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Video makes it better.

A good rule of thumb for high-precision assembly operations is that the assembly tool should be at least 10 times more precise than the part being manufactured. At Grumman Space Systems, however, part specifications demand 0.000 05" precision, putting the 10-times rule for assembly tool precision, according to one company official, virtually out of the question.

"We're already manufacturing the most precise part being assembled in the United States," explains George McCarty, Grumman's group head of design/drafting. "The technology simply doesn't exist to build a more precise assembly tool."

Aligning 7.8 million detectors

Grumman is building what it believes to be one of the world's most sensitive infrared detecting systems. One complete focal-plane array is comprised of nearly 130 individual sub-arrays, each of which contains 30 distinct eight-layer modules. Each sub-array carries two embedded microprocessors, allowing it to individually interpret and analyze signals.

Grumman selected Patford Inc, a company specializing in high-precision machines, to build the two work stations required to assemble the sub-arrays. According to Patford president Ray Geisinger, the machines are single-purpose devices precisely tailored to Grumman's assembly task.

Precision is of utmost importance throughout the assembly process. Accuracy of the infrared sensing system depends on knowing precisely where each sub-array is in focus to accurately interpret infrared detections.

"Patford was the only local source with the experience we needed to meet the established design criteria," Mr McCarty explained. "With a product design criteria of |+ or -~0.0001", the assembly machine obviously had to exceed that level of precision."

Precision assembly

Housed together in a Class 100 clean room, both assembly machines use a combination of 500-times magnification video cameras coupled with high-precision measurement systems. The video cameras allow the operator to align workpieces using electronically generated cross-hairs on video monitors. Measuring the exact position of the video cameras once the cross hairs are aligned is achieved using Heidenhain LS 403 linear scales and digital readout packages.

Manual assembly requires skill, patience

Presently, all assembly work is completed using specially designed manual machines. "Right now, we're simply proving that it is even possible to assemble to these specifications," says Mr McCarty. "When we go into full production, much of today's manual process will be fully automated."

According to assembly machine operator, Randy Binderup, Grumman associate engineer of micro electronics, building the sub-arrays is a complex process that requires skill and patience. To start, Mr Binderup places the first of the eight-layer modules in the assembly area of the first machine and aligns the devices' two video cameras. Once the cameras are focused on the workpiece, he records their exact position from the digital readouts.

The exact-position readouts serve as the standard for subsequent modules. The next 29 modules are aligned using position feedback provided by the Heidenhain system. Each is cemented using an extremely stable epoxy compound.

The first assembly machine moves on a total of eight axes, all of which are measured by scales. "Using the digital readouts is critical to making this machine work," Mr Binderup notes. "Without them, I would have to visually confirm the alignment of each module--a process that would take hours to scan with the high-power video cameras."

The process of inserting the stacked modules into the U-shaped holder uses a similar machine. This second assembly tool incorporates three video cameras, five Heidenhain LS 403 scales, and two Heidenhain Metro MT12 gages. With an accuracy of |+ or -~0.5 microns, the gages are used to measure the exact hole depth and placement on the carrier. The second machine moves on a total of 11 axes, seven of which are measured by scales and gages.

Automation to speed process

Because of the detailed manual work involved and the need to let the epoxy cure after each module is added to the stack, each unit requires about two weeks to complete, including electrical assembly. "This assembly process isn't appropriate for long-term production," Mr McCarty notes. "However, we're gaining valuable experience into the challenges involved in building parts to this level of precision. As we move out of the demonstration phase, we are confident that we'll be ready to build automated tools that will greatly streamline the process of building parts to meet the system requirements."

For more information on Heidenhain Corp, circle 261.
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Title Annotation:Quality Solutions; high-precision measurement systems with video cameras at Gruman Corp. Grumman Space Systems
Publication:Tooling & Production
Date:Oct 1, 1992
Previous Article:Getting started with QDC.
Next Article:Optics improve CMM.

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