Imagine a computer numerical control (CNC) system set free of proprietary hardware. Two shops discuss their experiences with software-based machine controllers.

The controller was as good as dead. The machine it was on couldn't even be traded in.

Almost every longstanding shop has had a machine like this - good iron but the controller is shot. The difference is that Melling Manufacturing Group in Jackson, Michigan, was able to bring their vertical machining center back to life. And it didn't take an expensive maintenance program. All it took was replacing the old hardware-based CNC with a software-based system, running on a mail-order personal computer. This PC control not only resurrected the machine tool, but it also lets the machine run better than it ever did with the old controller.

"The way it was, the most we could get for that VMC as a trade-in was its scrap value," said Melling President Dave Horthrop. This 45-man shop had owned the machine since 1985, and relied on it for years. But over time, the control unit had experienced its share of problems, with downtime getting longer and longer and repair bills going higher and higher.

Even when it was running, the machine was very difficult to program. The proprietary control was badly out-dated and no longer supported by the builder.

"Eventually it got to the point where, at best, we could use it only as a programmable drill press. Even at that, our machinists avoided using this machine as much as possible - most of the time it just took up valuable floor space," Mr. Horthrop recalls.

The shop found the new CNC, which made the turnaround possible, a little over two years ago. Called OpenCNC and produced.by Manufacturing Data Systems Inc. (MDSI) of Ann Arbor, Michigan, this CNC is a software-based system that uses a standard, off-the-shelf personal computer (PC) running a standard CNC software product. Only this software is proprietary; everything else is entirely independent of computer hardware. As soon as the old CNC was replaced with the new, this same machine was running, and running at faster feed rates and better accuracies than it did when brand new. It has become a very productive resource on the shop floor and there is no hesitation among operators when they are assigned to this machine.

"Besides turning what we thought was just a piece of junk into a valuable production asset, software-based CNCs are going to help us get into a new business that we couldn't otherwise compete in," says Mr. Horthrop.

A Different Kind Of CNC

Mr. Horthrop had heard about the new kind of CNC from Great Lakes Industry, Inc., a company also located in Jackson, near Ann Arbor in the southeastern corner of the state. GLI had retrofit several of its machine tools with these software-based CNCs. Almost out of desperation, Mr. Horthrop visited GLI to check it out. What he saw appeared to be the answer to the problems with his shop's VMC, even though the retrofit CNCs he looked at were radically different from the CNCs he was used to encountering. With little to lose, he had the old CNC replaced on the VMC in August, 1995.

The new CNC runs on an office-grade PC interfaced directly to the existing servo system of the machine tool. There is no separate motion control card, programmable logic controller, or other hardware in or attached to the PC. It has standard processors inside, exactly the way it came out of the box.

What it does have is software designed and written to allow an off-the-shelf PC to function as a CNC. This software handles everything that a CNC normally handles, such as the human-machine interface and I/O control, but it also closes the servo loop, a task usually relegated to add-on hardware such as a motion control card in some CNCs using a PC platform.

The reason Melling's VMC is now performing better than ever is that the old control did not have the processing power to keep up with the full potential of the axis drives on this particular machine. The drives' speed and accuracy were limited by the inability of the old CNC to process motion commands and position feedback signals very quickly. The new CNC is considerably faster.

"And when faster and more powerful PCs become available, we can upgrade by simply swapping out the old for the new," notes Mr. Horthrop. "The same control software will be loaded on the new PC. Likewise, if the control software is upgraded, the hardware will not prevent us from moving to the new version."

Hardware Independence

Having CNCs that are completely independent of hardware is an important consideration for both Melling and GLI, though their reasons are different. "We are interested in adding capacity to manufacture timing sprockets and similar workpieces for our parent company," explains Mr. Horthrop at Melling. "But we can't justify the cost of new equipment to produce these parts competitively in a mature market where margins are very low. The problem with used equipment is the controllers. They're either outdated to start with or if they're not, getting repair parts or future upgrades may not be easy or affordable."