Traditionally, only coplanar, or parallel, connecting rods have been ground on a double-disc grinder, which simultaneously grinds both sides of flat parts using opposing wheels, according to grinding machine builder C&B Machinery. That’s because the part path through the wheels only allowed grinding of coplanar rods, with additional operations required for stepped rods, and there was no method to dress a step into the grinding wheels.
However, General Motors sought a machine that could grind a noncoplanar connecting rod in a single setup for the auxiliary 1.4-liter engine in the Chevy Volt plug-in hybrid electric vehicle. GM requested a quote from C&B for two such machines. (The crank end is 1.2mm narrower than the pin end on the turbo version of the connecting rod.)
After analyzing alternate methods of double-disc grinding and material handling, C&B developed a CNC double disc grinder to remove 1.8mm of stock from the crank end and 0.6mm from the pin end in one cycle. According to the company, the process is unique and represents the first successful attempt to grind a stepped connecting rod with this technology on a production basis.
By grinding two different widths on one connecting rod in a single pass through the machine, GM saves the cost of buying another machine to reduce the crank end –and improves part quality, noted Chris Cox, C&B vice president. “When processing connecting rods of this type on two different machines, which is a more conventional approach, it is very difficult to hold the relationship, or symmetry, of the faces from crank end to pin end,” he said, adding that parts float between the wheels in a double-disc grinder and are not clamped. “By grinding the four faces simultaneously we were able to hold much tighter tolerances.”
The first grinder shipped to GM in 2010 produced flatness and parallelism of the rod faces within 4-microns on average, repeating within 1-micron! Size range is held within 25% of print tolerance and repeats within 3-microns. The step relationship from crank face to pin face is held within a range of 6-microns.
This state of the art Grinding System is equipped with a robot that places the ground parts into an automatic gauging system that 100% inspects the crank end thickness, pin end thickness and step relationship. The system is designed such that the grinder will automatically correct itself should any of the three characteristics drift beyond preset limits.
The entire cell which is comprised of the grinder, automation, robot, gauging system and coolant filtration system is monitored and controlled by a single Human Machine Interface (HMI).
Similar to the Volt getting better mileage than cars powered only by gasoline, Cox noted that the motors on C&B’s special double-disc grinding machines require less energy than conventional machines due to the process using 25% less horsepower.
For more information, contact C&B Machinery, Livonia, Mich., at (734) 462-0600 or www.cbmachinery.com.