Where diamond shines

Where diamond shines

Diamond tooling is a good fit for many nonferrous and abrasive applications.

Diamond tooling is a good fit for many nonferrous and abrasive applications.

Polycrystalline diamond tooling in a bimetal application for the automotive industry. The milling operation involves an aluminum engine block with cast iron liners.

CVD diamond-tipped cutting tools provide good tolerances and finishes.


When it comes to tooling materials, nothing is harder than diamond. And under the right machining conditions, it easily outlives high speed steel, tungsten carbide, ceramic, and polycrystalline cubic boron nitride in many nonferrous applications. Diamond does have drawbacks, for instance, it generally can't cut ferrous materials, but when the application involves high speeds, high volumes, or materials such as aluminum or graphite, diamond is often the most effective tool to use.

If an application calls for diamond tooling, users can choose between poly-crystalline diamond (PCD) and the relatively new chemical vapor deposited (CVD) diamond.

A proven performer
PCD offers the hardness, strength, and abrasion resistance of natural diamond without its susceptibility to fracturing. It is manufactured using man-made diamond particles that are grown together in a high pressure, high temperature process. At the same time, these particles are integrally bonded to a cemented tungsten carbide substrate for mechanical strength and impact resistance.

According to GE Superabrasives, Worthington, Ohio, PCD is well suited to high speed cutting of aluminum, particularly when good surface finishes are mandatory. It also excels in the machining of highly abrasive workpieces. Often, PCD is recommended for cutting high content silicon aluminum alloys. It is also used on brass, copper, carbide, and bronze in applications including turning, boring, profiling, grooving, milling, and holemaking.

Because of a chemical interaction between diamond and iron, PCD is not typically used to cut ferrous materials. However, it can tackle bimetal applications involving aluminum and cast iron. For example, an automotive supplier face milled an aluminum/cast iron engine block with PCD tooling, a 12-in. cartridge-style cutter with a 0.093-in. corner radius and wiper. The parts were machined at a 1,000-ft/min speed, 0.004-in. feed, and a 0.20-in. depth-of-cut. With the PCD tooling, the supplier made 5,000 engine blocks before it had to index an insert.

Tom Broskea, GE application program manager says that PCD use is driven by industries running high volumes — mainly the automotive sector, which is doing more high speed machining of aluminum. Also, in an effort to cut weight and save money, automakers are evaluating the use of metal matrix composites; materials that dictate the use of PCD. "You can't use carbide on those materials," says Broskea.

Although PCD's track record is already quite impressive, GE Superabrasives has been working to further improve its abrasion resistance, reports David Briggs, product manager for polycrystalline products.

PCD offers a number of advantages to manufacturing operations in terms of application range and productivity. Even though diamond is the hardest material known, there is also a material property, toughness, which must be considered. A factor that improves the toughness of PCD is the presence of cobalt in the microstructure along with the random orientation of the diamond particles. The tungsten carbide substrate also provides mechanical support for the diamond abrasive layer, increasing impact resistance and making it easier for braze attachment in tool fabrication.

Another benefit of PCD is the range of diamond grades available to fit any nonferrous application. Typically, fine-grain diamond is used for less abrasive applications requiring an excellent surface finish. Medium-grain diamond is considered a general-purpose machining grade. Coarse-grain diamond is used in rough machining and in extremely abrasive materials where surface finish may not be as important.

The newcomer
Chemical vapor deposited (CVD) diamond is a highly abrasion-resistant, pure diamond material that uses no binder. Instead, diamond is deposited in one of two forms: thick-film diamond, which is laid down as solid, free-standing wafers and then cut to size, and thin-film diamond, which is coated onto carbide inserts and rotary tools.

So far, CVD's most promising application has been graphite machining, but Norton Diamond Film, Northboro, Mass., is selling CVD diamond tooling for a number of nonferrous metal, plastic, and composite applications. "CVD is suited to almost all nonferrous metals," remarks Bela Nagy, manager, applied technology at Norton. "We're getting good results making interrupted cuts in high silicon aluminum alloys and cutting presintered carbides, brass, copper, and carbon fiber materials." Norton also believes that CVD can hold its own against PCD when machining a wide range of aluminum alloys, including 6061 and others.

"The main advantage of CVD over PCD is at the cutting edge," says Nagy. "The CVD edge is continuous. Even though it's polycrystalline, it is solid diamond, with no cobalt binder. Users can cut at higher speeds and get better finishes because the tool doesn't get hot," he explains. In addition, edge buildup is not a problem with CVD, he insists.

According to Norton, the thermal conductivity of CVD is 50% higher than PCD. "The CVD tool's solid diamond tip," explains Nagy, "instantly conducts heat away. With PCD, heat has to go through the cobalt-diamond matrix, so the thermal conductivity is not as good."

CVD also has a low coefficient of friction. "Material doesn't stick to it like it does with tungsten carbide or cobalt PCD," states Nagy. In addition, the low coefficient of friction lets CVD diamond tools take high chip loads, making for faster and more efficient cuts.

CVD diamond is thermally and chemically stable in applications where PCD and tungsten are limited by their metallic constituents. "You can drop CVD diamond in hydrochloric acid, for example, and nothing happens to it," says Nagy. "But if you drop in PCD, the acid eats out the cobalt binder." What this means is that CVD diamond tools can withstand materials that generate an acid by-product during machining. Examples of such materials include phenolic resin, urethane, and polycarbonate.

Although all forms of diamond (including PCD) are chemically reactive with ferrous and superalloy compositions at high temperatures, diamond is well-known for its chemical inertness with most materials. The binderless composition of CVD diamond bypasses the chemical degradation that can occur in cobalt metal sintered PCD or tungsten carbide, especially at high cutting temperatures. This behavior, coupled with high lubricity and thermal conductivity, is believed to be a key advantage for the use of CVD diamond in high speed and dry machining operations.

The final benefits CVD diamond offers are high temperature hardness and wear resistance, even at high cutting temperatures. "There is no spongy cobalt around the diamond," points out Nagy, "so the tool tip is uniformly hard."

A bright, shiny future
Both GE Superabrasives and Norton Diamond Film agree that shops aren't taking full advantage of diamond. The reason, according to both companies, is that many of today's machines just don't have the speed and torque to run the tools effectively.

Beyond mechanical issues, it is also hard to convince shops to look past the initial price of diamond — whether it be PCD or CVD — to the cost savings and performance benefits it can bring.

Says Broskea, "Depending on the machine, you can run diamond at higher rpm than conventional tooling and get increased throughputs. Also, fewer tool changes are required. If you're running 50 to 100 times tool life over carbide, you're not shutting machines down as often."

CVD keeps its cool in ceramic machining test

Presintered ceramic machining tests on a TiN-coated carbide end mill (top), an uncoated tungsten carbide end mill (middle), and a CVD diamond-coated end mill (bottom). The TiN-coated tungsten carbide and uncoated carbide tools failed within 6 in. of machining due to excessive wear and frictional heating. The diamond coated tool operated at estimated temperatures 500° to 1,000° C lower. In addition, it was still cutting after 264 in. of metal removal.


A recent ceramic machining test compared the performance of a Norton Diamond Film DF1000 CVD diamond coated end mill with that of an uncoated tungsten carbide end mill and a TiN-coated one of the same geometry ( 3 /8-in., four-flute). The test part was a SiC-containing bisque ceramic, which was presintered at temperatures greater than 1,200° C. Machining parameters were as follows: 10,000-rpm spindle speed and 20-ipm table feed.

The presintered ceramic workpiece was machined in alternating, 3-in.-long climb and conventional passes using 3 / 16-in. radial and 1 / 2-in. axial depths of cut.

The uncoated tungsten carbide and TiN-coated tungsten carbide tools failed within 6 in. of machining (two passes). The failure was highlighted by excessive abrasive wear, which was enhanced by frictional heating of the tools.

Both the TiN and uncoated carbide tooling showed a strong sensitivity to density variations in the presintered ceramic. In the case of one carbide tool, the wear and overheating reduced the 0.375-in. cutting diameter of a new tool by more than 0.100 in. in less than 9 in. of metal removal.

Optical pyrometry of the carbide and TiN-coated carbide tools indicated cutting temperatures in excess of 900° C. In contrast, a diamond tool that had already run 80 previous passes was barely warm to the touch after eight sequential passes.

PCD provides both cost savings and performance benefits

Application:
Automotive intake manifold milling

Material:
4227 aluminum (5.5 to 7% silicon)

Operation:
Finish milling flange face

Tool geometry:
6 7 /8-in.-diameter cutter 10 — 1 /2-in. square PCD inserts +5° rake, 11° relief (PCD) *Previous tool was tungsten carbide (WC) with a +20° rake, 25° relief

Machining parameters:
Speed: 2,200 sfpm Feedrate: 0.007 in./tooth Depth of cut: 0.030 in. Coolant: Yes

Tool cost:
PCD — $80 WC — $6

Tool life:
PCD — 250,000 pieces WC — 800 pieces

Other benefits:
No deburring 15% productivity increase Less machine downtime Tool cost per piece reduced 85%

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