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Flexible manufacturing demands flexible inspection.

Flexible manufacturing is a key to totally integrating metalworking manufacturing. In fact, an FMS approach is regarded by most strategic planners as the only way to meet conflicting goals of low-volume and low-cost production.

While most of the attention is on parts handling, machining, and systems management, ultimately inspection is required to qualify the set up. If we are to truly integrate our factories, then it's imperative that inspection equipment have the same flexibility and automation as the machine tools producing the parts. The long-term success of an FMS is predicated on a flexible inspection system (FIS) that can monitor and provide real-time feedback to keep processes in spec.

What are essential characteristics of an FIS?

* It must measure parts of virtually any configuration or complexity, and require little or no special fixturing.

* It must inspect single parts or small lots presented randomly in a shop environment.

* It must operate untended (or with relatively unskilled operators), communicate with remote host computers, and accommodate automated material handling (e.g., robotic load/unload, palletized workpieces, etc).

The central element of an FIS is a coordinate measuring machine (CMM). As NC is to production, coordinate measuring is to inspection, and for many of the same reasons, viz., increased throughput, minimized operator error, and repeatable quality. CMMs have proved especially useful in inspecting first parts off, and are suited for checking highly complex workpieces with tight tolerances. Surface-plate and height-gage techniques are typically too slow, too costly, particularly error prone, and, in many cases, not sufficiently accurate. The alternative of dedicated gages can be equally troublesome.

CMM configuration can be loosely grouped into two categories: vertical arm and horizontal arm. Specific application depends on part size and complexity, required inspection speed, etc.

Recent advancements that permit using CMMs in a flexible fashion include automatic probe changers, probe and fixture code recognition, automatic probe and fixture clamping, distributed microcomputer processing, and high-performance servosystems. For example, combining distributed data processing control systems (such as our MP-25 measurement processor) with multiaxes probe heads, dynamically stiff machine structures, and 3-D software can dramatically increase CMM throughput--average touch time can be as low as 1.75 sec/touch on prismatic parts measured on five sides.

Measuring-machine utility, however, largely depends on the nature of the probe. Electronic probes are simple, compact devices offering low gaging force, and they can be fitted with multiple styli to access various part features. The newer two-axis articulated probe heads contain two programmable motorized axes, permitting automatic positioning of the stylus to suit the part. This effectively converts a three-axis machine to five-axes, while eliminating the need for a rotary table and minimizing probe changes. Coming developments in surface finish, noncontact, and thickness probes will only enhance FIS.

Concerning computer control, we recently introduced operating system software for CMMs that has extensive data logging and statistical analysis capability. The program executes in a real-time multitasking host computer, enabling simultaneous servicing of multiple inspection stations and other terminal users.

FIS in action

A Cordax CMM module is integrated into Kearney & Trecker's FMS installation at Hughes Aircraft, El Segundo, CA, Figure 1. The FMS consists of nine machine tools, host computer, automated material handling and workpiece inspection, and in-floor chip-disposal system.

The CMM is mechanically interfaced to receive random palletized workpieces in the same manner as the machine tools. The CMM controller has a data communications link to the host computer's part program library, and can record and transmit inspection data for statistical processing or generating inspection reports.

In a second example, a joint development by Boeing Aerospace Co and the US Air Force (Wright Aeronautical Labs), demonstrates the feasibility of an untended inspection cell directly linking with an FMS. The FIS combines a host computer, staging area with data entry, load/unload stations, controlled environment, palletized holding fixtures, pallet storage rack, storage and retrieval vehicle, and vertical and horizontal arm CMMs. The project's objective is a 50-percent reduction in inspection costs.

An important system feature is its dynamic statistical sampling system with automatic severity adjustment. Automated statistical sampling plans determine part/feature acceptability at minimum risk and cost. Control software is modified automatically, then down-loaded to the CMM to reduce inspection frequency of consistently good characteristics; rejection of a part feature automatically resets the frequency to 100 percent.

In a final example, an FIS recently installed by an automaker is providing automatic measurement of body part fit up on a variety of models, Figure 2. The system permits randomly selecting cars from the assembly line.

An element of the FIS is an automatic locating fixture (ALF) that receives bodies diverted from the assembly line. The ALF transports, identifies the style, and positions the body for inspection. Positively aligning the body relative to the FIS isn't required since it corrects for misalignment. After inspection, data is sent to a host computer for storage, processing, and reporting. The ALF returns checked bodies to a parking area where they are subsequently reinserted in the assembly line. According to the automaker, the FIS is essential for further progress in body assembly automation.

What's next?

In the near fugure expect improvements in CMMs to come primarily from new lighter, stiffer materials, and by limited changes in machine configuration. Protective enclosures and factory hardening will permit more in-line use. Accuracy improvements will be achieved through real-time computerized error correction.

Also, expect more sophisticated, real-time data handling built into CMM controllers for tasks such as SPC, 3-D contour measurement, full ANSI Y14.5 capability, plus other still to be defined functions.

System performance will be improved by applying larger, faster microprocessors, and inspection part programming will be simplified greatly. Programs will be generated and verified off-line using a product design database. CAD/CAM graphics will be exploited in an approach similar to the way NC part programs are created. Inspection results possibly will be superimposed over the nominal product design. This will form a vital link between product design and inspection.

Further, advanced probing devices (both contact and noncontact) will be developed to significantly improve inspection cycle times, and increase system accuracy and number of parameters checked (including surface texture and hardness). Use of probe changers will be routine.

Flexible inspection systems eventually will be interconnected by a common computerized information network. They will share databases with regard to part programs and inspection results. The information then will be used for dynamic machine-tool capability analysis and process compensation.

For more information about FIS, circle E83.
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Author:Frank, G.L.
Publication:Tooling & Production
Date:May 1, 1985
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