Compacted Graphite Iron (CGI) has not only changed auto engine manufacturing beyond recognition, it has changed machining operations too.
The strength, stiffness and fatigue resistance of CGI is well documented – all of which are enhanced at far less weight when compared with traditional grey iron. The upshot is that an engine block, for example, can weigh around 20-22% less when made from CGI.
While the design engineer clearly reaps the benefits, the manufacturing engineer is left to solve the machining dilemma. CGI is speed sensitive – if the speed limit is exceeded, tool life decreases quickly and drastically. With this in mind, several machine tool builders have set about constructing machines able to accommodate the higher feed rates required to achieve suitable MRR (material removal rate) without cranking up the speed.
A machine such as the Makino a81M is a case in point, in which the spindle performance and rigidity of the horizontal machining centre is designed specifically for high power, high thrust CGI machining. The integral drive, high torque spindle offers many advantages over traditional gear-head designs. For instance, the elimination of gears and shafts allows for greater efficiency in transferring horsepower and torque from the spindle to the cutting tool. Furthermore, the low inertia spindle design is said to reduce cycle times due to fast spindle acceleration.
Since CGI features 2-3 times higher tensile strength than normal grey iron, milling processes induce higher cutting forces. Approximately 20% more spindle load will be consumed while machining CGI compared with grey iron. CGI has low thermal conductivity and hence trapped heat will be pushed into the workpiece causing rapid tool wear.
The microstructure of CGI is ferrite, causing material to stick to the edge of cutting tools, whereas in the case of grey iron the microstructure is pearlite. According to Kennametal, for efficient milling process, machinists should opt for coated carbide grades with aluminium- chromium or TiCN + AL2O3 coatings.
The company has released new generation of finish milling cutters for CGI applications. The Kennametal Dodeka milling platform offers 12 cutting edge inserts in three variants – Dodeka mini, Dodeka and Dodeka Max – based on stock removal requirements. Dodeka mini, the most recent development, offers very high feed capabilities because of its 15° approach design.
Another factor complicating the machining of CGI is that it doesn’t contain any manganese sulfide inclusions, which generally act as a lubricant in metalcutting processes on conventional engineering materials. As a result, the machineability of CGI depends heavily on the correct selection and application of metalworking fluids.
Quaker Chemical is among those to conduct a recent study in this area, focussing on CGI engine cylinder machining using coated tungsten carbide (at 250m/min) and PCBN (polycrystalline boron nitride) cutting tool inserts (at 700m/min). Two metalworking fluids were tested, one considered to be standard for conventional cast iron machining and one (Quakercool 7020-CG) engineered specifically for use in machining CGI.
Machining was performed on a Victor Fortune TNS-2 turning centre where multiple external turning passes were made on engine cylinders made from conventional grey cast iron and Grade 450 CGI, both supplied by SinterCast.
In assessing the relative performance of the two fluids, the enhanced performance offered by Quakercool 7020-CG over the conventional ferrous machining fluid was seen, says the company, resulting in a 33% increase in tool life on the CGI cylinder using the coated carbide insert. Quakercool 7020-CG also yielded an 18% increase in insert life over that obtained with use of the conventional machining fluid when using the PCBN insert, it says.
Arguably one of the biggest recent advances in this area has been delivered by MAG, which late last year made its cryogenic machining process commercially available for CGI applications.
This innovative, multi-patented process enables higher cutting speeds for increased material removal and longer tool life by transmitting liquid nitrogen at -161°C through a tool body, direct to the cutting edge. This mitigates heat generation during cutting, making it possible to increase cutting speeds and prolong tool life. In addition, the liquid nitrogen cooling system can be combined with MQL (minimum quantity lubrication) to reduce tool friction and adhesion, enabling even higher MRR or longer tool life.
As well as enhanced productivity, users are also reported to see a significant ROI from the reduction of postmachining finishing operations, as well as the elimination of water-soluble coolants and reduction in machine maintenance.
“We have achieved 60% speed increases in milling CGI with carbide, and up to four times using PCD (polycrystalline diamond) tooling,” says MAG cryogenics product manager George Georgiou. “With the addition of MQL we tripled speeds with carbide, but showed no further benefit to the four-fold increase with PCD. These tests focused on metal removal increases, while keeping tool life equal to what would be achieved with conventional coolants.
“Cost-wise, cryogenic machining becomes even more competitive when you consider it’s a non-issue environmentally,” he says. “There is no coolant mist collection, filtration, wet chips, contaminated workpieces or disposal costs, and certainly less energy consumption without all the pumps, fans and drives that go into handling coolant.”
According to Georgiou, 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 amount of liquid nitrogen. Our development work to date has focused on milling and boring, where consumption has been about 0.04 l/min per cutting edge. We believe drilling and tapping should be even less.”
Georgiou adds that tests by MAG have shown the range of capabilities for diamond tooling can be expanded significantly with cryogenic cooling, for example extending the heat limit in CGI by an impressive 3-4 times.