Servo Chatter

All CNC equipment works off error and error corrections. Think of servo chatter as a car’s cruise control, except that it’s more highly refined and accurate to a hundredth of a mile per hour. Imagine you set the cruise at 64 mph and this system taps the brakes just a tad if you hit 64.01, and punches the gas if it sees 63.99. Theoretically, if it reacted fast enough, you could be in for a jerky ride while staying well within even 0.10 mph of your target setting. Servo chatter is exactly the same type of condition. CNC systems use feedback to determine cutting speed. Servo chatter is the condition of the servo overcorrecting on both the high and low side, resulting in an almost faceted surface of the camshaft. The lift profile error is small, but the acceleration error is high. In other words, you are going the right speed, but your neck hurts at the end of the trip.

Breakthrough Technology

About 10 years ago, COMP Cams began closely working with Okuma on our grinding needs. Okuma had been trying to introduce their GC34Nh machine in the United States, but needed a partner in developing their machine more for a motorsports and aftermarket focus. Not only was Okuma able to get the servo chatter low enough to allow using the extremely stiff CBN wheels without chatter issues, but they were also able to develop software to take the setup time down from a few hours to about 15-20 minutes. An additional benefit of the Okuma GC34Nh is that these have about half the footprint of the large Landis grinders. It was not long before we had bought a second Okuma grinder, followed by a third and a fourth. This year, we decommissioned our two remaining Landis grinders and have purchased two more Okuma GC34Nh’s for a total of six units.

Maintaining Tolerances

Most of the aspects of cam design that can be measured well on a $3,000-$6,000 cam checking gauge may be far less important than you might first think. I’m not so sure the engine can tell if there’s a 1/2 degree change in duration or lobe separation from one cam to the next, but other aspects of the cam profile are extraordinarily sensitive to tiny variations in tolerances. These small aspects are probably what kept manual grinding machines so predominant in race camshafts for so many years. It helps to understand that the manual machines tend to try to smooth the master profile slightly just due to the slight compliance of the machine. This created very smooth profiles for use at high engine speeds. On a CNC machine, the profile durations may look near perfect, but the jerkiness of the servos as they respond to error can create an overlapping sawtooth waveform on the acceleration curve. This will excite the valvespring, which isn’t the part you want excited in your engine.

The biggest change at COMP Cams in grinding technology wasn’t the CNC machines that came in the late ’90s, but the first ADCOLE 911 cam gauge that we purchased in the mid ’90s. Management was vehement that we needed to have better measuring equipment before we upgraded our manufacturing equipment. We spent a significant amount on a camshaft gauge, fixtures, and software to bring in the best camshaft measurement system in the world. You cannot imagine how much we learned the first year! Being able to repeatedly measure a cam profile down to the sixth and seventh decimal place, in inches, opened our eyes to a whole new world of possibilities. Actually, we found out that our Berco cam grinders could be far better than we ever imagined if properly operated and maintained, and we learned that changes in the acceleration as small as a few percent could substantially change the dynamics as measured on our Spintron valvetrain dynamics measurement systems.