Ongoing development of tough, new workpiece materials is driving cutting tool manufacturers to create appropriate new cutter geometries, carbide grades and coating technologies. Shops serving the aerospace industry, for example, must find effective ways to machine 5553 titanium and composites. The same goes for medical shops being asked to machine PEEK polymer, stainless and other exotic materials. One machining-unfriendly material making inroads in the automotive industry is compacted graphite iron (CGI). This material is primarily used to create engine block, cylinder head and bearing cap castings typically used for large diesel trucks. The result is better fuel efficiency for over-the-road vehicles because CGI weighs half as much as conventional gray cast iron. In addition, it has twice the strength and stiffness of gray cast iron, allowing designers to minimize engine block wall thicknesses. As a result, an assembled CGI engine usually weighs about 9 percent less than one made of gray cast iron.
CGI has been used in Europe for some time and is gaining greater acceptance in the United States. It can handle peak firing pressure found in diesel engines—aluminum engine blocks with iron cylinder liners cannot. Some high-performance, V-style racing engines are also made of CGI not only because of reduced weight but also increased stiffness, especially in the valley between cylinders.
One reason why CGI is more challenging to machine is because it has two to three times the tensile strength of gray cast iron, notes Robert McAnally, industry specialist in automotive milling for Sandvik Coromant (Fair Lawn, New Jersey). The higher tensile strength translates to higher cutting forces during milling operations—approximately 15 to 25 percent more machining power is required to machine CGI versus gray cast iron. Therefore, shop equipment tuned to machine gray cast iron might not possess the power to handle CGI machining. Mr. McAnally points out there are other challenges in that:
- CGI has relatively low thermal conductivity, so heat generated during machining is pushed into the workpiece, adversely affecting tool wear. Conversely, gray cast iron possesses high thermal conductivity, which allows heat to be carried away with the chip during a machining operation.
- The casting crust on a CGI component has a ferritic structure, causing material to stick to the tool’s cutting edge. This doesn’t occur with gray cast iron because it has a pearlitic structure.
- Unlike gray cast iron, CGI does not contain sulfur. The sulfur in gray cast iron deposits on the tool’s cutting edge and acts as a lubricant that extends tool life.
- Titanium is used as an alloying element during the CGI casting process, creating a tougher casting skin. This also causes the formation of abrasive free carbides throughout the casting. The amount of alloying elements in CGI has a big impact on machineability and tool life.
Because of these factors, tools used to cut CGI generally last half as long as those cutting gray cast iron.
Milling And Boring
CGI does provide approximately 50 percent better milled surface finish (Rz) than gray cast iron, which means that either fewer machining passes may be needed or a separate finishing tool may not be necessary to deliver the requisite finish. During machining, CGI does not produce component edge breakout as the tool exits the cut. Gray cast iron can produce chipping, which could scrap the block with extreme breakout. CGI acts more like steel in that respect, producing a burr rather than a breakout.
Because of the reduced cutting speed required to machine CGI, it can take nearly three times as long as it would to cut gray cast iron using conventional processes. Sandvik has performed many tests to determine more effective ways to machine CGI. For milling operations, the tool material it has determined works best is carbide coated with thick layers of titanium carbon nitride (TiCn) and aluminum oxide (Al2O3). Mr. McAnally describes a thick coating as being 7 to 10 microns; thin coatings are typically 2 to 3 microns.
For turning and boring operations, the company recommends a carbide substrate with high abrasive wear characteristics coupled with wear-resistant thick coatings applied using medium-temperature chemical vapor deposition (CVD). It has found that boring CGI using a CBN insert offers only one-tenth the tool life of boring gray cast iron. A slightly positive geometry is appropriate (between 5 to 10 degrees), and it is recommended that CGI operations are performed sans coolant.
Sandvik worked with Makino to develop a boring process that can finish a rough-bored cylinder in one pass. The multiple-insert tool that was developed is called the Long-Edge Tool. The tool is fed in a helical path down a cylinder and is said to finish a bore in approximately the same amount of time as gray cast iron. A subsequent honing operation is all that’s required before engine assembly.
While developing this new finish-boring process, the companies determined that roughing is best attacked using a traditional, single-headed milling cutter with inserts having Si3Ni4 coating and geometry optimized for boring CGI.