Networked Machine Shop Learns The Value Of Proper Grounding

McAfee Tool & Die Inc. is a full-service tooling and manufacturing facility located in Uniontown, Ohio, a suburb of Akron. In business for more than 22 years, the company uses conventional machine tools, stamping presses and injection molding machines, as well as more than a dozen computer numerical control (CNC) machine tools, including three 3-axis machining centers, six wire electrical discharge machines (EDM), three laser-cutting machines and a CNC turning center.

CNC equipment is commonplace in tool and die shops, so to gain a competitive edge, McAfee began networking its equipment almost 8 years ago, when networks were still considered to be cutting-edge technology in the business. Networking its equipment enables McAfee to store more than 20,000 CAD/CAM programs centrally, making them instantly accessible to the CNC machine operators. The company’s Unix-based CAD/CAM workstations were first linked in 1992. In 1994, the control computers in ten of the company’s laser, EDM and CNC machines were networked.

That’s when the trouble started. Little things went wrong at first, but they got worse and more expensive over the next four years, especially whenever thunderstorms passed nearby.

“I used to call them ‘gremlins’ or ‘sunspots’ because our problems were very sporadic,” recalls John Stiles III, McAfee’s senior programmer and the man who sold management on the benefits of networking. “We had so many ‘little’ symptoms that finding the exact cause of the problem was difficult.”

In fact, the problems were anything but little when you added them up. Communication to a CNC machine would stop in mid-program or never start in the first place, CRT screens would flicker or distort, characters in programs would be missing or altered, ports on the computer side of the server box would quit working, communications boards in the CNC machines’ computers would burn out and have to be replaced, and an RS-232 cable resting against a particular machine would cause other machines not to work.

The company added or changed several CNC machines during the next 4 years. With each change on the shop floor, each machine’s ability to communicate would change. Networking problems continued, but neither their cause nor a solution was in sight. Scores of experienced technicians from OEMs, plus equipment repairmen and networking software suppliers, offered solutions. There were dozens of “Try this, let me know if it works” suggestions, though none of them worked. By January 1998, six of McAfee’s 12 CNC machines were disconnected from the network because of the numerous problems experienced. For awhile, the shop was reduced to operating from punched paper tapes and sharing one old computer to control its machines.

Mr. Stiles called in Ohio Edison, the local power utility. Their technicians confirmed that good quality power was entering the facility. Suspecting that poor grounds were the true source of McAfee’s networking difficulties, Ohio Edison recommended the company contact PowerEdge Technologies, Inc, a consulting organization that specializes in grounding problems. Mr. Stiles called them, and PowerEdge sent Tim Cookson, its senior electrical engineer. Mr. Cookson began his survey by measuring the ground rod resistance at each CNC machine. The results were revealing.

“We’ve done half a dozen machine shops in the Akron area over the years,” recalls Mr. Cookson, “and every one of them had problems like the ones we found at McAfee. Many of the people who install CNC machines don’t understand that these things are computers and computers need good grounding.”

PowerEdge Technologies recommends that installations housing sensitive electronics have a ground resistance to earth below a particular measurement; none of McAfee’s CNC machines met the criterion. Interestingly, two of the three machines with the highest ground resistance readings were the only ones to have both their communications circuit boards and the COM ports on the server computer destroyed during the previous thunderstorm. That was a strong tip-off that the problem lay in the installed grounding system.

“The electrical system in McAfee’s shop just wasn’t capable of supporting today’s sensitive electronic equipment,” says Mr. Cookson. “The system had one old primary ground rod, and its resistance was high. Ground resistance at all those unnecessary supplemental rods next to the CNC machines was too high, and some were very high. There were ground loop currents everywhere. On top of that, much of the building’s interior wiring was aluminum, and many connections had corroded over the years.

“We brought the company’s electrical and grounding systems back to basics. We set up the building steel as a reference, then bonded each CNC machine and all electrical equipment panels to it using #6 AWG copper and exothermic-type connections. We replaced all of the old aluminum with copper cable and copper connectors. Outside, we drove three new 3/4 inch by 10-foot ground rods in a triangular pattern at the main service entrance, burying them and their ground electrode conductors 2 feet below grade to ensure good contact and low resistance. For electrode conductors we used #4/0 AWG copper, bonding everything with exothermic connectors. In fact, in all of our jobs, we use nothing but copper.”

Mr. Stiles was more than satisfied. “Before PowerEdge came in, I was getting nothing but bad advice, and I was running out of ideas. When Tim Cookson first looked at our shop, he walked around with a grin on his face like he’d seen it all before, and when he said ‘Do this and it will work’ instead of ‘Try this and let me know,’ I knew he could help us.

“Since we installed the new copper grounding system, we haven’t had any problems with our CNC and CAD/CAM networks. Poor grounding was the problem all along, but it took us 4 years to realize it. We’re now so confident that our system is reliable that this year, we replaced our shared network with a high speed switched network, giving us even better efficiency. We want to stay one step ahead of the competition!”