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Assembly cell speeds riveting: workholding assembly tool serves family of aircraft structures for big payoffs in small lots.

Workholding assembly tool serves family of aircraft structures for big payoffs in small lots.

Aerospace manufacturing today requires a special hard-assembly tool for each assembly in the aircraft. Each assembly tool is generally a welded-steel structure, with fixed locators to position detail parts, and fixed clamps to hold them in place. Once detail parts are positioned, the assembly is hand drilled and tack riveted. Then it's removed from the assembly tool, and riveting is completed on a semiautomatic riveting machine.

Separate hard tools are required for different assemblies, resulting in a multitude of tooling. Cross-sectional bulkheads in the wings of airplanes are shaped the same and use webs, T-caps, and stiffeners. The major difference between bulkheads is the size changes for each station, from the smallest near the wingtip to the largest near the fuselage. Each bulkhead requires a separate welded hard tool. The large number of tools required represents enormous tool investment, storage, and maintenance costs.

To gain flexibility and reduce tooling costs, McDonnell Douglas, Long Beach, CA, developed an automated flexible assembly cell. The company replaced the separate tools with a single, flexible tool that can serve a family of assemblies and provide automated inspection. The cell automatically assembles, rivets, and inspects small, flat bulkheads.

To begin, an operator loads incoming detail parts into a kitting tray, and a machine-vision system inspects the parts to ensure that tolerance variations will not jam the loading equipment. A gantry robot configures a flexible jig and loads detail parts. The system eliminates manual tack-riveting, and automatic riveting equipment fastens the assembly together before it's removed from the jig. The robot then moves the completed assembly to a coordinate measuring machine (CMM), where it is inspected for accuracy.

The kitting tray holds individual detail parts in a known location and orientation. Inspection by the machine-vision system tells the robot where to pick up each detail as the part is loaded into the flexible jig. The cell controller uses a computer graphic of the kitting tray to show the operator which parts to load and how they are to be loaded. To minimize part-placement error by the robot, the part-holding mechanism holds each detail part in an established position within [plus or minus]0.0025". The tray is then moved on rails to the vision inspection station.

Vision inspection:

For any automated assembly system to work properly, it must be given the correct parts, and the quality of the parts must be good. An IRI SVP512 vision system inspects detail parts as they enter the cell. The vision system is programmed to verify part presence in the kitting tray, verify that the correct details are in the tray for the assembly being made, and measure the actual locations of the tooling holes.

To prevent jamming, the robot must position the tooling pins on the flexible jig to within 0.005" of the actual tooling holes on the detail part. The current fabrication tolerance for location of tooling holes is [plus or minus]0.030". Because the tolerance allows holes to move over a 0.60" range, they will frequently miss the jig tooling pins by more than 0.005", causing the part to jam.

To solve this problem without increasing fabrication cost, the vision system calculates the amount the actual holes are offset from the nominal dimension. The information is then passed to the robot controller to set the jig to fit the actual dimension of the next part.

A light box mounted under the kitting tray backlights the parts, eliminating the effects of factory lighting changes on the vision system. The camera is supported on X-Y rails over the kitting tray. Accuracy of the vision system depends on the sum of two types of errors: resolution error of the camera, and camera-positioning error.

The 512-pixel camera gives resolution of [plus or minus]0.0005" when the field of view is 1/2"sq. By using closed-loop encoder slides, the camera positioner can position the camera in the X-Y axis to within [plus or minus]0.0005" over the 2-ft x 6-ft kitting tray, giving a maximum total system error of [plus or minus]0.001" for vision measurements.

If vision inspection finds an error, it sends the kitting tray back to the operator and displays where the errors were found. If the system can correct an error, it sends the tray back to vision inspection. If it can't correct the problem, it aborts the assembly. Once the tray passes inspection, it moves to the robot-load station.

Flexible jig:

While vision inspection is taking place, the robot configures the flexible jig. The clamps are designed by Gemcor and modified by the Douglas PLACE robotic-simulation system. The system determines off-line what the clamp locations will be for a new assembly.

The simulation program electronically checks for clamp collisions before they are moved with the robot. The coordinates of each clamp are passed to the robot controller, and it positions the clamps on the fixture.

The robot end effector, designed and built by Cimcorp, Shoreview, MN, has several gripping stations mounted on an aluminum plate. By rotating around, different gripper stations can be used, eliminating toolchanger requirements. It also allows one end effector to move clamps and grip several different part configurations.

Each clamp can be moved in three axes: X, Y, and rotational axis. A clamp is locked in place by a wedge that is driven by a high-pressure pneumatic cylinder. Proximity sensors verify that the cylinder moves when commanded to do so. A two-stage 250-psi compressor provides high-pressure air to power the clamp locks and actuators. The air filters through a refrigerator dryer and special lubricator used to prevent malfunctioning of the clamp cylinders.

A next step in flexible assembly will be flexible jigs that position themselves using multiple-stepper motors. At this writing, clamps cannot position themselves. By using the robot to reconfigure the jig, the jig design and the cell-control program are simplified. Because of the robot's accuracy and repeatability, it makes sense to use this existing capability to handle positioning of the clamps.

Gantry robot:

The robot is a 21 -ft x 42-ft XR125 gantry, manufactured by Cimcorp. The gantry robot and the tooling within the work envelope are mounted on a special 28-ft x 44-ft x 5-ft-thick foundation. The foundation is reinforced by two layers of 7/8"-dia steel, and provides the required rigidity for dimensional stability. As the earth shifts, the foundation is designed to float as a unit. This is necessary to maintain accurate relationships between elements in the cell and to provide vibration damping of surrounding equipment.

A laser maps robot movement through the entire work envelope. Minor deviations in robot accuracy are recorded and entered into the robot's Mechanical Error Correction (MEC) system to compensate for mechanical inaccuracies.


Once the detail parts are loaded into the clamps, the assembly is riveted by a Gemcor G200 Drivmatic riveter. The riveter mounts on an X-Y positioner in front of the flexible jig, allowing the riveter to move over the assembly while it is held stationary in the jig. When the riveter moves, instead of the part, the relationship between assembly clamps and robot remains fixed, allowing clamp positions to be accurately set.

During riveting, the clamps and assembly can move up or down in the Z axis to allow the riveter to step around clamps, and to maintain a constant riveting work-height. When the riveting is completed, the clamp support arm moves back to the home position, so that the robot can reposition. The Z-axis table has 12" of travel and can position the support arm to [plus or minus]0.005".

Final-assembly quality verification:

The Flexible Assembly Cell does not verify tooling locations after positioning by the robot. Instead, it inspects the assembly directly. The robot moves the riveted assembly to a Sheffield Cordax CMM, where dimensions are verified. The CMM computer uses statistical process control to analyze trends and help predict possible problems before they create out-of-tolerance conditions. A 4-ft x 9-ft x 2-ft slab isolates the CMM from vibration and elevates it into the robot work envelope.

Using absolute-points-in-space dimensioning, an assembly is considered good if the stiffener falls anywhere between the two parallel planes that are established on each side of the nominal stiffener location. Using this method, the angle tolerance can be looser and still produce better-fitting assemblies. Completed assemblies have been modeled in a Unigraphics II (UGII) CADD system. A computer generates inspection paths and the actual machine-specific computer code to drive the CMM. When a new assembly is introduced to the cell, the CMM can automatically inspect it. If an error is found during the final inspection process, a printer near the assembly unloading station prints out a listing of what is wrong with the assembly.

Assembly controller and cell:

The assembly controller is an IBMAT computer that is part of the Cimcorp 4000 robot controller. It controls the real-time assembly coordination of the robot, end effector, flexible jig, and riveter. The controller communicates with the jig clamps via an RS-422 serial line to the jig PLC.

The PLC interprets and issues output commands to lock or unlock the clamps, and proximity sensors verify all moves. The controller also communicates with the riveter and the jig's Z axis via RS-422 serial line.

The assembly controller is equipped with a build program that automatically builds an RS-274 script file to run a new assembly. The build program generates computer codes required to process a new assembly, so a new assembly can be introduced into the cell without having to write a single line of computer code.

The cell scheduler is manufactured and programmed by Texas Instruments. It communicates with all other devices in the cell to track the progress of assemblies. The scheduler ensures each device has the correct programs and revisions in local storage. It's equipped with graphic screens that display cell status at a glance.

To solve communications problems between equipment, Texas Instruments developed translator boxes called Unilink Adaptors. These adaptors have a Personality Interface Module (PIM) that converts the format and protocol of each machine controller to the standard used on the local-area-network (LAN). The PIM can be programmed by artificial intelligence (AI) to speed changeover.

Cost savings:

It's often difficult to justify automation in the aircraft business because the lot sizes are relatively small. In this case, we estimated that one flexible tool can replace as many as 250 hard tools. If flexible tools were developed for parts that had a lower usage, we would have the capacity to replace even more tools.

In addition to the initial savings on hard-tooling costs, there are several other benefits. We reduced requirements for tool storage, transportation, and recalibration. Because it is just as economical to produce assemblies in lots of one as in larger lots, the cell can be operated in a just-in-time manner, improving quality while reducing inventory and handling.
COPYRIGHT 1990 Nelson Publishing
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Goodman, Harvey
Publication:Tooling & Production
Date:Dec 1, 1990
Previous Article:Robots automate body-panel fabrication.
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