Spindle on. Spindle off. Cycle on. Cycle off. Feed rate overridden by operator. Tool changed. Clamp activated. Macro routine initiated.

These are a few of the "significant events" that occur in the course of operating a machine tool. Information about these events is very meaningful—if the people who can use this information can get a timely record of it, that is.

Cessna Aircraft Company in Wichita, Kansas, has retrofitted two critically important machine tools with a software-based CNC system that creates a database of these events as they happen. Because the new CNCs use only standard PC (personal computer) hardware, and Windows-based software that is readily interfaced with other applications running on the plant's communications network, this record of machine tool happenings provides a level of information most plants can only wish for.

According to Phil Campbell, a supervisor in the manufacturing engineering department that is responsible for computer systems in the Cessna Components Manufacturing Facility, access to this data solves several long-standing problems. More important, it creates unprecedented possibilities for improving operations and increasing machine output.

"Because these CNCs keep a running log of events that normally occur during operation, we get an accurate, up-to-the-second picture of the machine's performance, " Mr. Campbell explains. "Not only can status of the machine be monitored more effectively during each shift, but at the end of the shift, reviewing the log shows us what is really contributing or not contributing to productivity." Figure 1 (below) shows what this record looks like.

Figure 1 — This file of events is automatically generated by MDSI's OpenCNC as the events occur. All of the messages are readily understandable; no cryptic codes are used. Cessna modified the default list of recordable events by adding new ones such as a message identifying the line of NC code being read at the moment.

Abnormal events are also recorded, Mr. Campbell points out. "An operator can determine what may have gone wrong and when. If it's something the operator can take care of, he can determine that right away. Otherwise, the maintenance department can come in and not waste time diagnosing the problem before fixing it."

Like the office and engineering software in use around the plant, these software-based CNCs are fully integrated with Cessna's intranet, which also doubles as a DNC system serving some 70 CNC machine tools in this plant alone. With the new software CNCs running on standard PCs using Windows NT, however, operators can download NC programs as quickly as transferring a file from the network file server. Engineering drawings, setup sheets, and other documents also can be downloaded and viewed. "It's very much like receiving e-mail in an office environment," says Mr. Campbell.

Downloading part program files from the intranet has been especially valuable. According to Mr. Campbell, it used to take 45 minutes or more to download a typical NC program with the old control units, which could not run one part program while another was being download. With the new system, downloading now takes seconds and can take place in the background during machining. "We're gaining about three hours a day as a result. That was an immediate payoff which, in itself, justified retrofittng these controls," he says.

But network integration is a two-way street. An authorized network user can "dial up" one of these CNCs and view the same displays that that the operator is watching—including the log of events as they are being recorded. According to Mr. Campbell, remote users can even take control of machine functions as if they were right at the machine—although he quickly warns that this happens only under extraordinary circumstances. "The big advantage is being able to see what the operator sees and to help him keep the machine running to meet production goals."

Figure 2 — This router was the first machine tool at Cessna to be retrofitted with a software-based CNC. A standard PC keyboard and monitor have replaced the old, propietary CNC in the pendant.

The two machine tools currently equipped with the new controls are large aerospace routers designed to cut parts from a stack of aluminum sheets. The PCs are standard, off-the-shelf Windows NT machines, but the software that allows them to operate as CNCs is OpenCNC from Manufacturing Data Systems, Inc. (MDSI) of Ann Arbor, Michigan. These two routers (there is a third that has not yet been retrofitted) are critical machines because they are relied on for cutting between 50 and 60 percent of the fabricated sheet metal parts Cessna needs to build its air-craft. One of these routers is shown in Figure 2 (at right).

And right now, Cessna is very busy building aircraft. As new orders continue to come in at a growing rate, the company expects to get even busier.

Flying High In Wichita

Founded over 72 years ago, Cessna Aircraft Company is a leading designer and manufacturer of light and mid-size business jets, utility turboprops and single-engine piston aircraft. For more than 50 years, Cessna set the standard for production of single and multi-engine piston powered aircraft, delivering more airplanes than any other commercial manufacturer. In 1967, the company announced plans to introduce its first business jet, the Citation, and it delivered the first of these in 1972. The Citation model line evolved through the 1980s, becoming faster, more comfortable and economical.

However, as Cessna's jet business was getting off the ground, its prop plane business was about to take a nosedive. In the 1970s, the proliferation of product liability litigation involving general aviation aircraft in the United States caused a downturn in the entire industry and eventually forced Cessna suspend light aircraft production during the late 1980s and early 1990s. Sluggish economic times also held down jet sales during this period. As production dropped, employment at the firm did also.

Things began to turn around in the mid-1990s. The passage of the General Aviation Revitalization Act of 1994, which established an 18-year Statute of Repose, immediately brought Cessna back into production of light aircraft after a ten-year hiatus. A new generation of single-engine piston-powered airplanes began rolling off the production line in mid-1996.

Multiple sheets of aluminum in a stack as thick as 1/2 inch can be machined at one time. A variety of parts can be nested together to meet production requirements.

In the meantime, the business jet side also got a boost. The Citation X business jet, which had been announced in 1990, was being delivered by August, 1996. Flying at 0.92 Mach (more than 600 mph), the Citation X is the world's fastest business jet. According to the builder, among non-military aircraft only the Concorde is faster. The jet proved to be very popular. Today, with the U.S. economy booming, sales of business jets have never been stronger. The 100th Citation X was delivered in August, 1999. The 3,000th Citation is expected to be delivered in late 1999. Likewise, among the general public, there has been a revival of interest in flying one's own plane. Orders for the Cessna Skyhawk, Skylane, and Stationairs are surging.

As production has been ramping up in recent years to keep pace, the company has invested heavily in a variety of new manufacturing technologies. At its Components Manufacturing Facility in East Wichita, for example, newly installed equipment includes a number of automated machining cells, multi-machine flexible machining systems, palletized machining centers, and mill-turn machines, to name a few. Almost every corner of the plant has been upgraded or renovated along the way.

A growing backlog of orders for aircraft means that Cessna must look continuously for ways to increase productivity and improve efficiency. At the components plant, the three sheet metal routers had the potential to become serious bottlenecks. Nearly 32,000 part numbers cross these three machines. Improving their throughput would have an immediate positive effect on the entire production cycle of these parts.

Routers To The Rescue

Fifteen years ago, when Cessna acquired the first of these three Trumpf BFZ3000 routers, the company recognized a need for more flexible manufacturing equipment. At the time, most sheet metal parts were stamped from sheet aluminum on large presses. The dies manufactured for this purpose were expensive and time-consuming to build, not easily modified, and required considerable storage space. Setup time on the presses was lengthy. The first router proved to be an effective alternative with the needed flexibility. The second router was installed in 1986, and the third in 1997.

These routers are designed to cut parts from a stack of aluminum sheets by milling the periphery of the part's inner and outer contours. Sheets as large as 4 by 8 feet in stacks as thick as one-half inch can be handled. Each router is equipped with five spindles in addition to the main spindle for routing, plus a riveting head. These auxiliary spindles include several for drilling holes of various sizes as well as a lighter duty spindle for routing with a smaller milling cutter. The riveting head inserts fasteners that prevent the sheets from delaminating or moving before all of the parts have been completed.

No dies, fixtures, or other hard tooling are required. Workpiece contours are stored as CAD files, an array of which can be nested to make maximum use of each sheet in the stack. Because Cessna has more aircraft models in more configurations than ever before, batches are smaller and the part mix is greater. Current production requirements can be more easily accommodated by changing the assortment of parts to be nested on the sheet. To expand this flexibility, parts designed before the advent of CAD systems are gradually being scanned and archived in electronic format.

In recent years, limitations in the capabilities of the control units on the two older routers were becoming a problem. The controls could not download NC programs from the plant-wide DNC system any faster than 2400 baud. Because these NC programs represented the contours of numerous complex shapes and hundreds of drilled hole positions, these programs were quite long, taking from 20 to 60 minutes each to download. During this time, the machines were idle because the controls were not multitasking. These aging controls were also prone to increased maintenance, with repair parts becoming more difficult and slower to acquire. Service costs were rising.

Given the critical role these routers play in the production of sheet metal parts, upgrading their CNCs became a priority. Early on, however, the manufacturing engineering team looked for alternatives to conventional, proprietary CNC systems. One of their main interests was avoiding the difficulties and limitations being encountered with such systems on other machine tools or machining systems, some of which are quite new. Although these systems are very advanced in many ways, the level of proprietary CNC software and hardware made interfacing with the plant's communication network difficult or limited. At best, these connections had communication speeds at baud rates considerably below the standard for other computer terminals on the network, such as PCs. The degree of integration with other systems was also limited by the proprietary hardware and software required.

Ken Stromberg, one the manufacturing engineers involved in this project, recalls other factors that the team considered. "We wanted to avoid the maintenance headaches associated with proprietary CNC systems—having single sources for specialized printed circuit boards, for example. Little commonality between systems was also a problem. It seemed like no two systems were alike, making it harder to diagnose problems and find solutions."

What attracted the team to MDSI's OpenCNC was its complete independence of proprietary hardware and its use of the standard Windows NT operating system. Other systems that were considered still required proprietary processors, motion control cards, customized communications interfaces, or essential software that could not be modified easily. In contrast, OpenCNC runs in the Windows environment like any other application software. The software is uniquely designed to run a machine tool entirely from software from a PC with a single processor, from a single operating system. Moreover, it is uniquely designed for controlling a machine as part of a network.

A Standard PC—No More, No Less

Figure 3 — A standard, office-grade PC and an uninterruptible power supply sit inside the machine's electrical cabinet. All CNC functions are fulfilled by MDSI's OpenCNC software running in the Windows NT operating system.

Retrofitting the first router with a new control system began in April, 1999. In addition to a new CNC, this machine received new spindle motor drives. The resolvers also were replaced with encoders to enhance performance. The PC that the CNC software runs on is a standard, office-grade PC interfaced directly to the existing servo system of the router. No other computer hardware, other than a standard battery-backup/surge protector unit, was installed. There are no added motion control cards, programmable logic controllers, or communications devices. The high-speed Ethernet network interface card in the PC serves as the link to the shop's intranet. A standard ribbon cable connects the PC with an off-the-shelf I/O card and data acquisition card to the I/O. When you open the electrical cabinet, the PC's processor box and the uninterruptible power supply are sitting on the bottom shelf, not unlike what might be found under a desk for the home or office PC, as shown in Figure 3 (above, right).

A standard monitor and keyboard were installed in a pendant-mounted console. A touch screen allows the operator to use a fingertip to change page displays, make menu selections, and change values with slide buttons. No mouse or mousepad are required unless the operator prefers them.

The single Intel Pentium processor inside the PC is standard-issue. The operating system is Windows NT. What does distinguish OpenCNC as a machine tool controller is the MDSI-developed software that handles all of the machine-tool related functions such as the operator interface, I/O control, programmable logic control, and the servo loop control. (Closing the servo loop within the software is the breakthrough that eliminates the need for an add-on motion control card, keeping the system entirely free of proprietary hardware.)

Installation took about eight weeks, although much of that time was devoted to the non-CNC upgrading and rewiring, which was complex due to the number of spindle drives involved. The second router was retrofitted over a period of less than four weeks and completed in August, 1999. In both cases, installing the CNC itself was simple, says Mr. Stromberg. "We loaded the OpenCNC software, plugged in the PC, booted it up, and were ready to go. After that, it was just a matter of fine-tuning the machine."

Because the control is software based, changes are easily made to the interface. Mr. Stromberg describes how useful this proved to be: "While the first router was being retrofit, operators mentioned how difficult it was to set tools when they were changed. The machine's layout requires them to crawl into 'the pit' in front of the machine and set the tool length with a piece of material while pressing several buttons on the control to activate the drill and plunge to depth. An easy-to-reach button was added to the control's touch screen that activated a drill setting macro. We were able to replace multiple commands with one easy button to push that helps the operators."

A Significant Event

According to Mr. Campbell, the new CNCs proved their worth quickly in terms of reduced download time, but other improvements also appeared. The new
CNCs allow smoother machining because the axis drives can be fine-tuned so that acceleration and deceleration follow a gentler S curve profile. Now, when the cutter changes direction along the machine's axes, the motion doesn't have the jerkiness characteristic of the old controls. Smoother motion reduces the risk that parts might break loose from their holding tabs before machining is complete.

Mr. Campbell also believes that feed rates can be increased somewhat because of the improved responsiveness of the servo system and the improved accuracy of the encoders, but testing is still underway. Currently, routing is performed with a two-flute, TiN coated HSS solid-body cutter. New tooling may be part of the effort to improve cutting speeds.

"Actually, our focus isn't on cutting speed," he explains. "Looking at the machine's performance log showed us that more time than we realized was being spent on the riveting operations. We have a much greater opportunity for productivity improvements by changing the riveting strategy or installing a faster riveter."

Figure 4 — Cessna engineers wrote a software application in Windows to summarize impotant production information automatically drawn from the data collected by the OpenCNC's Significant Events feature. Cessna's program totals the time each router spindle was in operation during the day.

This is a good example, Mr. Campbell points out, of how the log of machine events is a valuable management tool. (See Figure 4, at right.) "It's an electronic scorecard that lets us evaluate machine performance in a truly objective manner," he says. "We don't have to rely on what an operator or a maintenance technician thinks happened or how long something might have taken. We have a dated record to go by. The real causes and effects of what is going on at the machine become much more apparent."

This monitoring and reporting function of the CNC is not only important to Cessna engineers like Phil Campbell. It is also at the heart of the original concept of OpenCNC. When MDSI founders Chuck Hutchins and Bruce Nourse reunited in 1989 (they were two of the original founders of the first MDSI, the pioneering NC time-sharing company), they set out to develop a software-based machine controller that could be integrated in an open network environment so that manufacturing information could be collected and distributed in real time. Eliminating the "black box" approach to CNC, where hardware and software are bundled in an unalterable package tied to the machine tool, was not the end but the means to achieving this goal.

Having an open system opens the door to both collecting and sharing information about what is happening on a machine tool. MDSI developed (and patented) real-time data collection technology so that its OpenCNC product can collect a virtually unlimited range of data about the manufacturing process. This software feature, which MDSI calls Significant Events, essentially programs the CNC to detect the occurrence of any machining event predetermined by the user, then attach a human-readable label, to mark it with the date and time from a signal given by the processor's real time clock, and to store it in memory in a standard database format such as Microsoft SQL or Oracle. From this database, the data can be linked to or embedded in other applications running on the PC or on the network.

Most CNCs, especially the more recently developed ones, do create a data log, but the events recorded are usually limited to the activities of the PLC or to a few events signaling required maintenance as established by the control builder. With Significant Events on OpenCNC, the list of events monitored and recorded is extensive, and can include maintenance, quality and production information. New events can be added by the user.

At Cessna's routers, for example, the software was modified to record activation of the hydraulically operated pressure shoe that holds down the stack of sheets before a rivet is inserted. Likewise, Cessna programmers have defined several custom M-codes which are inserted into the part program to indicate that different sections of the program have been reached. These M-codes define program-start and program-finish, start and stop of drill, riveting and routing cycles, and start and stop of a "skeleton cut" cycle which cuts the web of material left after the finished parts are removed. Each time the program reads one of these M-codes, it is recorded as a significant event. These events are then used to determine how much time is spent in each machining cycle.

"We've only scratched the surface of what we'll be able to do with real time feedback from a CNC over a network," admits Mr. Campbell. One of the more useful aspects is remote access to the CNC on the company's intranet. (See Figure 5, below.) "We've already used this capability to troubleshoot the machines and keep their uptime high," he says.

Figure 5 — This is the main OpenCNC display screen as it appears on a laptop computer when connected remotely to the CNC on Cessna's network. An operator at the machine views the same screen and can use the active touchscreen to push buttons or change menus. Cessna customized the display to include displays and buttons tailored for operation of amulti-spindle router.

Beep! Beep! Fix Me

This Cessna plant operates 24 hours a day, seven days a week. Ken Stromberg and seven other manufacturing engineers in Phil Campbell's department are responsible for the computer and communications network side of machine tool operations in this plant, so they take turns carrying portable pagers around the clock for two weeks. Foremen or supervisors from the plant can page the engineer on call if a problem develops on the shop floor. Having remote access to the machine tool via the CNC has made life easier for the engineer on call.

Figure 6 — To access the router's CNC remotely on a laptop, Ken Stromberg clicks on the VNC icon appearing on the Windows Desktop, then enters his password when the menu box (top) opens. Clicking OK brings up the menu box (bottom), which allows him to select the networked CNC he wants to access. The next click will bring up the display shown in Figure 5 (above). If he wnats, he can use his cursor to activate start and stop functions, override feed rates, jog the axis positions, and so on, as if the display were the actual touch screen on the machine's monitor.

In one incident, a foreman called at 3 a.m. to report a malfunction with one of the routers. It had lost its homeposition and couldn't be reset. Mr. Stromberg describes how he was able to help solve the problem—using his laptop computer at home (Figure 6, at right):

"The laptop has a modem, so I logged onto the Cessna network via the Internet. My laptop has an application called Virtual Network Computing, a Windows utility program that allows one PC to interact with another PC and act as one. Of course, I needed my password to get on, but was then able to contact the control system on that particular machine.

"So I called up the same CNC screens that the operator would be viewing and opened the Significant Events file. One of the messages indicated that the Y axis had advanced past its travel limit in the jog mode, probably knocking off one of the alignment dogs. When that happened, the encoders could no longer report its position. I sent a message to the operator asking him to check out that possibility. He discovered that was, in fact, the situation. When a maintenance crew arrived, it was easy to show them the problem and get the limits reset. The machine was back in operation within the hour.

"The beeper and the network saved me from driving in to the plant and spending several hours with maintenance trying to figure out what the problem was," Mr. Stromberg summarizes.

Curtis Cook, another engineer, reported a similar case where accessing the CNC remotely helped solve a problem without a trip to the plant floor. From his laptop, Mr. Cook called up the CNC and went to the diagnostic page. An alert message indicated that an I/O point was not responding correctly. It appeared that a proximity switch was not closing as it should, creating an error in the control logic. He sent a command forcing that point to open and close several times. A metal chip was lodged under the switch; Mr. Cook's remote command forcing it to open and close dislodged it.

"We didn't even have to call in the maintenance department at all on this one," Mr. Cook remarks. "The machine was back to normal, losing only a few minutes of production."

On The Runway

As useful as remote monitoring is, Mr. Campbell believes that automatic monitoring has even greater potential. "We want to take advantage of the automatic real-time reporting capability of the MDSI control. Production data will be fed immediately to our scheduling system to update production schedules and work-in-process reports. Likewise, scheduling will be able to check what tools and drill sizes are currently in the spindles and re-sort the next mix of parts to avoid unnecessary setups or tool changes."

According to Mr. Campbell, implementing these plans "will happen pretty fast" because the CNC software is designed to facilitate this connectivity through the OpenCNC Application Programming Interface. The Application Programming Interface is a common set of programming functions and tools that allows users to develop their own programs for OpenCNC or to integrate third-party programs such as statistical process control (SPC).

In the meantime, other departments in the Cessna plant are evaluating OpenCNC for their machine tool control requirements. One of the big attractions is, of course, real time event reporting across the network. Regardless of what direction other parts of the plant might take, Mr. Campbell acknowledges that they share the same advantage with the Cessna workforce. "The positive attitude of employees is conducive to implementing new technology. People here are keenly interested in doing their jobs better, and they know we are looking for new tools that will help them do just that."

Write Your Own CNC Executive Software MPM Inc., a fast-growing job shop, has three things in common with Cessna Aircraft Company. Both companies are located in Wichita, Kansas. Both machine complex, high-precision aerospace components. And both now have software-based CNCs on critically important machine tools. The big difference is, at the job shop, the people who own the company not only run the shop, but also program the machines and sometimes even run the machines themselves. All three founders, Larry Watson, Don Gorges, and Stuart Stevenson, entered metalworking as machine operators and CNC programmers. After years of writing programs for complex, difficult-to-machine workpieces, the three decided that the real money was not in a programming company but in a machining company. So they formed MPM to machine the kind of parts other shops couldn't handle because the programming was too tough, the parts were too complex, or simply too big or long to fit on their machines. To this end, MPM's strategy is to have machines of the size or capability that most other shops don't have, with a particular emphasis on five-axis equipment in a variety of configurations. Having the programming systems and programming skills for this type of work is also part of the strategy. MPM rebuilt this five-axis milling machine and installed MDSI's OpenCNC software as the control system. The ten-foot bed of this machine lends itself to the unusual aerospace work that is this job shop's specialty. The shop now has a mix of new machines from such leading builders as Haas, Fadal, Mazak, Mori Seiki and others, as well as used machines, which the shop has refurbished or remanufactured in its own facility. (The shop has equipment large enough to machine the base or column of a machining center, for example.) "The more you can do for yourself like this, the more economical it becomes to find fixer-uppers on the auction block that would be out of reach purchased new," Stuart Stevenson explains. One thing that doesn't concern them too much when evaluating a used machine is the control unit. "We'll put our own control on it if we have to," he says. One recent acquisition was an older Hydrotel milling machine that had been rebuilt with a five-axis spindle head by Cincinnati Milacron in the 1980s. The control unit on this machine was no longer operational. MPM installed MDSI's OpenCNC software on a standard PC as a replacement. Although MDSI has an extensive library of machine logic to support a broad range of machine tool types, none of the programs matched the unusual configuration of this machine. "We take pride in being self-sufficient and in doing things ourselves, so we learned how to use MDSI's Integrator Toolkit and wrote our own executive program for this machine," says Mr. Stevenson. He attended a training class at MDSI in January, 1998. OpenCNC Integrator Toolkit is a software package designed to provide the machine tool rebuilder or remanufacturer with the software tools needed to integrate OpenCNC software with a machine tool. Although it is not primarily intended for the end user, the Toolkit was readily usable by Mr. Stevenson, who had his experience with CNC programming to draw on. "The Toolkit supplies sample logic programs for various kinds of machine tools, rotary tables, tool changers, and other accessories, in different configurations. In this case, I used a three-axis mill program as a template and copied the lines of code I needed to add two more axes. I edited the code so that the axes were identified properly and the coordinate positions oriented the right way," Mr. Stevenson explains. He tested the new program on a standalone computer using utilities in the Toolkit, then carried the new machine logic to the PC at the machine and installed it there. "This worked out so well that we went back and installed OpenCNC on a similar Hydrotel that we had acquired last year," Mr. Stevenson reports. "The software CNC is faster and now we don't have to worry about finding boards or parts for the old control any more."