Automotive Work
Machining compacted graphite iron
September 1, 2010
The unique challenges of machining compacted graphite iron (CGI) include machining at half the normal cutting speed and feed rate.
Over the past few years the number of compacted graphite iron (CGI) engine blocks in the marketplace has risen. It also has been used frequently in the production of disc brakes for high-speed trains. CGI is becoming popular due to its strength-to-weight ratio as well as its resilience to mechanical and chemical fatigue.
Described as a “gummy” material, CGI is also abrasive, and this combination of properties requires the use of cubic boron nitride (CBN) tooling. A common technique used to combat the unique challenges of machining CGI is running the machine at half the normal cutting speed and at half the normal feed rate.
History of CGI
The history of CGI has been a long one, if not a well-known one. In 1949 ductile iron was patented and so was CGI. While ductile iron became a very popular material for manufacturing, CGI was never utilized to its potential. Although CGI is not quite as strong as ductile iron, it is 75 percent stronger and up to 75 percent stiffer than gray iron.
It is these properties that make CGI suitable for engine manufacturing. For example, an engine can be made 9 percent lighter with CGI, with the engine block weight alone being reduced by up to 22 percent.
CGI blocks tend to hone more like steel than gray iron, according to Sunnen Product Manager Mike Murphy.
“While I think the use of CGI in engine blocks may have recently peaked, there still is a lot of it being used in the automotive sector in North America,” said Murphy.
Traditional honing stones can be used; however, a 150-grit stone should be the choice.
“CGI actually machines pretty easily,” said Murphy. “It’s not as brittle a material as gray cast iron, and it won’t react badly when touched by the tool. Proper tooling and machining techniques still need to be used.”
Also, most machine tool manufacturers suggest that when machining CGI, all speeds and feeds should be reduced by half of those used with gray iron.
New Machining Process
MAG Industrial Automation Systems recently introduced a new, patent-pending, through-tool design that the machine tool manufacturer says cools the cutting edge more efficiently than ever before possible.
This process, which was sponsored by the U.S. Navy, uses a through-spindle, through-tool cooling system designed to cool the cutting edge to enable higher cutting speeds for increased metal removal, longer tool life, or both.
The liquid-nitrogen (-321-degree-F) cooling system can also be combined with minimum quantity lubrication (MQL) to reduce tool friction and adhesion, enabling even higher metal removal rates or longer tool life. Ideal applications involve aggressive metal removal in very hard workpieces, such those made from titanium, nickel-based alloys, and nodular or CGI.
“We are still in development, but have achieved 60 percent speed increases in milling CGI with carbide, and up to four times using PCD [polycrystalline diamond] tooling. With the addition of MQL, we tripled speeds with carbide, but showed no further benefit to the fourfold increase with PCD,” said MAG Vice President of Engineering Doug Watts. “These tests focused on metal removal increases, while keeping tool life equal to what would be achieved with conventional coolants.”
According to Watts, early results indicate this technology could improve the life cycle cost model for machining in a hard-metal environment by reducing the required number of machines and associated plant infrastructure, or possibly increasing tool life beyond anything thought possible today. When it comes to costs, cryogenic machining can become even more competitive since traditional environmental issues are eliminated.
“There is no mist collection, filtration, wet chips, contaminated workpieces, or disposal cost, and certainly less energy consumption without all the pumps, fans, and drives that go into handling coolant,” said Watts.
The key to the new system's efficiency is its ability to concentrate the cooling effect in the body of the cutting insert.
“Through-tool cooling provides the most efficient heat transfer model and consumes the least amount of liquid nitrogen. Our development work to date has focused on milling and boring, where consumption has been about 0.04 liters per minute per cutting edge. We believe drilling and tapping should be even less,” explained Watts.
He added that tests done by the company have shown the capabilities for diamond tooling can be expanded significantly with cryogenic cooling. For example, the heat limit when machining CGI can be extended by three to four times. Carbide tooling, which is more affected by abrasive wear, responds best when MQL is combined with cryo cooling.
The through-spindle cryogenic cooling system is suitable for motorized, belt-driven, or geared spindles.
For more information, visit www.sunnen.com and www.mag-ias.com.
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