sfm (twice the speed of cemented carbide) and 230 minutes of cutting, the binder-free CBN inserts still performed stable. Carbide blades need to be indexed at the 45-minute mark. Image source: OMIC.

The biggest difference is speed. In repeated tests, all three CBN geometries were run at 400 sfm, while the cemented carbide was only 200 sfm. Nevertheless, OMIC has not yet been completed. The researchers pushed the highest performing CBN blade (with a medium rake angle) to over 45 minutes, and they extended the periodic wear measurement from 15 minutes to 30 minutes. After 230 minutes, the chip is almost indistinguishable from the chip produced at 45 minutes. In contrast, the carbide blade has worn twice as much, only half of the material is removed, and a rougher surface is left.

The test continues. Pushed to 600 sfm—three times the speed of cemented carbide—binderless CBN lasted more than 400 minutes, and then exhibited the same level of flank wear as a cemented carbide blade at 45 minutes (despite the use of CBN, The surface roughness is marked at 400-minutes). At 435 minutes, the chips began to grow longer and thinner. Finally, at 555 minutes, the tip of the blade was worn into a crescent shape, “like sanding a piece of wood on a belt sander,” said Cody Apple, a researcher at OMIC's processing solutions. "It's predictable, and it's easy to detect when it happens."  

Before chip control began to decrease significantly, the binder-free CBN inserts ran for more than 7 hours. Image source: OMIC

Urmaze Naterwalla, head of OMIC R&D, compares the traditional "bonding" blade to a road, where the individual particles embedded in the surface represent the cutting material (CBN in this case), and the tar represents the bonding material. The bonding material is soft, so as the surface deteriorates, it decomposes first. It tore into big pieces and left potholes. In the absence of tar, the particles will directly fuse with each other. There are no potholes because the surface wears at a relatively constant and predictable rate, resulting in two parallel pits being carved from the wheels of passing vehicles.

The binder in most CBN blades can act as a shock absorber, but only at the expense of some of the natural advantages of CBN. Photo source: Sumitomo Electric Carbide Corporation

However, adhesives can do more than just hold the insert material together. At the time of writing this article, Jason Miller, a national application engineer at Sumitomo Corporation, said that it also acts like a car suspension, using springs and other shock absorbers to drive smoothly. In fact, specially formulated adhesives can help other CBN inserts to withstand the forces associated with gear machining and other intermittent cutting applications that are difficult for the tool. He said that applying the new binderless plug-in for this type of work would be "similar to driving over a mountain with a trailer." "This won't work."

Nevertheless, the tractor is ideal for speeding on perfectly flat roads. For binderless CBN, the continuous surface of titanium aerospace parts (such as parts machined at OMIC) is essentially a runway. Interruptions are rare, the depth of cut is very shallow, and the recommended titanium alloy is only 0.020 inches.

The high-pressure, high-temperature sintering process can fuse sub-micron CBN grains together without a binder to maximize the hardness, thermal conductivity, and advantages of titanium and cobalt-chromium alloys. Photo source: Sumitomo Electric Carbide Corporation

This is not to say that titanium turning is the only potential application for binder-free CBN, nor is it that milling or other interrupted cutting is impossible. In contrast, Sumitomo reports that the insert has been successfully used for intermittent cutting of powder metal and cast iron, as well as hardened steel milling under certain conditions. At the same time, the development of different edge geometries and CBN particle size formulations continues to increase strength and toughness. These tools are also used to turn medical parts with materials such as cemented carbide and cobalt chromium alloy (Co Cr).

Research on other binder-free formulations is also continuing. In fact, Sumitomo first applied its new direct conversion sintering process to binderless polycrystalline diamond (PCD), which can be used to process tungsten carbide wire drawing dies, wear plates, and ceramic materials. The company expects that with the demand for new, highly durable but difficult-to-process materials such as spacecraft, aircraft, automobiles, medical and electronic components, the application of these tools and possibly other tools will expand. "We should realize that there will be an evolution, just like carbide," Mr. Naterwalla said. "The more we adapt to its abilities, the more evolution will move forward."

At the same time, advances in additive manufacturing may increase the focus on finishing and semi-finishing, rather than roughing. Compared with engraving solid blanks or blocks of material, processing 3D printed near-net shapes requires different techniques and different cutting tools. "This means turning requires less radial engagement but faster moving tools," Mr. Apple said.  

The two parallel horizontal lines represent the surface finish (gray) and blade wear (blue) of the carbide insert after 45 minutes of machining at 200 sfm. The blue vertical bars indicate the progress of wear of the binder-free CBN blades over time at 400 SFM (light blue) and 600 SFM (dark blue). The second pair of horizontal gray bars shows the deterioration of the surface finish over time when the binder-free CBN is used at 400 SFM (light gray) and 600 SFM (dark gray). Image source: OMIC

Simple process considerations can increase your productivity in milling titanium alloys.

Reducing the use of cutting fluid provides an opportunity to save a lot of costs. Tool life can even be increased.

When conventional drilling cannot complete the job, consider these alternatives.