Face milling of a compacted graphite iron (CGI) block at 1,250 m/min on a Lamb Technicon Jaguar horizontal machining center (a machine, incidentally, that is equipped with an exceedingly rigid traveling column: CGI machining requires high rigidity, large spindle motors, stiff tools, and even FEA of the fixtures to make sure that they can withstand the rigors). Lamb is working with SinterCast to develop high-volume machining solutions for CGI.
What you’re looking at is an electron microscope view of the coral-like shape of the graphite in compacted graphite iron.
When asked about the biggest development related to powertrain machining, Mark Tomlinson, vice president of Technology Integration, Lamb Technicon (Warren, MI), proffers an unusual answer. Lamb is a well-known supplier of machinery for powertrain. There are plenty of engine plants around the world with the Lamb placard affixed to a variety of transfer lines; the company, responding to requirements for more flexible equipment, has also built a number of programmable spindle machines that are either integrated into the more traditional transfer lines or serve as elements within flexible cells or systems.
All of which is to say that the expected answer is one that has something to do with spindles or controls or transport mechanisms or the like.
“Compacted graphite iron,” Tomlinson responds.
And he’s not talking about a material for machine bases, either. Rather, he points out that compacted graphite iron (CGI) is becoming the preferred material for use in diesel engine blocks. In fact, he estimates that by the end of 2005 it may account for as much as 80% of all the blocks made for diesels. (As you may have discerned from reading this magazine during the past several months, there are plenty diesel-related developments going on around the world, and although they are still the exception in North America for light vehicles, there are (1) still the significant market for trucks and (2) aborning opportunities for use in light vehicles if some environmental challenges can be overcome.) A reason why engine manufacturers are interested in the material is because casting processes have been developed that provide more consistent castings, which means less scrap and all of the associated problems with that. Also, it permits thin wall sections without property degradation, which means that blocks can be produced with less mass without giving up on the performance characteristics of the engine. Lamb and SinterCast (Stockholm) have entered into an agreement to support high-volume CGI machining. CGI is not a new material, but it’s just that companies are finally learning how to machine it in a cost-effective manner.
Looking at the Material.
Essentially, there are three types of iron associated with engine blocks:
- Gray iron
- Nodular iron
Each of these, because of their physical makeup at the microstructure scale, has different machining characteristics. In gray iron, the graphite is in the form of flakes; these flakes help machining because of the way that they fracture when being machined. While gray iron isn’t abrasive, nodular iron is. In nodular iron, the graphite is in spherically shaped nodules, which are surrounded by an iron carbide shell, which don’t have the same cracking behavior as the flakes in gray iron. During machining, the nodular iron, which has a higher shear strength than gray iron, doesn’t come off in small chips; the chips tend to be much more continuous. There are both more concentrated loads and heat generated by the cutting tool in cutting the nodular iron, which contribute to a reduction in tool life.
CGI has graphite that resembles coral; the form of the graphite structures, as is the case with nodular iron, is fundamentally controlled by the amount of magnesium in the mix. The microstructure of the CGI material is such that there aren’t the stress risers and fault lines associated with gray iron, nor are there the thermal requirements characteristic of machining nodular iron. So because CGI is in the middle of the easy-to-machine and the highly abrasive, one might appropriately imagine that machining should be a fairly straightforward thing.
Improving All Aspects.
Not so. Roger Cope, Lamb Technicon’s vice president of Business Development, observes, “Many people think that they can just slow their machines down and machine CGI. But it destroys inserts.” For one thing, unlike gray iron, CGI is a very low sulfur iron. What sulfur does in an iron is form, in effect, a lubricating layer (of manganese sulfide) that facilitates machining. CGI doesn’t have it. Cope cites studies that indicate that compared with machining gray iron, tool life for milling and drilling operations in CGI are half and tool life in CGI boring operations is just one-tenth. So there are lots of considerations related to setting up the machine, the tooling, and the fixturing to handle cutting.
For one thing, the machine tool power requirements for CGI are higher—on the order of from 10 to 30%. Appropriate machinery for handling CGI is equipped with large spindle motors, stiff spindles, and rigid fixturing. So it is not just a matter of lowering the speed and increasing the speed. And there is also the need to get production rates higher, so it is necessary to have optimized parameters. Tomlinson notes that no one can simply expect to take an existing block line and then running CGI castings down it. “You have to consider all aspects of the line,” he says.
Among the specific recommendations are to have larger spindle motors; higher torque spindle drives; larger spindles; increased feed unit and feed drive stiffness and load capability; stiffer tool holding; increased damping; and finite element analysis of fixtures. Roughing machines must certainly be setup to take the rigors of machining CGI, otherwise, production is likely to be down rather quickly.
All that said, however, CGI still looks promising for diesels. Not only is there a projected increase in the use of the material for blocks, but cylinder heads will also be moving to the material.