mfg. Magazine #1, Article 6

At the GE Aircraft Engines plant in Evendale, Ohio, the installation of a shop floor CMM has helped manufacturing personnel "visualize" process control in ways that aren't possible with traditional gages.

MicroVal PFxCELL at GE Evendale gives the operator real time graphic representation of dimensional tolerance data.

The coordinate measuring machine, a Brown & Sharpe MicroVal PFxCELL personal flexible gage, is installed in a manufacturing cell that produces four different, but very similar, airfoil-shaped high-pressure turbine nozzles. It replaced hard, or dedicated, gaging used to check the dimensional tolerances of the workpieces.

While the dedicated gages performed well, and quickly gave machine operators a pass/fail indication, the gages did not provide them with information that helped determine to what degree the process was in control.

With one part, for example, there are five blueprint characteristics to be checked by gaging after a finish operation. To evaluate machine performance, however, one of those locations might require two separate checks. The hard gages could check only one.

With the PFxCELL on the shop floor, operators can not only perform more measurements on the high pressure turbine nozzle than with hard gages, they also get a real time graphic representation of dimensional tolerance data and can make machine corrections accordingly. That lets them "see" how well the process is performing.

Information, Not Data

Rather than being bombarded with reams of tabular numerical data, machine operators see the process described by a process capability chart graphically displayed on the PFxCELL's computer screen.

"Our charter in putting a coordinate measuring machine on the shop floor was to use that equipment to control the process for this part," said Tim Taylor, GE Inspection Engineer. "While tolerances are important, our focus has been on Cp and Cpk values to show an operator to what degree his process is in control."

The MicroVal PFxCELL is operated by Micromeasure IV software with the DataPage/RT statistics package option. Micromeasure IV is a task-intelligent software system which guides the user through complex measurement routines using the Standard Operator Interface (SOI). The SOI is an icon-based menu screen that allows even an inexperienced operator to start up the system and execute a pre-programmed measurement routine.

Inspection programming at GE was developed by Michael Crowe, GE Inspector. "The SOI icon menu structure simplifies the task of programming inspection routines," Crowe said. "The software allows flexibility to package the application into a user-friendly interface for the shop floor." For example, process capability charts were generated through DataPage/ RT, but were customized to meet the needs of the end user. "Micromeasure IV subroutine programs were developed to modularize and standardize the application to build a shell for programming future applications," Crowe said.

A PFxCELL probe measures a high-pressure turbine nozzle. The CMM replaced dedicated gages on the shop floor.

A Typical Inspection Operation

Prior to inspection of parts, operators first calibrate the coordinate measuring machine's probe. Then, they calibrate the specific holding fixture for the part. These calibration routines, different for each family of parts, are selected by the operator by clicking on an icon. The system graphically illustrates where the fixture should be located on the PFx's granite table to allow fast and easy setup.

A workpiece is then placed in the dedicated holding fixture on the MicroVal PFxCELL, and the operator selects a pre-programmed measurement routine by clicking another icon on the system's screen. The PFxCELL automatically probes the part, recording data points called for by the measurement routine.

The software system displays the inspection results graphically on the screen in the form of control charts. Cp and Cpk are reported using two estimates of the process width, R-bar/d2 and S overall. Twenty parts are represented on the chart, the history of the previous 19, and the most recently inspected part. The operator only has to look at the right-hand side of the screen to see where the most recent part falls in relation to the others. Using this type of graphic data analysis, operators can quickly see if process variables are causing the production of out-of-tolerance workpieces.

Operators can get a real time graphic representation of dimensional tolerance data and make machine corrections accordingly.

For example, in a surface grinding operation, the taper of the workpiece is shown in four graphs, essentially the four corners of the surface. The operator can quickly see how variables in the grinding operation influence the taper characteristic. Variables such as infeed rate and spark-out can be adjusted by the operator to bring parts back into dimensional tolerance limits.

"It doesn't take a lot of time to gather this data either," Taylor said. In this operation, three surfaces in four different locations are measured for a total of 12 data points. The time to set up the workpiece and collect data from the 12 points is slightly under a minute.

Reducing The Cost Of Manufacturing

Reducing the cost of manufacturing also figures into GE's decision to install a coordinate measuring machine on the shop floor. Because the MicroVal PFxCELL is a flexible gaging system can accommodate many variations of high pressure turbine nozzles, each approximately four inches long, three inches wide and two inches high, with only a switch of holding fixtures. In a traditional measurement environment, each part variation would require its own dedicated, and costly, hard gage.

"There is an additional cost benefit in that the hard gages periodically have to be taken off line and calibrated to insure that they are holding the part correctly on the datums. The hard gaging does wear and has to be reworked in our toolroom to restore accuracy," Taylor said. "All of those costs add up and become significant over the life of a production run."

Calibration of the PFxCELL is done on the machine itself, using a self-contained, pre-programmed routine. Since the fixtures simply hold the part for probing, there is no need to be concerned about wear. Machine alignment is done relative to the part, not the fixture.

Programming For Quality

At the Evendale plant, a second Brown & Sharpe MicroVal PFxCELL is used in the coordinate measuring machine lab to develop and validate inspection programs for the shop floor PFxCELL. This procedure allows the machine on the floor to be used 100 percent for measurement.

"We had a vision of what we wanted to present to the operator on the shop floor," Taylor said. "We wanted to give the operator more than data. We wanted to give him a tool he could easily use to consistently produce quality parts." By programming in the CMM lab, highly sophisticated measurement routines can be developed using the system software.

"Data reporting can be modified too," he said. "Right now, we're concentrating on Cp and Cpk with capability charts, but we can have distribution statistics, a cumulative probability chart and other reports for quality assurance documentation if we desire," Taylor said.

The installation of the shop floor MicroVal PFxCELL has helped GE machine operators see process control as it relates to tolerance, not in terms of statistical data, but in a graphic format. As a bonus, manufacturing costs are also reduced since the need for costly dedicated gages has been eliminated by the flexibility of the coordinate measuring machine.